[TowerTalk] Changing Air Variables in an Antenna Tuner?

[K8RI]

----- Original Message ----- 
From: "Dan Zimmerman N3OX" <n3ox at n3ox.net>
To: <kd4e at verizon.net>; <towertalk at contesting.com>
Sent: Saturday, March 17, 2007 10:31 PM
Subject: Re: [TowerTalk] Changing Air Variables in an Antenna Tuner?


>> I'd like to improve performance on 80 & 160M
>> QRO and read of one Ham changing the C1 240pf
>> input series cap to 750pf and C2 114pf to 675pf.
>
> Why not add some 250 or 500pF doorknob caps that can be switched in
> in parallel with the current capacitors?
>
Hey! I was going to suggest that.
Actually it's a good way to increase the range without giving up the 
resolution of the smaller tuning cap.
There are a number of amps that take that approach with their final tuning 
one being the old HT33 series by hallicrafters that uses a simple knife 
switch tied into the band switch to add capacitance on 80.

> The actual switches will take some care, but maybe some vacuum relays
> could be used for an elegant solution, or knife switches with pull
> cords or push rods through the outside of the case for a cheap
> solution.

Take a look at how the old hallicrafters switches work in those HT33s. They 
are not much more than a plated copper arm that pivots on a screw through a 
piece of plexiglass

There's also a cam on a shaft tied to a knob, or push-pull switch, disk of 
plesiglass with arm mounted on one side.  In the case of the old B&W tank 
coils/band switch we tied the caps to the 80 meter position. Let your 
imigination be your guide.

and good luck,

Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
www.rogerhalstead.com (Use return address from home page)

>
> You are certain to give away some high-end tuning capability because
> the minimum capacitance of a big air variable will probably still
> present a fairly low reactance on upper HF.
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