[Antennas] Unused resonant antennas - shorted to ground or floating?

[Andy]

A reply to a question from a few weeks ago...

> Unused resonant antennas - shorted to ground or floating?
...
> Example: Do I ground an unused 40 meter vertical antenna to make it be
> "invisable" and not affect the pattern (mostly) of another antenna that is
> 1/4 wave away and also on 40 meters (or do I let it "float")?

In the ideal case, you have two requirements:

(1) DC ground, for static.
(2) At RF, detuned to minimize interaction.

At AM stations, unused towers are supposed to be detuned so that they have
zero current.  Not just zero current into the feedpoint, but actually zero
current up on the tower (as sensed by the remote current sense loop that AM
stations use).  Just having a stick there, grounded or not, will induce
currents in it and cause it to be a parasitic radiator.  To minimize that,
you want to tune them out.

Simply grounding the tower OR leaving it open (at RF) doesn't necessarily do
that.  There are remote control relays in the tuning box at the base of AM
towers, that switch in a different network when that tower is inactive.

I have since seen an article, somewhere on the Internet, that says you
actually want a specific, small non-zero current, in order to not distort
the pattern.  I'm not convinced, and it may be splitting hairs.

> Is it "good enough" to have it grounded by the Delta switch in the shack
> or does if have to be grounded AT the antenna?

For DC (static protection), it might suffice ... but much better if you can
shunt it to ground right at the base instead of bringing it into your shack.

For RF purposes, shorting it in the shack is VERY different than shorting it
at the base (assuming that's what you wanted to do).  A shorted length of
coax can look like an arbitrary inductive or capacitive reactance at the
other end, depending on how long it is.  If a quarter-wavelength long, it
looks like an open circuit to the tower.  I suppose you could probably
adjust the length of feedline, to get just the right impedance to null the
unused tower's current.

Andy