[CQ-Contest] Band reject filters

[Dick Green]

> Wonder if anyone out there uses 1/4 coax stubs as band-reject filters =
> for contesting, and if so, how effective are they? Are they broadbanded =
> enough to filter out rf across a whole band such as 3.5-3.9 mhz?

I use an automatically switched set of stubs for my SO2R station for both CW
and SSB contests. I cut the stubs for CW and that's where they seem to be
the most effective. That's because the bandwidth for maximum attenuation is
typically less than the width of the subband. However, on most bands I do
get enough attenuation in the SSB subband to be useful. At least I haven't
been motivated to cut another set of stubs for phone. Can't remember now
whether the 80M stub attenuates well on 75M, but I don't work a lot of phone
contests. You'll definitely need more than one stub to cover the entire 10M
band.

You will find lots of info on stubs at this URL:

http://www.berkshire.net/~robbins/technote/techref.html#filters

I should point out that maximizing antenna separation is typically more
effective than any kind of band reject filtering.

Hope this helps.

73, Dick WC1M



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>From Michael Tope <w4ef at pacbell.net>  Tue Mar 21 04:34:51 2000
From: Michael Tope <w4ef at pacbell.net> (Michael Tope)
Date: Mon, 20 Mar 2000 20:34:51 -0800
Subject: [CQ-Contest] Band reject filters
Message-ID: <005301bf92ef$1937f580$66efabce at neptune>



Hi Tom,

You should be able to get 25 - 30 dB peak attenuation out of a single
shorted 1/4 wave stub when operating it at the 2nd harmonic. I have
achieved 50 dB using a pair of stubs separated by a transmission line
which is a 1/4 wave at the reject frequency.  The bandwidth of a
single stub is fairly narrow (~3% @-20dB points, 10% @ -10dB points),
but you can broaden things out with the two stub system by staggering
the stub lengths a little.  Aside from cost, the other advantage over
the ICE/Dunestar type filters is power handling - you can run coax
stubs on the output side of your 1500 Watt PA, thus attenuating
harmonics generated in the amplifier.

At W6UE, we run a combination of ICE filters on the exciters, and
single coaxial stubs at the amplifiers outputs. This setup allows us
to run two 1500W stations on adjacent monobanders stacked only 10 feet
apart with no interference problems.

Good luck!

Mike, W4EF............................




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