[Elecraft] k2 KPA100/KPA100 schottky diodes

[AD6XY]

A proper static discharge path to ground is essential in all antennas likely
to see a high E field, which in areas with thunderstorms really means all
antennas. 

The RF choke is probably the easiest way to ensure a low DC impedance -
connect it across the centre of a dipole or from a vertical element to
ground - at the lowest impedance point you can - usually the feed point. A
high power high value resistor will work but not for higher fields like
nearby lightning. I use an air wound coil on my higher frequency antennas
4m/6m and a high power choke using an RF rated toroid for the lower bands.
As long as the impedance is 10 time higher than the antenna it should be OK.
That means 500 Ohms which at 7MHz is 12uH or more. Make sure you use a large
torroid able to handle the high RF currents that may occur on some bands
where the antenna match is poor. 

If your antenna is multiband but not trapped it will have a high impedance
at the second harmonic, e.g. 40m for an 80m antenna, 20m for a 40m antenna.
Apart from the obvious point that this antenna is not going to work well if
fed via a coax and balun, it does make things harder because you need a
higher impedance. A possible alternative is to use a 1:1 transformer to
couple the coax to the antenna, act as a balun and also isolate the static.
As high voltages would arc remember to ground the centre tap on the the
antenna side. If it is balanced, this should be a voltage minimum. Of
course, if you are already feeding it against real ground, then the vertical
will be already by grounded in the transformer fed configuration you will
not need a centre tap.

Mike


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