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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