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Convert old 27 MHz CB and marine transceivers to 28 MHz

27 MHz AM CB usage is a shadow of its former self, with most remaining CBers using the more efficient SSB mode and non-hobby users going to UHF. 27 MHz marine has gone the same way with more use made of the much less noisy VHF marine band around 156 MHz.

The silver lining for amateurs is a surplus of equipment which can be had for a song at hamfests and junk sales. 1970s era CB equipment and 1980s era marine transceivers were often crystal controlled. They had two crystals for each channel. The transmitter crystal was on frequency but the receiver crystal was 455 kHz off, reflecting the common superhet intermediate frequency.

The relatively low IF meant that receivers were susceptible to poor image rejection, causing off-frequency signals to spoil reception. This was a particular problem for 27 MHz marine transceivers (which operated around 27.9 MHz) because the image frequency was around 27 MHz, which at the time was populated with strong local CB signals (and still is when there's international propagation).

To overcome that problem suppliers started making 'high side' crystals for marine band users. That put the image in the much quieter 28 MHz amateur band, thus lessening interference risks.

The good thing for amateurs was that if you had a marine transceiver that had the high side receive crystals fitted you could just swap the transmit and receive crystals. Then you'd get a transceiver for 28 MHz AM with no other parts or effort required except perhaps for some minor tweaking to peak the transmitter and receiver. A crystal controlled mobile transceiver is best due to its ability to plug in a good external antenna and its 4 watt power output. However the better type of 27 MHz handheld (typically 1 to 4 watt output) is also suitable.

Quartz crystals are expensive to get custom made. But there's many floating around in peoples' junkboxes. You might be lucky and have suitable crystals. If you can cheaply buy a crystal controlled handheld or mobile CB or marine transceiver (mine cost $5 at the end of a hamfest) then a few minutes work will give it a new lease of life.

What crystals are suitable? Use the marine channel chart below. The left frequency is the receiver crystal (if low side), the middle frequency is the marine band transmit frequency and the right frequency is the receiver crystal (if high side). If you have one from the middle and one from the right in the same row then you are in luck (especially if they are in red).

Receive (lo) / Transmit / Receive (hi)
27.225 / 27.680 / 28.135
27.265 / 27.720 / 28.175
27.365 / 27.820 / 28.275
27.405 / 27.860 / 28.315
27.425 / 27.880 / 28.335
27.445 / 27.900 / 28.355
27.455 / 27.910 / 28.365
27.485 / 27.940 / 28.395
27.505 / 27.960 / 28.415
27.525 / 27.980 / 28.435

For example if you have a high side pair for 27.940 MHz (ie transmit 27.940 MHz, receive 28.395 MHz), just swap the crystals so 28.395 MHz is in the transmit socket and 27.940 MHz is in the corresponding receive socket for operation on 10 metres.

You're unlikely to have all frequencies but if you have a choice note that some are more suitable than others. Eg 28.135 and 28.175 MHz are in the CW segment so are unsuitable for AM activity. 28.275 MHz is good if you want to experiment with setting up a beacon as it's in the beacon segment. Otherwise, for regular AM talking, just install those for 28.315 MHz and above (ie the pairs in red).

If you only have low side crystals there is another trick you can do if you want interoperability with a standard 27 MHz CB radio. If you have pairs on the lower four frequencies then you can swap them to give access to channels in the legal 40. These relate to channels 22, 26, 36 or 40 respectively.

AM activity tends to be sporadic and being crystal controlled is even more of a challenge. But it's great fun if there are several in your area with matching channels. And you can get good distances if you go up on hills and put the word out that you are there. Interstate and even international contacts are possible if you're there at the right time.

This modification is not worth spending much money on. Only do it if you happen to have suitable crystals. Although the more inventive have successfully added DDS modules for frequency agility. If you do go down this path make sure you have added a switchable beat frequency oscillator for the receiver so you can resolve SSB signals. That way you'll be able to call SSB operators and multiply the contacts you get (as they will hear your AM provided you are on frequency).

The videos below recap the above and show the results possible with converted 27 MHz AM equipment on 28 MHz.

Crystal swap trick for 27 MHz walkie talkies


Five dollar marine radio converted to 28 MHz


28 MHz QRP AM pedestrian mobile


Antenna to use for 27 or 28 MHz pedestrian mobile


Learn more about what the 10 metre (28 MHz) band can do here.


Disclosure: I receive a small commission from items purchased through links on this site.


Books by VK3YE

Ham Radio Get Started (USA)

Australian Ham Radio Handbook (Aust)

Hand-carried QRP Antennas

More Hand-carried QRP Antennas

99 things you can do with Amateur Radio

Getting back into Amateur Radio

Minimum QRP

Illustrated International Ham Radio Dictionary

Make your Passion Pay (ebook writing)


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