Saturday, 26 September 2015

More musings on the Chinese Airband Receiver kit

After a fair bit of testing, several negatives have come to the fore with this kit. None is a really major problem, and for most builders just wanting to have a pootle about the air band they wont be of any consequence. These are as follows -

1. Sharp crackles from the loudspeaker if the antenna touches anything, or nearby switches are operated
2. IF bandwidth is about 800kHz!
3. A bit low on sensitivity
4. Squelch isnt very good
5. Tricky to tune - whole 118 - 136MHz in just a 270degree turn!

Addressing these in turn, no. 1 is of no real worry, and such noise susceptibility is to be expected working on AM. I expect a bit of extra front end protection is needed, maybe a high value resistor to ground or an RF choke. No. 2, well, ok, it means I receive several channels at once, but lets face it where I live the band isnt exactly crowded, I might have the ceramic filter in the wrong way around, although I thought they were bidirectional! No. 3, only really an issue picking up the ground station, and I can hear both my local airfields towers on it. No. 4 is a bit annoying, but really, see no.3!

So, no. 5 is the issue that is most problematic. With the supplied 10k pot the whole coverage is in a single turn, which means its very easy to miss a signal by tuning right over it. I considered replacing the oscillator with a DDS unit, but mine is only good to about 40MHz, far too low. A simple fix, which ive done, is to replace the 10k pot  with a 10k 10-turn precision potentiometer.

Tuning is now much cleaner, but more time consuming! And, since the knob goes around ten times, impossible to know what channel your on except from ID'ing the traffic! I could add a turns counter. But, instead, my thought is to add the el-cheapo LCD frequency meter module I have! I have yet to see if connecting it to the oscillator will cause unacceptable pulling, but will try it out. It might need a little FET buffer circuit.

With the addition of a multipole switch, and a bank of 10-turn presets, I can tune in several local frequencies to switch between.  And from there, its not much more effort to add a simple timer and stepping system to allow scanning.

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