[TowerTalk] Twin-coax balanced line and Ant Sys

[Chuck Counselman]

"Chris BONDE" <ve7hcb at rac.ca> wrote:
>I think that I remember something from long time ago.  Twin lead was 
>inexpensive, coax was expensive, dipoles multi dipoles and 
>non-resonant dipole were fed with twin lead or home-brewed twin 
>lines.

I remember this, too; and it remains true today; doesn't it?


>Now, I think that this was terminated by an open air coil with multi 
>taps outside of the house.  The tapes were moved for the best 
>connexion via two pieces of short length coax to the tuner or pie 
>net work next to the Tx.  Is this correct?

I don't remember hearing about an open-air coil outside the house; 
and, anyway, an open-air coil outside seems to me a bad idea.  It'd 
be unsafe; insects and small birds would nest in it (Yeah, I know, 
not for long; but there'd be residue.); and, where I live, it'd be 
full of snow and/or ice for much of the winter.

So let's assume that we enclose it.  Changing bands would require a 
trip outside (Bummer!) unless you had a remotely operable switch or 
relay arrangement.

Now removing my Grinch hat....

I have a very nice remote antenna tuner, made by RCA apparently for a 
military or commercial application, and which I picked up  as 
surplus, which does more or less what you describe.  It's in a large 
weather-tight metal box with a removable, gasketed, access door and 
two big beehive ceramic feed-throughs for connecting balanced 
open-wire line to go to the antenna.

Inside is a 3-inch-diameter tank coil with several balanced pairs of 
taps, and a big ceramic rotary switch for selecting a tap-pair.  The 
taps can be repositioned on the coil manually, and continuously 
rather than just discretely, with a screwdriver.  The switch is 
operated by a stepper motor.

An additional deck of switch contacts selects a resonating capacitor 
for each pair of coil taps.  The capacitors are big ceramic 
transmitting types.  A screw-terminal strip is provided for changing 
capacitors.  (There's enough room in the box that I've considered 
installing one or more continuously variable capacitors, for speedier 
tuning.  However, a continuously variable capacitor is not strictly 
necessary, because the coil taps are continuously variable.)

The input to this tuner is by a single coaxial cable and is 
link-coupled to the big tank coil.  I suppose that you could connect 
the coax shield directly to the midpoint of the tank coil, and the 
coax center-conductor directly to a point on one side of center; 
however, link coupling is superior because it provides excellent 
common-mode isolation.

I haven't used this tuner yet, but some day....

73 -Chuck, W1HIS