What is it?

I built this gadget around 20 years ago. It’s been in use on a mountaintop ever since, until being brought down to my shop today. Three of the cylinders are marked “C” and the other three are marked “L”.

I remember having it the back of my 1966 Chrysler Town and Country station wagon parked across the street from the Federal Building in Moscow, Idaho right after the Oklahoma City bombing and wondering if I was going to get hassled for it.

The Gadget

The Gadget


18 thoughts on “What is it?

  1. Well, eriko, it certainly looks similar to the cans I’m slightly familiar with.

    Tuning the repeater cans. The cans are a resonant cavity, with the resonance frequency tunable by changing how far the rod is inserted into them. I’m going to refine the guess by tossing out cellular phone as the application.

    Not sure about “C” and “L” labelling. C could be for capacitance, and L for inductance, but the cans offer only inductance — I think. Maybe C and L are band identifiers?

    • I replied to erico initially, but you are almost entirely correct. C and L do indeed stand for capacitance and inductance. Those are components shunted across the input and output of each cavity, to modify the frequency response from a simple passband to a passband with a deeper notch (more attenuation) on one side of resonant freq. An air variable “trim” capacitor on each of three cavities, and a one half turn inductor on each of the other three. One puts a notch (more attenuation) in the response below the resonant freq and the other outs the notch above resonance, so the transmitter side notches at the receiver freq. and the receiver side notches at the transmitter freq. In short, they are ultra high Q filters.

      There are thus two assemblies of three cavities. Each would be referred to as a “series resonant filter” and the assembly as a whole is a “duplexer”. The repeater station’s transmitter is fed into one set of three cavities, and the receiver is fed from the other set of three. On the other side of the assembly the transmit and receive cables are coupled and fed to the same antenna. You can see the two feed lines coupled in the image.

      A well-built commercial duplexer may have only two cavities on each side, as they would be more efficient. This project is designed around being able to use readily available copper pipe.

      You got the frequency pretty far off. This (as one could guess by the length of the cavities) runs in the two meter ham band (around 144 MHz). The length of a cavity is roughly one quarter wave, or about half a meter.

      A duplexer allows the use of a single antenna for simultaneous transmit and receive on two frequencies that are VERY close together, which is pretty frickin’ amazing when you think about the amount of power going out from the transmitter verses the teeny, weeny, ethereal amount of power coming in from a 1.5 watt handheld using a crappy omni-directional antenna five or ten miles away. The frequency pair in this case was 146.100 & 146.700 MHz

      Having been in the music business I likened it to being able to hear an A 440 Hz signal through the filter clearly from across town, but being unable to hear a virtually “in tune” A 441 Hz blast from a trumpet in your face.

      I built it from instructions in a book, so I can’t begin to take credit for the design. The better it is tuned the more sensitive your repeater station will be (the better range it will have, being that receive sensitivity is generally far more important than transmitter output power when it comes to repeaters), so careful tuning of the assembly is paramount.

      • Well, I don’t know how I managed to not use the term ‘duplexer’ in my reply. The gory details of which are beyond my small electronics knowledge. I am a technician class operator, but not an EE, though I do know the dirt-level basics about filters.

        Can I claim I was thrown off by what looks like TNC connectors and something like RG58? When I’ve talked to folks about duplexers for a 2M repeater, they’re saying they use … I think it’s RG213.

        • The interconnects are RG55/U, using BNC connectors, which is all what the plans called for. About the size of 58 (goog eye) but quite different. I don’t recall its properties, it’s been so long, but you can look it up if you’re interested. It’s an ARRL project, so it should still be readily available. The antenna connection (and its Tee) are PL 259 / SO 239 “HF” connectors.

          I don’t expect I’ll be doing anything like that again, but it was fun.

          The local sheriff at that time was VERY interested in the project (this was before the proliforation of the cell phone) because that repeater had an autopatch (we had a phone line on it, so you could make regular telephone calls with it if you had the codes). The same man just brought it down to me today, twenty years later after the repeater radio went TU. The duplexer may end up on Elk RIver Butte, is the word. If not, I suppose I’ll be wanting to sell it.

  2. Yes! But what does it DO? More hints; It contains no active devices, i.e. no chips or individual transistors or diodes, and uses no power supply. It is entirely passive. The cylinders are copper (even though it’s home-built it wasn’t cheap. Commercially built versions are a bit smaller but far more expensive).

    • The Cylinders being copper, I think of how the apartment Rosa Parks lived in with her husband in 1955 when she was arrested for refusing to give up her seat on the bus in Montgomery Alabama was vandalized over the weekend by thieves looking for copper. If they knew such things as this were located on mountaintops copper thieves would surely risk microwave radiation to steal copper.

  3. I am always impressed by things like this, handmade things made by people whose day job isn’t to do such things. In my family there were quite a few such handmade things that either couldn’t be afforded, or the application required something so customized that it was the same thing. Unfortunately skills ran more to the “homemade” end of the craftsmanship spectrum, sort of like wartime Mosin-Nagants and Mausers, and the last-ditch Arisakas. They would work as designed, but would not win ANY beauty contests. Things made by Physicists out of plywood are always things of beauty for me for that reason.

  4. Well given that what little background I have in electronics is digital I am going to at least be satisfied with getting on paper. I think on analog electronic as being like calculus. Supremely usefully hard to keep in my head. If I work at it I can gain some understanding but if I do not use the knowledge frequently it fades quick.

    It is quite elegant.

  5. As a side bar, and because this stuff fascinates me so I like to prattle on about it, there is a law of rapidly diminishing return when it comes to repeater output power. First, more output power doesn’t help in two way communications unless it is accompanied by an equally more efficient receive capability (you may be able to BE heard, but it doesn’t help you communicate unless you can HEAR from the same distance). Given that the mobile stations for which a repeater is generally suited are not going to be super powerhouses (1.5 to 5 watts being the typical handheld power, and 50 or 100 watts being the typical car-mounted transceiver output). All else being equal, increasing the repeater’s output power will REDUCE the repeater’s receiver sensitivity, as more output will bleed over into the receiver and overwhelm weak signals from afar, thus reducing its effective range or coverage area. If you want to use power to boots the coverage area, you must ALSO boost receiver sensitivity accordingly, which would mean a far better duplexer and really high-end receive circuitry. The easiest method, rather than more output power, is a better antenna and/or better location. This repeater put out about 15 watts as I recall, but there are plenty of 50 and 100 watt repeaters. I talked to one guy who was working our little 15 watt repeater from Spokane, some 80 miles away, using a highly directional “beam” antenna and 50 watts out, the mountaintop location of the repeater being the primary facilitating factor.

  6. What does it do? It’s a pair of narrowband filters, one tuned to the transmit frequency, one to the receive frequency, both connected to the antenna. Its job is to keep transmitted signals out of the receiver so effectively that the receiver can be receiving at the same time as the transmitter is transmitting, AND the transmitter can transmit what was received, without the electronic equivalent of the “feedback squeal” so well known from PA systems. Given that the two frequencies are 600 kHz apart at around 145 MHz, i.e., about 0.4%, that requires very sharp and very effective filters. Cavity filters are among the best for that job.

    ni1d / ex-pa0pkg

    • Precisely! That thar be right.

      I refer to each cavity as the RF equivalent of a flute.

    • That analogy occurred to me too, last night a few hours after I wrote my previous comment. Actually, I was thinking about organ pipes (because they don’t have holes and only have one resonant fundamental frequency). It would be interesting to build a cavity duplexer for sound; it would show the analogy and also would be easier for people to perceive directly what is going on.

  7. You know, I’ve noticed a fairly decent overlap between ham radio operators, and people who like to shoot. Maybe because both involve pure physics.

    • You opened a can of worms there, my friend. I moved away from the musical instrument business and into the gun accessory business because I believed the same thing, that surely with firearms there is no room for emotional evaluation, “witchdoctorism”, cultural bias, et al. I was totally wrong. Further; the act of gun handling and shooting is very much psychological in nature, as is the motivation for getting into it in the first place.

      In both music and shooting, physics is of course extremely important, and yet in both PRACTICES, at the highest levels, the physics are forgotten to a greater and greater extent. I joked, during my pinnacle as the music instrument technician, that I was by necessity as much or more of a psychologist or guru than a pure technician. I still believe that. In fact, customers are/were often put off, or even become hostile in a way, toward a purely technical presentation.

      That’s a long-winded way of approaching the point, which is that, while both amateur radio and guns are very technical pursuits, those who shoot or use a radio may or may not have any real interest in, or comprehension of, the technical aspects. Some do and some don’t, and there is a continuum in between.

      What, I believe, motivates every one of us in ham radio and shooting however is the idea of self sufficiency and independence. There are other motivations, but I think these two are universal.

      AND SO, I am increasingly of the belief, lately, that physics cannot be separated from meta-physics (spirituality, reason and conscience verses emotion, i.e. the personal journey), that such is axiomatic, and that belief to the contrary is a denial of something uncomfortable.

      At one time I sought the purely technical, I now see, as an escape from the “human condition” or the tribulation into which we are all born, but after much struggle found it impossible. The technical and the spiritual (or meta physical) are one in the same, like space, time, mass and gravity, or like ice cream and cold. Or, to put it very simply; Science is an art.

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