Expanding Channels with Crystals
The time was back in the early 1970's, during the initial growth in hobby users which preceded the big rush when CB became the latest social fad. Back then, there were already a growing number of people who were interested in the wireless nature of the radio hobby and these early experimenters continually looked for ways to optimize their setups. At this time, one of the most sought after performance modifications was expansion of channels. The appeal of such a mod had many facets. Some users wanted "private" channels to conduct semi-personal business or to exchange information that they didn't want the general public to hear. Others wanted an escape from radio agitators, who jammed the locals on a semi-regular basis. There were also those who wanted to reduce the number of competing stations (heterodyne) so as to increase their chances of making rare DX contacts. There was also a sort of "King of the Hill" mentality at work as well, as many guys sought to achieve some sort of status or notoriety amongst the locals by owning the biggest, loudest, or the most capable radio on the block. It was pretty much the radio equivalent of NASCAR racing.
In the earlier article "Putting Channel 22A in the blank spot", I gave a basic explanation of how a crystal synthesized 23 channel radio derived its channels, and how to access the "hidden" 24th channel. When someone wanted to further expand channel capability beyond simply adding Channel "22A", there were a few additional options that they could explore.
One of the easiest and cheapest ways to gain additional frequencies in a standard AM CB, was somewhat of a technical kludge, but provided satisfactory results for local communications. All that was needed to accomplish this mod, was a DPDT switch and some hookup wire. This method was a simple crystal bank reversal. On a basic AM 23 channel radio, in addition to the main bank of 6 crystals, there were either two synthesizer banks of 4 crystals; one set for transmit, and the other for receive, or a single bank of 4 synthesizer crystals with two individual offset crystals for transmit and receive. These two separate sets of mixer crystals were normally spaced 455 Khz apart, to coincide with the frequency of the Intermediate Frequency (I.F.) of the receiver. Recalling frequency mixer theory, if you mix 2 frequencies together, you get both sum and difference frequencies on the output. The converse is also true. If both the sum and difference frequencies are introduced at the input, either one will mix with the local oscillator to form a single output frequency. So if you input the desired channel frequency for Channel 23 (in this example) of 27.255, and mix in the local oscillator frequency of 26.800 (27.255 minus .455 Mhz I.F), the result will be a frequency of .455 Mhz (455Khz), which will pass through the I.F. to the detector and on to the speaker. But remember I said the input could mix both sum and difference frequencies? It so happens that if you were to input a frequency of 26.345 Mhz, the difference of that frequency and the local oscillator of 26.800, would also equal .455 Mhz. This is called the "image" frequency in receiver terms. Normally the band pass of the receiver is narrow enough that the receiver sensitivity at the image frequency is significantly lower than on the desired frequency, so it isn't usually a problem. Also, at the time, the frequencies which were images to the 23 channel CB radios were not in wide use, and therefore were not deemed to be a high interference potential (It was this image frequency interference issue which forced the the FCC to trim back the original frequency expansion for the CB band from 99 to 40 channels in the 1976 proposal. Local oscillators for many the 23 channel radios started at 27.420 Mhz).
Now for the neat part: If you swap the transmit and receive offset crystals, and using the Channel 23 example from above, you are now transmitting on 26.800 Mhz. The receiver local oscillator is now running at 27.255 Mhz, which makes the primary receiver frequency 27.710 Mhz. At first it looks as if you're transmitting and receiving on two different frequencies. But the IMAGE frequency is now at 26.800 Mhz, which coincides with your new transmit frequency. You can now communicate on a new set of 23 frequencies. The receiver sensitivity will not be as great, and the frequencies will end in "0" instead of "5", but it will work ok for those local chats which call for a semi-private channel. The banks of crystals could be wired to a DPDT switch and switched at will, whenever the need arose.
However, as in life itself, nothing is ever 100% straight up and clean cut, and to make matters even more confusing, not all radios used the same crystal mixing scheme. Some (like the SBE Cortez) used a "low side" local oscillator injection frequency which was .455 Mhz lower than the normal transmit frequency. While other radios (Like the SBE Trinidad) utilized a "high side" local oscillator injection, which ran .455 Mhz higher than the normal transmit frequency. When you reversed the crystals on these two radios, you had the Cortez transmitting (on Channel 23) at 26.800, while receiving 27.710 Mhz plus 26.800 Mhz as an image. The Trinidad meanwhile, would be transmitting on 27.710 Mhz, while receiving 26.800 Mhz, plus 27.710 Mhz as the image. Are you confused yet? In practice it was not so bad. What happened was that the two radios could still talk to each other, thanks to the image phenomenon. Plus, there was an added measure of security since you were actually working split frequency duplex, as the transmit frequencies on the two radios would be different. Anyone trying to listen in on a conventional tunable receiver, would only be able to hear one side of the conversation at a time. This little factoid became apparent to me during one such foray to these channels with my friend Mitch in late 1975. I had a Cortez, and Mitch had a Trinidad. We were in the middle of talking about something, when another station jumped in and started giving Mitch a hard time for talking AM on a "sideband channel" (back then the frequencies above Channel 23 were "reserved" by loose gentleman's agreement for SSB, and the channels below Channel 1 were for AM use). Mitch started arguing back at the guy, and I chimed in as well. But after a few minutes, it dawned on me that the guy was not responding to anything I had said, or even acknowledged my presence. Even when I fired up the amp, he was still not responding. I thought it odd that he could hear Mitch and not me, as close as we were to each other, and our similar signal potentials. It was only later on that it occurred to me that the complainer was running on the upper channels where he could hear Mitch, while I was transmitting on the lower channels. Since the complaining station was running a ham rig, which used a different I.F. frequency, he could not hear me. Ain't radio a fun hobby?
In contrast to this rather cumbersome (but cheap!) and somewhat difficult to explain crystal swap method for fudging in some "private" channels, the usual straight up method of crystal frequency expansion involved simply purchasing additional crystals for new frequencies. If you had a SSB radio, the swap method usually didn't work anyway. When adding crystals, you typically concentrated on the main bank of 6 crystals, each one was responsible for generating 4 consecutive channels (3 in a row, then a 20 Khz jump for the 4th) a piece. All one needed to do to gain some additional frequencies was to find the highest (or lowest as the case may be) frequency crystal, and figure out what frequency will give you the channel(s) that you want. Then you swapped out the existing crystals, of the channels that you seldom used, for the new ones. Or you could wire up a switch or two to change between the stock crystals and the added ones. Since each crystal would only give you 4 additional channels (5 if you had a means to fill in the "gap" channel), the total amount of frequency coverage was not all that great for the people who employed this method. You also had to be careful as to not use a wire lead length which was too long, or you could affect frequency accuracy. Many times, the radios had to be "coaxed" with small capacitors in order to reach the proper frequency. A frequency counter is a must for anyone attempting this modification, especially if you obtained your crystals from "bargain" crystal manufacturers.
Usually people went this route when they only wanted a few quiet channels to escape from the noise, or they were members of "exclusive" freeband SSB clubs who had their own "club" frequency away from the legal channels. Many people also added the 3 additional crystals needed to give them the means to access the 17 new expanded channels, when the 40 channel band plan became legal in 1977, rather than purchasing new rigs. For those who were not satisfied with only a few extra channels, and wanted even more, they usually opted for an external VFO unit, which allowed a tunable band coverage which could extend all the way up to the edge of the 10 meter band. More on this in a later article........