Date: Mon, 28 Apr 2014 08:32:04 +0200
From: peter chadwick <g8on@fsmail.net>
To: Bill Turner <dezrat1242@wildblue.net>, amps@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
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|