Re: slightly off topic feel free to delete
IIRC some early German RF cables and test equipment impedance was nominally 60 Ohms for that reason.? The book "Transmision-Line Theory" by Prof. RWP King analyses this and many other transmission line minutiae in depth.
PeterS??? ??? G8EZE
-----Original Message-----
Jeff Green <Jeff.L.Green1970@...>? Tue, 28 Mar 2023 2:17 wrote:
"I doubted the claimed loss due to mismatch, until I worked out the Telegraphers Equation for myself."
|
Re: Philips PM3070 Operator Manual
I just emailed you the service manual so check your email. I also have the manuals for the PM3065 and 3067.
Sam
W3OHM
|
Re: Philips PM3070 Operator Manual
I have used a PM3050 quite a while ago, there may be a switch on the rear panel for the channel, but mor likely there is something in the soft menu about configuration, or channel select. Do check online for this model no., operator¡¯s guide.
Don Bitters
|
Re: Philips PM3070 Operator Manual
So I don't know if any one as this issue, I bought a used PM3070 scope and the 2chs are summed together and I cannot figure out how to clear that to make them independent.? I went threw the menus and looks like maybe it is something other , thanks for the helps so far and any other ideas out there
|
Re: slightly off topic feel free to delete
Great discussion, very informative.
I just want to point out that a good portion of the article describes the counterpoise and ground part of the system.
In my opinion, this is the main difference between this design and the mini-whip, which uses whatever cable is feeding it as the ground/counterpoise.
The amplifier and the feedline are also important, but the biggest impact on repeatability of the antenna system is the ground environment around it.
73, Mike AF7KR
|
Re: slightly off topic feel free to delete
Thanks for the clue, Alan. I dug up the links to the article so I
could see the schematic.
looks pretty good; I believe that is what the missing link in your
email was intended to cite and he has links to other of his work
there.
73,
Donald.
On 2023-03-24 08:01, Alan wrote:
The original design has a very unusual source, an Australian
Government Sigintel agency. This is a very unusual source of
information and may be subject to? unusual conditions that have
been not picked up on so far.
[snip]
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Re: slightly off topic feel free to delete
The original design has a very unusual source, an Australian
Government Sigintel agency. This is a very unusual source of
information and may be subject to? unusual conditions that have
been not picked up on so far.
- Active whips are very useful in multi element broadband
direction finding and directional intercept work- greatly
preferable to passive antennas
- Such arrays need very stable active elements that are
reproducible in numbers, you might need hundreds of identical
active antennas at one site.
- Very good coax matching is needed with the other elements such
as hybrid couplers in beamforming networks
- The active devices are from the 1970's being then close to mil
spec hermetic metal cans but today hard to find/discontinued.
Readily available cheaper alternatives around.
- The circuit design is typical of other active antennas in the
1960-1980 period.
- The design needs to cope with very large temp ranges, a
plastic box in a hot dessert can go from -10C to +70+C when
internal dissipation is added.
- The paper - although released in 2018- perhaps talks about a
design produced 50-60 years earlier, now published as a long
obsolete design.
- Many improved designs exist, in the last decade many ultra
fast Opamps have replaced discrete part designs but they need
good plated through PCB's and careful decoupling
There are some design details worth comment. The biasing of the
U310 is sub optimal, JFETs have very broad Vgs spreads- at least
2:1 so the iD is not well controlled, better results follow from
positive gate bias and a higher source resistor. That could remove
the need for RFC's. But if you want RFC's wind them differently to
avoid adding identical resonances.
I agree with Robin, the biassing of the output stage is weird,
the source resistance of the base drive is around 15K? which is
way too high. If the output bias is 20mA then that produces a base
current of 0.2 to 0.1 mA for HFE 100-200, that? results in far
more sensitivity to device HFE than is needed. If you lower the
27K 68K 27K ladder by 1/10 and insert an emitter follower between
the FET and the output pair then you end up with a four device
circuit similar to Steve's? and to that produced by Jack Smith of
Clifton Labs.? That's good enough for nearly all hobby users.
The elegant design award must go to PA3FWM for a two device
active antenna which replaced the PA0RDT active antenna at the
Twente SDR in 2017. His article is?
The two links to comments about how the device works are very
well written, some very misleading rubbish about grounding of
E-field antennas abounds, any active E-field antenna has to have a
local ground and decoupling chokes if the other end of the coax is
grounded in any way. The gotcha is that the wonderful BF862 was
stopped ( along with the bfq149/19 ) a few years back.
73,
Alan G8LCO
Simple and better circuit for MiniWhip antennas
Pieter-Tjerk de Boer, PA3FWM pa3fwm@...
On this page I propose a simple circuit for use in MiniWhip and similar e-field probe antennas.
Since December 2017 a version of this circuit is in use at the
receiver.
The circuit
The original PA0RDT MiniWhip is a very simple and effective active receive antenna for the longwave, medium wave and
shortwave range. It consists of a small metal plate and a buffer amplifier.
The amplifier has a very high input impedance (implemented using a JFET as a source follower)
to sense the voltage between the metal plate and ground, and a low output impedance
(implemented using an NPN transistor as an emitter follower) to drive the
50 ohm coax cable.
I've written more about how it works here and here.
PA0RDT's circuit can be easily found on the internet, as can various more complicated circuits
promising better intermodulation performance.
![[schematic]](/g/Test-Equipment-Design-Construction/messages/miniwhip-pa3fwm.png)
On this page, I introduce an alternative circuit for this application which is
simpler, yet has good (probably better) intermodulation performance.
See the diagram for the circuit.
The new circuit has only the JFET source follower between input and output.
Compared to the usual circuit, this avoids the emitter follower's contribution to the intermodulation.
The output impedance of the JFET by itself is not low enough to drive a 50 ohm load
(without lots of distortion).
In the new circuit, this is solved by the PNP transistor: when the FET needs to supply
some current into the load, this current also flows through the base of the PNP transistor,
causing a proportionally larger (e.g., 50 times larger, if the transistor's hFE is 50) current
to be supplied to the load. Effectively, the FET sees a 50 times higher load impedance.
For the FET, I prefer the BF862 because of its exceptionally low input noise.
Unfortunately, it is only available in SMD.
The non-SMD J310 (as used in most active e-field antenna designs) probably will work just as well in most situations,
but I haven't tried it.
[Update January 2018: I've tried a J310 in otherwise the same circuit, and found it gives somewhat more intermodulation: IP2 +60 dBm, IP3 +37 dBm.]
There aren't many PNP transistors which can both handle some power and work at high frequencies.
One suitable transistor is the BFQ149, with an fT of 5 GHz, but unfortunately only available in SMD.
An alternative is the non-SMD 2N5583, with an fT of 1.3 GHz.
Those fT values may look ridiculously high for shortwave use, but at the fT the transistor's
current gain has reduced to 1; the circuit stops working well at a much lower frequency.
Possibly a plain 2N2905 or 2N2907 (fT=200 MHz) can also work acceptably well in some cases,
but I haven't tried.
Measurements on various realizations of this circuit, using a BF862 for the FET and either a BFQ149 or 2N5583 for the
PNP transistor, gave OIP3 values between +40 and +43 dBm, and OIP2 around +70 dBm (tested with signals of 7 and 8 MHz).
I did not invent this JFET/PNP principle myself.
I've seen such circuits in a few active antenna designs, but never
with specifications of achieved intermodulation performance, and usually with a DC bias that seemed
far from optimal for low intermodulation.
The new WebSDR antenna
![[miniwhip as built for the WebSDR]](/g/Test-Equipment-Design-Construction/messages/miniwhip-pa3fwm.jpg)
The above circuit forms the basis for the new antenna for the Twente WebSDR,
installed there on December 18, 2017.
See the pictures.
Compared to the basic circuit, there are a few notable differences:
-
Some passive components have been put between the collector and the source.
Doing this allows the circuit to have some gain (it effectively acts as an amplifier
with feedback), and even to make that gain depend on the frequency.
In the WebSDR antenna, we make it 0 dB at low frequencies, and increase to 10 dB
above a few MHz, with a dip back to 0 dB around 26.95 MHz, on which we have a
very strong local pager transmitter.
We need the gain because of our SDR board being somewhat insensitive, and the cable
being long.
- There are two relays (the grey blocks) which can be remotely controlled
(via the IC at the left). With these we can switch the input coil and the feedback circuit,
which was useful for testing.
- There's quite some shielding, in the form of copper foil (bent down in the picture,
but normally covering the circuit board). This was needed
to prevent oscillations. Probably this wouldn't be needed with the basic no-gain circuit.
A practical problem is that the collector of the PNP transistor is connected to its
heatsink, so the entire heatsink is electrically "hot" and can easily
capacitively couple to the antenna element.
- The actual antenna element is not a metal plate, as usual in MiniWhip antennas,
but a bent thick copper wire. Since most of the charge on a metal plate sits at the edges
(see here),
it still works almost as well if only the edges are there, in the form of this wire.
The wire looks like a coil of two turns, but it isn't: at the bottom, all wires
are connected.
Text and pictures on this page are copyright 2017, P.T. de Boer, pa3fwm@... .
Republication is only allowed with my explicit permission.
|
Re: slightly off topic feel free to delete
Assuming this comment is directed at me, years ago I did the IMD
tests for Roelof's Mini-Whip before he had that capability. His
original Mini-Whip with the jfet front end and bipolar output
measured about +50 dBM OIP2 and +30 dBm OIP3, drawing about 50 mA
at 12 volts. Note that for several years now Roelof Bakker PA0RDT
is using a new design with a single high speed opamp in his
Mini-Whip active whip antennas.
My experience is well-matched betas for the output transistors
don't affect overall IMD performance very much vs. not well
matched transistors. Good match would have little to no effect on
OIP3 but could help with OIP2. Years ago I used to make a "high
performance" active whip using a similar complementary-symmetry
output stage, running at 24 volts. The output PNP and NPN
transistors were never matched at all, it was impossible, yet I
was able to get around +105 dBm OIP2 and +46 dBm OIP3 IMD
performance. That circuit used the J310 input, bipolar midstage
and NPN/PNP output stage.
73,
Steve AA7U
On 3/23/2023 2:41 PM, Jeff Green wrote:
toggle quoted message
Show quoted text
Thank you for the detailed report on construction and performance.
Would matched PNP and NPN output transistors help the OIP2/3?
Just for reference, what OIP2/3 does the typical PA0RDT have?
|
Re: slightly off topic feel free to delete
Thanks to Robin for the detailed comments. I built the Aussie
active whip today, testing it on the bench. I used a plastic J310
with no heat sinking. The use of just a 39 ohm resistor plus
whatever resistance the source choke had pretty much set the bias
at the IDss of the jfet--my measured resistance was 40 ohms. Mine
ran about 21 mA after warm up (it gets pretty warm, as
expected--most active whip jfets are not biassed to run so much
current). They used the metal can U310 with some sort of heat
sinking--since the can is the gate, the heat sink must have been
well elevated above ground. I used PZT2222 and PZT2907 bipolars;
one had beta of 175 the other beta of 127 so not well-matched--I
measured a bunch and that was the best I could do. I used just the
dual 100 ohm emitter resistors without the added 2.7k and cap. I
used a bench supply at 15 volts, without the extra chokes and
regulator the original circuit used. My circuit output drew right
at 20 mA, and as mentioned the jfet about 21 mA. I used a 15 pF
input cap for bench testing with 50 ohm gear (51 ohm to ground at
the BNC input as usual. The circuit has pretty decent performance,
I have to admit. Gain is about -7.3 db. IMD performance at 15
volts +61 dBm OIP2, +40 dBm OIP3. I tried it at 12 volts, more
useful for most hobbyists--IMD still decent +58 dBm OIP2, +37 dBm
OIP3. Note that gain is not needed for an active whip; most active
whips have some negative gain up to 0 dB gain.
73,
Steve AA7U
On 3/23/2023 5:39 AM, G8DQX list wrote:
toggle quoted message
Show quoted text
Jeff and others,
the need for a well defined output impedance from the active
antenna/aerial is explained on pp22-24. The graph in figure 2.16
shows the variation of gain with frequency when around 2 ¦¸ of
output impedance drives a 30 m (~100') length of 50 ¦¸ coax which
is terminated in 21 ¦¸. Figure 2.17 below shows what happens when
the active antenna offers a source impedance of 50 ¦¸. ~7dB of
ripple is replaced by about 0.05 dB of ripple in this case. (And
the longer the cable between active antenna and receiver, the
more the effect will be.)
Does that matter? In a professional (or semi-professional)
context factors such as repeatability, consistency, and accuracy
are important. On p.56, section 4.3, Calculated and
Measured Antenna Factor shows that Martinsen is trying to
develop an antenna that can be used for accurate off-air
measurements. The report is from the Australian Department of
Defence, for whom these sorts of issues¡ªwhen spending taxpayers'
AUD¡ªare important.
Do SDR receivers always have a 50 ¦¸ input impedance? The short
answer is no, for all sorts of good reasons, but principally that
of cost. For most applications a well defined 50 ¦¸ input impedance
is simply not necessary.
The output amplifier is not class B, but rather a
two-active-device class A amplifier. The second active device
(transistor) tends to cancel even order distortion products
(mostly intermodulation distortion (IMD)) compared to a similar
design with a single active device. The better that the ?
(current gain) of the two transistors are matched, the better
the cancellation. The substantial standing current is necessary
to assure linearity. In rough terms, more current gives better
linearity. [That said, the bias circuitry looks odd. The
standing current will be dependent on the output devices' ?.]
The sensitivity of most active antennas is more-than-enough to
overcome the natural noise level. ITU recommendation P.372-16 ()
goes in to far more detail than most of us would ever wish to
see! What matters most is signal-to-noise, not absolute
signal level in to a receiver. Too much signal simply gives rise
to intermodulation and thus an excess noise floor.
So how does this design compare, taking it as a touchstone, with
the classic PA0RDT active antenna? [A description may be found at
.]
Martinsen's description demonstrates designed-in consistency?.
For knockabout purposes both designs are pretty good. In terms
of accurate measurement, then the PA0RDT design is
susceptible to inaccuracies due to a non-cable-match output
impedance. Do those inaccuracies matter? Probably not to most of
us, but for those who need to know an antenna factor, then they
certainly do. As to which copes better with strong signals¡ªthe
IMD problem¡ªthe more expensive design has the potential to give
better results. But, as always, the achieved results with either
antenna design strongly depend on care in installation and local
circumstance.
HTH, 73, Stay Safe,
Robin, G8DQX
? Except possibly for that output amplifier bias
circuitry. However, unless one has access to impeccable test
equipment to check for any changes in IMD, one would be strongly
advised to leave it as it is. Overall, it is a well
thought-through and well documented piece of work.
|
Re: slightly off topic feel free to delete
Many thanks to Robin, G8DQX and Martin, G8JNJ
for your insightful comments - I'm glad that others have
had a chance to read Martinsen's paper in more
detail than I have yet had.
I fear that my original comment regarding my
surprise at seeing a circuit based on the U310
JFET and BJTs 2N2222 and 2N2907 was taken
as a criticism of Martinsen's design - it is not that
at all, but merely surprise.? I have all of those
devices, many 50 years old, in my own parts
stock and have used them in various projects
over the years with good success, so again no
criticism intended of Martinsen's use of those
parts.
I believe Robin has voiced a key distinction
between end user objectives, namely that
in professional/military applications the
accuracy and consistency across frequency
of signal strength measurements using an
active antenna may be paramount, whereas
those of us looking for a compact, wideband
receiving antenna for casual monitoring may
not emphasize accuracy or consistency.
I have a very old Sony active antenna intended
for casual SWL use, and am now curious to
see how its buffer amp is designed.? I don't
expect it to be accurate or consistent across
the HF range.
Mike, K8CN
toggle quoted message
Show quoted text
CAUTION: This email originated from outside of the University System. Do not click links or open attachments unless you recognize the sender and know the content is safe.
Jeff and others,
the need for a well defined output impedance from the active antenna/aerial is explained on pp22-24. The graph in figure 2.16 shows the variation of gain with frequency when around 2 ¦¸ of output impedance drives a 30 m (~100') length of 50 ¦¸ coax which is
terminated in 21 ¦¸. Figure 2.17 below shows what happens when the active antenna offers a source impedance of 50 ¦¸. ~7dB of ripple is replaced by about 0.05 dB of ripple in this case. (And the longer the cable between active antenna and receiver, the more
the effect will be.)
Does that matter? In a professional (or semi-professional) context factors such as repeatability, consistency, and accuracy are important. On p.56, section 4.3,
Calculated and Measured Antenna Factor shows that Martinsen is trying to develop an antenna that can be used for
accurate off-air measurements. The report is from the Australian Department of Defence, for whom these sorts of issues¡ªwhen spending taxpayers' AUD¡ªare important.
Do SDR receivers always have a 50 ¦¸ input impedance? The short answer is no, for all sorts of good reasons, but principally that of cost. For most applications a well defined 50 ¦¸ input impedance is simply not necessary.
The output amplifier is not class B, but rather a two-active-device class A amplifier. The second active device (transistor) tends to cancel even order distortion products (mostly intermodulation distortion (IMD)) compared to a similar design with
a single active device. The better that the ? (current gain) of the two transistors are matched, the better the cancellation. The substantial standing current is necessary to assure linearity. In rough terms, more current gives better linearity. [That said,
the bias circuitry looks odd. The standing current will be dependent on the output devices' ?.]
The sensitivity of most active antennas is more-than-enough to overcome the natural noise level. ITU recommendation P.372-16 ()
goes in to far more detail than most of us would ever wish to see! What matters most is signal-to-noise,
not absolute signal level in to a receiver. Too much signal simply gives rise to intermodulation and thus an excess noise floor.
So how does this design compare, taking it as a touchstone, with the classic PA0RDT active antenna? [A description may be found at
.]
Martinsen's description demonstrates designed-in consistency?. For knockabout purposes both designs are pretty good. In terms of
accurate measurement, then the PA0RDT design is susceptible to inaccuracies due to a non-cable-match output impedance. Do those inaccuracies matter? Probably not to most of us, but for those who need to know an antenna factor, then they certainly do.
As to which copes better with strong signals¡ªthe IMD problem¡ªthe more expensive design has the potential to give better results. But, as always, the achieved results with either antenna design strongly depend on care in installation and local circumstance.
HTH, 73, Stay Safe,
Robin, G8DQX
? Except possibly for that output amplifier bias circuitry. However, unless one has access to impeccable test equipment to check for any changes in IMD, one would be strongly advised to leave it as it is. Overall, it is a well thought-through and
well documented piece of work.
On 23/03/2023 00:33, Ed Breya via groups.io wrote:
I just quick-looked at this, and don't see any particular problems. Here are some comments:
The document is dated 2018 - it is likely not some ancient work, although the principles and components have been around for quite some time.
I don't see any problem at all in using the classic old-school parts - I do it all the time, instead of buying new stuff - to help use up some of the millions (not an exaggeration) of components I have accumulated.
The part about impedance matching seems to just be lamenting that sometimes people don't pay attention to it. But they will have to, when it causes problems. I think in this instance it was about the mistake of connecting the nearly zero Z of the buffer emitters
directly to a 50 or 75 ohm line. The "6 dB" thing is encountered all the time - if you have a low Z thing, say nearly zero, the simplest broadband match is a 50 or 75 R in series to the line, giving up half the available voltage. For a short run, you can do
tricks to make it OK, but for a long line, it's best to just do it right and take the loss in exchange for better signal integrity. It depends on the frequency content. So, I don't think this was about different cable types, but simply the need for proper
matching.
Ed
|
Re: slightly off topic feel free to delete
On Thu, Mar 23, 2023 at 12:33 AM, Ed Breya wrote:
The document is dated 2018 - it is likely not some ancient work, although the principles and components have been around for quite some time.
The design is now dated as more modern devices have become available and our understanding of active antennas has also improved. The choice of coax is not that important from the perspective of loss, as the antenna amplifier circuit sets the system noise figure. Like wise the impedance match, as the receiver will usually terminate the coax with something close to its characteristic impedance. However it is important to have good screening, in order to reduce the possibility of noise and interference ingress along the cable run. 75 ohm satellite TV cable with a copper foil and screen (not aluminium which has to small a skin depth ) and solid copper inner (not copper plated steel which is ferromagnetic) offer a good balance between cost and performance. Some notes on the subject of active antennas. Regards, Martin
|
Re: slightly off topic feel free to delete
Jeff and others,
the need for a well defined output impedance from the active
antenna/aerial is explained on pp22-24. The graph in figure 2.16
shows the variation of gain with frequency when around 2 ¦¸ of
output impedance drives a 30 m (~100') length of 50 ¦¸ coax which
is terminated in 21 ¦¸. Figure 2.17 below shows what happens when
the active antenna offers a source impedance of 50 ¦¸. ~7dB of
ripple is replaced by about 0.05 dB of ripple in this case. (And
the longer the cable between active antenna and receiver, the more
the effect will be.)
Does that matter? In a professional (or semi-professional)
context factors such as repeatability, consistency, and accuracy
are important. On p.56, section 4.3, Calculated and Measured
Antenna Factor shows that Martinsen is trying to develop an
antenna that can be used for accurate off-air
measurements. The report is from the Australian Department of
Defence, for whom these sorts of issues¡ªwhen spending taxpayers'
AUD¡ªare important.
Do SDR receivers always have a 50 ¦¸ input impedance? The short
answer is no, for all sorts of good reasons, but principally that of
cost. For most applications a well defined 50 ¦¸ input impedance is
simply not necessary.
The output amplifier is not class B, but rather a
two-active-device class A amplifier. The second active device
(transistor) tends to cancel even order distortion products
(mostly intermodulation distortion (IMD)) compared to a similar
design with a single active device. The better that the ?
(current gain) of the two transistors are matched, the better the
cancellation. The substantial standing current is necessary to
assure linearity. In rough terms, more current gives better
linearity. [That said, the bias circuitry looks odd. The standing
current will be dependent on the output devices' ?.]
The sensitivity of most active antennas is more-than-enough to
overcome the natural noise level. ITU recommendation P.372-16 ()
goes in to far more detail than most of us would ever wish to see!
What matters most is signal-to-noise, not absolute signal
level in to a receiver. Too much signal simply gives rise to
intermodulation and thus an excess noise floor.
So how does this design compare, taking it as a touchstone, with the
classic PA0RDT active antenna? [A description may be found at .]
Martinsen's description demonstrates designed-in consistency?.
For knockabout purposes both designs are pretty good. In terms of
accurate measurement, then the PA0RDT design is susceptible
to inaccuracies due to a non-cable-match output impedance. Do
those inaccuracies matter? Probably not to most of us, but for
those who need to know an antenna factor, then they certainly do.
As to which copes better with strong signals¡ªthe IMD problem¡ªthe
more expensive design has the potential to give better results.
But, as always, the achieved results with either antenna design
strongly depend on care in installation and local circumstance.
HTH, 73, Stay Safe,
Robin, G8DQX
? Except possibly for that output amplifier bias
circuitry. However, unless one has access to impeccable test
equipment to check for any changes in IMD, one would be strongly
advised to leave it as it is. Overall, it is a well
thought-through and well documented piece of work.
On 23/03/2023 00:33, Ed Breya via
groups.io wrote:
toggle quoted message
Show quoted text
I just quick-looked
at this, and don't see any particular problems. Here are some
comments:
The document is dated 2018 - it is likely not some ancient work,
although the principles and components have been around for quite
some time.
I don't see any problem at all in using the classic old-school
parts - I do it all the time, instead of buying new stuff - to
help use up some of the millions (not an exaggeration) of
components I have accumulated.
The part about impedance matching seems to just be lamenting that
sometimes people don't pay attention to it. But they will have to,
when it causes problems. I think in this instance it was about the
mistake of connecting the nearly zero Z of the buffer emitters
directly to a 50 or 75 ohm line. The "6 dB" thing is encountered
all the time - if you have a low Z thing, say nearly zero, the
simplest broadband match is a 50 or 75 R in series to the line,
giving up half the available voltage. For a short run, you can do
tricks to make it OK, but for a long line, it's best to just do it
right and take the loss in exchange for better signal integrity.
It depends on the frequency content. So, I don't think this was
about different cable types, but simply the need for proper
matching.
Ed
|
|
|
Re: slightly off topic feel free to delete
I just quick-looked at this, and don't see any particular problems. Here are some comments:
The document is dated 2018 - it is likely not some ancient work, although the principles and components have been around for quite some time.
I don't see any problem at all in using the classic old-school parts - I do it all the time, instead of buying new stuff - to help use up some of the millions (not an exaggeration) of components I have accumulated.
The part about impedance matching seems to just be lamenting that sometimes people don't pay attention to it. But they will have to, when it causes problems. I think in this instance it was about the mistake of connecting the nearly zero Z of the buffer emitters directly to a 50 or 75 ohm line. The "6 dB" thing is encountered all the time - if you have a low Z thing, say nearly zero, the simplest broadband match is a 50 or 75 R in series to the line, giving up half the available voltage. For a short run, you can do tricks to make it OK, but for a long line, it's best to just do it right and take the loss in exchange for better signal integrity. It depends on the frequency content. So, I don't think this was about different cable types, but simply the need for proper matching.
Ed
|
Re: slightly off topic feel free to delete
Mike,
Thanks for the reading on the article. I¡¯ve read over it but not studied it. ?I agree with what you said about the devices but one point there. The characteristics of these devices are very well known and documented so that may have played in his choice.
My question came when looking at the buffer amplifier. While a direct parallel to high performance audio may not be appropriate here I wonder about the lack of the 3 (somewhat typical) diodes that are run from base to base. My understanding is that these are used to minimize crossover distortion. Could this be a factor at RF frequencies also possibly leading to induced distortions?
toggle quoted message
Show quoted text
On Mar 22, 2023, at 10:31, Mike - K8CN <k8cn@...> wrote:
?
Correction to my previous post - the
JFET front end in Mr. Martinsen's
active antenna design is the U310 -
typing glitch!
Mike, K8CN
CAUTION: This email originated from outside of the University System. Do not click links or open attachments unless you recognize the sender and know the content is safe.
Thanks, Jeff, for posting your questions
and the link to the paper (and to another
poster who gave a viable link).
I only quickly scanned Mr. Martinsen's
paper and will return to a more careful
reading soon.? My impression from looking
at the circuit for the active antenna was
surprise that the devices used are the
venerable U130 JFET and BJTs 2N2222
and 2N2907.? The use of old device designs
does not by itself invalidate the choice
of circuit topologies nor the accompanying
analyses, of course.
The issue you raised about matching the
output impedance of the active antenna
buffer amplifier to the characteristic
impedance of the coax to the receiver (SDR)
is valid.? Your experimental results confirm
that there is no effective loss of sensitivity
as a result of using RG6 vs LMR-100 coax
for your receiving setups.
My sense of Mr. Martinsen's analysis and
attention to IMD in a wideband receiving
scenario is that he is concerned with inherent
nonlinearities in the active antenna circuit
being stimulated by reflected energy at
the active antenna to coax junction, which
is also due to the wideband impedance
profile of the receiver input.? In a receiving
scenario where there may be very strong,
nominally out of band, emitters present,
the output buffer amplifier's nonlinear
effects may be worsened by the impedance
presented to it at the frequency(ies) of
very strong emitters.?
While one could via circuit simulation
estimate IMD levels for specific receiving
scenarios using Mr. Martinsen's circuit
topology, a general analysis is not
really meaningful.
I wouldn't worry about your choice
of coax between your active antenna
and your SDR (or other receiver).?
Determining where the IMD is principally
generated and mitigating it there would
be my first line of investigation.
Mike, K8CN
CAUTION: This email originated from outside of the University System. Do not click links or open attachments unless you recognize the sender and know the content is safe.
Thank you, that worked.? I will read it and see what I make of it.? However, I am not an expert
? Mostly just lurk here?
-------- Original message --------
From: cricri <cricri2002@...>
Date: 3/22/23 1:16 AM (GMT-08:00)
Subject: Re: [Test Equipment Design & Construction] slightly off topic feel free to delete
Le 22/03/2023 ¨¤ 05:24, Richard Knoppow a ¨¦crit?:
Can't open the pdf on my galaxy
Try this :
73
Xtian
-------- Original message --------
Date: 3/21/23 7:47 PM (GMT-08:00)
Subject: [Test Equipment Design & Construction] slightly off topic feel free to delete
Based on the threads regarding "RF stuff," I suspect many of the members here are experts on RF.
?
Please review this design:
?
I know just enough to be confused.
?
Mr. Martinsen makes a big deal of importance of matching the output of his active antenna design to the impedance of the coax.
?
He covers his ideas starting on page 30.
?
Many (all?) modern SDRs have a RF impedance of 50 ohms over their frequency coverage. Older, traditional, general coverage receivers exhibit wildly varying
input impedances.
?
I use a PA0RDT active antenna as my portable test antenna to test the local ambient RF levels.
?
I compared 100 feet of LMR-100 Times Microwave coax with 100 feet of Lowe's
Southwire RG6.
?
I used Perseus SDR, SDRplay RSP1a, R2000, and Sangean ATS-909 as test receivers.
?
I used the Lexington airport NDB and outer marker, local and distant MW stations, WWV, various amateur radio stations and CB (11M in the USA.)
?
There was no
discernible?difference between coax cables.
?
Am I missing something here or is Mr. Martinsen making a mountain out of an anthill?
?
?
?
|
Re: slightly off topic feel free to delete
Mike,
?
Thanks for the reading on the article. I¡¯ve read over it but not studied it. ?I agree with what you said about the devices but one point there. The characteristics of these devices are very well known and documented so that may have played in his choice.
?
My question came when looking at the buffer amplifier. While a direct parallel to high performance audio may not be appropriate here I wonder about the lack of the 3 (somewhat typical) diodes that are run from base to base. My understanding is that these are used to minimize crossover distortion. Could this be a factor at RF frequencies also possibly leading to induced distortions?
?
Steve, KC4SW
|
Re: slightly off topic feel free to delete
It is curious that the document is undated. The design and construction photo makes it look like it is from the last millenium. Impedance matching does help as he describes in that paper as far as noise, phase shift? etc. However, those transistors are not low noise to start with.
Since this is an untuned design, it is less of a problem, but the design of the SDR input is unknown. My background includes CATV systems, and high end Telemetry where impedance match was quite critical. We often used a 3dB pad at the input and output of an LC tuned filter to provide some isolation. Without this, it was difficult to achieve a reasonably acceptable signal. Instead, it had too much ripple, and in some designs, it had deep notches or spikes because of reflections. Since both use many cascaded amplifiers, flatness is critical. There are 50 ohm MMICs these days that are quite cheap.There is a simple resistive 50 to 75 ohm converter with a fairly low loss that can be used at each end to match the CV coax, as well. Here is a page that describes the pad, and its loss is 5.72 dB so a pair would be 11.44 dB, which can be eliminated with the MMIC. provided most of what we used. They have a wide range of excellent components and a wealth of Application data. Digikey is a stocking distributor, and in stock units start at under $2 in single quantities.
On Wed, Mar 22, 2023 at 1:31?PM Mike - K8CN < k8cn@...> wrote:
Correction to my previous post - the
JFET front end in Mr. Martinsen's
active antenna design is the U310 -
typing glitch!
Mike, K8CN
CAUTION: This email originated from outside of the University System. Do not click links or open attachments unless you recognize the sender and know the content is safe.
Thanks, Jeff, for posting your questions
and the link to the paper (and to another
poster who gave a viable link).
I only quickly scanned Mr. Martinsen's
paper and will return to a more careful
reading soon.? My impression from looking
at the circuit for the active antenna was
surprise that the devices used are the
venerable U130 JFET and BJTs 2N2222
and 2N2907.? The use of old device designs
does not by itself invalidate the choice
of circuit topologies nor the accompanying
analyses, of course.
The issue you raised about matching the
output impedance of the active antenna
buffer amplifier to the characteristic
impedance of the coax to the receiver (SDR)
is valid.? Your experimental results confirm
that there is no effective loss of sensitivity
as a result of using RG6 vs LMR-100 coax
for your receiving setups.
My sense of Mr. Martinsen's analysis and
attention to IMD in a wideband receiving
scenario is that he is concerned with inherent
nonlinearities in the active antenna circuit
being stimulated by reflected energy at
the active antenna to coax junction, which
is also due to the wideband impedance
profile of the receiver input.? In a receiving
scenario where there may be very strong,
nominally out of band, emitters present,
the output buffer amplifier's nonlinear
effects may be worsened by the impedance
presented to it at the frequency(ies) of
very strong emitters.?
While one could via circuit simulation
estimate IMD levels for specific receiving
scenarios using Mr. Martinsen's circuit
topology, a general analysis is not
really meaningful.
I wouldn't worry about your choice
of coax between your active antenna
and your SDR (or other receiver).?
Determining where the IMD is principally
generated and mitigating it there would
be my first line of investigation.
Mike, K8CN
CAUTION: This email originated from outside of the University System. Do not click links or open attachments unless you recognize the sender and know the content is safe.
Thank you, that worked.? I will read it and see what I make of it.? However, I am not an expert
? Mostly just lurk here?
-------- Original message --------
Date: 3/22/23 1:16 AM (GMT-08:00)
Subject: Re: [Test Equipment Design & Construction] slightly off topic feel free to delete
Le 22/03/2023 ¨¤ 05:24, Richard Knoppow a ¨¦crit?:
Can't open the pdf on my galaxy
Try this :
73
Xtian
-------- Original message --------
Date: 3/21/23 7:47 PM (GMT-08:00)
Subject: [Test Equipment Design & Construction] slightly off topic feel free to delete
Based on the threads regarding "RF stuff," I suspect many of the members here are experts on RF.
?
Please review this design:
?
I know just enough to be confused.
?
Mr. Martinsen makes a big deal of importance of matching the output of his active antenna design to the impedance of the coax.
?
He covers his ideas starting on page 30.
?
Many (all?) modern SDRs have a RF impedance of 50 ohms over their frequency coverage. Older, traditional, general coverage receivers exhibit wildly varying
input impedances.
?
I use a PA0RDT active antenna as my portable test antenna to test the local ambient RF levels.
?
I compared 100 feet of LMR-100 Times Microwave coax with 100 feet of Lowe's
Southwire RG6.
?
I used Perseus SDR, SDRplay RSP1a, R2000, and Sangean ATS-909 as test receivers.
?
I used the Lexington airport NDB and outer marker, local and distant MW stations, WWV, various amateur radio stations and CB (11M in the USA.)
?
There was no
discernible?difference between coax cables.
?
Am I missing something here or is Mr. Martinsen making a mountain out of an anthill?
?
?
?
|
Re: slightly off topic feel free to delete
Correction to my previous post - the
JFET front end in Mr. Martinsen's
active antenna design is the U310 -
typing glitch!
Mike, K8CN
toggle quoted message
Show quoted text
CAUTION: This email originated from outside of the University System. Do not click links or open attachments unless you recognize the sender and know the content is safe.
Thanks, Jeff, for posting your questions
and the link to the paper (and to another
poster who gave a viable link).
I only quickly scanned Mr. Martinsen's
paper and will return to a more careful
reading soon.? My impression from looking
at the circuit for the active antenna was
surprise that the devices used are the
venerable U130 JFET and BJTs 2N2222
and 2N2907.? The use of old device designs
does not by itself invalidate the choice
of circuit topologies nor the accompanying
analyses, of course.
The issue you raised about matching the
output impedance of the active antenna
buffer amplifier to the characteristic
impedance of the coax to the receiver (SDR)
is valid.? Your experimental results confirm
that there is no effective loss of sensitivity
as a result of using RG6 vs LMR-100 coax
for your receiving setups.
My sense of Mr. Martinsen's analysis and
attention to IMD in a wideband receiving
scenario is that he is concerned with inherent
nonlinearities in the active antenna circuit
being stimulated by reflected energy at
the active antenna to coax junction, which
is also due to the wideband impedance
profile of the receiver input.? In a receiving
scenario where there may be very strong,
nominally out of band, emitters present,
the output buffer amplifier's nonlinear
effects may be worsened by the impedance
presented to it at the frequency(ies) of
very strong emitters.?
While one could via circuit simulation
estimate IMD levels for specific receiving
scenarios using Mr. Martinsen's circuit
topology, a general analysis is not
really meaningful.
I wouldn't worry about your choice
of coax between your active antenna
and your SDR (or other receiver).?
Determining where the IMD is principally
generated and mitigating it there would
be my first line of investigation.
Mike, K8CN
CAUTION: This email originated from outside of the University System. Do not click links or open attachments unless you recognize the sender and know the content is safe.
Thank you, that worked.? I will read it and see what I make of it.? However, I am not an expert
? Mostly just lurk here?
-------- Original message --------
From: cricri <cricri2002@...>
Date: 3/22/23 1:16 AM (GMT-08:00)
Subject: Re: [Test Equipment Design & Construction] slightly off topic feel free to delete
Le 22/03/2023 ¨¤ 05:24, Richard Knoppow a ¨¦crit?:
Can't open the pdf on my galaxy
Try this :
73
Xtian
-------- Original message --------
Date: 3/21/23 7:47 PM (GMT-08:00)
Subject: [Test Equipment Design & Construction] slightly off topic feel free to delete
Based on the threads regarding "RF stuff," I suspect many of the members here are experts on RF.
?
Please review this design:
?
I know just enough to be confused.
?
Mr. Martinsen makes a big deal of importance of matching the output of his active antenna design to the impedance of the coax.
?
He covers his ideas starting on page 30.
?
Many (all?) modern SDRs have a RF impedance of 50 ohms over their frequency coverage. Older, traditional, general coverage receivers exhibit wildly varying
input impedances.
?
I use a PA0RDT active antenna as my portable test antenna to test the local ambient RF levels.
?
I compared 100 feet of LMR-100 Times Microwave coax with 100 feet of Lowe's
Southwire RG6.
?
I used Perseus SDR, SDRplay RSP1a, R2000, and Sangean ATS-909 as test receivers.
?
I used the Lexington airport NDB and outer marker, local and distant MW stations, WWV, various amateur radio stations and CB (11M in the USA.)
?
There was no
discernible?difference between coax cables.
?
Am I missing something here or is Mr. Martinsen making a mountain out of an anthill?
?
?
?
|
Re: slightly off topic feel free to delete
Thanks, Jeff, for posting your questions
and the link to the paper (and to another
poster who gave a viable link).
I only quickly scanned Mr. Martinsen's
paper and will return to a more careful
reading soon.? My impression from looking
at the circuit for the active antenna was
surprise that the devices used are the
venerable U130 JFET and BJTs 2N2222
and 2N2907.? The use of old device designs
does not by itself invalidate the choice
of circuit topologies nor the accompanying
analyses, of course.
The issue you raised about matching the
output impedance of the active antenna
buffer amplifier to the characteristic
impedance of the coax to the receiver (SDR)
is valid.? Your experimental results confirm
that there is no effective loss of sensitivity
as a result of using RG6 vs LMR-100 coax
for your receiving setups.
My sense of Mr. Martinsen's analysis and
attention to IMD in a wideband receiving
scenario is that he is concerned with inherent
nonlinearities in the active antenna circuit
being stimulated by reflected energy at
the active antenna to coax junction, which
is also due to the wideband impedance
profile of the receiver input.? In a receiving
scenario where there may be very strong,
nominally out of band, emitters present,
the output buffer amplifier's nonlinear
effects may be worsened by the impedance
presented to it at the frequency(ies) of
very strong emitters.?
While one could via circuit simulation
estimate IMD levels for specific receiving
scenarios using Mr. Martinsen's circuit
topology, a general analysis is not
really meaningful.
I wouldn't worry about your choice
of coax between your active antenna
and your SDR (or other receiver).?
Determining where the IMD is principally
generated and mitigating it there would
be my first line of investigation.
Mike, K8CN
toggle quoted message
Show quoted text
CAUTION: This email originated from outside of the University System. Do not click links or open attachments unless you recognize the sender and know the content is safe.
Thank you, that worked.? I will read it and see what I make of it.? However, I am not an expert
? Mostly just lurk here?
-------- Original message --------
From: cricri <cricri2002@...>
Date: 3/22/23 1:16 AM (GMT-08:00)
Subject: Re: [Test Equipment Design & Construction] slightly off topic feel free to delete
Le 22/03/2023 ¨¤ 05:24, Richard Knoppow a ¨¦crit?:
Can't open the pdf on my galaxy
Try this :
73
Xtian
-------- Original message --------
Date: 3/21/23 7:47 PM (GMT-08:00)
Subject: [Test Equipment Design & Construction] slightly off topic feel free to delete
Based on the threads regarding "RF stuff," I suspect many of the members here are experts on RF.
?
Please review this design:
?
I know just enough to be confused.
?
Mr. Martinsen makes a big deal of importance of matching the output of his active antenna design to the impedance of the coax.
?
He covers his ideas starting on page 30.
?
Many (all?) modern SDRs have a RF impedance of 50 ohms over their frequency coverage. Older, traditional, general coverage receivers exhibit wildly varying
input impedances.
?
I use a PA0RDT active antenna as my portable test antenna to test the local ambient RF levels.
?
I compared 100 feet of LMR-100 Times Microwave coax with 100 feet of Lowe's
Southwire RG6.
?
I used Perseus SDR, SDRplay RSP1a, R2000, and Sangean ATS-909 as test receivers.
?
I used the Lexington airport NDB and outer marker, local and distant MW stations, WWV, various amateur radio stations and CB (11M in the USA.)
?
There was no
discernible?difference between coax cables.
?
Am I missing something here or is Mr. Martinsen making a mountain out of an anthill?
?
?
?
|
Re: slightly off topic feel free to delete
Thank you, that worked.? I will read it and see what I make of it.? However, I am not an expert ? Mostly just lurk here?
toggle quoted message
Show quoted text
-------- Original message -------- From: cricri <cricri2002@...> Date: 3/22/23 1:16 AM (GMT-08:00) Subject: Re: [Test Equipment Design & Construction] slightly off topic feel free to delete
Le 22/03/2023 ¨¤ 05:24, Richard Knoppow
a ¨¦crit?:
Can't open the pdf on my galaxy
Try this :
73
Xtian
-------- Original message --------
Date: 3/21/23 7:47 PM (GMT-08:00)
Subject: [Test Equipment Design & Construction]
slightly off topic feel free to delete
Based
on the threads regarding "RF stuff," I suspect many of the
members here are experts on RF.
?
Please
review this design:
?
I
know just enough to be confused.
?
Mr.
Martinsen makes a big deal of importance of matching the
output of his active antenna design to the impedance of the
coax.
?
He
covers his ideas starting on page 30.
?
Many
(all?) modern SDRs have a RF impedance of 50 ohms over their
frequency coverage. Older, traditional, general coverage
receivers exhibit wildly varying input impedances.
?
I
use a PA0RDT active antenna as my portable test antenna to
test the local ambient RF levels.
?
I
compared 100 feet of LMR-100 Times Microwave coax with 100
feet of Lowe's
Southwire
RG6.
?
I
used Perseus SDR, SDRplay RSP1a, R2000, and Sangean ATS-909 as
test receivers.
?
I
used the Lexington airport NDB and outer marker, local and
distant MW stations, WWV, various amateur radio stations and
CB (11M in the USA.)
?
There
was no discernible?difference between coax cables.
?
Am
I missing something here or is Mr. Martinsen making a mountain
out of an anthill?
?
?
?
|
![[circuit board close-up]](/g/Test-Equipment-Design-Construction/messages/miniwhip-pa3fwm-board.jpg)
Sam Reaves
Michael A. Terrell