[Amps] low pass filter fail ;)

[peter chadwick]

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


========================================
 Message Received: Apr 28 2014, 05:48 AM
 From: "Bill Turner" <dezrat1242 at wildblue.net>
 To: amps at contesting.com
 Cc: 
 Subject: Re: [Amps] low pass filter fail ;)
 
 ORIGINAL MESSAGE:          (may be snipped)
 
 On 4/27/2014 5:21 PM, Benedikt Sveinsson wrote:
 > 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
 
 REPLY:
 
 I wonder if the problem with the filter overheating might be that the 
 cutoff frequency is too close to the operating frequency?
 
 That's just a guess - I have no personal experience with this kind of 
 problem but that's what jumps out at me.
 
 73, Bill W6WRT
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