Micromatch SWR meter for balanced feedline

[k4kyv]

April, 1947 QST has an article on how to build a symmetrical SWR meter for a flat balanced feedline.  It has a range of something like 70 to 300 ohms, and pretty much functions like a coax SWR meter, once calibrated.

Has anyone tried building or using one of these?  Looking over the theory of how it works, I see no reason why its range couldn't be stretched to cover up to 450 ohms or maybe 600 ohms, if stray capacitances were kept under control.

I have been thinking lately about replacing the buried co-ass feedline between the shack and my remote antenna tuning units with homemade open wire feeders, using #8 copperweld spaced 1 9/16" apart, which would run about 325 ohms Zo, and operating it as an untuned flat line.  Of course, it would be elevated off the ground and not buried. The open wire line up the tower to the dipole would still function as a tuned feedline.  The micromatch would be used  in the shack to indicate SWR on the untuned open wire line out to the tuners.

I am thinking I might be able to get away with covering a larger chunk of each band without having to retune the ATU's this way, plus it would matter less if the open wire line did run a high SWR if I didn't readjust the tuner upon QSY'ing.  I may also  resurrect my remote tuning control for the ATU's with the reversible DC motor, this time using some of that low voltage direct burial cable designed for outdoor lighting, for the control cable.  If I buried it, maybe I wouldn't run over it with the lawn mower like I did last time.

Before making the changes, I would insert an rf ammeter at the output of the tuners and measure the feedline or antenna current at a known power and frequency in each band, to be able to compare the new system with the present, making the change permanent only if the efficiency is close to the same or better than what I have now.

Anyone have any experience using a "micromatch"?

[N3DRB The Derb]

don,

they made those units with both 300 ohm and 600 ohm couplers factory made. they are rare as hell, but I've seen them over the years.

[WA1LGQ]

I may still have one of those Don, its lost in the pile or gone. I will keep an eye out for it and hook it up again if I find it and see what happens. It was a commercial made thing........LB

[WA1GFZ]

I wonder if you could just run each feeder through 2 current transformers  as a pickup type bridge with high voltage isolation.

[W3SLK]

I have a commercially built Micro Match. It has SO-239 connectors on it but I think it had two binding posts for ladder line originally.

[k4kyv]

I think I have all the stuff here to build one.  The variable capacitor would have to be pretty wide spaced, 2 kv or so rating for full power modulated 100+% into 350 ohms flat feedline.  I have a neut. condenser that should do the trick, plus enough 10 ohm 2-watt carbon resistors to make up the special 1-ohm resistor per the 1947 article.  I think getting it to work at higher Z is just a matter of shuffling the fixed capacitor value a little and carefully keeping stray capacitances to a minimum.  It doesn't have to be extremely precise; just need something to indicate the reflected power null while adjusting the remote controlled antenna tuner.

[Todd, KA1KAQ]

I have one of the Jones MicroMatch units for balanced line, Don. Unfortunately, it's still stored up north. The box is maybe 6-8" across and has twist-on terminals on each end that seem to be spaced for TV twinlead or similar.  No external coupler or moving parts that I recall, just the coupling components/meter inside (if that's of any help).

[W4RFM]

Don, I have these. You are welcome to borrow either or both, and check them out.

Bob

[k4kyv]

Don, I have these. You are welcome to borrow either or both, and check them out.

Bob,

What impedance ranges are yours?  I see from the photos that the meter has a power scale and SWR scale, with a sensitivity option, but I don't see any adjustment for the surge impedance for the line it is designed to measure.  The one in the QST article uses a variable capacitor for the smaller of the two in the bridge, something like 5-15 pf, while the larger capacitor is a fixed silver mica about 800 pf.  You adjust the variable cap to set Zo, the surge impedance, which runs in the range of 70-300 ohms for the meter in the article. 

Surge impedance is determined by the ratio of C1 (the variable) to the fixed (mica) as a function of the ratio of the line series resistor to Zo:

C1/C2 = R1/Zo.

R1 is fixed at 1Ω.  So for a 450Ω line, the ratio is 1/450.

The fixed cap is 800 pf.  That means the variable cap should be adjusted to 1.78 pf, which might be difficult to achieve because that is less than the minimum capacitance of most small variables, and the inevitable stray capacitances.

But if C2 is increased to 2000 pf, C1 would be 4.44 pf for 450Ω line, which is more within the range of a small wide-spaced trimmer or neutralising cap.  C1 and C2 form a capacitive voltage divider, so the sensitivity would be reduced, but the original article calls for a 0-1 ma meter.  I  have several microammeters, some down to 50 microamps, so meter sensitivity shouldn't be a problem.

I would be interested in seeing how they did it in any commercial units designed to measure lines as high as 600 Ω as Derb mentioned.  I suspect the unit described in the 1947 article was meant for use with 300Ω TV ribbon, since most hams in those days who used 600Ω open wire line, ran it as tuned feeders and didn't worry about SWR (if they even knew what SWR was).

Why am I  concerned about this?

I am contemplating running open wire line from the shack to the base of the tower as a flat line to feed my existing antenna tuners at the base, which would continue to feed the tuned feeders going up the tower.  The reason I wouldn't simply extend the existing tuned line all the way back to the shack and run the whole thing as a tuned line is that the resulting tuned line would be over 260 ft. long.  Long tuned feeders @ several quarter wavelengths in length, become very sharp and critical in tuning, leaving little bandwidth to QSY without having to reset the tuner.  I have calculated that in my case, on 75/80m, the tuner would need to be switched from series to parallel feed or vice-versa when going from one extreme end of the band to the other, for any overall length compatible with other frequency ranges on other bands I want to run.  I suspect it would be extremely touchy to tune on 40m.  Plus I have figured out that I would need to add some additional length to the line and run it in some fashion like a zig-zag, to keep the total length away from an odd 1/8 wavelength at all frequencies, or else add additional lumped inductances or capacitances in the line to tune out the reactance, both of which would be added complications I don't want.

The present setup using coax line to the tower has reasonable bandwidth, +/- about 50 kHz without serious SWR, and I can get away with QSY'ing almost 100 kHz.  I  would simply replace the 50Ω RG-213 buried coax with elevated homemade open wire line, made of #8 copperweld and those little EF Johnson ceramic spreaders designed for the "Johnson Q" matching stub using 1/4" copper tubing.  Those insulators would give 1 9/16" spacing, and with #8 wire, would result in a surge impedance of roughly 375 ohms according to the charts in the handbooks.  With the open wire line, that  would allow much more QSY bandwidth, since open wire would tolerate substantially higher SWR with less loss and danger of arc-over on modulation peaks, than the existing coax. 

Plus I already have the wire and insulators on hand, and about everything I think would need to make the micromatch, as opposed to trying to locate a quantity of heliax or some other form of air-dielectric coax hardline, get it here, and then worry about keeping moisture out of it once it is installed.

[Steve - WB3HUZ]

Couldn't you determine if your line was flat with a bulb? Constant brightness across the length equals flat. Or how about a grid dip meter looking for peaks on the meter. No peaks, flat line.

Don, I have these. You are welcome to borrow either or both, and check them out.

Bob,

What impedance ranges are yours?  I see from the photos that the meter has a power scale and SWR scale, with a sensitivity option, but I don't see any adjustment for the surge impedance for the line it is designed to measure.  The one in the QST article uses a variable capacitor for the smaller of the two in the bridge, something like 5-15 pf, while the larger capacitor is a fixed silver mica about 800 pf.  You adjust the variable cap to set Zo, the surge impedance, which runs in the range of 70-300 ohms for the meter in the article. 

Surge impedance is determined by the ratio of C1 (the variable) to the fixed (mica) as a function of the ratio of the line series resistor to Zo:

C1/C2 = R1/Zo.

R1 is fixed at 1Ω.  So for a 450Ω line, the ratio is 1/450.

The fixed cap is 800 pf.  That means the variable cap should be adjusted to 1.78 pf, which might be difficult to achieve because that is less than the minimum capacitance of most small variables, and the inevitable stray capacitances.

But if C2 is increased to 2000 pf, C1 would be 4.44 pf for 450Ω line, which is more within the range of a small wide-spaced trimmer or neutralising cap.  C1 and C2 form a capacitive voltage divider, so the sensitivity would be reduced, but the original article calls for a 0-1 ma meter.  I  have several microammeters, some down to 50 microamps, so meter sensitivity shouldn't be a problem.

I would be interested in seeing how they did it in any commercial units designed to measure lines as high as 600 Ω as Derb mentioned.  I suspect the unit described in the 1947 article was meant for use with 300Ω TV ribbon, since most hams in those days who used 600Ω open wire line, ran it as tuned feeders and didn't worry about SWR (if they even knew what SWR was).

Why am I  concerned about this?

I am contemplating running open wire line from the shack to the base of the tower as a flat line to feed my existing antenna tuners at the base, which would continue to feed the tuned feeders going up the tower.  The reason I wouldn't simply extend the existing tuned line all the way back to the shack and run the whole thing as a tuned line is that the resulting tuned line would be over 260 ft. long.  Long tuned feeders @ several quarter wavelengths in length, become very sharp and critical in tuning, leaving little bandwidth to QSY without having to reset the tuner.  I have calculated that in my case, on 75/80m, the tuner would need to be switched from series to parallel feed or vice-versa when going from one extreme end of the band to the other, for any overall length compatible with other frequency ranges on other bands I want to run.  I suspect it would be extremely touchy to tune on 40m.  Plus I have figured out that I would need to add some additional length to the line and run it in some fashion like a zig-zag, to keep the total length away from an odd 1/8 wavelength at all frequencies, or else add additional lumped inductances or capacitances in the line to tune out the reactance, both of which would be added complications I don't want.

The present setup using coax line to the tower has reasonable bandwidth, +/- about 50 kHz without serious SWR, and I can get away with QSY'ing almost 100 kHz.  I  would simply replace the 50Ω RG-213 buried coax with elevated homemade open wire line, made of #8 copperweld and those little EF Johnson ceramic spreaders designed for the "Johnson Q" matching stub using 1/4" copper tubing.  Those insulators would give 1 9/16" spacing, and with #8 wire, would result in a surge impedance of roughly 375 ohms according to the charts in the handbooks.  With the open wire line, that  would allow much more QSY bandwidth, since open wire would tolerate substantially higher SWR with less loss and danger of arc-over on modulation peaks, than the existing coax. 

Plus I already have the wire and insulators on hand, and about everything I think would need to make the micromatch, as opposed to trying to locate a quantity of heliax or some other form of air-dielectric coax hardline, get it here, and then worry about keeping moisture out of it once it is installed.

[W4RFM]

I really don't know about their impedance, I assumed 50 ohms, and you know what happens when you assume? I was actually more concerned with the frequency than the impedance.
Again, you are welcome to look them over.

[k4kyv]

Couldn't you determine if your line was flat with a bulb? Constant brightness across the length equals flat. Or how about a grid dip meter looking for peaks on the meter. No peaks, flat line.

What I want the indicator for is to remotely adjust the tuner at the tower base when I QSY.  I have a 12V reversible DC motor that is capable of rotating the variable caps in the antenna tuner, and changes direction of rotation by reversing the DC polarity.  The idea is when I QSY, say from the Ghetto to 3685, to put the transmitter in tune mode and inject a little rf at the new operating frequency into the antenna, enough to get an indication on the micromatch, then remotely adjust the tuner until the reflected power dips to a null.  Running around outside with a bulb on the feedline, moving variable taps on a tuning stub, or sweeping across the band with a variable signal generator would be impractical for a quick re-tune for QSY purposes.  This is to be an operating, not a test, procedure.

This is how I remotely tuned my 80m dipole on 160 before the ground radials for the vertical were installed.  It worked fine with the coax transmission line until one  day I forgot and ran over the DC control cable with a lawn mower. The tuning had to be done remotely because that setup was so sharp tuning.  I had to readjust the tuner to QSY more than about 3 kHz in either direction.

[Steve - WB3HUZ]

Roger. That's different.