This information reprinted with the kind permission of the author, Tom N4KG
The FIRST thing you need to know is how much power is coming in to the second radio (and vice
versa). Connect a dummy load and wattmeter to EACH of the alternate antennas while transmitting
full power into the other antennas. Orient all antennas for maximum coupling.
I found about 5 watts between high band antennas on separate towers at 75 ft spacing. Around 50
watts between 80M slopers attached to the same tower. Those slopers have been replaced by
separate high dipoles at right angles to each other between towers.
My "Protection Boxes" consist of a series 2W 22 Ohm Carbon Composition (non-inductive) Resistor
followed by 2 stacks (opposite polarization) of 4 diodes. Use any fast switching diodes such as
1N914 which will have very low capacitance. These are placed in the RX antenna line of each
(Kenwood) Radio using the Transverter Plug which conveniently interrupts the RX line with input
and output terminals available on the plug.
The series resistor absorbs the excess incoming power. One of my resistors showed signs of
excessive heating. I'm not sure if that was from my other transmitter or a lightning event. In either
case, it DID protect the receiver.
Yes, 4 diodes connected in series with cathodes to ground in parallel with another 4 diodes
connected in series with anodes to ground. The diodes clamp the voltage to 2.8 V peak to peak. I
wanted to keep the voltage high enough to prevent IMD generation from strong RF signals yet low
enough to protect the small wire in the 7mm input coil of my RX.
Without the resistor, you would be dumping the excess power into the small signal diodes. You
could exceed their current rating and if they open up, then your RX is the next load. :-(
[N1EU note: 1N914 is spec'd at 400ma peak, 1A one sec surge]
ANT -------RESISTOR------------------RX Input
22 Ohm | |
2W D D D= 1N914 or any
| | fast switching diode
etc (4 series diodes in each leg)
Cheap and easy. NO more blown input transformers. The TS830 manual warns not to exceed 7 V at
the antenna input. My old Collins 75A3 has a stamped warning on the back not to exceed 50 V
RMS at the antenna input. You could dump your exciter into it (short term) without ever damaging