[Amps] low pass filter fail

[Jim Thomson]

Date: Fri, 25 Apr 2014 21:49:47 +0000
From: Benedikt Sveinsson <benedikt at ok.is>
To: Amps <amps at contesting.com>
Subject: [Amps] low pass filter fail ;)

I have been building a 2M SSPA - final part in the puzzle has been the low pass filter.

After building this filter, copied the design from W6PQL - but I have limited resources here in TF land, so I don't have access to good PCB materials. But the filter was measured using a mini VNA Pro analyser with good results.  1:1.2 SWR on the operating frequency and cuttoff starts around 150Mhz

well, after transmitting about 10s I noticed a drop in output power, I took the cover off and aimed the camera while transmitting

This is what happened on youtube:  https://www.youtube.com/watch?v=9lU3I3BgmkE

I suspect there is something else wrong, as this happened very fast 10-15s - and solder melts!

Guess I will go and find teflon sheets and make my own capacitors with copper ;)


73, Benni TF3CY

##  Do you really require a LP filter ?   You could also use a .25 wave long coax stub..with far end shorted.   You can also add additional stubs etc. 
Paralleling caps is the real answer to your exploding cap problem.  2-4 caps in parallel  would be the ideal ticket.   Your typ 2m ant will only be resonant on
its 3 and 5th harmonic.   You can make stubs that will kill odd harmonics if I remember correctly. W2VJN has a good book on stubs. 

## Parallel some caps..and you wont have any more issues.   I have used the teflon sheeting sandwiched between 2  aluminum plates,,and that works superb...
provided it is constructed correctly.  

##  I like the idea of a LP filter......... then it kills everything beyond the cutoff freq.   A bandpass filter would even be better.    You could also use a LP filter
for TX....and a low power bandpass filter...for RX only.  

Jim  VE7RF