Equal Currents On Feedline

[N7ZDR]

I am wondering what would be the simplest way to check feed line current on my balanced line? I dug around in the junk box and found a couple to these meters. I could mount both meters on the wall right near the feed point on the secondary coil?? Could I just wrap a few equal turns of small gauge wire around each line to pickup any current. Or maybe incorporate a couple small Snap-On cores into the mix.----- Just thinking out loud. 
Larry

[W7TFO]

Two little toroids with a turn or two secondary may do the trick for sampling.

Swamp your diodes with as high Ohms as possible so the meters will read more linear.

Have a switching arrangement so you can isolate the meters during storms.

73DG

[N8CMQ]

Here is one idea, another from the old days used neon lamps on each side of the line.

http://www.philipstorr.id.au/radio/seven/three/transline%20test%20box.pdf

[N7ZDR]

Thanks for the suggestions----- I had been using the light bulb trick but it seemed like I could really never find the right spot on the feed line- Oh I found it ok, but with three (3) tuners on the table it gets to be a bit of a challenge. The feed lines coming in the shack are coated (not bare wire) and this also doesn't  help in finding the sweet spot easily.

All my wire antennas are strung through heavy foliage/trees so I know I would benefit from something a bit more permanent.

Regards,
Larry 

[Steve - K4HX]

If the currents are not equal, what will you do?

[W1ITT]

If the line currents turn out to be unequal, it's a matter of asymmetry, either at the load or in the coupling network.
One would look at both the line and the antenna for obvious signs of proximity to something that isn't mirrored on the other side.  For instance if one half of a dipole is in the clear, and the other half runs low over a steel roofed barn, you have a problem.
The other place to look is in the tuner/coupler. Assuming it's link coupled, is the link in the middle?  If the input link is resonated with a capacitor to ground, physical center may not correspond with electrical center.  If the output taps are off center, or if their wiring to a switch is cattywompus, that may be a clue.  If the antenna has asymmetry, you may be able to cadge it into balance by offsetting the tuner symmetry.  This works for amateur radio, but I wouldn't be able to get away with it in the shortwave broadcast world.  As I recall, most of the customers who think to ask for line balance measurements look for -20 db or so. The toroid measurement method referenced above will give you the most reliable results.  Do the best you can. 

[K1JJ]

If the currents are not equal, what will you do?

It's not a pretty thought.

[KB2WIG]

The seas of the four directions—

all are born of one womb:

why, then, do the culombs and swrs rise in discord




klc

[N7ZDR]

Here is a picture of one the tuners. What to do if the current is not equal??-- using the light bulb trick I was able to get both lights equal with use a small adjustment on the tuner (usually in or out 1 additional coil space) However that did not always indicate the lowest SWR.

[Steve - K4HX]

Interesting. I would think that unless it is severely out of balance, it's probably not worth worrying about. This would be especially so if you are stuck with a particular geometry/arrangement of the antenna wires.

I've heard some changes can be made with the tuner. I never tried with my balanced tuner. Probably due to laziness.

[KA2DZT]

The currents in a balanced feed line are governed by the antenna at the end of the feed line.  If the antenna is not balanced or equal in all respects the currents in the feed line will not be balanced.  The feed line can also be thrown out of balance by nearby objects too close to the feed line along its route.

Problem is trying to accurately measuring the feed line current.  The only way to do this is with two accurate antenna current meters place at the same location in the feed line.  Other methods using some sort of coupling on the the feed line may not be too accurate as the amount of coupling may be unequal.

The OP pictured two microamp meters in his first post, never thought it would work to any degree of accuracy.  Those meters would most likely need shunts to start with, which can be made to a high degree of accuracy.  The coupling still remains a problem.  If the meters pictured are DC movements, this creates another problem.

Solution, use two thermal couple type antenna current meters.  The two meters would have to be calibrated to see if they both read the current equally, this is relatively easy to do.

Fred

[Opcom]

The 50 Ohm setup makes some sense.

I thought it might be interesting to use with an open wire feed and random impedance and a zero center meter.. Not sure of the circuit is right for a zero center meter or not.

but at any given power level the great variation in current depending on whatever the place on the untuned 'balanced' feeder would mean range switches, etc.. may as well go with thermocouples if something's to be kept inline. But those have frequency range limits as well.

As a test instrument, fine.. but not something I'd like for hands off automatic use.

[KA2DZT]

That circuit would work FB.  But, you still have to check the coupling of the feed line with the coils.  Unequal coupling for any reason would give false balance results.

[w1vtp]

If'n I ever get ready to build a LCT, this design looks great to me.  The current read transformers look pretty good to me.

http://www.wz5q.net/index/shack_data/tuna.htm

[AB2EZ]

This thread piqued my interest in balanced lines and antennas fed with balanced lines.
The more I thought about it, the more I realized that the subject of balanced line-fed antenna systems is more complicated than I thought it was.

Some examples:

1 It is possible to have a perfectly symmetrical dipole (including all the the surrounding objects... and a perfectly symmetrical balanced line feeder (including all of the surrounding objects)... and still end up with unbalanced currents in the two wires of the balanced line feeder.

2. It is possible to have perfectly balanced RF currents (equal in amplitude and 180 degrees out of phase) at a point along the balanced feed line where you measure the currents, and have unbalanced currents at another point along the feedline that is closer to the antenna. Therefore, measuring the balance of the currents at a point along the balanced feedline that is close to the transmitter may not tell you much about whether the currents are balanced further along the feedline.

Note that the presence of unbalanced currents in some portion of the feedline implies that there is some common mode current... and therefore some RF being radiated by that portion of the feed-line.

As an aside, it is often convenient to describe the currents in the two feedline wires as the sum of a common mode current in each wire, plus a balanced current (equal and opposite currents in the two wires). If the currents in the wires are I1 and I2, then:

I1 = Icom/2 + Idiff
I2 = Icom/2 - Idiff

Where:

Icom = (I1 + I2)
Idiff = (I1 - I2)/2

Stu

[K4RT]

  it seemed like I could really never find the right spot on the feed line- Oh I found it ok, but with three (3) tuners on the table it gets to be a bit of a challenge. The feed lines coming in the shack are coated (not bare wire) and this also doesn't  help in finding the sweet spot easily.  

Interesting, but I don't understand the why of it.  Does your transmitter require an exact balance?

[W1ITT]

Brad..
I'm glad you asked!  The transmitter doesn't require, but may prefer the situation where its load is balanced.  If we are running push-pull finals, and unbalanced lod will translate to unbalanced operation of the tubes.  With a single-ended setup, it doesn't matter as much.
As far as the antenna goes, unbalanced line implies common mode currents, which will translate to feeder radiation and RF in the shack, which brings up its own bag of worms.
Most important, if things are unbalanced at the antenna, that perfect textbook figure-8 pattern will be skewed.  In most practical ham installations, with other stuff in the near field, it's already skewed.  However, at a real short wave broadcast installation, with acres of clear field, the skew will be noticeable in that the azimuth pattern will be twisted, maybe enough to lower signal levels in the target direction.  Many of the short wave broadcast antennas I have built have switchable slews, like plus and minus 15 and plus and minus 25 degrees, besides the boresight direction.  If feed is unblanced these slews not only go "somewhere else", but also their pattern gets distorted, again with less than the expected microvolt level at the target.
In ham radio, we mostly put up our antennas and let the RF fall where it may.  But it's good engineering to do the best we can with what we have.  I work very hard to keep the big curtain arrays balanced.  I don't work quite as hard with my wire arrays at home.  The pay is better when I'm working on the big stuff!
Norm W1ITT

[W3GMS]

Brad..
I'm glad you asked!  The transmitter doesn't require, but may prefer the situation where its load is balanced.  If we are running push-pull finals, and unbalanced lod will translate to unbalanced operation of the tubes.  With a single-ended setup, it doesn't matter as much.
As far as the antenna goes, unbalanced line implies common mode currents, which will translate to feeder radiation and RF in the shack, which brings up its own bag of worms.
Most important, if things are unbalanced at the antenna, that perfect textbook figure-8 pattern will be skewed.  In most practical ham installations, with other stuff in the near field, it's already skewed.  However, at a real short wave broadcast installation, with acres of clear field, the skew will be noticeable in that the azimuth pattern will be twisted, maybe enough to lower signal levels in the target direction.  Many of the short wave broadcast antennas I have built have switchable slews, like plus and minus 15 and plus and minus 25 degrees, besides the boresight direction.  If feed is unblanced these slews not only go "somewhere else", but also their pattern gets distorted, again with less than the expected microvolt level at the target.
In ham radio, we mostly put up our antennas and let the RF fall where it may.  But it's good engineering to do the best we can with what we have.  I work very hard to keep the big curtain arrays balanced.  I don't work quite as hard with my wire arrays at home.  The pay is better when I'm working on the big stuff!
Norm W1ITT

Norm,

If the push pull final is link coupled, the balance of the antenna has no effect on the tubes.   The tubes pull current based on the isolated load through the link.  If you did not have a link and capacitively coupled into the push pull tank coil, that would be a different story.

Balance feed line current is good to keep RF out of the shack and a worthy goal to shoot for, but has little practical differences in the performance of the antenna unless you have a sizable amount of imbalance in the feeders.     

Joe-GMS 

[W1ITT]

Joe..
About a decade ago, I was in a post cold war Eastern European country which shall remain nameless installing a modern broadband shortwave curtain.  The transmitter was an ancient commie rig with big glass finals and peekaboo windows, in push pull, link coupled output.  The crew there maintained concern about output balance affecting the finals. I agree with you that the effect through a link is small, but I would add that it's not zero.  And of course one doesn't want to argue with the customer over the small stuff, so I kept my trap shut.
As far as skew effects on the radiation pattern from unbalanced feeders, the effect is not inconsequential.  If the department of propaganda, whoever it may be, wants to dump a strong signal on a population center a thousand or two miles away, remember that higher gain arrays make fairly tight patterns, and a divergence of 100 miles at the target can translate to quite a few decibels. 
For hammy hambone, with a dipole hanging from the apple tree, it doesn't make a darn bit of difference as long as we manage to keep RF out of the audio chain and don't get nipped by a hot microphone.  But it's fun to do the best we can, and try to understand what we are giving up.  I often wonder about the effort put into really good audio when most of us are now at the age where our eardrums are fossilized.  It's for the technical challenge and the pride of "doing it right".  I have set up my network analyzer on my own balanced feeders, with toroidal coupling transformers  to make good measurements, but most of the time, I just set a book on the key and see how much arc I get with a pencil on each side of the line.  Close enough for non-government work...
Norm

[Opcom]

If'n I ever get ready to build a LCT, this design looks great to me.  The current read transformers look pretty good to me.

http://www.wz5q.net/index/shack_data/tuna.htm

That would be the way to do it!

[N7ZDR]

Thanks All-- This has also turned in a great thread that really makes one think about it a bit?

I tend to agree with all the posts in that we all want to achieve the best we can do. At a point we say "That's good enough" and try to just be happy. But we are really never happy because there is something in our brain telling us "you know- that could be better".

On a different note--My wire antennas are all over the place in the trees around the house, I just try to get them up and straight as possible. I am sure I am just worrying about something I should loose any sleep over.
Thanks
larry