[VHFcontesting] 222 Activity

[David Olean]

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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