Construction Of Uhf Cavity Filters

Paul Franson WA1CCH Peterborough, N.H.

The Uhfit

A UHF wavemeter, detector, field strength meter, monitor, multiplier, filter, converter etc.

The other day, a visitor told me that he wanted to get on 432 mc, but hadn't because it's too expensive, too hard, too much trouble, too complicated, too difficult to find equipment and too time consuming.

That's ridiculous. Anyone who can solder can sit down for a few minutes with a few scraps of metal and a few other parts and build himself useful pieces of UHF equipment. This article describes one I call the uhfit (for UHF unit).

The uhfit can be used for finding bands, tuning transmitters and antennas, monitoring signals, filtering out unwanted signals (and filtering ¡N wanted ones), mixing for converter use, multiplying signals for local injection or

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Uhf Cavity Wavemeter

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Fig. 1. The Uhfit—□ UHF multi-purpose unit. This is the basic wavemeter-field strength meter. Modifications for other uses are described in the text.

Fig. 1. The Uhfit—□ UHF multi-purpose unit. This is the basic wavemeter-field strength meter. Modifications for other uses are described in the text.

test generator use and many other tasks.

So what is the uhfit? It's a simple tunable trough line cavity tuning 210 to 470 mc. Various "accessories" such as antenna jacks, diodes, wire links, transistors, feedthrough capacitors and other components are added for specific purposes as outlined in this article. The uhfit covers the 220 mc band for individualists, UHF aircrafts, the 420-450 mc ham band, the 460 mc citizens band and lots of whining radar.

Construction of the uhfit

The uhfit is most easily made from copper clad laminate board with the copper on the inside, but it can also be made from other materials. Probably brass makes the prettiest box; copper is nice but a bit soft. You can even use old tin cans. Tinjsn't the best of conductors, but good solder is 63%. tin and not many people bother to lose sleep over that. An advantage of solid metal over the copper clad board is that you can put a cover on the uhfit for neatness, higher Q (probably), and to keep the bugs out. There will be a small change in frequency range if you use a cover. I built my uhfits from the copper clad board and didn'-t put a cover on because of the mechanical problem of getting a good electrical contact.

The center conductor is a piece of copper or brass rod about 3" long. Yon can often find pieces of it in old TV sets as shaft extensions and most hardware stores carry it too. Be sure to tin the ends of the rocl and the matching part of the box before you try to stick them together. The best soldering tool for this—or any soldering—is an Ungar 47Ji watt long chisel tip.

The variable capacitor is not too critical. I used a small Johnson or Hammarlund miniature variable in mine, and some surplus capacitors are real nice. If you use a smaller capac itor, the minimum frequency will be higher, but the maximum won't change very much since most of the small capacitors have about the same minimum capacitance. Conversely, if you use a slightly larger capacitor, the minimum frequency will be lower than 210 mc, but the maximum frequency will decrease quickly since miniature capacitors over about 15 pf aren't very small. See the section on other frequencies if you want to fiddle.

We're now to the point where you have to decide what you're going to use the uhfit for. Chances are you'll want and need the basic wavemeter-field strength meter most, so I'll describe that first.

Fig. I and Fig. 2 show this basic uhfit use. In addition to the trough line cavity, you'll need an rf connector, some heavy wire, a diode and a feed through capacitor. I like BNC connectors. They are excellent electrically, easy to connect and disconnect, and plentiful. You can pay 35(f apiece for them surplus, but most hams don't. Fairly modern surplus seems to be covered with them and a good scrounger can get all he needs for very little. If you want to use other connectors, RCA phono type are probably as good as any for this use.

The feedtbrough bypass capacitor is not critical. Values from 100 to 2000 pf seem fine. There are many different types available from surplus, new, and from old TV tuners. Be fairly gentle with them since they're fragile. If you want to buy a new one, the Centralab MFT-1000 for 30(f from most distributors seems about as cheap as any.

I used a 1N82 diode in some of the uhfits, a 1N295 in others, and some unmarked detector diodes in others. I also tried many miscellaneous computor types out of curiosity and satisfied my curiosity. Stick to diodes designed for VHF and UHF use. Transistors Unlimited and others sell them at very reasonable prices. Solder these diodes quickly as thev're not fond of heat. VHF transistor collector-base and emitter-base junctions can work well. too. if you happen to have more VHF transistors than VHF diodes.

The loops for the antenna input and the diode aren't critical for most uses. You can even eliminate the loops and tap directly on the center conductor if you prefer, though that will broaden the tuning and change the frequency range somewhat.

Using the uhfit wavemeter

It's very simple to use the wavemeter once you have it calibrated. For that, see the section on calibration. Connect an antenna—usually a

The Uhfit. Note that the diode is tapped on the tuning fine. It was later found that a link was better for most uses.

short piece of wire—to the antenna jack and a meter to the bypass terminal and case of the uhfit. Use a sensitive meter or meter and amplifier (see the article in the January 73 for some excellent amplifiers) for low power oscillators and transmitters or a low range voltmeter or milliammeter for high power transmitters, but don't try to get too much or you may bake the diode. Then just tune the wavemeter for maximum output. You'll quickly notice that tuning is very sharp. In fact, you'll probably try to figure out a vernier arrangement or at least use a big knob after you've used the uhfit for a while.

The uhfit as a field strength meter

The same arrangement is used here. Connect an antenna to the antenna jack and tune for maximum meter reading. Use any old piece of wire for rough tuning 'err-a carefully made, balanced, isolated, matched dipole for antenna measurements.

The uhfit as a monitor

The uhfit can be used as a monitor of signal strength by connecting it to an antenna near your transmitter antenna. Then a meter in your shack wHITel! you instantly if something goes wrong with the transmitter, transmission line, or antenna.

You can use it as an AM monitor by connecting a pair of headphones to the output jack if your signal is strong. If not strong enough, use a 1 k to 1" M load resistor across the output jack and amplify the signal there with an audio amplifier such as one of the Lafayette $5 transistor ones. The feed through capacitance and load resistor should be compatible in regards to time constant; if that statement doesn't mean anything to you, don't worry about it unless the audio sounds "bassy". Then try a larger load resistor, or smaller feed


Fig. 2, Two uses of the Uhfit: o high Q tank to help eliminóte unwanted frequencies, and the basic wavemeter,


Fig. 2, Two uses of the Uhfit: o high Q tank to help eliminóte unwanted frequencies, and the basic wavemeter, through or check your transmitter which may be bassy. You can also use the uhfit as an audio detector for the horizontal plates of your scope.

You can monitor SSB signals with the uhfit if you happen to have a very stable oscillator to inject a carrier, but I wouldn't build the uhfit just for that.

The uhfit is good for monitoring CW if you have enough voltage output to key a small transistor oscillator.

ATVP Sure. Use the same scheme as monitoring AM if you like to listen to video signals. Come to think of it, they tell me that some devoted RTTee's can copy teletype: in their head. Maybe ATV'ers see the pictures when they hear video. . .

More useful might be to connect the output of the uhfit to the Z axis of your scope. You might have to reverse the diode if you don't like negative pictures. For horizontal and vertical sweep, you might be able to use the scope itself since you may be synched on the 60 cycle AC line in one way or another in this age of interconnected power lines. You'll probably have to clip off some sync pulses with diodes or transistors and reduce the feed-through capacitance. I haven't hied it.

You can also use the uhfit as a simple AM receiver for nearby planes or hams. Don't forget that that's not legal in the VHF contest anymore, though.

RF filter

This use of the uhfit requires a slight modification from the above uses. It's shown schematically in Fig. 2 as a hi-Q tank. Use another rf connector and loop instead of the feed-through and diode. Then the filter can be used in receiving to help keep unwanted signals—especially images and strong locals— out of your converter. Use large coupling loops for minimum degradation of your noise figure, and small loops for maximum rejection of spurious signals. These small filters don't work as well as large, silver plated coaxial ones, of course, but they do a pretty good job.

The same considerations apply for transmitting. The filters will help prevent UHF TV1 or AFI (air force interference, very bad) if you use a varactor multiplier on 432 mc without an amplifier. Don't try to feed too much power into the uhfit. of course.

The filter is also good for tuning up transmitters. Put it between the transmitter or multiplier and the dummy load to make sure that the power you're measuring is on 432 mc and not 288 mc or somewhere else.

Diode multiplier

In the basic uhfit, use a varactor or UHF diode (such as the Amperes 1N3182 at 85{) and rf connector instead of the detector diode and feedthrough. Here's one case where it's good to tap onto the line instead of using a loop. Feed rf into the varactor rf connector (input) and time the capacitor to twice or thrice its frequency. This provides a low level multiplier for local injection in a converter or a test signal. A resistor of 47 k to 1 M from the low frequency side of the diode to ground improved results for me.

NI ixer- converter

To make a simple converter from the uhfit. you'll need two loops with rf connectors, and one loop with a UHF diode and a low capacitance feedthrough capacitor (say 50 pf). You can easily make a capacitor of that value from a small piece of double coppci; clad fiber glass laminate or what have you. While it isn't critical since this converter isn't going to set the moon on fire, a convenient arrangement is to put the other loop behind the center connector.

One rf loop is for the antenna. The other is for the local injection. The.loop with the diode is the mixer and the output from the feed through is your if. Resonate a coil of wire with the feedthrough capacitor at the if you choose and use a loop of wire to couple to your if receiver.

For narrow hand work, you can use a simple crystal controlled local oscillator and multiplier. The best if for this would be a low noise six or two meter converter. For ATV, use a simple one transistor local oscillator and your TV set as an if. For wide band, modulated oscillator use, a superregenerative receiver such as the Sixer or Twoer or a cheap FM receiver is a good if. Good FM tuners (like my Scott) do too good a job of rejecting AM and you lose part of the modulation.

No one claims that this type of converter is the ultimate. However, with fiddling and/or a transistor preamplifier or two, it can do a very good job.

Tunable oscillator

The iihlit. can also be used as an oscillator in a. simple adaptation of the UHF dipmeter in the December 73. Simply use two feed-throughs (as in that article) and connect the collector lead about an inch or two up on the center line. Use a small loop for output. That gives you a simple UHF signal source, tunable local oscillator or low power transmitter. If you want to modulate the oscillator, you'll get mostly FM. Transistor oscillators love to FM. Don't expect too much.

Different frequencies

II you want to cover lower frequencies, your best bet is to make the box larger, the line longer and use a larger capacitor. I didn't try to get lower than two meters.

Higher frequencies are more fun. For 1215 to 1300 mc, make the box about one-third as long, but the same width and depth. Use a brass screw or brass core from a broken coil form with suitable nut or bushing as a capacitor, It provides just the right amount of capacitance as it goes into the open top of the line. Be careful that it doesn't short or you'll have a half wave instead of quarter wave line and twice the frequency.

For fixed frequency use in converters, etc., a glass or ceramic piston trimmer is far easier to tune than a variable air one since they provide at least a ten to one bandspread.


Calibration of the uhfit is easiest with a tunable calibrated oscillator such as the UHF dipmeter in the December 73. Of course, you need a wavemeter to calibrate that. . . Chances are that someone around you has one or the other. If people who volunteer to help calibrate wavemeters or dipmeters will send me their names and addresses, and those who want help will send a self addressed envelope or postcard, I'll try to get you together properly.

And finally, I make no claim for any originality for any of this, though none is taken from any rememberable source as I have a spongy mind. I also don't claim that the uhfit-is the ultimate in UHF equipment, but it's simple, cheap versatile—and fun! Why else build anything, . . . WAICCH

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    How to make uhf cavity filter?
    3 years ago

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