W6PQL Amplifier's intermittent RX issue troubleshooting and sequencer adjustment

 W6PQL amplifier is great modular amplifier design. Since I had homebrew assembled in 2015/16, it has proven very solid and reliable. I have some issues with Directional coupler and detection of signals to represent reverse power accurately as per Jim's design. However, other than this aspect, i have used the amp in many contests and activations. Last year(2022) during CQWW CW contest, I realized that sequencer is slightly slow in switching back to RX from TX mode. Some of the stations I did not copy either first character or sometimes missing dit or dashes of the first character. This was not serious issue, but anyways, once you know what adjustments to do internally, you tend to tweak it towards perfection. I tweaked the timing so that its faster switching in sequence. 

During the CQWPX SSB contest i did not realize any issues. And after all SSB signals are delay start by human nature of speaking into mic. Also, problem of hot switching might have started during the WPX SSB itself , but did not come to surface. During WPX CW, My entry was bit delayed due to exhausting MQTH work till previous day late night and tr avel related fatigue.  VU2VTI was the early starter at VU2REC club station for the contest. When I started on the bands, at first it seemed all ok. But, suddenly started feeling the RX is weak sometimes and delayed to full signals. I thought it was due to some ionospheric disturbance. But when it continued for some hours, i sensed it unusual. Asked VU2VTI to be on my system and run the contest for few minutes. He confirmed that it is unusual and something might be wrong with my Amp or Radio. I then disconnected amp from the path and observed no issues with RX drop. This confirmed me that Relays inside the Amplifier might be damaged. No time to fix and no spare relays on hand. I packed my amp to carry back and continued the contest with TS590 barefoot.

Back at home, after opening the amplifier lids, first thing visually checking the three LEDs in sequence i.e. Hold off ALC line, Trigger Relay line, Release ALC line. Initially it was almost instant, I adjusted PCB variable pot to have some visible lag between the events. There was no RF input given to the amplifier, all testing was done using a SPST switch to PTT in RCA plug. This allowed me to rapidly check the working. Next came checking the Input and Output  Relays. While input relays typically take less than 10Watts or less in the path (my drive power), output relays handle upwards of 350W or more.

I had a hunch that output relays might be the issue due this factor. While switching PTT line, keeping my fingers on the relay, I sensed no movement in the output relay. Thanks to modular design of the amp,  i could desolder the output relay PCB and test it separately. Found that relay coil was showing higher resistance and was not energizing. This is Omron G2RL-1E relay and fortunately, i had few of them in my spares. I did replace the relay and test OK.

This incident has taught me something very important about sequencer lines and never to be too greedy beyond the design specs. By properly adjusting PTT delay for Amp (i.e RF hold back after PTT switching), we can safely control the TRX path for signals. Typically 100ms is what W6PQL suggests (Hold ALC line + 50ms delay+ trigger relays+50ms delay + release ALC line). 

Now will have to wait for next contest or event for my setup testing in real situation.

Hope this helps someone in similar situation.

VU2XE

Triband VDA antenna update (V2)

 Last year during CQWW SSB (Oct 2020), I had tried to tune my own designed triband VDA antenna. Whatever I tried, it seemed that antenna was not tuning or getting out well. Youtube video on that effort is at https://youtu.be/tJb-NoXO6V0

Couple of months ago, I was pondering about the issue with VDA and realized that my design was not considering PVC jacket dielectric loading (velocity factor). That  made me to load the model(4NEC2) with PVC jacket and check once again with same dimension. Oh. My.. Yes, I was way too off from the design frequency and no matter what I did on ground, it would not have given right results. 

So, a month back during Covid19 native place stay, I carried the VDA to be installed at my garden and check with PVC adjusted dimensions. And I was finding the spot on tuning. I know, triband on a single pole is compromise due to spacing requirements and height above the ground etc. But this is something I could live when Sea is washing its base, when I travel solo etc.

With this test done at home, I got opportunity to test it right on the beach when I was invited me to operate at Kapu Lighthouse AT8KLH near my hometown. It is a week long activation of lighthouse with many events and teams planning to operate from the location. I did quick recce of the place on my way to Udupi where I had kept most of the cables and poles from last WPX contest. Found very friendly neighborhood and a school on the beach next to lighthouse - very ideal for the operating. There is greater collective story to be compiled once the activation is complete. I am sharing my short 2 day participation experience with VDA.

Putting any antenna on beach during monsoon season is bit of pain, we took 2-3 hours to just put the antenna and secure with guy due to heavy rains. Once it was erected with help of Lakshminidhi VU2WFN, SWR plots were checked to be on CW portion of the band where myself and VU2VTI Murthy wanted to operate. Though I had carried some heavy filters etc. to operate multi, we could not due to lack of electric power at the premise.

Anyone who has operated the VDA from right on the water edge know how signals come. I can make a quick contrast with eating food right out of frying pan (still sizzling) vs somewhat cold but still fresh on the table Hi Hi!. Signals with incredibly low noise and crisp right out of the operator's shack on the other side of the globe. We made a 650+ QSOs with many breaks and change of operators etc. till we closed later evening on 15th. Epitome of thrill when one could be in the center of massive pileup and in control of it!.

Following is the 4nec2(freeware) design file if someone is interested in replicating/improvising.

https://drive.google.com/file/d/155F-yHVCxiO9XVuinu8rqDsIFrOxilXo/view?usp=sharing

There is lot of opportunity to optimize further especially for F/B ratio. Here are some plots from 4NEC2

Note: This VDA requires 10mtr tall support (I used 12mtr spider pole with last two segments collapsed). At the Cross section, I used a cutting board material with drilled holes for wire ties. For cross arms (total length of 2 meter), I could have used PVC pipes, but I had couple of Caperlan fishing rods (1 meter collapsed length). This antenna did not hear anything from back. Just to note, there was a small ridge (15ft tall) at the back side towards south east/east. which might have helped in reducing noise/signals from back. Following reverse beacon report and QSO stats shall summarize the efforts well.

Website for AT3A contest and all homebrew projects

I have created a new website for compiling all the projects and tiny contest efforts:

https://vu2xekiran.wixsite.com/at3acontest

It all started with couple of small projects and contesting is means put those projects under real test. AT3A was the call used by me in CQWW contests in 2018.

Cheers

VU2XE

VHF PowerDevider for Stacking Yagis

This build project was published on ARSI Ham Radio News 2018 Q4  edition

Please check it out.
Kiran VU2XE

Low Pass Filter(LPF) and Application in Amplifiers

Recently I came across few comments on whether LPF is really required for Solid State Amplifiers.
I am no professional or expert in this area, however, my W6PQL amplifier built has taught me something to state here.

Any solid state design has some non linearity and thus introducing harmonics. Harmonic distortions are main reason why one hears spatter on few bands even when other parameters are conducive for HF. There are regulations in some part of the world to keep such distortions limited to lower than -40dB. It does not matter what power we are running, we need to keep this as a best practice guideline and make things better for all others on the band.

If you have an amplifier for 1 to 7Mhz, having one LPF for 7Mhz band is not sufficient for operating the amplifier on 160mtr and 80mtr. We have second and third harmonic of 160mtr falling in pass band range of 7Mhz LPF. Similarly second harmonic of 80mtr falls in 7Mhz. So essentially such LPF will only be good for operating 7Mhz band (where its harmonics are on 20 and 15Mtrs will be filtered).

Next time someone says amplifier works well because of its efficiency etc. Also pause and talk about signal purity and distortion characteristics. No modern solid state amplifier is complete without LPF sections to cover each band of its operating frequency. Else you might be splattering all over other bands even with 100 watts of power output on fundamental operating frequency!.

Spending some time on understanding these aspects will keep us serene on the bands :)

VU2XE
Kiran