Hi Howard and 222 MHz geeks.
Having a good receiver these days is harder than you think. The advent
of digital TV stations has altered the landscape by causing interference
that you might not even notice. If you plug in a spectrum analyzer to
your antenna, you will probably see TV signals that are in the -20 dBm
range. This is more than enough to overload most any preamp. Consider
that there are multiple big TV signals and the number of strong signals
in the preamp passband all add up to cause overload. Digital RFI sounds
just like white noise. Also realize that peak signals of noise extends
about 10 dB above what you see on a screen. The bottom line is that very
few VHFers are NOT affected by TV RFI. I was getting hammered by a 200
MHz VHF TV station about 20 miles away on 222, and on 432, I am
clobbered by a UHF TV station 40miles away.
Any loss is noise and raises your noise floor. On 222 MHz with noise
temps in the mid double digits, you are wasting your energy in
purchasing a low noise preamp and then having 2 or 3 dB of feedline
loss. All noise is additive so the very low antenna temperatures
obtained by aiming up off the horizon can really skew the results from
what you would see at HF or 50 MHz. If you have a 3 dB feedline loss
with a 0.25 dB preamp, the noise temp will be 323 degrees K, while
having the 0.25 dB preamp at the antenna with no loss will produce a
noise temperature of only 17 degrees K. Adding in a 50 degree sky
temperature gives you 67 degrees vs 373 degrees K. The S+N to N ratio
would be about 5.57 times or 7.4 dB better than having 3 dB of feedline
loss on 222 MHz when the sky temps are low. Why give up 7.4 dB?
OK, I am not quite fair as I have neglected some other losses that
happen in a real ham VHF system (small jumpers, coax relays etc) but the
trend is there for the cold sky high VHF bands. When you introduce
feedline loss, you lose much more S/N ratio than the amount of your
feedline loss. At low frequencies, as the sky temps increase, that
seeming anomaly disappears. On 222 MHz, adding a preamp at the tower
brings you huge benefits. I even saw a noted improvement with 0.4 dB
feedline loss on horizon signals that had sky temperatures in the 290
degree range.
Dave K1WHS
On 11/21/2024 3:58 PM, Howard Reynolds wrote:
From Dave's post: I guess I got lucky to have 100 feet of 1-5/8" heliax
going up the tower. There's probably more loss in the pigtails and antenna
selector switches in the shack and at the top of the tower. I'm sure that
the rx preamp at the antenna helps a lot. I attempted to work one station
running a kW who was around S7 here who couldn't hear my signal which
couldn't have been more than two S-units less there; said I needed more
power. As the saying goes, "If you can't hear them, you can't work them".
WA3EOQ
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