Re(2): [Antennas] Wideband Receive preamp for Screwdriver.

[Chris Trask]

    For dealing with man-made noise (QRM), you can relieve the problem to
some degree by using a second antenna.  If the noise is a point source, you
can aim the second antenna at that source and by way of phase shifting and
attenutation reduce (and possibly cancel) the noise received by the first
antenna.  If the noise is more widespread (such as terestrial or galactic
backgound theremal noise), then your options are very limited.  There are
some cases where the second antenna is omnidirectional and it is somewhat
effective.

    In the receiver itself, reducing the IF bandwidth to the minimum needed
to receive the intended signal will reduce the total noise power to it's
theoretical detected minimum.  Beyond that, there's some very rigorous
numerical processing that can be done, but you don't want to go there.

Chris

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----- Original Message -----
From: "DavidE Benedict" <iam at pmug.org>
To: <w4tg at bellsouth.net>; <antennas at mailman.qth.net>
Sent: Friday, June 30, 2006 3:21 PM
Subject: Re(2): [Antennas] Wideband Receive preamp for Screwdriver.


> w4tg at bellsouth.net writes:
> >In general, preamps do not prove useful (on any antenna) for use in the
> >HF part of the spectrum. There is a good reason for this: In the UHF and
> >VHF spectrum the noise level, which determines the signal-to-noise
> >ratio, comes from within the receiver, so amplifying the input signal
> >improves the signal-to-noise ratio. Whereas in the HF spectrum the the
> >noise floor, which determins the S/N ratio comes in with the signal as
> >picked up by the antenna. Any pre-amp used, therefore amplifies the
> >noise along with the signal. (Does not improve S/N ratio.) You can
> >easily prove this to yourself: disconnect the antenna from a working HF
> >receiver and note that the noise, as well as the signal, disappears.
> >Whereas, removing the antenna from a working VHF or UHF receiver and
> >only the signal is gone, the noise is still there!
> >I hope this helps.
> >73,
> >Harvey/w4tg
>
> Good insight!
> So, it must follow that any receive help about  has to be in noise
> cancellation somewhere in the receive system of: geographic location, rec.
> antenna, rec. feedline, and actual receiver... is that all true?
>
> David B.
> W7DBH
> Mobile-Only ...in NW Oregon
>
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