Facts and Fibs about Preamps

Two do-it-yourselfers.

When a satellite downlink bareiy makes it through the noise, one immediately think.s of using a preamplifier I have built two samples each, one tuing a bipolar transistor (MRF901), and ihe other using an FET (BF981)[

Both types have a switched bypass to allow transmitting. It's important that the switching works well- have 30 dead preamps sitting on a shelf. All hail RF up their rear ends," noted friend and fellow ham Irvin Spackirum ZL1MO.

Switching

1 use a type of single relay That loc al radiotelephone manufacturers also use, Common lore has it that this is a no-no; you must use a rvlay at each end. Figs, ] and 2 show that I have added shunt trimmers ai the antenna and at the transmitter end of the relay. These are adjusted lor maximum power on a power meter There is no discernible difference in fxm.LT between that arrangement and a direct piece of coax.

1 have used this switching for both types of preamp, and there is no instability. The amplified output docs not talk to the input from the antenna.

"'Bipolar transistors have a higher noise figure than FHTs." we hear. So they have. But if you use ihem with an omnidirec-

Henry Falkner ZL1AAN 8 Kapai Road Devonport Auckland 9 New Zealand improvement in recovered signal. You st;irt worrying about noise ligures when >ou use a helical array into ^ cavit) filter.

I used the MRF901 with resistor biasing only, with Pi- matching, 1 can t measure gain, but the S-mcteron the FT-290 arid TR9130 goes from 0 to 9 when the preamp is switched on.

"You cannot use Vero-board construction at VHF/' is another objection. My preamps are dashed o1ï on Vero-board. !

"The modem synthesized transceivers have untuned front ends, The mixer therefore has to discriminate from signals off the desired frequency."

tional antenna into an untuned front end, the received noise will mask the noise of the active device, even at VHF levels. Hence, having an improvement of 0.6 dB in noise reduction is not going to make an did need lo solder together all unused tracks and tie them to ground through the stand-offs. The signal paths were kept short, as in "dead bug" construction. On (he first FET preamp I had the two tuning

0 0

Post a comment