[hpsdr] strange SWR issues

[Ramakrishnan Muthukrishnan]

I built a 50db sampler and looked at the output of the Munin2 (without connecting the LPF). It looks awful. For the first few seconds of switching on the radio, I see signals other than the main signal and harmonics. After a few seconds, it is only the signal and its harmonics, but the signal jumps up and down a lot. Looks like the amplifier is the culprit, now I need to go deeper into it.

A short video..

<https://photos.app.goo.gl/thieGdoqTGv75se97>

73
Ram

On Sat, Nov 25, 2023, at 6:11 PM, Ramakrishnan Muthukrishnan wrote:
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>
> Thanks John. Thanks for the tip. 
>
> I also observe the behaviour in all bands, which leads to the suspicion 
> that parts common to all the bands -- like the relays in the path could 
> be the culprit. The power observed fluctuates and the SWR keeps 
> jumping.. 
>
> I will try to check the caps/relays in the transmit path. I did a bunch 
> of tests and so far, everything points to a bad T/R relay. I am hoping 
> to disconnect the SMA connector going into the Alex RX and then short 
> the pins going to the input of the T/R relay with output so that its 
> effect is bypassed and then try transmitting into a dummy load. In the 
> meantime, I have ordered some replacement relays.
>
> 73
> Ram VU2JXN
>
> On Fri, Nov 24, 2023, at 7:29 PM, John Ackermann N8UR wrote:
>> ***** High Performance Software Defined Radio Discussion List *****
>>
>> 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: <http://openhpsdr.org/support/Alexiares/ALEX-P1.1.4-TX%20Sch.pdf>
>>> 
>>> 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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>
> -- 
>   Ramakrishnan
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-- 
  Ramakrishnan