[Amps] low pass filter fail ;)

[Jim Thomson]

Date: Mon, 28 Apr 2014 08:32:04 +0200
From: peter chadwick <g8on at fsmail.net>
To: Bill Turner <dezrat1242 at wildblue.net>, amps at contesting.com
Subject: Re: [Amps] low pass filter fail ;)


With a VSWR of only 1.2:1, I wouldn't have thought it was too close to the cut off frequency. Passive LC filters (other than the absorptive types) work by having a high VSWR in the stop-band. According to the table in Geffe's book (Simplified Modern Filter Design), a VSWR of 1.22:1 represents a ripple of 0.04dB. I would suspect a lossy capacitor - we don't know the details of the filter designs and exactly what the capacitor was, nor the power level. You can have some high currents at quite low power levels.

vy 73

Peter G3RZP


##  2m is  144-148 mhz.   So where is the optimum cutoff freq ?      Heck, even with a 200 mhz  cutoff freq.... it will still kill the 2nd and up harmonics. 
Most of the problems I have had with LP filters for HF   is the caps..and their current ratings.   U are  better off to use say  2 x 100 pf cap   vs  a single 200 pf cap. 

##  I have one of last built henry radio  10 kw HF  LP filters..with  7-16 dins on each end.   They used a mess of 25 +  50 pf  NPO caps...and all are  HT-57 types.
They used a single 50 pf at each connector ..and in the middle of the filter, between the 2 x .25” OD  tubing coils, they used  8 x 25 pf doorknobs  in parallel. 
Cut off was at aprx 33 mhz. 

## HT-57 caps are rated for 15 kv...and in low values..are not only NPO..they also handle one helluva  lot of current.   LP Box was  aprx  5.5 inch square x 10-12 inches long.
Of course on the lower freqs, like  160-12m..the size of the LP box becomes huge. 

##  Those 5 kw band pass filters  made in the uk are slick.  band pass is better than  a LP.    They were band pass on all HF bands  except 160m.  
160m is a LP.  

##  LP filters  for  VHF is not that big a deal.    You can see the various LP  filters  available  for  88-108 mhz FM broadcast.   Those filters  are available  with any
cut off freq u want..and  any power rating too...like  25 watt  up to the  sky is   the limit. 

Jim  VE7RF