Posted by
W8JI on
Jun 14, 2010; 10:15am
URL: http://elecraft.85.s1.nabble.com/K3-Kite-antennas-and-static-protection-tp5174542p5176809.html
The most important thing in this is to always have a dc leak path to earth
on the antenna side of any "T" network or anything else that might add a
series capacitance. (Some lighting suppressors are a bad design with "dc
isolation" by a series capacitor on the center conductor.) We never want
series capacitance that prevents or blocks a bleed-off path to earth.
>From my measurements here on a 300-foot tall well-insulated tower, the
current is microamperes even in inclement weather. The ground path doesn't
have to be low resistance to hold the antenna to reasonable voltages.
It's the charging of antenna and any feeder or equipment capacitance (like
the antenna capacitor in a T network) that is the big problem, because when
voltage gradually builds and eventually becomes high enough to arc over,
that charged capacitance can dump a lot of current into other equipment.
This fast dumping of charge buildup is the major cause of damage to diodes
in SWR detectors and directional couplers. In a T network tuner it is the
output capacitor that charges and eventually dumps a spike back though the
other components.
The bleeds on the radio input ports are a great idea for stations with poor
or non-existent charge drains on antennas, but won't do anything once an
antenna tuner or some other series capacitance is in line.
A small high impedance RF choke, or even a 10K to 100K resistor (careful of
normal operating voltage and dissipation) is an adequate drain according to
measurements I made on a 300-ft very well insulated tower. (Without a drain
that tower would charge enough to knock me on my backside in just a very
gentle breeze on a nice clear day!)
73 Tom
> GW0ETF wrote:
>> Does the K3's built-in protection (surge arrestor and bleed resistor on
>> each
>> rx input) obviate the need for external protection when using a big kite
>> antenna?
>>
>> If it does, and in view of the inevitable advice *to* arrange for a DC
>> path
>> to earth at receiver input whenever using a kite, I suppose an implied
>> question is....is the K3 unusual in providing static protection at it's
>> inputs?
>>
>> 73,
>>
>> Stewart Rolfe, GW0ETF
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