[ARC5] Balanced antenna coupling...

[Kenneth G. Gordon]

On 25 Dec 2014 at 8:38, AKLDGUY . wrote:

> Some of us have not enough space for a full size dipole, so
> a 50 or 72 ohm match is impossible. In my case, there's only
> about 6 meters,

A little over 18 feet for us "'muricans".

> from fence to fence so I'll be forced to set up
> a very short dipole.

You might consider making that an inverted-vee dipole, or even going 
"diagonally" from fence corner to fence corner. That would give you a bit 
more length to play with. I used that combo, at a height of about 30 feet (10 
meters) for many years as my only station antenna when I lived in Missoula, 
Montana. I was quite pleased with its performance.

Mine was about 130 feet long, however.
 
> What I want to try is an (unbalanced) 2-wire feed from the
> BC-230 to a link coil wound at the centre of a matching coil.

Yes. I used that too. 

However, if you make the link tunable by adding a fairly high capacitance 
tuning cap in series with the coil to ground, it will make it considerably 
quicker to set up for a particular frequency or band. You may wish to make 
the link coil adjustable for inductance, less inductance for higher frequencies.

In my case, I wound the link-coil out of 1/4" copper tubing and mounted it 
INSIDE the main balanced coil. That coil had two sets of taps: one for the 
open-wire line, and a second set to adjust the coil "length".

> The bottom end of the link (connected to the G terminal of
> the BC-230) will form a 'ground' terminal.

Or to the rotor of a tuning cap, with the stator grounded.

> There will be a split stator capacitor across the coil with its
> frame connected to the 'ground'. Taps on each side of the
> coil's centre will go out via 300 ohm ribbon cable to the dipole.
> The capacitor may need to be made up of 2 separate units
> and a gearing arrangement, as split stator capacitors of the
> expected size are not readily available.

VERY true. I had a main tuning split-stator capacitor from a Wilcox 96A 
which I used in my coupler. The Wilcox 96A used a pair of 450TLs at over 
3KW plate modulated AM, so it is pretty substantial. It is a real "bread slicer".

> This classic balanced matching system of the 1930s before
> coax came into widespread use should work,

Yes. It will work very well. The only thing making it less common these days 
is that it is very "un-handy", requiring a lot of manual energy entry to make it 
work correctly.

> but I'm not
> expecting DX contacts. I'll be happy with local NVIS contacts.

I expect you will find that it will work better than you suspect it would.

Actually, if you diagram the entire link-coupling system, including the coax 
and the final tank Pi-network circuit, you will find that it is actually two 
parallel-tuned circuits that only LOOK series tuned where two of the tuning 
capacitors in the link circuit (the Pi-network loading cap and the link-tuning 
cap) are in series across the link coil, with the center-connection of those two 
capacitors grounded.

The Pi-network is built the same way, in fact.

I would think that a circuit-analysis of what amounts to two Pi-networks, 
connected end-to-end, might prove to be very entertaining. ;-)

Or maybe it is simply a Pi-L network. I dunno. Anyway, it worked extremely 
well for me.

The way I operated my coupler was to center the Pi-network loading cap, the 
splt-stator cap, and the link cap, then resonate the final with its plate tuning 
cap, then adjust the taps on the balanced output coil for a minumum SWR, 
then go back and tweak the other three caps for maximum output and 
minimum SWR.

I went through all the bands and MARS frequencies on which I expected to 
operate, and wrote up a chart with all the appropriate settings. Moving from 
band to band didn't take long after that.

In point of fact, the link-tuning cap is simply an extension of the loading cap, 
and can be used as a loading cap.

Ken W7EKB