Topband: Wire antennas and antenna analyzers

David Gilbert xdavid at
Thu Sep 28 12:44:05 EDT 2006

Tom Rauch wrote:
> <<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.>

I agree totally.  I spent most of my career in discrete semiconductors 
(including schottky diodes), first as a product engineer and then as a 
manager at various levels.  The metal-to-semiconductor contact area on a 
high frequency schottky is about the same as the cross section of a 
human hair.  It takes very little ESD to zap it, and as Tom says, the 
failure is not always a full open.  If the ESD is small, it can alloy 
only a portion of the contact area and change the diode's 
characteristics ... and thus the analyser's calibration.  Schottky's 
make great fuses, and we occasionally had customers who used them for 
that purpose.

Out here in dry Arizona, even a mild wind on a clear day can build up a 
significant charge on a long piece of wire.  I've experienced the jolts 
to prove it.  I can also draw an arc from my body (or from any 
instrument I may be carrying) to a grounded antenna after walking across 
a carpet.  I always short the connector on the coax before connecting it 
to my AEA HF-CIA analyser.  An even better approach would be to use a 
T-connector between the coax and the analyser with the unused port 
shorted until the measurement is taken.

Dave  AB7E


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