Wednesday, August 12, 2009

HF and VHF preselector

Here's a simple circuit for the sunny days

Found a simple HF preselector circuit on the web and decided to try it out.

Basilcaly it's a variable tuned LC circuit between input and output...

I tested with 14 turns acording to schematic but only got preselection from 18 to 28Mhz then I add some more turns to 25 and got the 40m band also, tuning is very sharp, my variable capacitor is from one of those cheap radios and with all the plates in parallel to give about 240pF.
Take not that my tests were only against the antenna analyser, circuit input to signal out on the meter and circuit output to a dummy load, tuning was made until get lowest swr on the metter.

I decided to try an "air core" to see if it will scale to vhf, it works but in VHF work things must be well planed and it need's for sure an blindage of some kind since I placed between the antenna and my TM-D700 and I got more 1 "s" unit.... could also be because of metering circuit on the radio but probably was the air core was getting also signal. anyhow will test again against the airband receiver. Since my last testing on the airband receiver now I have too much signal...from a near by broadcast station and have to get rid of that.

The HF preselector:

The VHF preselector (same circuit but air core an less turns, 1-5-1):

Since it's still summer and very hot around the post's will slow down...

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