[hpsdr] strange SWR issues

[John Ackermann N8UR]

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