Monday, March 14, 2011

New shack + New VHF TX

I'm currently making some tests with the 21.4Mhz filters bought in last ham fair for an Si570 based VHF FM transmitter. Tests are promising and looking good so probably a final schematic in the next few days.
Here's some images on progress so far:
The original schematic, bellow.
Right now after some changes the tuned circuit on the 10.7Mhz oscillator/ FM modulator (21.4Mhz) is now on a red toroid. The output of the ne602 has an output coil of 6 turns on the NE602 side (pin 4 and 5) and 1 turn for antenna matching (amplifier strip later) hopping to give 1500/50 impedance conversion...or near :)
21.4Mhz filter is connected to the NE602 via a 10nF cap, so does pin 3 to ground. The 100K on the oscillator collector was removed. A small cap was added from the top the tuned circuit to the ground, there's a 56pF cap in parallel with the ferrite ring, resonance viewed by the dip meter is around 22Mhz, the red variable cap should put it on 21.4 after tuning.
Even with the 21.4 filter and the output coil I still have small harmonic leakage on the Si570 frequency - 21.4 * 2, that is: the frequency I hope to transmit (Si570 frequency - 21.4Mhz) minus 21.4Mhz, or Si570 frequency - 4 * 10.7Mhz :) Did not make yet tests on other harmonics. Si570 oscilation frequency is on the high side for that matter.

Mean while I arranged a secondary shack at my parents house, they were kind enough to prepare some extra electricity plastic tubing, when the house was built, from the shack to the rooft. That way it's easier to install antenna cabling. Unfortunately not all weekends I pay them a visit but it's nice to know that I am radio ready there case needed. At the moment I only have an VHF antenna tuned for the marine band but I left there a balun and some wire ready to deploy on a simple dipole form.
here's the setup:

Later on will get a wood rack on top of the FT-102.

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