As Tom points out, DC grounding doesn't make much
difference except in the case of electrostatic discharge.
This can manifest itself in the form of rain or snow
static. I recently saw an anecdotal account from Mike, N7MH
up at the Stanford Radio Club, W6YX. Seems that all their
KLM monobanders were wiped out by rain static in the ARRL
CW contest while their Mosley PRO-67 stayed quite.
I suspect, but I am not sure that this has something to
do with the fact the KLM antennas use insulated elements
whereas the Mosley elements are grounded to the boom.
Mike, W4EF.................
----------
From: Tom Rauch[SMTP:w8ji@contesting.com]
Reply To: W8JI@contesting.com
Sent: Saturday, March 11, 2000 9:59 AM
To: Amps Reflector; Carl Clawson
Subject: Re: [AMPS] Re: Static discharge caused NOISE
> > All antennas should be DC shorted to ground at the receiver input
> > because
> of
> > another reason: a sometimes much lower level of receiving back-ground
> noise.
> > de Hans
DC grounded antennas are no different in noise level that DC leak
antennas, or DC floating antennas with one exception.
If the antenna charges up enough to arc something over, you will
hear a loud POP in the receiver. Other than that, they are just as
"quiet".
My lowest noise receiving antenna for 160 meters are arrays of
eight verticals and four verticals that are NOT dc grounded in any
way at all. They are quieter than my arrays of Beverages, which are
dc grounded.
The reason the choke makes no difference is because the noise
outside the frequencies where the receiver is tuned does not make
it through the receiver, unless the receiver is very poorly designed.
At the operating frequency, the coke is a high impedance and so it
doesn't affect noise at that frequency at all.
> And we certainly need to keep the choke on the tank side of the output
> relay. Otherwise the tank floats during receive. With HV on there are
> probably enough ions to charge it up and cause arcs.
Yep, that annoying tick tic as the leakage in the blocking cap
allows the variables to charge until the flash over! That can be
shocking.
> It makes sense to put the rf choke at the output side of the relay
so that
> it shorts anything that is not supposed to be there to ground -
and, as
> Hans has suggested, improve receiving back-ground noise.
There is one
> minor caution, though, in that the rf choke will drop in impedance
at some
> point and affect received signals. A 2.5 mh choke would be fine
through
> 160 meters when using a low impedance antenna - but suppose
you also use
A 100 microhenry choke is enough for 160 meters in a 50 ohm line.
> Sorry to digress, but it really has to do with design of power
amplifiers
> - and whether the rf choke should be hung on the output side of
the relay.
A choke should always be used on the tank circuit end of the relay.
73, Tom W8JI
w8ji@contesting.com
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