Don:
Nice report!
Could you address the difference between "tuned" and "untuned" feeders to which
refer.
I have yet to read a description of this phenomena or technique that I can
honestly say I fully understand, or that I can repeat to any other interested
party.
Other sages on this forum are invited to respond.
Thanks all,
73
Ed McCann
AG6CX
Sent from my iPhone
> On Aug 7, 2016, at 6:33 PM, Donald Chester <k4kyv@hotmail.com> wrote:
>
> 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 varia
ti
> 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 t
he
> 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 no
n-
> 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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