Manufacturing

WAYLAND, MASSACHUSETTS

Frank Jones W6AJF-ÀF6AJF 850 Donner Avenue Sonoma, California 95476

New VHF Circuits for Transistors

This article, the first in a series, describes a simple but effective 50 mc antenna tuner and low noise 50 mc converter using three 52 ç transistors.

Some sad experiences with transistor converters and receivers at my station have resulted in the se of several ideas for protection of transistors which now seem to be satisfactory for most overload conditions. These overloads in the past abruptly ended the useful life of the front end transistor or caused a gradual deterioration of receiver noise figure and loss ol weak Signals. Four general forms i_* \_.-*

of overload were present at W6AJF, First, a [iigh powered VHF transmitter connected to a beam antenna which was too close to another VHF band antenna and perhaps pointing across the other antenna at times. This would produce a damaging voltage across the input of the first transistor thru the antenna feeder,

coax relay and input circuit (low Q) into the transistor even though that unit was not in operation, that is, with no battery connection. For a long time the only protection was to disconnect all antenna coax fittings except for the band in use- A good VHF contest with all antennas connected usually resulted in a frantic search for a new transistor or two.

The dual antenna couplers shown in this article cured this problem since two very lug: Q circuits added enough selectivity to the transistor converter front ends to knock out this problem. To get very high Q circuits these units have to be large physically, so a second benefit results from their use. Transmitter spurious frequencies are greatly attenuated and the rf energy reaching the particular antenna is confined to that particular band with a reduction of TVI problems in the neighborhood. These dual circuits were built into standard aluminum chassis and fastened on the wall for connection between the coax line of each beam antenna and its coaxial antenna relay. Very high Q is needed to not only reduce trai ism it ting power loss but to

Frank Jones is one of the best-knoten and most capable of VHF hams and authors; He has written over WO articles and radio handbooks, including "VHF for the Radio Amateura staple on any VHF*er s book shelf. This is Frank's first article in 73; we hope that it will be followed by many more>

Dual circuit antenna filter for 50 mc used in both transmitters and receiving.

keep from losing NF in the receiver. Any loss here reduces the weak signal capabilities so the losses should be kept well below one DB in the two circuits of each coupler.

If each circuit is eon pled so as to have a working Q of perhaps 20 and the unloaded Q is perhaps 500, the total coupler circuit loss would be 8% or an efficiency of 92%. The loss in NF would then be less than db. Small circuits cannot be built with high enough Q for low losses, and as much as 2 db is sometimes lost in NF if these selective circuits are built into the converter unit-

The second cause of transistor failures is fairly rare at this location, lightning storms in the area* These onl\ occur once or twice a m year here and the best protection is still to have all antennas disconnected from all receivers during these storms. Lightning protectors in the antenna feeders to a good outside ground may save the transmitters except in a direct hit, but transistors aren't tough enough and out they go even though the bolt of lightning may hit a few miles away. Low capacity fast diodes connected back to back across the first transistor circuit help a little.

The third cause of transistor failure has to do with improper antenna and power control relays. No matter how good an antenna relay is for isolation between transmit and receive positions, an arc at the points will sure put a lot of rf voltage across that first transistor. The answer to tliis problem is to manually control the relay switching or to use timed delay sequence by electrical means so the antenna re lay will be in transmit position before the power relays are in transmit position—and most important* the power relays are "off"' long enough for all transmitter rf energy to dissipate in the antenna before the antenna relay is restored to receive position. Some circuits which accomplish this properly have been or will soon be published. Antenna relay switching still seems to be the most effective way of getting those weak signals into the receiver in the VHF region.

A fourth cause of trouble is in the antenna relays because of lack of isolation between the transmit and receive coax connections. Nearly all VHF transistors will break down if the peak input voltage is much over a half volt. Some antenna relavs ontv have about 20 db isolation at 144 mc, or a power isolation of 100 to 1- li you have 100 watts peak transmitter power output, this means 1 watt down

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