Thank you, JC:
I don't know where this coming from ,but the gain you need for a VWF
modest
size is 20db for vertical polarization and for horizontal HWF you need
40db
on 160m, on 80m divide this by 2, you need only 20 dB and on 40m 10 dB a
NORTON preamp is enough. All situations you need a band pass filter.
20 dB is a realistic gain figure.
The very low sensitivity of horizontal polarization, because at low heights
in wavelength the earth "shorts the electric field", and because at low
heights the earth's reflection nulls the antenna peak response, causes great
difficulty.
If local site noise is high, and if care is taken in balance, the horizontal
system can be built but 40 dB gain is unlikely to be needed unless the
receiver is dead.
The reason is pretty simple. Most receivers are in the minus 130-140 dBm
noise floor range. If you added 40 dB gain to that, the noise figure of the
required front end would be an impossible negative noise figure in the -20
dB or more noise figure range. Of course anything less than 1 dB is very
difficult, and below 1/2 dB starts to be impossible. Even if you obtain that
noise figure, cable leakages and common mode would overwhelm the low antenna
level.
20 dB is about the limit for most receivers, although a dead receiver could
use 40. If the receiver is stone deaf, 40 dB would allow a workable noise
figure at the front end. :-)
This low sensitivity is why K6STI's antenna met with such limited reports of
success. If the site is very noisy with local distant noise, then the
antenna's noise floor is high enough to limit system noise floor. Otherwise,
the cables and input amplifier would set noise floor.
I have a similar thing here with a commercial loop antenna. Even though
vertically polarized, it is noise limited at my location by internal
amplifier noise. Now if I move it into a noisy location, it limits by
outside noise.
No matter what we try to do, we are not going to have a 0 dB noise figure.
When we start making the antenna sensitivity so low it requires gain with a
normal receiver so unrealistic that it limits on the front end noise, it is
useless. What good would seeing the S meter at S-2 or S-4 from amplifier
noise do? That is what the popular commercial loop I have does. In a quiet
location, it limits on its own internal amplifier noise. Six dB less gain
does not change S/N ratio one bit.
We should all question systems that need 40dB with normal receivers. 20 dB
is more rational.
73 Tom
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