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?
?
?
?
