Balancing Act

[flintstone mop]

Hello everyone,
I installed a couple of RF ammeters on my balanced line and was going to experiment with the K1JJ tuner taps, etc. At first glance the one meter read 2.5A and the other leg was almost 3A. This was with 250 watts TPO.
I juggled the antenna taps on the coil and the Var cap taps and obtained almost perfect balance with the meters now reading 3A.
The transmitter did not like what it saw on its output. The Bird read over 400 watts and the reflected was 75W. The PA current was 450Ma.
I was able to re-tune the final and the loading back to the 275ma Plate current, and the Bird still reads almost 400 watts and 75W refl.
What did I gain or lose by obtaining a balance? The meters now read 2.5 amps.
Apparently the impedance on the TX side is no longer 50 ohms and who knows what the real power out is? My version of this famous tuner does not have the series cap connected to the link coil. Is this the culprit?
Go back to the original set-up? Or wait when MaNature gets a little quieter and use the ant system and ask for sig reports. Do you think I gained another S-unit??

[w4bfs]

hi Fred ... I would first want to know just how accurate the meters are ... go back to original tuning with 3A one meter and 2.5A the other and good match ... then without changing  anything else, swap the meters ... if js connections are required, but them in first ... interesting ...73...John

[WA1QHQ]

Fred,

The so called K1JJ tuner does not require a tuning cap on the link coil if the antenna you are matching is resonant or close to it on the frequency you are checking it out on.

The link coupled antenna tuner can be viewed as a balanced resonant auto-transformer with two taps on it, one for the 50 Ohm link and the other for the feed-line. Once you have resonated the auto transformer with the cap across the coil then what ever remaining reactance you have from the antenna that could not be tuned out by the main tuning cap/coil combination will be presented at the link.

Having used a balanced feeder with link coupled tuner for many years my experience tells me that you have an unbalance in your feed line assuming the antenna is cut for the band you are testing it on and is not highly reactive.

Did you factor in the internal resistance of the RF ammeter when you were doing your measurements?

[WA1GFZ]

Fred consider a common mode choke on the coax to keep RF currents off the shield  My fugly tuner has balanced currents and low VSWR. I would think the turns ratio of the JJ tuner critical for low VSWR.

[K1JJ]

Fred,

I'd agree with what the guys said.

It's good you're now fine tuning the system, OM.


In addition....

Remember that this tuner is simply a transformation or ratio match between two impedances. For example, if the tuner ratio is set at say, 10:1, and if the antenna load happens to be 750 ohms, then the input will be 75 ohms. This will appear as a 1: 1.5 swr to a fixed 50 ohm Bird wattmeter.  However, your transmitter may have a pi-net and will simply match this 75 ohms with no swr loss.  But, it is nice to use 50 ohms as a standard for meter accuracy, etc. You might JS in an input tuning cap as a test. Maybe there is some reactance that needs to be tuned out. I've found most situations did not need an input cap, but some did. 


Remember that this tuner was designed for simplicty first -  and uses an easy to find single section tuning cap, thus the floating components. About feedline balance -  It's not an easy thing to erect a perfectly balanced open-wire-fed dipole.  For example, if one antenna flat top leg is closer to the ground (on average) than the other, this will create an unbalanced condition to some degree. The same holds true for the feeders as they work their way into the shack. If the feeder run has one leg biased closer to the house or Earth, or tower, control lines, etc, then this will unbalance things too. By adding a slow twist in the line all the way through hairy areas, this will tend to randomize the effect. It all adds up.

I used to use small 6V flashlight bulbs across EACH feeder line in the shack to see brillance, thus balance.  500W would require about 3" of tapped feeder.  Make the bulbs JUST bearly lit. The eye is most sensitive to light magnitudes at low levels vs: bright bulbs.  This can be a comparison against your RF amp meters. You can come pretty close to perfect balance using this method by tapping the feeders on the coil in or out a few turns. I found most every antenna I've erected (open wire) was never perfectly balanced. As I mentioned, by biasing one tapped leg in or out a turn or two on the coil, I "forced" the feedline antenna currents  back to equal or into cancellation/balance.

Also, be sure your tuner tuning cap is floating high above the tuner chassis and the coil itself is horizontal and does not look physicallly closer to objects on one side than the other, just like your feeders.  Remember that everything after that 50 ohm link - everything - is floating and needs to maintain balance all the way to the ends of the antenna insulators. Even your insulators should be matched... :-)

But, off hand, I'd say that 2.5 amps vs: 3 amps is not bad for a begining situation.  BTW, the on-air difference between 2.5/3amps and perfect balance would be unperceptible. The antenna pattern will remain basically the same as well as the power out. It's splitting hairs, but always a good thing to do anyway.  Balanced antennas are very critical when using phased or stacked arrays, etc.

Good luck -

Tom, K1JJ

[W1VD]

I've purchased a number of new and used rf ammeters...the new ones (N.O.S) have  always been spot on but not so with the flea market specials.

Ac line current can be used to test them for accuracy. A digital voltmeter and suitable load are all that's required. 

 

[flintstone mop]

This was a nice time to get to know what goes on beyond the feedline. I'll try the series cap for the link coil for any matching issues and try to get the system back to 50 ohms as possible on the TX side. The original finding of 2.5A/3A will be ignored. May not be an issue after all.
I thought there might be some dramatic findings that would improve my signal on the air. I have not yet realized that there is a mileage difference from the East Coast and the Mid-West/ Western Pa. I had a prime QTH with a pure flat top dipole about 90 feet high.
My present QTH is in a small valley but the general elevation, here in Western Pa., compared to Oxon Hill Md. is much higher. And it probably doesn't make any difference, as we are most likely warming clouds with our VEEs and dipoles, on 160M
120 feet high diople is the beginning of an antenna system.

Fred

[AB2EZ]

Fred

Hi: see the attached .jpg

If (after to swap the meters to see if the readings are accurate) the currents in the two wires are not equal then (by definition) some of the current must be passing through a third path. The current in one one wire must be equal and opposite to the current on the other wire (the thermocouple-type rf ammeters don't care about the polarity) unless there is another path from one of the wires back to the tuner (without passing through the other of the two wires).

This usually means that one of the two halves of the antenna is coupling to ground more strongly than the other of the two halves of the antenna... and the associated unbalanced current is getting back to the tuner through a ground that is connected directly to the secondary of the balanced tuner's output coil.

Assuming the antenna is reasonably symmetrical, if you want to get the balanced antenna to work like a balanced antenna... you need to disconnect the extra ground path

[flintstone mop]

Thanks Stu
Tom also pointed out that even the insulators on the dipole can give the effect shown in your schematic. In my system I am aware that the wires on one side are draped in some tree limbs, but it's insulated wire....so far there's no smoking tree limbs. If there was 5KW or RF out there, there might be a problem.
The antenna edifices/trees at this present QTH are scarce. I miss the oak trees from the Maryland QTH.

Fred

[W2DU]

I too, have used a link-coupled tuner for many years. Guess I was lucky, because with RF ammeters in the open-wire line the balance was routinely within 2 or 3 percent. This was with the taps for the feedline at exactly equal distance from the center of the coil.

However, I could never get the input to the link to be 50 ohms without a careful adjustment of the cap in series with the link to cancel the reactance in the link.

I'm using a Drake MN2000 now, with an external 4:1 balun, but I still like the inherent balun effect of the link coupling better than the external balun.

Walt, W2DU