Topband: Wire antennas and antenna analyzers
w8ji at contesting.com
Thu Sep 28 11:32:02 EDT 2006
The consensus seems to be that there is no need to worry
about static build
up when using the analyser on large antennas. >>
Having been involved in the design of antenna analyzers I
strongly disagree with the consensus.
In order to maintain detector linearity the diodes in the
MFJ analyzers are low bias schottky microwave diodes with
very small junctions. The manual (assuming it is left as I
submitted it) warns about ESD from large antennas.
Even on a calm day the natural electrical field gradient
surrounding the earth can create very high voltages on a
large antenna that is well insulated from ground. The proper
procedure with a large high ungrounded antenna is to short
the antenna to ground before connecting to the input port,
and to make the transfer as fast as possible.
Once connected, assuming there are no storms or other
sources of rapidly increased voltage, there is no issue. The
low resistance of the bridge will keep the antenna
Although I don't know what diodes other people use, the only
way to have good detector linearity and maintain accurate
readings at higher frequency is to use a diode with low
voltage threshold and low junction capacitance. That always
translates to a diode that is sensitive to ESD.
By the way, the failure mode isn't always an abrupt total
failure of a diode. The result is often a change in diode
characteristics that ruins calibration.
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