[RFI] Balanced-line antennas and couplers

[Rob Atkinson]

It's a bit tricky because the current range depends on the impedance
of the line at the test frequency and where you are in the line.  I
don't now how to predict what that will be at the insertion point.
What works for me is assuming a nominal Z of 600 ohms for example,
which would require high power to provide a usable meter deflection,
if you use a 1/2 or 1 amp meter.  5 or 10 amp meters won't work well
because at high Z they require too much ham power to provide a useful
reading.   Keep them for the feedpoint of an unbalanced 1/4 wave
vertical or 50 ohm dummy load.  Initially it is a gamble about the
power needed to get a usable reading so a test source of RF that is
easily variable like a 200 watt solid state transceiver that offers
continuous variation of the PA collector current is excellent for
this.  Start out feeding the line with around 20 or 30 watts and take
a look at the meters.  increase the power until you see a reading that
is high enough to offer some comparison between the two meters.  If
you are putting out around 100 watts you may need to look for meters
that have a lower current range, maybe 250 ma.
This kind of work is great for a warm summer afternoon in a dead spot
around 3600 kc.  With a fist full of clip leads and banana plugs, and
50 ohm carbon resistors you can go around and try grounding, floating,
or terminating near field metal objects and other antennas.   I have a
160 m. inverted L and I found that it unbalanced the line on the 75 m.
dipole, unless I connected it to an open run of 70 feet or so of 50
ohm coax cable.  I guess the cable capacitance de-tuned it.  The
dipole is held up on one end by a tree and the other end by an
aluminum mast.  Grounding the mast unbalanced the line.  Floating the
mast above ground restored the balance or nearly does.  100% on one
side, 90% on the other.   It was surprisingly fun to experiment, but
take notes or you forget what works.

I guess one good thing about the poor band condx at least in the
daytime now on the low bands is that it should be easy to find a
no-man's land frequency to test on.  I usually do a CW ID.

73
Rob
K5UJ


On Fri, Aug 5, 2016 at 9:30 AM, Kenneth G. Gordon
<kgordon2006 at frontier.com> wrote:
> On 5 Aug 2016 at 5:19, Rob Atkinson wrote:
>
>> Balanced line operators need a pair of identical thermocouple amp
>> meters.
>
> Yes. Got any recommendations on which to use? There are at least a couple of schematics
> and articles on home-built units on the web. What do you use?
>
> I have a link-coupled home-brew balanced coupler I built many years ago which I am going to
> use with my "new" Vertical Vee-Beam antenna as soon as the Swainson's Hawk family which
> has taken up residence in my only support (a 110' tall Grand Fir Tree) leave this fall.
>
> So, I want to install a pair of RF ammeters to help me achieve balance.
>
> Ken W7EKB