[RFI] Balanced-line antennas and couplers

[Edward McCann]

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 at 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 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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