Topband: Filters for Detector

[Mike Waters W0BTU]

--- On Sat, 6/20/09, Richard (Rick) Karlquist <richard at karlquist.com> wrote:

> From: Richard (Rick) Karlquist <richard at karlquist.com>
> Subject: Re: Topband: Filters for Detector
> To: "Wolfert, William R." <WWolfert at columbuspolice.org>
> Cc: Towertalk at contesting.com, Topband at contesting.com
> Date: Saturday, June 20, 2009, 1:46 PM
> Wolfert, William R. wrote:
> 
[snip]
> 
> > #2 I'm building the W1MK detector (LBDX Ch. 11, pg.
> 26) and am including
> > the antennascope (6 dB Hybrid) on the same board. I
> also thought to just
> > build a BCB reject filter and PB filter into the same
> box. Less to carry
> > to the site is good, right? Since there's 2 channels,
> filters for each
> > channel would be indicated. For simplicity, a rotary
> switch to select
> 
> The gotcha here is that the filters would have to be
> matched, or
> they will decrease the accuracy.  Matching high
> selectivity BCB
> filters is a tall order, and no one sells matched sets.
> 
> Rick N6RK
>

Hello Rick,

Maybe I'm missing something that should be obvious, but I can't see where filter impedance matching should necessarily be a 'tall order'.

There are many ways to match impedances, and my favorite way is always the simplest one. For example, what about a simple PI or T resistive attenuator pad? Yes, there is a minimum loss depending on the difference between the input and output impedances.

Here's a link to a site with several impedance matching network design calculators, resistive and otherwise.

http://www.raltron.com/cust/tools/t_attenuator.asp

73 Mike