[Elecraft] Remote Controlled Balanced Antenna Tuner

[Ron D'Eau Claire]

If you take a look at the schematic you will note that the ATU is a
simple pi-network with a balun at the input.

The fact is, you can use just about ATU in this way by putting a balun
at the input. A solenoid coil of good coax will work nicely. Use solid
dielectric to keep the center conductor from migrating through the foam
and shorting over time.  It may be necessary to decouple any other wires
(power, remote control, etc.) for RF using ferrite cores or rods and/or
bypass caps, since the "case" of the ATU may be "hot" with r-f. 

The inductances and capacitances in the ATU cancel out the reactance
presented by the radiator, and convert the resulting resistive value to
the 50 ohms your rig wants to see. Unless some of these components have
a great deal of loss, the matching network does NOT need to be
"symmetrical" or "balanced" for a good balanced feed.
 
I have done this sort of thing with manual ATU's and have been surprised
at how hard it is to produce an unbalanced feed when the feeders present
a low impedance load to the ATU.. I have found that it can make NO
difference in the balance whether I put a balun at the input or forget
about it. Indeed, I can hook an earth ground to the side of the feedline
attached to the ATU case and not see any significant difference in the
current flowing in that side of the balanced feeder. The reason is
obvious if you consider that when the feeder is presenting a very low
impedance load to the r-f most of the r-f will flow in the feeder
instead of any alternative paths. The balance is hardly affected even
when a "ground" wire is attached (so much for the quality of the r-f
ground provided by an 8-foot copper rod in wet soil outside the shack
window). 

When you feed higher impedances, connections to the "ground" side of the
ATU can cause an unbalance. These "connections" might be by direct
contact or capacitive. But with the ATU well isolated from ground there
shouldn't be any such effects. That's why it is a good idea to decouple
any power or control lines and the outside of the coax from any stray
r-f with "chokes" to keep the ATU "floating" for r-f. 

My experiences are with manual ATU's, as I said. I might question
whether small differences in the r-f potential within an automatic ATU
might have an effect on logic circuits that are not well bypassed or
otherwise well isolated. But I'd expect such problems to turn up with
ANY sort of antenna it might be used with. The ATU designer would be
very naive to assume that the case of the ATU was always held at a good
"r-f ground" in any Ham installation! 

Ron AC7AC
K2 # 1289


On my lot the most practical antennas are simple multi-band wire
designs. Unfortunately, there seams to be no way to avoid a lengthy run
of coax to get into the shack. With the KAT2 I get a good match to the
system I have now, but I am dissipating a lot of energy in the coax. I
also have problems with the coax radiating.  It seams what I need is a
remote tuning unit.

The perfect tuner for my needs would be:
1.   Remote mountable.
2.   Auto Tuning.
3.   Balanced output.

One and Two are available in several commercial units, but a balanced
output is rare. I recently found this unit
http://www.hamware.de/hardware/tuner402/at402-e.htm it is remote mounted
and has balanced output, but does not auto tune. Has anyone seen this
unit? What do you think? Jeff Burns AD9T