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Locked John Crabtree

 

My apologies guys for posting this to the list, but I'm trying to get
in touch with John Crabtree and my mail to his aol address has bounced.
If this gets to you, John, can you please get in touch with me.

Many thanks

Tracey
Locked Re: Active Antenna Comparisons

 


My active whip used a U-310, not a J-309. Intercepts of my active whip
in the MW band were typically +43 dBm OIP3 and +83 dBm OIP2. They
might be slightly lower at the frequencies Steve used, but unlikely to
be 10 dB lower. Perhaps the J-309 gives lower intercepts than a U-310,
or circuit or measurement mistakes were made. Dallas
That's not at all unlikely. It would be very useful to do a bench
comparison to see how various devices compare. I've gone to great lengths
in the past to select bipolar transistors for linearity by testing a few of
each on a curve tracer to see how they compare, and many years ago wrote an
extensive article in QEX on the linearity of bipolars. I should perhaps due
a similar study of JFETs, especially since the J309/J310 datasheet from
Fairchild only shows one curve family and it's supposedly applicable to both
devices.

But, I'm not entirely fond of FETs, mostly due to the wide range of
cutoff voltages which makes them more difficult to deal with than bipolars
when it comes to biasing and coupling them directly to other devices, and
especially when using discretes as differential pairs.

Although I believe that I understand the mechanism that is the root
cause of the rise in IMD when feedback is used for linearization, I'm not
entirely convinced that a source/emitter follower pair is a good overall
solution for an active antenna as it lacks gain, despite the fact that it
provides a high IP3 and IP2. It does provide a suitable interface between
the antenna and the cable or subsequent amplification, but it does so as the
expense of signal loss.

I'm now looking at using dual-gate MOSFETs as they are inherently far
more linear than JFETs. There are some interesting circuitry configurations
derived from pentode vacuum tubes that can be applied to both source
followers and balanced amplifiers that improve the linearity even further.

Chris

,----------------------. High Performance Mixers and
/ What's all this \ Amplifiers for RF Communications
/ extinct stuff, anyhow? /
\ _______,--------------' Chris Trask / N7ZWY
_ |/ Principal Engineer
oo\ Sonoran Radio Research
(__)\ _ P.O. Box 25240
\ \ .' `. Tempe, Arizona 85285-5240
\ \ / \
\ '" \ IEEE Senior Member #40274515
. ( ) \
'-| )__| :. \ Email: christrask@...
| | | | \ '.
c__; c__; '-..'>.__

Graphics by Loek Frederiks
Locked Re: Trask Active antenna design #2 tested

 


My own circuit is relatively simple--a jfet emitter-follower feeding a
bipolar emitter follower which feeds a final bipolar balanced output
stage.
Properly biassed, such a circuit is capable of very high intercept points.
In fact, I shared my circuit with Dallas Lankford a number of years ago,
and
I notice he has a recent article out with that circuit featured in both a
monopole and dipole version for which he claims very high intercepts.
Trying to get positive gain out of similar circuits is proving to be a
real challenge, and it's interesting to see how the 10pF whip modeling
capacitor makes significant changes in the reverse transmission.

I have found one thing in the modeling that does give positive forward
gain while at the same time decreasing the reverse transmission, which is to
couple the J310 source to the 2N5109 base with a 1:2 transformer. This also
eliminates the need for a voltage adjustment on the J310 gate, and the bias
current for it can now be controlled with just the source resistor. The
reverse transmission is actually reduced while at the same time the forward
gain increases to about 4.0dB. The circuit is at:



This circuit is fairly convenient for my present active antenna
amplifier and tuning arrangement. I send 8.0V to 20V up the coaxial cable
for both bias supply and tuning voltage. At the amplifier there is a 5V
regulator, which can supply drain supply for the JFET and base bias for the
NPN, which eliminates a few parts. The NPN collector then goes to the 8-20V
supply.

Chris

,----------------------. High Performance Mixers and
/ What's all this \ Amplifiers for RF Communications
/ extinct stuff, anyhow? /
\ _______,--------------' Chris Trask / N7ZWY
_ |/ Principal Engineer
oo\ Sonoran Radio Research
(__)\ _ P.O. Box 25240
\ \ .' `. Tempe, Arizona 85285-5240
\ \ / \
\ '" \ IEEE Senior Member #40274515
. ( ) \
'-| )__| :. \ Email: christrask@...
| | | | \ '.
c__; c__; '-..'>.__

Graphics by Loek Frederiks

----- Original Message -----
From: "Steve Ratzlaff" <steveratz@...>
To: <loopantennas@...>
Sent: Sunday, June 10, 2007 8:31 PM
Subject: Re: [loopantennas] Re: Trask Active antenna design #2 tested


Hi Chris,
Thanks for your detailed comments. It's clear you've put some thought into
this problem of a high dynamic range untuned high impedance amplifier, for
use as an active whipamp.

I've been building such an active whip circuit, primarily for LF DXers,
for
about 10 years now, and it's very robust, and it's of course very wideband
for more than just LF reception, and some folks use it for 6 meter
reception, as long as the coax cable from whipamp to coupler and receiver
is
fairly short. In very high urban RF environments it needs to be used with
a
shorter whip, but otherwise copes with nearby 50 kW AMBCB stations just
fine. And actual sensitivity doesn't change very much with a 20" whip vs.
a
56" whip (the usual range of whip lengths that I recommend for mine).
And an active whipamp doesn't need much if any voltage gain; anything from
about -12 to +3 dB gain works fine in actual practice.
And one person, Roelof Bakker, PA0RDT, is building his own active whip
design with extremely short whip lengths, 1-2", and is finding that
satisfactory sensitivity results. He calls his product the "Mini-Whip". A
number of LF and HF DXers are using it.
I
don't have an IMD test set that can match the system intercepts he claims
for his own IMD test set, but my own more modest setup gives very good
intercept results for my circuit, and I'm pleased to hear that he can
measure even higher intercepts than I had achieved--the circuit is even
better than I'd thought.

73,
Steve

----- Original Message -----
From: "Chris Trask" <christrask@...>
To: <loopantennas@...>
Sent: Sunday, June 10, 2007 5:52 PM
Subject: Re: [loopantennas] Re: Trask Active antenna design #2 tested


Yes, I'm beginning to see why there is such a substantial difference
between the two. I've spent the entire efternoon with my PSpice models
trying to locate the source of the IMD problem and correct it.

I took the PSPice models for both circuits and did a reverse
transmission test, discovering that the reverse transmission for my
circuit
is twice that for the Lankford circuit regardless of whether a 0.1uF or
10pF
coupling capacitor is used. Thinking that this would be due to no small
part by the signal voltage on the J309 drain being coupled to the gate,
I
devised a third circuit which adds a 2N2222 common base stage between
the
J309 and the 2N2907 (making it a cascode section), which substantially
reduced the J309 drain signal voltage when the coupling capacitor was
10pF.
This combination now makes the reverse transmission of the first and
third
circuits virtually identical.

My reasoning here was that in the second circuit (my first), there
will
be substantial IMD voltages at the J309 drain that are a result of
correcting for the differences between the J309 gate and source signal
voltages, which will then be coupled to the gate and subsequently
amplified.
The 10pF coupling capacitor makes this signal path more substantial
since
it
unloads the gate and lets more of the IMD signal at the drain be coupled
to
the gate.

I then took all three circuits and replaced the J309 with a 2N2222.
For
the Lankford circuit, this resulted in little performance change for
either
forward or reverse when using the 10pF coupling capacitor but improved
the
performance when using the 0.1uF coupling capacitor. For my two
circuits,
when using the 0.1uF coupling capacitor the forward gain was almost 0dB
and
the reverse isolation for my second circuit was improved.

I can see two things from all this. First, a JFET is the lesser
choice
for the first stage in an amplifier such as this as it results in lower
forward gain. Secondly, the use of a cascode for the first stage
reduces
the reverse transmission and may subsequently reduce the IMD problem by
reducing the IMD voltages at the input transistor drain (or collector)
which
are fed back to the unloaded gate (or base).

This is something that I had not considered in low power amplifier
design as I'm generally dealing with 50-ohm terminations. Now, I can
see
that there are at least two things that need to be considered in the
design
of active short dipoles (or monopoles), which was what I was focused on
when
this began. The first of these is that high antenna impedances, such as
from an untuned short dipole, can cause additional IMD problems in the
amplifier by way of improperly loading reverse IMD products that are
transmitted to the amplifier input. The second is the design of the
amplifier itself, requiring that the source of the IMD products that can
be
conducted to the input need to be reduced.

The low power amplifier design itself is not that much of a problem
as
for now it appears that the IMD source can be controlled by using a
cascode
first stage and the overall reverse isolation can be improved by using
bipolar devices. This will be especially true if it is intended that
the
amplifier have signal gain.

The matter of the high antenna impedance which results in unloaded
reverse IMD products makes the interface between the antenna and the
amplifier difficult. Given the ramifications of high input impedances,
parallel tuning (or no tuning) is out of the question. And the remote
adjustment of variable inductors can be a bit of a task, especially for
wide
bandwidths since transductors (aka saturable reactors) have a limited
practical range of variation.

All of this being the case, I'm going to have to take all the notes
I've
made for remotely tuned active short dipoles and just start all over
again.
Looking at all of this, designing active loops was simple as you are
dealing
with low impedances and inductive reactances.

I'm glad, though, that this all took place because I would have spent
a
lot of time in designing these and would have come up with something
that
was far less than ideal. Less than ideal is a practical goal.

Chris

----------------------------------------------------------------------------
----


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Locked Active Antenna Comparisons

hpmwphaser
 

My active whip used a U-310, not a J-309. Intercepts of my active whip
in the MW band were typically +43 dBm OIP3 and +83 dBm OIP2. They
might be slightly lower at the frequencies Steve used, but unlikely to
be 10 dB lower. Perhaps the J-309 gives lower intercepts than a U-310,
or circuit or measurement mistakes were made. Dallas
Locked Re: Trask Active antenna design #2 tested

Steve Ratzlaff
 

Hi Patrick,
For the optimum intercepts for an active whipamp, one should optimize it for the actual whip length that will be used. But for a circuit such as mine that has such high intercepts to begin with, losing 10-15 dB is probably not going to make a lot of difference in actual usage of the antenna. I've observed in the 10-15 dB difference for one of my circuits before it's optimized for best 2IMD performance. I have little experience for other whipamp circuits. (3IMD performance is primarily a function of the output stage current; more current gives better 3IMD performance, as a general rule.) Most of the antennas I've sold have been to LF DXers, and if they experience IMD due to strong nearby BCB stations, they simply reduce the whip length until the IMD goes away. The most extreme reduction that I'm aware of for one of my own antennas is a reduction to a 22" whip. I recommend a basic 36-39" (approximate one meter) whip to start with. My whipamp has -2 to -3 dB overall voltage gain (monopole version), and is more than sensitive enough. I live in a rural area with no strong stations anywhere around; I've used as much as a 56" whip, but could tell only a very slight improvement in sensitivity over a 36" whip. I know there are commercial whipamp circuits, such as the one by DX Engineering, that recommend use of a 102" whip, but that circuit also has quite a bit less voltage gain, around -12 dB, I believe. Perhaps with such a circuit a much longer whip is worthwhile, but not with my own circuit, in all the tests I've done.
I lived in the very-urban San Francisco Bay area for a number of years (Palo Alto), with a high level of RF from numerous 50 kW BCB stations, and used my own whipamp with no problems from BCB IMD, with a 36" whip, mounted about 10' off the ground.

So, given a choice, I recommend to try to optimize the whipamp bias for the actual whip length to be used.

73,
Steve

----- Original Message -----
From: "Patrick Reynaert" <preynaert@...>
To: <loopantennas@...>
Sent: Sunday, June 10, 2007 5:17 PM
Subject: Re: [loopantennas] Re: Trask Active antenna design #2 tested


Hi Steve,

so basically, there is no difference in the IMD performance with a different antenna capacitance.

This is a good thing, since I observed a dependancy on the typical common-source jFET + common-emitter BJT circuit with the antenna capacitance.

Do you have an idea how the IMD of your circuit changes with antenna capacitance?


Patrick.
Locked Re: Trask Active antenna design #2 tested

Steve Ratzlaff
 

Hi Chris,
Thanks for your detailed comments. It's clear you've put some thought into this problem of a high dynamic range untuned high impedance amplifier, for use as an active whipamp.

I've been building such an active whip circuit, primarily for LF DXers, for about 10 years now, and it's very robust, and it's of course very wideband for more than just LF reception, and some folks use it for 6 meter reception, as long as the coax cable from whipamp to coupler and receiver is fairly short. In very high urban RF environments it needs to be used with a shorter whip, but otherwise copes with nearby 50 kW AMBCB stations just fine. And actual sensitivity doesn't change very much with a 20" whip vs. a 56" whip (the usual range of whip lengths that I recommend for mine).
And an active whipamp doesn't need much if any voltage gain; anything from about -12 to +3 dB gain works fine in actual practice.
And one person, Roelof Bakker, PA0RDT, is building his own active whip design with extremely short whip lengths, 1-2", and is finding that satisfactory sensitivity results. He calls his product the "Mini-Whip". A number of LF and HF DXers are using it.

My own circuit is relatively simple--a jfet emitter-follower feeding a bipolar emitter follower which feeds a final bipolar balanced output stage. Properly biassed, such a circuit is capable of very high intercept points. In fact, I shared my circuit with Dallas Lankford a number of years ago, and I notice he has a recent article out with that circuit featured in both a monopole and dipole version for which he claims very high intercepts. I don't have an IMD test set that can match the system intercepts he claims for his own IMD test set, but my own more modest setup gives very good intercept results for my circuit, and I'm pleased to hear that he can measure even higher intercepts than I had achieved--the circuit is even better than I'd thought.

73,
Steve

----- Original Message -----
From: "Chris Trask" <christrask@...>
To: <loopantennas@...>
Sent: Sunday, June 10, 2007 5:52 PM
Subject: Re: [loopantennas] Re: Trask Active antenna design #2 tested


Yes, I'm beginning to see why there is such a substantial difference
between the two. I've spent the entire efternoon with my PSpice models
trying to locate the source of the IMD problem and correct it.

I took the PSPice models for both circuits and did a reverse
transmission test, discovering that the reverse transmission for my circuit
is twice that for the Lankford circuit regardless of whether a 0.1uF or 10pF
coupling capacitor is used. Thinking that this would be due to no small
part by the signal voltage on the J309 drain being coupled to the gate, I
devised a third circuit which adds a 2N2222 common base stage between the
J309 and the 2N2907 (making it a cascode section), which substantially
reduced the J309 drain signal voltage when the coupling capacitor was 10pF.
This combination now makes the reverse transmission of the first and third
circuits virtually identical.

My reasoning here was that in the second circuit (my first), there will
be substantial IMD voltages at the J309 drain that are a result of
correcting for the differences between the J309 gate and source signal
voltages, which will then be coupled to the gate and subsequently amplified.
The 10pF coupling capacitor makes this signal path more substantial since it
unloads the gate and lets more of the IMD signal at the drain be coupled to
the gate.

I then took all three circuits and replaced the J309 with a 2N2222. For
the Lankford circuit, this resulted in little performance change for either
forward or reverse when using the 10pF coupling capacitor but improved the
performance when using the 0.1uF coupling capacitor. For my two circuits,
when using the 0.1uF coupling capacitor the forward gain was almost 0dB and
the reverse isolation for my second circuit was improved.

I can see two things from all this. First, a JFET is the lesser choice
for the first stage in an amplifier such as this as it results in lower
forward gain. Secondly, the use of a cascode for the first stage reduces
the reverse transmission and may subsequently reduce the IMD problem by
reducing the IMD voltages at the input transistor drain (or collector) which
are fed back to the unloaded gate (or base).

This is something that I had not considered in low power amplifier
design as I'm generally dealing with 50-ohm terminations. Now, I can see
that there are at least two things that need to be considered in the design
of active short dipoles (or monopoles), which was what I was focused on when
this began. The first of these is that high antenna impedances, such as
from an untuned short dipole, can cause additional IMD problems in the
amplifier by way of improperly loading reverse IMD products that are
transmitted to the amplifier input. The second is the design of the
amplifier itself, requiring that the source of the IMD products that can be
conducted to the input need to be reduced.

The low power amplifier design itself is not that much of a problem as
for now it appears that the IMD source can be controlled by using a cascode
first stage and the overall reverse isolation can be improved by using
bipolar devices. This will be especially true if it is intended that the
amplifier have signal gain.

The matter of the high antenna impedance which results in unloaded
reverse IMD products makes the interface between the antenna and the
amplifier difficult. Given the ramifications of high input impedances,
parallel tuning (or no tuning) is out of the question. And the remote
adjustment of variable inductors can be a bit of a task, especially for wide
bandwidths since transductors (aka saturable reactors) have a limited
practical range of variation.

All of this being the case, I'm going to have to take all the notes I've
made for remotely tuned active short dipoles and just start all over again.
Looking at all of this, designing active loops was simple as you are dealing
with low impedances and inductive reactances.

I'm glad, though, that this all took place because I would have spent a
lot of time in designing these and would have come up with something that
was far less than ideal. Less than ideal is a practical goal.

Chris
Locked Re: Trask Active antenna design #2 tested

 


Reconfiguring with a 1 uF monolithic input cap, resetting jfet gate bias
for
minimum 2IMD:
+25 dBm OIP3, +48 dBm OIP2. Vcc 12.87 volts, 26 mA. Drain 4.51 volts,
source
1.80 volts, emitter 5.16 volts (almost the same as before). -10 dBm output
tone levels, at SA, same as before.

Gain/frequency response: 20 kHz -1.6 dB, 50 kHz -0.2 dB, 100 kHz 0 dB, 1
MHz
0 dB, 10 MHz -0.4 dB, 20 MHz -0.3 dB, 30 MHz -0.2 dB, 40 MHz -0.9 dB.

(Of course the circuit is no longer equivalent to an active whipamp
circuit.
I can build a jfet-input active whipamp circuit with greater than +45 dBm
OIP3, +85 dBm OIP2---tested with the same testing configuration using 10
pF
input capacitor simulating a one-meter whip.)
Yes, I'm beginning to see why there is such a substantial difference
between the two. I've spent the entire efternoon with my PSpice models
trying to locate the source of the IMD problem and correct it.

I took the PSPice models for both circuits and did a reverse
transmission test, discovering that the reverse transmission for my circuit
is twice that for the Lankford circuit regardless of whether a 0.1uF or 10pF
coupling capacitor is used. Thinking that this would be due to no small
part by the signal voltage on the J309 drain being coupled to the gate, I
devised a third circuit which adds a 2N2222 common base stage between the
J309 and the 2N2907 (making it a cascode section), which substantially
reduced the J309 drain signal voltage when the coupling capacitor was 10pF.
This combination now makes the reverse transmission of the first and third
circuits virtually identical.

My reasoning here was that in the second circuit (my first), there will
be substantial IMD voltages at the J309 drain that are a result of
correcting for the differences between the J309 gate and source signal
voltages, which will then be coupled to the gate and subsequently amplified.
The 10pF coupling capacitor makes this signal path more substantial since it
unloads the gate and lets more of the IMD signal at the drain be coupled to
the gate.

I then took all three circuits and replaced the J309 with a 2N2222. For
the Lankford circuit, this resulted in little performance change for either
forward or reverse when using the 10pF coupling capacitor but improved the
performance when using the 0.1uF coupling capacitor. For my two circuits,
when using the 0.1uF coupling capacitor the forward gain was almost 0dB and
the reverse isolation for my second circuit was improved.

I can see two things from all this. First, a JFET is the lesser choice
for the first stage in an amplifier such as this as it results in lower
forward gain. Secondly, the use of a cascode for the first stage reduces
the reverse transmission and may subsequently reduce the IMD problem by
reducing the IMD voltages at the input transistor drain (or collector) which
are fed back to the unloaded gate (or base).

This is something that I had not considered in low power amplifier
design as I'm generally dealing with 50-ohm terminations. Now, I can see
that there are at least two things that need to be considered in the design
of active short dipoles (or monopoles), which was what I was focused on when
this began. The first of these is that high antenna impedances, such as
from an untuned short dipole, can cause additional IMD problems in the
amplifier by way of improperly loading reverse IMD products that are
transmitted to the amplifier input. The second is the design of the
amplifier itself, requiring that the source of the IMD products that can be
conducted to the input need to be reduced.

The low power amplifier design itself is not that much of a problem as
for now it appears that the IMD source can be controlled by using a cascode
first stage and the overall reverse isolation can be improved by using
bipolar devices. This will be especially true if it is intended that the
amplifier have signal gain.

The matter of the high antenna impedance which results in unloaded
reverse IMD products makes the interface between the antenna and the
amplifier difficult. Given the ramifications of high input impedances,
parallel tuning (or no tuning) is out of the question. And the remote
adjustment of variable inductors can be a bit of a task, especially for wide
bandwidths since transductors (aka saturable reactors) have a limited
practical range of variation.

All of this being the case, I'm going to have to take all the notes I've
made for remotely tuned active short dipoles and just start all over again.
Looking at all of this, designing active loops was simple as you are dealing
with low impedances and inductive reactances.

I'm glad, though, that this all took place because I would have spent a
lot of time in designing these and would have come up with something that
was far less than ideal. Less than ideal is a practical goal.

Chris

,----------------------. High Performance Mixers and
/ What's all this &#92; Amplifiers for RF Communications
/ extinct stuff, anyhow? /
&#92; _______,--------------' Chris Trask / N7ZWY
_ |/ Principal Engineer
oo&#92; Sonoran Radio Research
(__)&#92; _ P.O. Box 25240
&#92; &#92; .' `. Tempe, Arizona 85285-5240
&#92; &#92; / &#92;
&#92; '" &#92; IEEE Senior Member #40274515
. ( ) &#92;
'-| )__| :. &#92; Email: christrask@...
| | | | &#92; '.
c__; c__; '-..'>.__

Graphics by Loek Frederiks

----- Original Message -----
From: "Steve Ratzlaff" <steveratz@...>
To: <loopantennas@...>
Sent: Sunday, June 10, 2007 4:25 PM
Subject: Re: [loopantennas] Re: Trask Active antenna design #2 tested


Hi Chris,
Reconfiguring with a 1 uF monolithic input cap, resetting jfet gate bias
for
minimum 2IMD:
+25 dBm OIP3, +48 dBm OIP2. Vcc 12.87 volts, 26 mA. Drain 4.51 volts,
source
1.80 volts, emitter 5.16 volts (almost the same as before). -10 dBm output
tone levels, at SA, same as before.

Gain/frequency response: 20 kHz -1.6 dB, 50 kHz -0.2 dB, 100 kHz 0 dB, 1
MHz
0 dB, 10 MHz -0.4 dB, 20 MHz -0.3 dB, 30 MHz -0.2 dB, 40 MHz -0.9 dB.

(Of course the circuit is no longer equivalent to an active whipamp
circuit.
I can build a jfet-input active whipamp circuit with greater than +45 dBm
OIP3, +85 dBm OIP2---tested with the same testing configuration using 10
pF
input capacitor simulating a one-meter whip.)

73,
Steve

----- Original Message -----
From: "Chris Trask" <christrask@...>
To: <loopantennas@...>
Sent: Sunday, June 10, 2007 3:14 PM
Subject: Re: [loopantennas] Re: Trask Active antenna design #2 tested


Prior to breadboarding this, I went back and forth with the PSpice
models to determine where the discrepancies. I went so far as to remove
the
2N2907 emitter bypass capacitor, but that resulted in a loss of -6dB.
So,
I
rearranged the input to place the 50-ohm termination ahead of the
blocking
capacitor, changed the blocking capacitor to 10pF, then added a 100K
resistor from the JFET gate to ground, and then I saw the -3dB of
midband
loss.

So now I see why my model gain and your measured gain are so far
apart
this last time around. I was measuring the gain grom the JFET gate to
the
output, and you're measuring it from the 50-ohm load AHEAD of the 10pF
capacitor to the output.

That being the case, I now have to wonder what effect this 10pF
coupling
capacitor has on the IMD performance. So, could you take both of these
circuits, replace the 10pF capacitor with 0.1uF, and then measure again?
My
suspicion here is that the JFET drain voltage in my circuit is feeding
back
to the gate and combined with your 10pF coupling capacitor is causing
the
IMD performance to degrade.


Chris

,----------------------. High Performance Mixers and
/ What's all this &#92; Amplifiers for RF Communications
/ extinct stuff, anyhow? /
&#92; _______,--------------' Chris Trask / N7ZWY
_ |/ Principal Engineer
oo&#92; Sonoran Radio Research
(__)&#92; _ P.O. Box 25240
&#92; &#92; .' `. Tempe, Arizona 85285-5240
&#92; &#92; / &#92;
&#92; '" &#92; IEEE Senior Member #40274515
. ( ) &#92;
'-| )__| :. &#92; Email: christrask@...
| | | | &#92; '.
c__; c__; '-..'>.__

Graphics by Loek Frederiks

----- Original Message -----
From: "Steve Ratzlaff" <steveratz@...>
To: <loopantennas@...>
Sent: Sunday, June 10, 2007 9:17 AM
Subject: [loopantennas] Re: Trask Active antenna design #2 tested


Hi Chris,
Your revised circuit is just built and tested.
Same testing configuration as before (10 pF series input cap with 51
ohm
termination at input, to generator; 1 uF output cap; jfet gate bias
adjusted
for best 2IMD. -10 dBm each tone at the output to the spectrum
analyzer.)
+21.5 dBm OIP3, +45 dBm OIP2.
Vcc 12.82 volts, 26 mA.
Drain 4.58 volts, 12 mA approx jfet current
Emitter 5.21 volts, 13.6 mA bipolar current
Source 1.80 volts

Gain/Frequency response
20 kHz -8.5 dB, 50 kHz -4.1 dB, 100 kHz -2.9 dB, 1 MHz -2.5 dB, 10
MHz -3.0
dB, 20 MHz -3.2 dB, 30 MHz -3.7 dB, 40 MHz -4.9 dB

73,
Steve


----- Original Message -----
From: "Chris Trask" <christrask@...>
To: <loopantennas@...>
Sent: Saturday, June 09, 2007 12:24 PM
Subject: Re: [loopantennas] Re: Active antenna design


Okay, I see what I did here that made a mess of things. I had
hurredly
adapted a two bipolar transistor circuit used for low power
applications
by
substituting a JFET for the NPN device and then changing the bias
resistors
without properly thinking about it. With the JFET, it turns out that
the
emitter of the PNP needs to be degenerated so as to accomodate the
wide
range of Vgsoff of the JFET. Doing so allows me to increase the JFET
drain
resistor so as to improve the open loop gain. The gain is now
around -0.5dB. And you can now vary the bias current from 5mA each
leg
to
12mA by applying a voltage to the JFET gate instead of changing out
the
resistors as before:



Chris
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Locked Re: Trask Active antenna design #2 tested

Patrick Reynaert
 

Hi Steve,

so basically, there is no difference in the IMD performance with a different antenna capacitance.

This is a good thing, since I observed a dependancy on the typical common-source jFET + common-emitter BJT circuit with the antenna capacitance.

Do you have an idea how the IMD of your circuit changes with antenna capacitance?


Patrick.

Steve Ratzlaff <steveratz@...> wrote: Hi Chris,
Reconfiguring with a 1 uF monolithic input cap, resetting jfet gate bias for
minimum 2IMD:
+25 dBm OIP3, +48 dBm OIP2. Vcc 12.87 volts, 26 mA. Drain 4.51 volts, source
1.80 volts, emitter 5.16 volts (almost the same as before). -10 dBm output
tone levels, at SA, same as before.

Gain/frequency response: 20 kHz -1.6 dB, 50 kHz -0.2 dB, 100 kHz 0 dB, 1 MHz
0 dB, 10 MHz -0.4 dB, 20 MHz -0.3 dB, 30 MHz -0.2 dB, 40 MHz -0.9 dB.

(Of course the circuit is no longer equivalent to an active whipamp circuit.
I can build a jfet-input active whipamp circuit with greater than +45 dBm
OIP3, +85 dBm OIP2---tested with the same testing configuration using 10 pF
input capacitor simulating a one-meter whip.)

73,
Steve

----- Original Message -----
From: "Chris Trask"
To:
Sent: Sunday, June 10, 2007 3:14 PM
Subject: Re: [loopantennas] Re: Trask Active antenna design #2 tested


Prior to breadboarding this, I went back and forth with the PSpice
models to determine where the discrepancies. I went so far as to remove
the
2N2907 emitter bypass capacitor, but that resulted in a loss of -6dB. So,
I
rearranged the input to place the 50-ohm termination ahead of the blocking
capacitor, changed the blocking capacitor to 10pF, then added a 100K
resistor from the JFET gate to ground, and then I saw the -3dB of midband
loss.

So now I see why my model gain and your measured gain are so far apart
this last time around. I was measuring the gain grom the JFET gate to the
output, and you're measuring it from the 50-ohm load AHEAD of the 10pF
capacitor to the output.

That being the case, I now have to wonder what effect this 10pF
coupling
capacitor has on the IMD performance. So, could you take both of these
circuits, replace the 10pF capacitor with 0.1uF, and then measure again?
My
suspicion here is that the JFET drain voltage in my circuit is feeding
back
to the gate and combined with your 10pF coupling capacitor is causing the
IMD performance to degrade.


Chris

,----------------------. High Performance Mixers and
/ What's all this &#92; Amplifiers for RF Communications
/ extinct stuff, anyhow? /
&#92; _______,--------------' Chris Trask / N7ZWY
_ |/ Principal Engineer
oo&#92; Sonoran Radio Research
(__)&#92; _ P.O. Box 25240
&#92; &#92; .' `. Tempe, Arizona 85285-5240
&#92; &#92; / &#92;
&#92; '" &#92; IEEE Senior Member #40274515
. ( ) &#92;
'-| )__| :. &#92; Email: christrask@...
| | | | &#92; '.
c__; c__; '-..'>.__

Graphics by Loek Frederiks

----- Original Message -----
From: "Steve Ratzlaff"
To:
Sent: Sunday, June 10, 2007 9:17 AM
Subject: [loopantennas] Re: Trask Active antenna design #2 tested


Hi Chris,
Your revised circuit is just built and tested.
Same testing configuration as before (10 pF series input cap with 51 ohm
termination at input, to generator; 1 uF output cap; jfet gate bias
adjusted
for best 2IMD. -10 dBm each tone at the output to the spectrum analyzer.)
+21.5 dBm OIP3, +45 dBm OIP2.
Vcc 12.82 volts, 26 mA.
Drain 4.58 volts, 12 mA approx jfet current
Emitter 5.21 volts, 13.6 mA bipolar current
Source 1.80 volts

Gain/Frequency response
20 kHz -8.5 dB, 50 kHz -4.1 dB, 100 kHz -2.9 dB, 1 MHz -2.5 dB, 10
MHz -3.0
dB, 20 MHz -3.2 dB, 30 MHz -3.7 dB, 40 MHz -4.9 dB

73,
Steve


----- Original Message -----
From: "Chris Trask"
To:
Sent: Saturday, June 09, 2007 12:24 PM
Subject: Re: [loopantennas] Re: Active antenna design


Okay, I see what I did here that made a mess of things. I had
hurredly
adapted a two bipolar transistor circuit used for low power
applications
by
substituting a JFET for the NPN device and then changing the bias
resistors
without properly thinking about it. With the JFET, it turns out that
the
emitter of the PNP needs to be degenerated so as to accomodate the wide
range of Vgsoff of the JFET. Doing so allows me to increase the JFET
drain
resistor so as to improve the open loop gain. The gain is now
around -0.5dB. And you can now vary the bias current from 5mA each leg
to
12mA by applying a voltage to the JFET gate instead of changing out the
resistors as before:



Chris

----------------------------------------------------------------------------
----


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Checked by AVG Free Edition.
Version: 7.5.472 / Virus Database: 269.8.13/843 - Release Date: 6/10/07
1:39
PM



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---------------------------------
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Locked Re: Trask Active antenna design #2 tested (addendum)

Steve Ratzlaff
 

Just for completeness, I should mention that I use 3 and 4 MHz tones for all my IMD testing, and for these circuits, set for -10 dBm each tone at the output, as seen on the spectrum analyzer.
Steve

----- Original Message -----
From: "Chris Trask" <christrask@...>
To: <loopantennas@...>
Sent: Sunday, June 10, 2007 3:14 PM
Subject: Re: [loopantennas] Re: Trask Active antenna design #2 tested


Prior to breadboarding this, I went back and forth with the PSpice
models to determine where the discrepancies. I went so far as to remove the
2N2907 emitter bypass capacitor, but that resulted in a loss of -6dB. So, I
rearranged the input to place the 50-ohm termination ahead of the blocking
capacitor, changed the blocking capacitor to 10pF, then added a 100K
resistor from the JFET gate to ground, and then I saw the -3dB of midband
loss.

So now I see why my model gain and your measured gain are so far apart
this last time around. I was measuring the gain grom the JFET gate to the
output, and you're measuring it from the 50-ohm load AHEAD of the 10pF
capacitor to the output.

That being the case, I now have to wonder what effect this 10pF coupling
capacitor has on the IMD performance. So, could you take both of these
circuits, replace the 10pF capacitor with 0.1uF, and then measure again? My
suspicion here is that the JFET drain voltage in my circuit is feeding back
to the gate and combined with your 10pF coupling capacitor is causing the
IMD performance to degrade.


Chris
Locked Re: Trask Active antenna design #2 tested

Steve Ratzlaff
 

Hi Chris,
Reconfiguring with a 1 uF monolithic input cap, resetting jfet gate bias for minimum 2IMD:
+25 dBm OIP3, +48 dBm OIP2. Vcc 12.87 volts, 26 mA. Drain 4.51 volts, source 1.80 volts, emitter 5.16 volts (almost the same as before). -10 dBm output tone levels, at SA, same as before.

Gain/frequency response: 20 kHz -1.6 dB, 50 kHz -0.2 dB, 100 kHz 0 dB, 1 MHz 0 dB, 10 MHz -0.4 dB, 20 MHz -0.3 dB, 30 MHz -0.2 dB, 40 MHz -0.9 dB.

(Of course the circuit is no longer equivalent to an active whipamp circuit. I can build a jfet-input active whipamp circuit with greater than +45 dBm OIP3, +85 dBm OIP2---tested with the same testing configuration using 10 pF input capacitor simulating a one-meter whip.)

73,
Steve

----- Original Message -----
From: "Chris Trask" <christrask@...>
To: <loopantennas@...>
Sent: Sunday, June 10, 2007 3:14 PM
Subject: Re: [loopantennas] Re: Trask Active antenna design #2 tested


Prior to breadboarding this, I went back and forth with the PSpice
models to determine where the discrepancies. I went so far as to remove the
2N2907 emitter bypass capacitor, but that resulted in a loss of -6dB. So, I
rearranged the input to place the 50-ohm termination ahead of the blocking
capacitor, changed the blocking capacitor to 10pF, then added a 100K
resistor from the JFET gate to ground, and then I saw the -3dB of midband
loss.

So now I see why my model gain and your measured gain are so far apart
this last time around. I was measuring the gain grom the JFET gate to the
output, and you're measuring it from the 50-ohm load AHEAD of the 10pF
capacitor to the output.

That being the case, I now have to wonder what effect this 10pF coupling
capacitor has on the IMD performance. So, could you take both of these
circuits, replace the 10pF capacitor with 0.1uF, and then measure again? My
suspicion here is that the JFET drain voltage in my circuit is feeding back
to the gate and combined with your 10pF coupling capacitor is causing the
IMD performance to degrade.


Chris

,----------------------. High Performance Mixers and
/ What's all this &#92; Amplifiers for RF Communications
/ extinct stuff, anyhow? /
&#92; _______,--------------' Chris Trask / N7ZWY
_ |/ Principal Engineer
oo&#92; Sonoran Radio Research
(__)&#92; _ P.O. Box 25240
&#92; &#92; .' `. Tempe, Arizona 85285-5240
&#92; &#92; / &#92;
&#92; '" &#92; IEEE Senior Member #40274515
. ( ) &#92;
'-| )__| :. &#92; Email: christrask@...
| | | | &#92; '.
c__; c__; '-..'>.__

Graphics by Loek Frederiks

----- Original Message -----
From: "Steve Ratzlaff" <steveratz@...>
To: <loopantennas@...>
Sent: Sunday, June 10, 2007 9:17 AM
Subject: [loopantennas] Re: Trask Active antenna design #2 tested


Hi Chris,
Your revised circuit is just built and tested.
Same testing configuration as before (10 pF series input cap with 51 ohm
termination at input, to generator; 1 uF output cap; jfet gate bias
adjusted
for best 2IMD. -10 dBm each tone at the output to the spectrum analyzer.)
+21.5 dBm OIP3, +45 dBm OIP2.
Vcc 12.82 volts, 26 mA.
Drain 4.58 volts, 12 mA approx jfet current
Emitter 5.21 volts, 13.6 mA bipolar current
Source 1.80 volts

Gain/Frequency response
20 kHz -8.5 dB, 50 kHz -4.1 dB, 100 kHz -2.9 dB, 1 MHz -2.5 dB, 10
MHz -3.0
dB, 20 MHz -3.2 dB, 30 MHz -3.7 dB, 40 MHz -4.9 dB

73,
Steve


----- Original Message -----
From: "Chris Trask" <christrask@...>
To: <loopantennas@...>
Sent: Saturday, June 09, 2007 12:24 PM
Subject: Re: [loopantennas] Re: Active antenna design


Okay, I see what I did here that made a mess of things. I had
hurredly
adapted a two bipolar transistor circuit used for low power applications
by
substituting a JFET for the NPN device and then changing the bias
resistors
without properly thinking about it. With the JFET, it turns out that
the
emitter of the PNP needs to be degenerated so as to accomodate the wide
range of Vgsoff of the JFET. Doing so allows me to increase the JFET
drain
resistor so as to improve the open loop gain. The gain is now
around -0.5dB. And you can now vary the bias current from 5mA each leg
to
12mA by applying a voltage to the JFET gate instead of changing out the
resistors as before:



Chris

----------------------------------------------------------------------------
----


No virus found in this incoming message.
Checked by AVG Free Edition.
Version: 7.5.472 / Virus Database: 269.8.13/843 - Release Date: 6/10/07 1:39
PM



If you've got links, post them in the Links section!


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Yahoo! Groups Links
Locked Re: Trask Active antenna design #2 tested

 

Prior to breadboarding this, I went back and forth with the PSpice
models to determine where the discrepancies. I went so far as to remove the
2N2907 emitter bypass capacitor, but that resulted in a loss of -6dB. So, I
rearranged the input to place the 50-ohm termination ahead of the blocking
capacitor, changed the blocking capacitor to 10pF, then added a 100K
resistor from the JFET gate to ground, and then I saw the -3dB of midband
loss.

So now I see why my model gain and your measured gain are so far apart
this last time around. I was measuring the gain grom the JFET gate to the
output, and you're measuring it from the 50-ohm load AHEAD of the 10pF
capacitor to the output.

That being the case, I now have to wonder what effect this 10pF coupling
capacitor has on the IMD performance. So, could you take both of these
circuits, replace the 10pF capacitor with 0.1uF, and then measure again? My
suspicion here is that the JFET drain voltage in my circuit is feeding back
to the gate and combined with your 10pF coupling capacitor is causing the
IMD performance to degrade.


Chris

,----------------------. High Performance Mixers and
/ What's all this &#92; Amplifiers for RF Communications
/ extinct stuff, anyhow? /
&#92; _______,--------------' Chris Trask / N7ZWY
_ |/ Principal Engineer
oo&#92; Sonoran Radio Research
(__)&#92; _ P.O. Box 25240
&#92; &#92; .' `. Tempe, Arizona 85285-5240
&#92; &#92; / &#92;
&#92; '" &#92; IEEE Senior Member #40274515
. ( ) &#92;
'-| )__| :. &#92; Email: christrask@...
| | | | &#92; '.
c__; c__; '-..'>.__

Graphics by Loek Frederiks

----- Original Message -----
From: "Steve Ratzlaff" <steveratz@...>
To: <loopantennas@...>
Sent: Sunday, June 10, 2007 9:17 AM
Subject: [loopantennas] Re: Trask Active antenna design #2 tested


Hi Chris,
Your revised circuit is just built and tested.
Same testing configuration as before (10 pF series input cap with 51 ohm
termination at input, to generator; 1 uF output cap; jfet gate bias
adjusted
for best 2IMD. -10 dBm each tone at the output to the spectrum analyzer.)
+21.5 dBm OIP3, +45 dBm OIP2.
Vcc 12.82 volts, 26 mA.
Drain 4.58 volts, 12 mA approx jfet current
Emitter 5.21 volts, 13.6 mA bipolar current
Source 1.80 volts

Gain/Frequency response
20 kHz -8.5 dB, 50 kHz -4.1 dB, 100 kHz -2.9 dB, 1 MHz -2.5 dB, 10
MHz -3.0
dB, 20 MHz -3.2 dB, 30 MHz -3.7 dB, 40 MHz -4.9 dB

73,
Steve


----- Original Message -----
From: "Chris Trask" <christrask@...>
To: <loopantennas@...>
Sent: Saturday, June 09, 2007 12:24 PM
Subject: Re: [loopantennas] Re: Active antenna design


Okay, I see what I did here that made a mess of things. I had
hurredly
adapted a two bipolar transistor circuit used for low power applications
by
substituting a JFET for the NPN device and then changing the bias
resistors
without properly thinking about it. With the JFET, it turns out that
the
emitter of the PNP needs to be degenerated so as to accomodate the wide
range of Vgsoff of the JFET. Doing so allows me to increase the JFET
drain
resistor so as to improve the open loop gain. The gain is now
around -0.5dB. And you can now vary the bias current from 5mA each leg
to
12mA by applying a voltage to the JFET gate instead of changing out the
resistors as before:



Chris

----------------------------------------------------------------------------
----


No virus found in this incoming message.
Checked by AVG Free Edition.
Version: 7.5.472 / Virus Database: 269.8.13/843 - Release Date: 6/10/07 1:39
PM
Locked Re: Trask Active antenna design #2 tested

 

I'm going to build and test this later. The gain should have improved to about -0.5dB.

-----Original Message-----
From: Steve Ratzlaff <steveratz@...>
Sent: Jun 10, 2007 9:17 AM
To: loopantennas@...
Subject: [loopantennas] Re: Trask Active antenna design #2 tested

Hi Chris,
Your revised circuit is just built and tested.
Same testing configuration as before (10 pF series input cap with 51 ohm
termination at input, to generator; 1 uF output cap; jfet gate bias adjusted
for best 2IMD. -10 dBm each tone at the output to the spectrum analyzer.)
+21.5 dBm OIP3, +45 dBm OIP2.
Vcc 12.82 volts, 26 mA.
Drain 4.58 volts, 12 mA approx jfet current
Emitter 5.21 volts, 13.6 mA bipolar current
Source 1.80 volts

Gain/Frequency response
20 kHz -8.5 dB, 50 kHz -4.1 dB, 100 kHz -2.9 dB, 1 MHz -2.5 dB, 10 MHz -3.0
dB, 20 MHz -3.2 dB, 30 MHz -3.7 dB, 40 MHz -4.9 dB

73,
Steve


----- Original Message -----
From: "Chris Trask" <christrask@...>
To: <loopantennas@...>
Sent: Saturday, June 09, 2007 12:24 PM
Subject: Re: [loopantennas] Re: Active antenna design


Okay, I see what I did here that made a mess of things. I had hurredly
adapted a two bipolar transistor circuit used for low power applications
by
substituting a JFET for the NPN device and then changing the bias
resistors
without properly thinking about it. With the JFET, it turns out that the
emitter of the PNP needs to be degenerated so as to accomodate the wide
range of Vgsoff of the JFET. Doing so allows me to increase the JFET
drain
resistor so as to improve the open loop gain. The gain is now
around -0.5dB. And you can now vary the bias current from 5mA each leg to
12mA by applying a voltage to the JFET gate instead of changing out the
resistors as before:



Chris

Chris

,----------------------. High Performance Mixers and
/ What's all this &#92; Amplifiers for RF Communications
/ extinct stuff, anyhow? /
&#92; _______,--------------' Chris Trask / N7ZWY
_ |/ Principal Engineer
oo&#92; Sonoran Radio Research
(__)&#92; _ P.O. Box 25240
&#92; &#92; .' `. Tempe, Arizona 85285-5240
&#92; &#92; / &#92;
&#92; '" &#92; IEEE Senior Member #40274515
. ( ) &#92;
'-| )__| :. &#92; Email: christrask@...
| | | | &#92; '.
c__; c__; '-..'>.__

Graphics by Loek Frederiks
Locked Re: Trask Active antenna design #2 tested

Steve Ratzlaff
 

Hi Chris,
Your revised circuit is just built and tested.
Same testing configuration as before (10 pF series input cap with 51 ohm termination at input, to generator; 1 uF output cap; jfet gate bias adjusted for best 2IMD. -10 dBm each tone at the output to the spectrum analyzer.)
+21.5 dBm OIP3, +45 dBm OIP2.
Vcc 12.82 volts, 26 mA.
Drain 4.58 volts, 12 mA approx jfet current
Emitter 5.21 volts, 13.6 mA bipolar current
Source 1.80 volts

Gain/Frequency response
20 kHz -8.5 dB, 50 kHz -4.1 dB, 100 kHz -2.9 dB, 1 MHz -2.5 dB, 10 MHz -3.0 dB, 20 MHz -3.2 dB, 30 MHz -3.7 dB, 40 MHz -4.9 dB

73,
Steve

----- Original Message -----
From: "Chris Trask" <christrask@...>
To: <loopantennas@...>
Sent: Saturday, June 09, 2007 12:24 PM
Subject: Re: [loopantennas] Re: Active antenna design


Okay, I see what I did here that made a mess of things. I had hurredly
adapted a two bipolar transistor circuit used for low power applications by
substituting a JFET for the NPN device and then changing the bias resistors
without properly thinking about it. With the JFET, it turns out that the
emitter of the PNP needs to be degenerated so as to accomodate the wide
range of Vgsoff of the JFET. Doing so allows me to increase the JFET drain
resistor so as to improve the open loop gain. The gain is now
around -0.5dB. And you can now vary the bias current from 5mA each leg to
12mA by applying a voltage to the JFET gate instead of changing out the
resistors as before:



Chris
Locked Re: Active antenna design

 


Hi Chris,
OK, I'll be around, ready to try a new configuration/bias, etc. Hope you
get
it figured out eventually.
Have a good trip.
73,
Steve
Okay, I see what I did here that made a mess of things. I had hurredly
adapted a two bipolar transistor circuit used for low power applications by
substituting a JFET for the NPN device and then changing the bias resistors
without properly thinking about it. With the JFET, it turns out that the
emitter of the PNP needs to be degenerated so as to accomodate the wide
range of Vgsoff of the JFET. Doing so allows me to increase the JFET drain
resistor so as to improve the open loop gain. The gain is now
around -0.5dB. And you can now vary the bias current from 5mA each leg to
12mA by applying a voltage to the JFET gate instead of changing out the
resistors as before:



Chris

,----------------------. High Performance Mixers and
/ What's all this &#92; Amplifiers for RF Communications
/ extinct stuff, anyhow? /
&#92; _______,--------------' Chris Trask / N7ZWY
_ |/ Principal Engineer
oo&#92; Sonoran Radio Research
(__)&#92; _ P.O. Box 25240
&#92; &#92; .' `. Tempe, Arizona 85285-5240
&#92; &#92; / &#92;
&#92; '" &#92; IEEE Senior Member #40274515
. ( ) &#92;
'-| )__| :. &#92; Email: christrask@...
| | | | &#92; '.
c__; c__; '-..'>.__

Graphics by Loek Frederiks
Locked Re: half size G5RV

N4CQR
 

The 1/2 size G5RV-style antenna is a simply the full
size antenna with the dimensions halved. two 25.5 foot
wires connected to 15.5 feet of twin-lead and feed
with a length (probably 35-foot) of coax.

It ain't rocket science.

Take care



Craig
SWL WA4128SWL
ARS N4CQR
Shortwave Radio
Hustler Antenna Products
AEA / Timewave Products




____________________________________________________________________________________
Looking for a deal? Find great prices on flights and hotels with Yahoo! FareChase.
Locked Re: half size G5RV

[email protected]
 

In a message dated 09/06/2007 04:13:38 GMT Daylight Time, frtglz@...
writes:

I would like to request a complete design
of a half size G5RV that covers 7-28mhz (not sure)
My space between supports is 55feet. I will also
try it as a loop antenna.
------------------------
PS

Please do share your findings as to how well it functions as a loop, that
should make interesting reading !!
Locked Re: half size G5RV

[email protected]
 

In a message dated 09/06/2007 04:13:38 GMT Daylight Time, frtglz@...
writes:

I would like to request a complete design
of a half size G5RV that covers 7-28mhz (not sure)
My space between supports is 55feet. I will also
try it as a loop antenna.



------------------------------------------------------

This is where you'll find all the info you need....
_
()

This was found after a 10 second google search and it probably took longer
posting your request here than it would have done looking for yourself !!!

Is that crazy or what?
Locked half size G5RV

 

I would like to request a complete design
of a half size G5RV that covers 7-28mhz (not sure)
My space between supports is 55feet. I will also
try it as a loop antenna.
Locked Re: Active antenna design

Steve Ratzlaff
 

Hi Chris,
OK, I'll be around, ready to try a new configuration/bias, etc. Hope you get it figured out eventually.
Have a good trip.
73,
Steve

----- Original Message -----
From: "Chris Trask" <christrask@...>
To: <loopantennas@...>
Sent: Tuesday, June 05, 2007 8:04 PM
Subject: Re: [loopantennas] Re: Active antenna design
Something here just isn't right. Regardless of whether you are
applying
a bias voltage to the JFET gate or changing the resistors, the JFET drain
current remains virtually unchanged.

The values were determined by way of PSpice. I'll test this myself,
but
I have to be out of town all day tomorrow so I won't have an opportunity
to
look at it until Thursday.

Chris
Locked Re: Active antenna design

Steve Ratzlaff
 

Hi Rob,
The gate voltage/pot bias method is a nice way to vary the bias; I've used that method for many years on the various active whips I've built, both for hobby and at the company before I retired. I used to do a lot of IMD tests at work; now I do it for fun. :)
I haven't tried either circuit in a balanced/differential arrangement though.
73,
Steve

----- Original Message -----
From: "Rob Moore" <rmoore5@...>
To: <loopantennas@...>
Sent: Tuesday, June 05, 2007 9:10 PM
Subject: [loopantennas] Re: Active antenna design


Thanks Steve,

I didn't have the article open at the time and had forgotten about
the pot that Dallas had. It amounts to sort of the same thing -
varying the DC bias on the gate - but keeps the impedance relatively
constant.

I'm aware of IP2 and IP3 although I've never measured them. One of
these days I'll set up the proper equipment to do that. Have you
ever tried either of these circuits in a balanced/differential
configuration?

Rob
17441 - 17460 of 21030