The FCC rules state that repeaters are not legal for use on the CB band. But that small detail never stopped those of us who had some knowledge, and a lot of free time, from experimenting with them when the need arose, or when curiosity overrode any desire for legality.
The first time that this subject was explored was in the late 70's, when I was a member of the Channel 13 group. At that time, I had a Radio Shack 3 watt, 4 channel walkie-talkie which I used to monitor and talk on the channel when I was out in the yard, working on the car, or hanging out at a neighbor's. Most of our CB group were clustered in an area which was about 2 to 3 miles away from my house. While I could listen just fine with the W/T, my transmit signal was relatively weak and subject to being "taken out" by any skip interference, distant chatter, or errant carrier thrower that happened on frequency. I had thought about running a coax lead from my radio room to the outside, so I could use the base antenna on the walkie-talkie. While the gain of the base antenna would certainly help my signal, it turned out to be impractical as it "tied" me to the cable, and I lost mobility. So now what?
I then thought about making a repeater. A repeater is a device which takes a signal, which it receives on one frequency, and re-transmits, or "repeats", it on another frequency. Sounds like a fairly simple concept right? You take one radio, which receives on one channel, and then couple its audio output to the mic input of another radio in order to re-transmit it on another channel. You control when the transmitter keys with a voltage from the receiver which becomes active when the squelch breaks. However, nothing in life is ever as easy as it first sounds. The main problem with using a repeater, would be one of receiver de-sense. Due to the close proximity between the transmitter and receiver, every time the output transmitter comes up, it will overload and swamp out the input receiver. Most commercial and amateur radio repeaters solve this problem by either seperating the transmitter and receiver by some distance, or using an extremely narrow bandwidth filter device called a "duplexer", to provide the necessary isolation between the transmitter and receiver. This then allows both receiver and transmitter to use one antenna, without the transmitter de-sensing the input receiver. Due to the frequency wavelength of the CB band though, the size of each can of a cavity duplexer (and you would need at least 4 cans) would be close to 9 feet tall! Not too practical, nor would it be cheap. After rejecting the idea of using a duplexer, the only other practical option was to use separate antennas, and placing them as far apart as practical. While separate antennas will not provide as much isolation between the transmitter and receiver as a cavity duplexer, they can provide some isolation. Usually, the greater the distance and polarity between the antennas, the less the de-sense. Also, the farther apart the transmitter and receiver frequencies are spaced, the less de-sense. Finally, limiting transmitter power will also reduce de-sense. But the fundamental problem remained, how could I run either the receiver or transmitter off of my base antenna, and rig up a second antenna which would work almost as well as the base antenna? Experimenting with separate antennas revealed that the only way that I could make this work at all, was to run the receiver frequency as far away from the transmitter frequency as practical. (Within the availability of standard synthesizer CB crystals) I also found that I could only get away with using a small, back of the set antenna for the receiver. Considering all these factors, as well as the remaining de-sense, and the useful range of the input receiver was only good for less than a 1/4 mile. Pretty feeble for a true duplex repeater. With few options left, I abandoned the idea of making a full duplex repeater and decided instead, to make it a one-way "transmit extender" only. I would receive Channel 13 directly on the walkie-talkie, but would transmit back through the repeater, (Since I should be close enough to overcome the de-sense). I would use an input frequency that was as far removed from Channel 13 as practical, and the repeater's transmitter would be hooked to the base station antenna, which would then hopefully provide me with a competitive signal, that would not be taken out by a little on-channel interference.
Now for the details of the actual construction. I took a basically junk "parts" 23 channel radio to use for the input receiver. I reversed the synthesizer 2nd oscillator crystals, and aligned it for maximum sensitivity on the image frequency. I tapped a voltage from the squelch switch transistor in the receive radio, to provide a carrier operated switch (COS) to operate the transmitter. I coupled the low level audio from the receiver's detector, to feed into the mic jack of the transmit radio. To help minimize de-sense, I pointed my beam antenna away from where the receiver antenna was located, and I oriented the receiver's antenna for the least amount of interference. I also lowered the power of the transmit radio to around 2 watts, as I felt this was a good compromise between minimizing de-sense, and still having a fairly strong signal on Channel 13 (my whole purpose for building this). I chose the receive crystal frequency for Channel 3 for the input, as this was the farthest frequency away from Channel 13 which I had for the walkie-talkie. I put the Channel 3 receive crystal (26.530) in the transmit slot of the walkie-talkie, and put a normal receive crystal for Channel 13 in the receive slot. I set the input receiver radio to channel 3 (26.530 image).
Once built, I had to tweak a few things like audio level, and receiver performance. But I soon had it working. I could walk around outside, up to a block or two away and have the ability to listen to Channel 13 directly, while transmitting through the "repeater" so that the locals could also hear me fairly well. I used this setup for the rest of that summer. But like most of my other experimental novelties, I eventually lost interest in it and moved on to other things. But these early repeater experiences laid the foundation for when I would do it for real, on the 220 Mhz ham band, almost 10 years later.
The sequel, or "Return of the CB Repeater" saga, came about a few years later around 1983. Interest in strange, and increasingly off the wall projects had become much more intensive. This was due mostly to Art who, thanks to a second shift work schedule and a steady diet of Budweiser, was totally into the outer limits with the things that he was doing, and he had the shocks and burns to prove it. His projects and experiments provided a sort of competitive drive which kept me focused on similarly bizarre escapades of my own. Also, at that time, we had been plagued by some clever and persistent carrier throwers and agitators. They would appear, almost nightly, and attempt to cause trouble with the locals on channel 30. When one of us would jump to the car to attempt to track them down, they would sense what was happening and disappear before any of us could get close. The thought had occurred to me, that it might be helpful to give them the illusion that I was still on my base, when in fact, I was out looking for them in the car. But how to do this? I once again thought about using a repeater. Remembering the past problems that I had had with de-sense on the last repeater attempt, and also not wanting to bleed over the agitators and blow my cover when I got close, I decided that using the CB band as an input would not be an ideal solution. With my recent acquisition of a ham license, and with a growing cache of 2 meter VHF equipment, I decided that a cross-band configuration would work out much better. There was enough frequency separation between VHF and CB, that de-sense would not be a problem. And since I had a base station antenna for VHF, as well as CB, I could count on fairly good range. I could use VHF-FM as the input, and then retransmit it out over CB frequencies. Like before, I needed to take the low level audio out of the VHF receiver, and apply it to the CB transmitter. I also needed a COS. It turns out that this was a much simpler task this time, as the VHF radio had both of these points conveniently wired to an external accessory jack, so interfacing was easy. I also opted to use CTCSS (PL), as an added security measure, to prevent a stray VHF user from accidentally bringing up the VHF/CB crossband repeater. I could also turn off my transmit CTCSS briefly, in order to legally I.D. on the ham band, without bringing up the repeater and re-broadcasting this over CB. Since this was a one way repeater, and the CB signals did not get re-broadcast over the ham band, I figured that I should stay out of legal trouble.
I fabricated the interface cable, and once I was done tweaking the setup, I placed it into service. I was pretty certain that the agitators knew us and might get suspicious if my crossbanded audio did not have the same acoustical signature as my normal D-104, combined with my somewhat hollow room acoustics. But since I was also known to frequently switch between radios quite often as I worked on them, and many of those radios had hand held mics, I hoped that this fact would be enough to fool the agitators. So I made a point of running the hand-held mic on my main CB for a few days before trying out the repeater, so as to not arouse any suspicion.
The first night that the carrier throwers came back, I tried the crossband out. I quietly left in the car and began to search, while keeping up the facade of trying to continue the casual conversation that I was involved with previously. It looked like the repeater was working fabulously, at least initially. But as I started getting closer to the carrier thrower, I realized that I could no longer hear the locals over the carrier. Guys who lived down the street from me, who normally should have no trouble overcoming a carrier, were now being covered over. While the one-way repeater made my transmit signal appear to be coming from my home base, my received signal was still coming direct to me from my now remote location. It was tough trying to have a conversation under these conditions, while keeping up the appearance that I was still at home, and not actually searching for the agitators. Another CBer/ham, who knew what I was doing, came to my rescue, by simulcasting his transmissions over both the CB and on the VHF channel that I was using. I was then able to carry on a conversation while getting real close to the carrier thrower. Things started looking better and I continued my search. But for some reason, as I was closing in on their location, the agitators abruptly stopped and didn't come back. Based on their signal, I had gotten pretty close, within a block or two. So it's possible that they saw my mobile, and realized just how close I had been, and stopped before their identities were finally discovered. I had a feeling that the jammers were people that we knew, and this just added fuel to that theory. Oh well, since they didn't come back (in this form at least) I never found out for sure. But at least that problem was solved.
I soon found another interesting use for the repeater. I could drive away to a more distant spot, and by bringing up the crossband repeater, I could compare the signal and sound of my base station, to that of the the other local's stations. That was fun for a short while. But once the novelty wore off, I didn't have much of a need for it anymore, so the crossband repeater went into history in much the same way as the other repeater before it.