or: No pain, no gain

In the beginning…
Long, long ago, far, far away, we used to fly one model in the air at a time. That wasn’t due to any politeness, or awe at the other pilots’ skill – it was because the super-regenerative design used in the receiver could not filter out other transmissions, so we did it one at a time!

Then came crystal-controlled super-heterodyne receivers so that we could fly numerous models at one time – well, six actually, as that’s all the frequencies (spaced 50 KHz apart) technology would allow at the time, within the bandwidth the powers-that-be allocated to us (27 MHz)

Receiver technology advanced to allow us to use intermediate frequencies spaced 20 KHz & 30 KHz between the existing six. This did cause some problems with older radio gear, as their receivers were not able to separate out these closer frequencies. Towards the end of the 27MHz hey-days, FM radio gear was available that could operate 10 KHz apart – the shape of things to come!

The dark days…
Citizen Band became popular and sounded the death-knell of 27 MHz radio for flying model aircraft. Many a good model was lost to the ‘Breakers’ as they chatted their way up and down the Queens highway. Fortunately, from the Continent came another, models only, frequency – 35 MHz. The U.K. powers-that-be allowed use of the 35 MHz band in this country, and with FM transmissions, we could operate on a spacing of 10 KHz between frequencies. In the beginning, we had channels 60 to 80 allocated to us and we were in ‘hog-heaven! More available frequencies AND no interference!

Too much of a good thing…
We then were homogenised with Europe, adding a further 5 frequencies to our band (81 to 85). This however brought with it a problem of its own – 23-channel interference. Basically, a transmitter on a frequency twenty-three channels below you could interfere with your receiver (channel 60 interfering with channel 83, 61 with 84 & 62 with 85) more technically minded people will be able to explain the reasons why the standard receiver’s I.F. strip will allow these second-order harmonics through to spoil the day of your logic decoder!

As there were only a few channels affected, it wasn’t a big problem – the times you would get those particular permutations on a flying field were very few and far between.

However, now we have gained even further frequencies at both the bottom and top of our bandwidth (55 to 59 & 86 to 90). With this gain comes the pain of extra channels falling into the 23-channel interference ‘bucket’, affecting channels 77 up to 90, rather than just 83 to 85. So now we have these upper 14 frequencies that are prey to interference from the lower 14 (60 to 74).

What to do? Where do we go? Should we all take up control-line again?

Technology is your friend…
The answer, my friend, is to use a filtered receiver.

The radio manufacturers have all but eliminated this potential problem by adding a further set of filters into some of their receivers – Futaba call these receivers ‘Dual Conversion’ (DC), Multiplex call them ‘Double Superhet’ (DS). JR calls them ‘Automatic blocking circuit & window’ (ABC&W). So if you’ve one of these, the 23-channel interference problem isn’t a problem for you. PCM receivers are of the filtered variety as a matter of course.

If you don’t have a filtered receiver (a lot of after-market & miniature receivers are not) consider using frequencies below channel 77 and be a problem to others!

Tom Gaskin
A satisfied JR user…