Regarding thermocouple RF ammeters, from my experience the calibration between
two identical meters (same range, same manufacturer, same type number) can vary
considerably. The best way to check is to put one of the meters in one side of
the line and measure the current, then replace it with the other meter and
measure again, to see if the readings agree. Best to do this several times
since random line voltage variations and even heating up of components may
cause the actual line current to vary a few percentage points in a matter of
seconds. Another way is to put the two meters in series on the same side of
the line and see how they compare, then exchange positions to make sure the
readings are consistent. Or else, connect the two meters in series and feed
60~ a.c. through them, using a variac, filament transformer and current
limiting resistor, and compare readings. The meter reading should be the same
at 60~ as it is, at say, 4 mHz; at 60~ you wouldn't get phase variati
ons when moving the insertion point of the meter a few inches.
If the meters are slightly off calibration with one another, make note of the
variance and use a conversion factor or make up a calibration chart to
determine identical readings. Once you are sure the meters are properly
calibrated (or readings corrected with conversion factor) don't worry if the
absolute readings is off by a few percentage points, or even 10%-20%, as long
as the two meters have identical calibration errors; what you are seeking is
any *difference* in currents in each conductor at a certain point along the
line.
With a symmetrical, balanced open wire line, tuned or untuned, feeding a
balanced load, unbalance in the readings is caused by common-mode currents
superimposed on the differential-mode currents. With no common mode currents,
the line current *has* to be the same in each conductor, since the outgoing and
return currents in any closed loop must be identical. If no common mode
current exists, the voltage loops and current loops on balanced tuned tuned
feeders will occur at the same points along the line. If a common mode current
on a transmission line (sometimes called "antenna current") exists, it may
shift the voltage/current loops and nodes of one conductor relative to the
other so that they are offset from each other along the line. At certain
points along the line the currents may read identical with the RF ammeters, but
if the meters are shifted up or down the line a significant fraction of a
wavelength, the current readings could be quite different. Imbalance in the
load will cause unbalanced readings for one reason and one reason only:
common mode current induced onto the transmission line.
Take my quarter-wave 160m vertical as an example. I use a 450-ohm UNTUNED
open-wire transmission line from shack to the base of the tower, feeding the
base of the vertical through a coupling coil wound over the cold end of a
parallel tuned circuit, the cold end grounded to the radial system and the
insulated base of the vertical tapped down on the coil to achieve optimum
match. Although I tried to eliminate electrostatic coupling between the
coupling coil and the main coil as best I could, some common mode current
still shows up, so that right at the coupler at the base of the tower, RF
current meter readings are the same, and a neon lamp lights up equally bright
when brought near either one of the OWL feeders. Further back towards the
shack, I can find points along the line where the neon lamp is very bright when
brought near one feeder, but I can practically touch the other feeder with it
and it won't light up, at least at lower power levels. OTOH, using a 450-ohm
non-
inductive resistor as a dummy load, the currents read the same in both feeders
and at any point along the line the neon lamps glow with equal brightness at
each feeder.
Even with the residual common mode current in my transmission line, the OWL is
still more efficient than a piece of fresh RG-213 feeding the vertical through
a matching L-network. Running the same DC input to the final and an rf ammeter
in series with line running to the base of the tower, I get a noticeably higher
RF current reading with the OWL than I get with the coax. A couple of years
ago I met the retired chief engineer at WSM, and mentioned the balanced
two-wire feedline they used to used with their big Blaw-Knox tower (the
feed-through insulators are still mounted on the walls of both the transmitter
building and ATU shelter). He said when they used that system, there always was
some unbalance in the two-wire transmission line, but it never caused them any
great concern.
Although not a problem in my case, it is possible that common-mode current (aka
antenna current) in a nominally balanced transmission line could be a source of
RFI from the transmitter, since what is happening is that the supposedly
balanced OWL is acting like a single-conductor long-wire antenna as far as the
common mode current is concerned, thereby increasing the RF field in the
vicinity of the transmission line.
Don k4kyv
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