VHF FM Antenna Booster: New horizons in 3-metre band DXing 3

Published:2011/8/23 2:32:00 Author:Amy From:SeekIC

N. S. Hori Sankar VU3NSH

Setting up

Give the PCB a final, thorough inspection for bad soldering, wrong components, etc., and resolve all problems before proceeding.

Set all presets and trimmers to the centre of their travel. Apply power to the amplifier and check the various test voltages indicated in the circuit diagram. These voltages are typical and should not be taken to mean the ’law’ ’Tune your VHF FM radio to a weak signal at around 98 MHz. Tune the amplifier back to front, that is, first CM. then C5 and thon C2 for best reception — your ears and the tuner’s S meter will tell you what’s happening. If necessary reduce or increase the gain using PI. A good way to simulate a weak signal is to turn the antenna away from the transmitter direction.

The amplifier’s 3-dB bandwidth will be about 10 MHz with all tuned elements at the centre frequency. By careful retuning of the three trimmer capacitors (‘staggering’), this may be widened to 20-25 MHz at the cost of some gain. Fine tuning may also be achieved by stretching of compressing the three air-spaced inductors. This method may be for specialists only, however.

Other bands

The amplifier may be modified for use at slightly higher frequencies like120 MHz (VHF airband), 145 MHz {2-m radio amateur band or even 146-174 MHz (PMR band). Some experi­ence may be required in tweaking the inductors for resonance at their new frequency however. If you find that a particular trimmer no longer ’peaks’ then the L/C combination is out of range. Lacking specialized test equipment like a grid dipper to see where it does resonate, go for trial and error by fitting an inductor with fewer turns and see what happens then. Whatever the frequency, the booster will not fail to give the kiss of life to a duff receiver.

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