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Re: [TowerTalk] Stub Filters

To: "'Robert Pack, NX5M'" <>,<>
Subject: Re: [TowerTalk] Stub Filters
From: "Gary Schafer" <>
Date: Wed, 22 Mar 2006 18:04:14 -0500
List-post: <>
I am no expert on HF stubs but I can tell you how it is done at UHF.
On the 160 receiver you can only use a stub that will provide a high
impedance at 160. The line would have to be a shorted 1/4 wave 160. 
A shorted 1/4 wave length at 160 presents an open to the 160 receiver (high
impedance) and a short to the 80 meter frequencies as it looks like a
shorted 1/2 wave length to the 80 meter frequencies.

If you need more rejection than one stub will supply you can add a second
one on the 160 receiver line. You would want to put the second one 1/4 wave
length (80 meter 1/4 wave length) away from the first.

At the 80 meter transmitter you could install a 160 meter open 1/4 wave
length stub which would look like a short to any 160 meter noise. It would
be an open 1/2 wave circuit at 80 meters and look like a high impedance to
the 80 meter signals.

Again if you need more rejection you can add a second stub of the same
length 1/4 wave length away from the first. In this case the 1/4 wave length
apart distance would be a 1/4 wave length at 160 meters.

When adding multiple stubs you want each stub to see a high impedance to the
other stub. This gives the most effective rejection as the stub is working
at a higher impedance point rather than extremely low impedance that the
other stub presents to the line.

The idea is to null the transmitter frequency at the receiver and at the
transmitter null the transmitter broadband noise that falls on the receiver
frequency. You may not find any improvement by nulling the transmitter noise
as it may not be part of the problem.

It may be more effective to build a multiple stage L/C notch filters for the
receiver. Or add several stubs to the receiver as described above all with
1/4 wave length separation between each.

Gary k4FMX

> -----Original Message-----
> From: [mailto:towertalk-
>] On Behalf Of Robert Pack, NX5M
> Sent: Wednesday, March 22, 2006 2:33 PM
> To:
> Subject: [TowerTalk] Stub Filters
> If I were to put stubs on my 80 and 160 beverage jumpers (between the
> switch in the shack and the radio), would this create nulls for the
> beverages?
> My scenario....I have different sets of beverages for both bands.  But qrm
> between the 80 and 160 stations, when on at the same time, is bad enough
> that weak signals can be covered up.  I have bandpass filters in line with
> all stations as well as bandpass filters on each beverage jumper but there
> is still a bit of a problem.
> With that said, I know that a 92 foot shorted stub will null 80m and all
> bands above and a 46 foot shorted stub nulls 40m and up.  So what is best
> for nulling 80m signals from the 160m station?  46 foot open, 92 foot open
> or what?
> Like to hear from some experts on this subject.
> 73,
> Bob NX5M
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