[RFI] IN-band Filters

Jim P jvpoll at dallas.net
Tue Nov 7 23:11:58 EST 2006


Low Q?

What are you using - RG58?

Q, related to cable loss, is much reduced for the larger 
diameter coaxial cables, with measured Q's of over 1000 
obtainable with 'stubs' made of Heliax at Six meters (for
instance), but I must confess, not everyone has the ability 
to obtain, construct and transport a "stub-based in-band 
filter" made out of 1 1/4" or 1 5/8" Heliax for use on Forty 

A couple of years back I went though the exercise of designing,
building and testing an in-band filter for use on the PSK31 seg-
ment of the 20 Meter band against the majority of the phone 
portion of that same band; I ended up fabricating the capacitors 
from Duroid uWave board material since the ESR of dipped 
silver Mica caps caused them to quite literally smoke at the 5 - 
10 W power level of a PSK31 rig owing to the very, very high 
currents appearing in the filter elements given the design and 
'sharpness' of that particular filter.

Jim P // WB5WPA //

----- Original Message ----- 
From: "Tom Rauch" <w8ji at contesting.com>
To: "Bryan Swadener" <bswadener at yahoo.com>; <RFI at contesting.com>
Sent: Tuesday, November 07, 2006 5:26 PM
Subject: Re: [RFI] IN-band Filters

> >  I'm looking for some form of filter for the purpose of 
> > reducing interference between stations operating on the 
> > same band (ie, fone-cw).  This would be used in a planned 
> > multi-transmitter contest setup during Field Day 2007. 
> > The bandwidth and slope would need to be good enough to 
> > prevent fundamental overload of (and possibly damage to) 
> > the receiving station.
> You typically won't be able to have significant attenuation 
> even with very good L/C components and stubs with their 
> inherent low Q are out of the question.
> I was somewhat successful using toroids with a Q unloaded of 
> 500 or so in a well designed filter, but tuning of the 
> filter was critical.  Stubs actually have terrible Q, 
> especially down on 80 or 40 meters.
> The real key is antenna separation and good radios.
> 73 Tom 

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