The AM Forum

The SplatterView discussion reminded me...

Does anyone know of a revised circuit for the Micro Match whose range goes all the way up to 450Ω?

The Micro Match is an instrument that samples the RF passing through a balanced feed line and provides an SWR reading at full transmitter power. This is a balanced SWR meter, for parallel conductor line, not one designed for 50Ω or 75Ω coax. One of the first amateur SWR meters, the article was published in QST in the late 1940s. I have the original article, but the unit as described has a range only from 50Ω to 300Ω. I have seen later articles, but they all have it revised for unbalanced coax line.

After studying the theory of operation and schematic, I don't believe it would be too difficult to make it work up to 450Ω or higher, particularly at the lower amateur frequencies, which is all I am interested in for this project. Because of the capacitance and resistance ratios required, this might be problematic for frequencies higher than the 40m band because stray capacitances could swamp out the low-C leg of the bridge.

This would involve building up a prototype and getting it to work properly. If somebody somewhere has already published a successful revision, or designed a similar unit that works at the higher Z range, that would save me from expending a considerable amount of time re-inventing the wheel.

I tried a web search, but all I could find were designed for typical Hammy Hambone 50Ω coax feed lines.

Hi Don. Let me know if the attached is the correct network. I remember building one as a kid and had to dig into my notes. If this is the correct version, then I think I can help you quick. If not, let me know the correct version or sketch.

As I recall, two ports, J3, J4 dependent on frequency out to the ant. I placed a 450 ohm terminations there. A set of mutual coupled L's (at right angles to each other), a ganged 250 pF cap, a 340 pF var coupling cap and of course the SWR bridge system. The TX port to the radio is on the left.

Regards, Alan

That looks like a matching network, 50Z unbalanced to 450Z balanced. The Micromatch is only a measuring instrument, 450Ω in, 450Ω out, balanced all the way.  The meter indicates reflected power. You use it as a tuning aid, by adjusting the load at the far end of a flat, untuned parallel conductor line, for a null in reflected power.

The old fashioned way for measuring SWR, before WWII, was to independently measure rf voltage or current at two points along  he feeders, 1/4 wavelength apart, and actually determine the difference as you move through a quarter wave of feed line.

I've got one of the original open wire/ribbon line MicroMatch boxes here, Don. It came down from the northern storage recently. Has the original box and manual with it, so I can scan and post once I figure out which box it's in. If that's of any help.

Ah, my misunderstanding. I'll see what I can find in my files.

Leacher Lines? Is that what they were called?

A Lecher line for 40, 75 or 160m would be long. You would have to build the Lecher wire analyzer outside.

Don,

Maybe one of these will do?

Figure 20 SCHEMATIC OF BRIDGE FOR BALANCED LINES:
http://www.super-phantom.ru/arh/111-eng/246/

or
       -
Figure 17 twin lamp SWR Indicator
http://www.super-phantom.ru/arh/111-eng/245/

Jim
WD5JKO

Good Evening. Not quite what you are looking for, but what I originally built a few years back to handle both my coaxial and open wire antenna feeds. What I built is essentially as shown in the attached figue except that I added a 3 position switch to allow for it's use as an antenna switching unit for each of the appropriate antennas. I still use the unit on HF, but now have a Micranta 3 Range Power/Modulation/ SWR Tester inserted in the line from the transmitter to it's input. The attached is from the 1978 Radio Amateur's handbook, page 584. Sorry about the quality of the copy. While not automatic in tuning, it is relativly simple and easy to use. In closing, why not just use an input balun, 450 to 50, feed via coax into a conventional SWR/Power Meter and output from the Meter via coax to an output balun, 50 to 450. Just a thought. Thanks for letting Me comment.   Tony W1ADR

Tony, your attachment apparently didn't come through. I would like to see exactly what you have; I'll check and see if I have that handbook. Maybe you  could re-send the attachment?

The configuration this would be used with has no 50Ω TX output feeding a 9:1 step-up balun or tuner; the final tank is inductively coupled to a balanced, resonant coil that feeds the 450Ω line directly. If I didn't have +HV on the tank coil, I could omit the inductive coupling altogether and simply clip the 450Ω line right to the balanced tank coil in the push-pull final. Or a balanced pi-network could match the push-pull final stage directly to the 450Ω untuned balanced line.

Jim, the "bridge for balanced lines" shows a 250Ω resistor in series with each side of the feed line.  Wouldn't that generate a lot of insertion loss plus burn up the resistors? I suspect that circuit is meant to be used as a test instrument running at very low power rather than an in-line reflected power monitor.

I have considered something similar to the twin lamps, but modified to accommodate 450Ω open wire line instead of TV ribbon. The Micromatch seems like a better idea, since the rf sampling unit can be contained in a small box instead of consisting of a cumbersome section of transmission line.

Leacher lines work at VHF and UHF. A somewhat similar device has been used for HF that has a meter inside a frame, designed to hang on the OWL, and you slide it along the line to observe changes in the  rf current/voltage at each point; once the reflected power has been nulled out, the reading remains constant as the device is slid along the line. I recall seeing a description, in the LaPort antenna handbook maybe. Again, a test instrument, not practical for use as a reflected power monitor during normal operation.

Don, yes that balanced bridge would be built for a "tune" power, maybe 10 watts depending on the resistor wattage used. Could still do that with a big remote 4PDT relay bypass switch. Then use the twin lamp concept sized for "full strap" power. Once the SWR on the line is 1:1 (non resonant feeder - since your tuner is at the dog house), a twin lamp setup at the shack end of the feeder should only have one light glowing.

Tuning at lower power is a good thing.  ;D

That Russian site I linked too has some interesting discussions after you let Google translate it. I liked the one about a combined bathroom where toilet and sink are in the same room.. :'(

Jim
WD5JKO

Good Morning. Sorry about the picture, will try again. Although; not quite what you are looking for, the details for the 9 to 1 balun in the article might be appropriate for Your application. As previously noted, the use of two baluns, one (9 to 1) at the input of a conventional 50 ohm coaxial SWR/Power meter and the second (1 to 9) at it's output might be appropriate. Particularly since You are link coupled and can easily be inserted between the link output and line. Secondly insertion losses should be at a minimum like what would be expected without the step down and step up provided by the baluns.

Still having trouble with picture file, will try later   Tony W1ADR

http://www.storm.ca/~nshepherd/VE3OWV/450%20ohm%20VSWR%20Bridge.pdf

This link may be helpful. There was another paper written by a VK2 that also treated a 450 ohm balanced bridge but was challenged from common mode signals upsetting the reading. I will forward if I can find.

One possibility is to use the lower power instrument to make tuning charts for each band, and afterwards just replicate the settings.  However, I would like to be able to monitor on-line reflected power in real time, which might be useful in the event of icing, rain or antenna/feedline/insulator failure. I am not particularly interested in absolute SWR readings, just monitoring the null in reflected power. One possibility would be a twin lamp type of deal, substituting a resistor and diode rectifier to feed a remote meter for the lamp.  But then, there is the certainty of lightning destroying it within a week of completion.


Outside of North America, in the home the toilet is usually a separate room from the bathroom, although there may be a small sink in the toilet for washing up after the job is done. The bathroom has a tub and/or a  shower, a larger sink and often a bidet. In older hotels, even expensive ones, it is not uncommon for the toilet and bathroom facilities to be located in the corridor, shared by all guests on that floor. Sometimes you are charged a small fee for using the bathroom, and the employee hands out towels when you make the request. A room with private bath might cost 50% or more extra.  Newer hotels usually have private bath in every room. If you ask someone in the street where a "bathroom" is, you will likely be pointed towards a public bath house. You still commonly see high-tank toilet fixtures with pull chain.

Don:
Here is a link with a little tid-bit of information.
http://www.qsl.net/vk5br/MeasBalLine.htm
Chuck