K.W (E-ZEE) match balanced output unbalanced

[G4_Pete]

I measured the current on the back of my K.W (E-ZEE) Match feeding a ladder line into an offset dipole. Using a TEK P6021 high frequency current probe on 80meters, 3.6Mhz I expected balanced currents but that is not the case I get more than double current from one terminal , swapping the ladder line still gives more current from that same terminal. For those not familiar with the K.W (E-ZEE) the balanced output coil is fully floating ,not tied to the ground terminal or using any ferrite unbalanced to balanced devices.

I can get a match for the transceiver but the unbalanced current is not what I expected , normal searches have not helped explain this. Has anyone done a similar test or can explain what is happening?

Pete

[Radio Wrangler]

Originally Posted by G4_Pete

still gives more current from that same terminal.

Same ATU terminal or same feeder terminal?

Capacitive coupling tends to unbalance the E-ZEE match somewhat. I've got a KW 109 in use at home, open wire feeders up to an as-symmetrical-as-my-place-allows doublet. There are some current probes, somewhere.

I'm away from home, but if I come across the probes, I'll have a look. Current transformers can be done easily enough, I suppose.

David

[G6Tanuki]

Offset dipoles [where the feeder is joined elsewhere than to the centre point] are known to have issues with asymmetric feeder currents (or 'outer braid' currents when coax is used). There's quite a bit about this in the Les Moxon antennas book.

[GW3OQK Andrew]

Pete, I have done that same test with a Z-Match, same circuit as E-zee match. I have RF ammeters in each line, so I could see (example) left line low. Swapping the feeder at the Z-match terminals maintained the unbalance as expected, left line still low.

I didn't like the Z-Match and now have a homebrew balanced tuner and the unbalance is maintained. As it happens the balance is fine on some bands and unbalance reversed on others. Its a doublet with 450 ohm line. Look forward to hearing what you find with the E-zee match.
73, Andrew

[G6Tanuki]

Offset-feed antennas are always problematic - the big issue being that the RF field they generate is presented assymetrically to the feeder [which acts as a vertical antenna and so gets secondary currents induced in it assymetrically].

Even with a proper centre-fed antenna this can be a confounding issue if the feeder is not led 'squarely' away from the radiating part

Unpredictable capacitance-to-ground between the two 'sides' of an open-wire-fed dipole [or its "This cannot Work" - G6XN - end-fed Zepp] brother.

Just because you can get ~good RF current~ flowing into an 'antenna' from your transmitter does not mean it will radiate well; I can design you a short bit of open-wire feeder that will give you a nice SWR and draw lots of RF-Amps but really won't radiate anything worthwhile.

[G3PIJpeter]

The KW ATUs were based on the old ARRL Handbook multiband no-switches 'Z-match' circuit, in which both output links have one end near the earthy ends of the main tuning coils. The circuit is inherently unbalanced. Balanced lines require a balanced ATU.
Peter G3PIJ

[G4_Pete]

Thanks for the responses to date. I have done some more tests .
Two 220 Ohm resistors to ground and the currents look balanced.
Apologies I may have got the feeders crossed even though I marked them as it seems the same lead has the greatest current now.
There is obviously some issues here that I need to research /test a bit more carefully.

I guess any capacitance effects due to inherent Hot/Cold end coil imbalance will be more pronounced depending on the feeder impedance presented to the match unit.

Pete

[G4_Pete]

I went back and re tested with some photos to avoid any errors using two identical current probes to remove any probe unbalance effects. This was done at low transmit power with the probe/scope calibration set to 200mA per division.

One leg of the ladder line was designated as feeder A and the other as B .The first two checks were with the probes both clipped to A then both to B to prove the current probe system was working correctly.
The bottom trace probe was then clipped to feeder A and the top trace to B. The bottom probe on feeder leg A shows less current.
The feeders on the back of the KW match unit were then swapped over without unclipping the probes. This eliminates probe swap errors.

The results show feeder A takes less current either way it is connected to the KW coil, slightly less one way round to the other as just visible in the photo.

So in conclusion my opening post was incorrect in that I must have got confused trying to do the measurement with just one probe.

However I had thought the "balanced" coil would have forced the currents to be equal at the back of the KW match, particularly at the relatively low frequency of 3.6Mhz unit but this turns out to be an incorrect assumption.

I am still struggling to understand this fully as in my mind if this was completely down to "Hot" end vs "Cold " end coupling one terminal on the balanced coil would always have a greater potential relative to earth and hence always try to "Push" more current.

I have tried to be careful in the measurements but unless I have made more measurement errors can anyone explain further why the currents appear to be imbalanced across this isolated floating coil.
Pete

[GW3OQK Andrew]

Pete, glad you got your test set up going now. Your offset-dipole is an unbalanced aerial. The short side has less capacity to ground than the long side, likewise inductance, so the currents in the feeder can not be equal.
73 and MX, Andrew

[Radio Wrangler]

If the ATU behaved as a perfect floating source, there could be no imbalance in the feeder currents because there would be nowhere else for current to flow, and Kirchoff gets in on the act. What your unbalanced antenna would then do would be to have unbalanced VOLTAGES on the two wires of the feeder. Ever get that feeling you just can't win.

The KW ATU isn't perfectly balanced, but it's quite useful never the less. Attempts to add a balun after an unbalanced ATU like the transmatch design have worse issues.

It is hard to design a balun with good bandwidth. It is very hard to design a balun to work over a wide range of impedances. The ATU into Balun approach needs a balun good over the frequency range AND the impedance range you want and you can only make things to handle smaller portions of that territory.

Anyway, your measurements look to be making more sense now.

David

[G6Tanuki]

While theorising is all very well, the pragmatist in me says "does your antenna let you work the stations you want to?" - if so it's fit-for-purpose and so go on and natter on-air rather than worrying about RF current-imbalances!

[most of my HF antennas would not survive any kind of detailed analysis - but I can happily work into Greenland on 80M with only 50 Watts of SSB]

[Radio Wrangler]

For the past several years I've been using an Icom IC7700 into a KW 109 supermatch (EeZee match with metering) driving the doublet over my house.

It works.

I'm not aware of anything that would be noticeably better on the ATU front, or of any better antenna that would fit into the space. G3ROO on a visit was trying out a K3 he's bought, using my tuner and antenna. Worked Japan within minutes of setting-up.

David

[Peter G4OIM]

The E-Zee match works for me.
I have a tiny doublet in my tiny garden. It is just 6m of wire fed in the middle with 300 ohm ribbon cable so just 3m each side. The 300ohm comes into the window then crosses the room to the atu.
I use this on 17m, 15m, 12m and 10m. It is only resonant on 12m where it is a half wave but loads up on all these band without trouble. I have made a lot of contacts on all of these bands using about 60w ssb or 20w cw.
I have a TV in the room with a set top aerial and it doesn't like 2m but there's not a flicker on it when I'm on hf even if I put the 300ohm next to it. This suggests that the is little radiation from the feeder. Recently, there was a problem with the TV when I transmitted and I found that one of the connections at the back of the atu have come out. Put it back-problem solved. I guess that with just one side of the 300ohm connected there WAS radiation. Bottom line is that for me, the E-ZEE match works.
Cheers,Peter.