Re: NEC-LIST: High Frequency Signal to Noise problem

At 05:30 PM 1/25/2006, zvi frank wrote:
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>-----Original Message-----
>From: zvi frank
>Sent: Monday, January 23, 2006 11:57 AM
>To: nec-list_at_robomod.net
>Subject:
>
>Hallo All,
> I have a problem at H.F. Somebody told me he improved the
>S/N of an HF antenna. As far as I understand usually the S/N of an HF
>antenna is determined by external noise. This would mean improving the
>efficiency of the antenna would not increase the S/N. I suppose if the
>antenna was made more directional than one might be able to decrease
>noise coming from some directions. Has anyone any comments or experience
>with this sort of problem ?

This is a common thing on HF...

1) As you say, you can avoid getting noise from the "wrong direction", BUT
2) The noise is a "distributed source"...

Consider an antenna with a beam width of, say, 20 degrees (and would have a
gain of about 20dBi) receiving a signal at some power, call it -50 dBm at
a SNR of, say, 10 dB, so the total noise power is -60dBm. We assume a
uniformly distributed noise flux over the sphere, of which we are looking
at about 1% (i.e. a solid angle of about 400 square degrees)

Now, shrink the beamwidth to half, so now it's 10 degees. The gain of the
antenna goes up to 26dBi. The desired signal is now at -44 dBm (6 dB
more). The noise power seen by the antenna has gone down by 6 dB (because
the solid angle of the distributed noise source is reduced to about 100
square degrees), but, combined with the 6dB increased antenna gain, means
that the noise power into the receiver is the same as for the other antenna.

6 dB more signal, same noise, so the SNR is now +16dB

Very non-intuitive, I know...

James Lux, P.E.
Spacecraft Radio Frequency Subsystems Group
Flight Communications Systems Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
tel: (818)354-2075
fax: (818)393-6875

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