[RFI] IN-band Filters

Tom Rauch w8ji at contesting.com
Wed Nov 8 08:09:29 EST 2006

> Thanks for the quick replies.  In the interest of
> maintaining Q, I have access to tight tolerance
> porcelain (very low ESR) capacitors and up to #10
> polyimide-coated wire.  However from your experiences,
> it sounds like my idea would be wasted effort.

Most good capacitors are far above what you can do with the 
inductor. The highest Q reasonable size inductors I have 
ever measured just approach Q=1000. Those are large 
transmitting inductors used in high power BC transmitters 
like the big edge wound or copper tubing coils you see in 
5-10kW transmitters. #8 tinned copper buss wire in good form 
factor can provide a Q of 500 or more.

I even tried using 1/2 inch silver plated pipe inductors two 
feet tall and 18 inches diameter with vacuum caps for lower 
HF without success.

Every inductor design program I have tried greatly 
overestimates Q.  At least according to my HP4191A, which is 
often consider the standard of component measurement 

> We'll use your suggestions: wide antenna separation +
> good radio equipment.

That's about all that will work, other than bandpass crystal 
filters or other expensive systems. By the way, if you use 
cross polarization be SURE the dipole is broadside to the 
vertical. As we move from perfectly broadside the 
polarization of a dipole tilts more and more vertical. 
Nearly off the ends the dipole will have maximum vertical 

73 Tom 

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