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Re: [TowerTalk] TowerTalk Digest, Vol 88, Issue 45

Subject: Re: [TowerTalk] TowerTalk Digest, Vol 88, Issue 45
Reply-to: "Tower and HF antenna construction topics." <>
Date: Tue, 13 Apr 2010 14:30:49 -0400
List-post: <">>
 Hi Steve,

On the higher frequencies, >144 MHz, the noise received at the antenna is 
usually (unless you have problems with QRM and QRN) close to or lower than the 
thermal noise. Your possibility of receiving a signal is then limited by the 
noise factor in the receiving system (your antenna, feeders and receiver). If 
you don't have an antenna with good directivity (to get a good signal), a low 
loss feeder (not to loose signal and add thermal noise) and a low noise 
receiver (not to add noise to your signal) you will have problems receiving 
weak signals. At HF the received signal already comes with a good amount of 
noise (the noise temperature is high, to use a "professional" term). If you 
loose some signal in your feeder you loose noise just the same so your 
signal/noise figure stays about the same. Naturally, if you have plenty if 
looses you will at some point loose too much signal to hear your station.

Think about it: You rarely hear about anyone putting a pre-amp at the antenna 
when working HF but you find it frequently when working VHF/UHF such as 
moon-bunch, TV reception etc. I don't think anybody would even think about 
adding even a short coax between the antenna and receiver working 10 GHz.

Hans - N2JFS




-----Original Message-----
Sent: Tue, Apr 13, 2010 1:58 pm
Subject: Re: [TowerTalk] TowerTalk Digest, Vol 88, Issue 45

In a message dated 4/13/2010 10:20:10 A.M. Pacific Daylight Time, writes:

>>Some potential problems  with the amp as already  mentioned but you don't 
>>get the real advantage for  long coax  runs with low loss coax which is 
>>loss on the received  signals. If  you can't hear them - you can't work 
>>no  matter how much power you're  running. 

>  Yes, BUT --  coax loss on RX only matters if the limitation is noise in 
receiver.  That is VERY RARE on the HF bands, where the major limitation is 
either  QRM or noise picked up on the antenna. In other words, on HF, coax 
loss  only affects TX. 

    I'm not splitting hairs about noise limitations  of your rx. 
    Are you saying that the practical effect of  a gain (less loss) of 3 dB 
on the air due to lower loss coax is not heard  on your receiver? So your 
dipole, vertical, coat hanger, just as good  on receive as your 
multi-element yagi if it was all fed with the same  coax? Isn't gain gain and 
Steve    K7LXC


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