need to make a lightening arrestor as mine has had it , was thinking of below design, could i fit a resistor from the center conductor to ground to bleed off static ? if so what value ? i was reading people strapping resistors across the antenna sockets inside radios so though about doing it in this box..


billy

Billy,

As long as it's much higher than any impedance you are likely to encounter, it should be OK. Say 100k Ohms.

73,
Steve G3TXQ

ok steve, i read a few sites stating a lot less than that which i didn't think sounded right but also some sites stating much higher,upto 1M ohm .... could the lower values be for instance resonant antennas ? and the higher for multibanders .

for the purposes of static bleed off does it need to be any particular power rating resistor ?


also one builder reccommended 1/4 inch gap for the lightening spark gap ?? surely the rig would be knackered by the time enough charge was there to jump the gap , i was thinking of a very small gap , maybe 1mm , im only 2 watts out so that shouldn't jump the contact on tx .. maybe im wrong.

billy

I remember seeing a spark plug as the gap on a long wire.
Dont ask me where, but i was shown one years ago and it was documented in a magazine as well.

Another grey moment.

Phil

A rule of thumb for the resistance value would be a minimum of twenty times the highest line impedance. So if you know it's a 50 Ohm dipole you might be able to go as low as 1kOhms, but if you're in multiband doublet territory 100kOhms is more like it.

Wattage rating will depend on the resistance value you choose, the highest signal voltage that will appear across it, and the power it needs to dissipate when bleeding static. Typically folk seem to choose something in the range 2W to 5W.

Obviously the gap depends on the voltage at which you want it to strike; 1/4 inch sounds way too big and wouldn't strike until about 21kV. For maximum protection you would want it to just not strike on your biggest Tx signal. With your QRP levels, the gap could be really small.

Strike voltage depends on many things - air temperature and pressure, shape of the electrodes etc - but a very rough guide is 33kV/cm.

73,
Steve G3TXQ

cheers for that steve .. much appreciated, just one more thing before i forget , do they have to be carbon resistors or will wire wound do ??


billy

Wire wound resistors would have inductance, and, in conjunction with any stray capacitances, a resonant frequency.
It's like dummy loads which need to be purely resistive.

Cheers

Tony

cheers Tony , wasn't sure for this application..


billy