At 20:47 8/10/1998 +0200, Giulio IK2DED wrote:
>
>Then I have put a ... low dipole about 1.5 meters up. Incredible,
>the noise was absolutely less than 1 on my S meter, the S/N was
>really much more better than my vertical system.
It's amazing how often you hear of low antennas giving better SNR, and not
just for brief periods around sunrise, as discussed (and explained) here
several weeks ago. How is this so, when low angle DX signals ought to be
absorbed close to the ground by buildings, trees etc?
Several years ago I brought home a prototype receiver for testing. It
performed fine at work on the small rooftop antennas, but I wanted to see
how it coped with signals from my own much larger antennas.
The reception was reasonable but not outstanding, but as I increased the
attenuation in the antenna circuit (I had brought an attenuator home for
that purpose), the bands sprang to life with signals popping out all over
the place. What sounded like white background noise was in fact immense
broadband IMD! By adjusting the attenuator you could peak the SNR, and from
memory the best setting was around 25-30 dB I think.
Therefore it wouldn't surprise me if some instances of better reception on
small antennas, compared to higher and more efficient ones is, in fact, due
to less IMD. In fact 160m carries a double whammy: not only are our
antennas tuned close to the AM broadcast band, which has the strongest
signals, but the narrow frequency separation produces low order products,
which are the worst by far.
Even though my own station receiver is quite resistant to IMD, I still had
to fit a high pass filter in the antenna circuit to suppress IMD from AM
broadcast stations. In CW and SSB modes the IMD sounded like normal band
noise, and it was only when AM was selected that the IMD became recognisable.
Check it out, you might be surprised.
By the way, I'm really envious about those super-quiet locations!
73,
Peter VK3APN
pnesbit@melbpc.org.au
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