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Date: Sat, 19 Dec 2009 17:23:12 +0000
Message-ID: <fc7eccec0912190923m768fd16egb96178a2667357c2@mail.gmail.com>
From: Andy Talbot <andy.g4jnt@googlemail.com>
To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: Noise cancelling by using optic transmission of RX signals
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Like the sound of that.

How did you keep common mode interference induced on the power lead
from getting anywhere near the high impedance antenna probe ?

Andy
www.g4jnt.com

This email has been scanned for damaging side-effects by the health
and safety police



2009/12/19 Roelof Bakker <roelof@ndb.demon.nl>:
> Dear James,
>
>> I think the role of the ground with active whips is often neglected - the
>> output of the whip preamp is the voltage differential between the whip
>> element and the circuit ground, so the ground connection is just as much
>> part of the antenna as the whip element itself is. Clearly, an electrically
>> quiet ground is needed for the active whip, whether this is provided by the
>> coax feeder or a seperate local ground connection.
>
> It seems that the requirements for a ground connection are quite relaxed.
> I have tested a completely isolated system with the coax running horizontal
> tied to a non conduction mast and shoved out of an upstairs window. Signal
> strength received with the same system and the mast vertical mounted in the
> garden to put the antenna at the same location in space, was within one dB
> for a vertical polarised signal.
>
> I have also build a system with CAT5 cable as a balanced feeder. One twisted
> pair was used for RF, the other for power feed. At both ends of the RF pair
> rf-isolating transformers were used. This resulted in a very low local noise
> pick up.
>
> Using a common mode choke at the antenna end does not change anything at all
> either. My guess is that the antenna element is so small, that even a small
> (capacitive) coupling is good ebnough for a ground.
>
> Best regards,
> Roelof Bakker, pa0rdt
>
>
>