[Elecraft] Bandpass Filters

[Don Wilhelm]

Narrow tunable filters are amazingly simple to construct.  A variable capacitor and an inductor forming a parallel tuned circuit will do the job with good to great selectivity depending on the Q of the inductor.  If you form the inductor with a toroid, coupling into and out of the filter is relatively easy - in most cases a 1 to 5 turn link at the input and another at the output will provide 50 ohm connectivity.  

You can figure that the parallel tuned circuit will have about 2000 ohms impedance, and the turns ratio to couple into a 50 ohm load is SQRT Z1/Z2 or 6.3 - if you have a 12 turn winding on the inductor for the parallel resonant circuit, a 2 turn input and output link would allow connection into a 50 ohm system. 

With a 365 pF variable capacitor (with a minimum capacity of 40 pf), you can expect an octave or more of frequency coverage - just two inductors should be sufficient to tune the entire HF band spectrum 80 thru 10 meters - 6 uH will tune 3.5 to 10.2 MHz and 0.7 uH will tune 10.0 to above 30 MHz.  6 uH is 35 turns on a T50-2 core (10 turn links) - use #26 wire, and 0.7 uh is 13 turns on a T50-6 core (2 turn links) - #22 or #24 wire should do fine.  

You should find that the selectivity is sharp enough that you will have to re-peak it over a single ham band for maximum signal (particularly 80, 40 and 30 meters).  You can expect a few dB loss, but the the rejection of off frequency signals may be more benefit than the trouble of re-tuning.

Bandpass filters are complex, but simple tuned circuits are easy to implement.

73,
Don W3FPR

> -----Original Message-----
> This link might be of interest to you.
> 
> http://www.bobsamerica.com/swl
> 
> Endless  data for building a receive preselector, that could
> easily be bypassed on transmit. While bandpass filters maybe offer
> better covenience and performance  to signals  outside the band you
> operating on, a preselector  also helps protect your front end from
>  nearby close  stations. Theres Pro's and Cons for both filters.
> 

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