[hpsdr] Hpsdr Digest, Vol 211, Issue 8

Fri Nov 24 13:20:56 PST 2023

In the schematic only the components to the right of the directional coupler will cause the actual SWR to change.

  1.   Use a different antenna port.
  2.   Put a dummy load on the troubled port.
  3.   If the PA power is actually intermittent the software might be confusing things because two different measurements are required.

73,
John K2ox
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Subject: Hpsdr Digest, Vol 211, Issue 8

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Today's Topics:

   1. strange SWR issues (Ramakrishnan Muthukrishnan)
   2. Re: strange SWR issues (John Ackermann N8UR)

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

Message: 1
Date: Fri, 24 Nov 2023 11:21:16 +0530
From: "Ramakrishnan Muthukrishnan" <ram at rkrishnan.org>
To: hpsdr at lists.openhpsdr.org
Subject: [hpsdr] strange SWR issues
Message-ID: <640295ce-4194-471a-bf92-74464c4f0640 at app.fastmail.com>
Content-Type: text/plain

Hello HPSDR Hackers!

I have an Atlas based system with Munin2 and I have been happily using the system for a few years now.

Last week, I noticed high SWR with my 10m Moxon. And it also fluctuates a lot. I have an external tuner and it struggles to match the feeder+antenna. So, the values shown by the Alex coupler is also high since tuner has not matched.

My first suspicion was of course the antenna, but antenna seem intact, I checked with a VNA and things look fine. I also changed the feeder with a brand new cable.

Here is the strange thing. If I bypass Alex and directly give the 100W PA output to the tuner, everything looks great. Power is going out, tuner shows a steady power output and SWR. Also, I used a 20W PA (PennyWhistle) with Alex and could make a QSO and got good reports, no issues with SWR. But if I use 100W + Alex Tx filters, I observe SWR fluctuations. I opened the hammond box that has the Alex filters, no burns or obvious failures. I also tried the "bypass" button on powersdr and still observe problems.

Now, my only suspicion is the relays and connectors in the path: K17, K18 and K8. The only variable I can now change is K5, I can try a different Antenna port, so K11/K14 (ant2 and ant3) is used.

Schematic here for handy reference: <https://urldefense.com/v3/__http://openhpsdr.org/support/Alexiares/ALEX-P1.1.4-TX*20Sch.pdf__;JQ!!I5pVk4LIGAfnvw!lgKUV8vrSx_23Dsp1CTmMlz3Y8jgBqsyyhhIRwhIet0kpOL559wfxaxFG1saHKlxdH8HArdEZ9Oc6ys6WcMvmuAeqtNtQkeS36vy$>

Anyone else faced similar issues? I have also tried various patch cables etc just to rule out the cables in the path... Pretty much exhausted all possibilities now. Interesting problem and a lot of new learning, so amateur radio's purpose of self learning is served!

73
--
  Ram VU2JXN

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

Message: 2
Date: Fri, 24 Nov 2023 08:59:48 -0500
From: John Ackermann N8UR <jra at febo.com>
To: hpsdr at lists.openhpsdr.org
Subject: Re: [hpsdr] strange SWR issues
Message-ID: <f5b88884-52e0-4681-aa9b-6d3fc57be050 at febo.com>
Content-Type: text/plain; charset=UTF-8; format=flowed

Hi Ram -

For what it's worth, I have an external switchable bandpass filter we
used when running multiple bands at the Field Day exercise.  It's rate
for 100W and normally is just fine.  But two separate times when set to
10M and keyed by mistake with no antenna, the high voltage caps in the
tuning circuit blew.  There was no visible damage but the caps were no
longer caps.

My point is that at 10M even short transmission into bad SWR could
generate high voltage that could pop a cap or relay contact, without
leaving external sign of burning.  So it would be good to check those
components if you can.

73,
John
----
On 11/24/23 00:51, Ramakrishnan Muthukrishnan wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> Hello HPSDR Hackers!
>
> I have an Atlas based system with Munin2 and I have been happily using the system for a few years now.
>
> Last week, I noticed high SWR with my 10m Moxon. And it also fluctuates a lot. I have an external tuner and it struggles to match the feeder+antenna. So, the values shown by the Alex coupler is also high since tuner has not matched.
>
> My first suspicion was of course the antenna, but antenna seem intact, I checked with a VNA and things look fine. I also changed the feeder with a brand new cable.
>
> Here is the strange thing. If I bypass Alex and directly give the 100W PA output to the tuner, everything looks great. Power is going out, tuner shows a steady power output and SWR. Also, I used a 20W PA (PennyWhistle) with Alex and could make a QSO and got good reports, no issues with SWR. But if I use 100W + Alex Tx filters, I observe SWR fluctuations. I opened the hammond box that has the Alex filters, no burns or obvious failures. I also tried the "bypass" button on powersdr and still observe problems.
>
> Now, my only suspicion is the relays and connectors in the path: K17, K18 and K8. The only variable I can now change is K5, I can try a different Antenna port, so K11/K14 (ant2 and ant3) is used.
>
> Schematic here for handy reference: <https://urldefense.com/v3/__http://openhpsdr.org/support/Alexiares/ALEX-P1.1.4-TX*20Sch.pdf__;JQ!!I5pVk4LIGAfnvw!lgKUV8vrSx_23Dsp1CTmMlz3Y8jgBqsyyhhIRwhIet0kpOL559wfxaxFG1saHKlxdH8HArdEZ9Oc6ys6WcMvmuAeqtNtQkeS36vy$>
>
> Anyone else faced similar issues? I have also tried various patch cables etc just to rule out the cables in the path... Pretty much exhausted all possibilities now. Interesting problem and a lot of new learning, so amateur radio's purpose of self learning is served!
>
> 73

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

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