[hpsdr] strange SWR issues

[Ramakrishnan Muthukrishnan]

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
> _______________________________________________
> HPSDR Discussion List
> To post msg: hpsdr at openhpsdr.org
> Subscription help: http://lists.openhpsdr.org/listinfo.cgi/hpsdr-openhpsdr.org
> HPSDR web page: http://openhpsdr.org
> Archives: http://lists.openhpsdr.org/pipermail/hpsdr-openhpsdr.org/

-- 
  Ramakrishnan