I employ several Beverage antennas at both the home and summer locations; each is selected and switched remotely. My first encounter with a potential problem occurred at home, using my FT990 and ClippertonL amplifier. One evening I selected the a Beverage antenna to copy a DX station and keyed the transmitter. When I pressed the key, the amplifier "took off", drawing heavy plate current and putting great stress on the power supply. I quickly unkeyed and tried again with another Beverage antenna which was further away from the TX antena. This time there was no problem, nor was there with any other Beverage except the one which ran close to the vertical. I thought at that time that the problem was the antenna, so I moved its feedpoint further away from the TX antenna and the problem disappeared.
A few months later I had a similar occurrence at our summer place, this time using the FT990 and an AL80A amplifierSome time earlier I had built a relay switching box to enable me to use an ICOM 751A with an external receive-only antenna, and I now had this box attached to my FT990. Somehow the relay activation line from the transceiver had become disconnected; the RX port was not isolated during transmit and the AL80 took off, just as the Clipperton L amplifier had, when I pressed the key. This time I realzed that the problem lay with the FT990.
Inspection of the schematic revealed that the RX port was switched but not grounded during transmission. The relay switching between antennas in the FT990 transceiver is a PC board type with very close contact spacing. RF from the TX antenna was being coupled via the Beverage antenna across the "open" relay contacts back into the transceiver and amplifier, causing the latter to break into oscillation. To eliminate this condition, an external relay would have to be employed to disconnect the Beverage antenna and ground the transceiver's RX port when in transmit whenever a RX only antenna was used.
In both cases above I had a preamp in the line; by-passing it minimized the condition but did not entirely eliminate it. The circuit developed for my old IC751 was modified to include a linear amplifier keying circuit.
When power is applied by turning the function switch to "ON", RLY-1 is activated, pulling the RX line off ground and interrupting the TX keying line. When a ground is applied from the TX keying line, RLY-2 pulls in, lifting the ground for RLY-1 which drops out and grounds both the RX port and RX antenna. At the same time RLY-2 applies a ground to the linear keying line. With the switch in the by-pass position, the linear can be keyed but the RX port which is not in use stays grounded as it is in the "OFF" position.
This is the interior of one "Bevbox". The 2n5109 preamp is to
the left
while the relay to remove power to the preamp and ground the Beverage
feeds is to the top right. The relay has a 5 amp contact rating and
is a
4PDT type with one set of contacts not used. It is what I had on hand
and is by no means the only style of relay which can be used. The
imp-
ortant thing is that it switches the RX line to ground when the linear
is
being used.