Topband: Wire antennas and antenna analyzers

Tom Rauch w8ji at
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.

73 Tom 

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