[RFI] IN-band Filters

Tom Rauch w8ji at contesting.com
Wed Nov 8 06:00:06 EST 2006

> Stubs?
> Low Q?
> What are you using - RG58?

I replied in the context of the question, HF filters.

I have some 2 meter duplexers that work very well at 600kHz 
spacing, but they aren't quite practical to scale for 80 
meter for the next  field day. I wouldn't suggest they are 
practical for HF filters, especially on 80 and 40 meters.

> 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
> Meters!

You are forgetting the cable length problem also, not just 
the diameter scaling.  On lower bands the cable length is 
longer, adding loss. For example my own measurements of open 
circuit 1/4 wl stubs have shown the lowest terminal 
impedance of good quality RG-213 is over 1.2 ohms on 80 
meters. The same cable cut for 28MHz had less than 0.45 

I have an 80 meter pass trap 40 meter reject harmonic "trap" 
made from a shorted 1/4 wave LMR-400 cable, and it is about 
1.4 ohms on 40 meters. It also passes the entire 80 and 75 
meter band on 80 without a switch in length. That isn't that 
good for field day with two transmitters onm the same band, 
is it? I actually use it because it is cheap, simple, and 

Anyone thinking coaxial cable stubs made from typical coax 
at HF have high Q better think again. Coax makes a poor 
capacitor, poor trap, and poor inductor on HF...especially 
lower HF...compared to conventional transmitting-style 
inductors and capacitors.

73 Tom 

More information about the RFI mailing list