[TowerTalk] TowerTalk Digest, Vol 88, Issue 45

[hanslg at aol.com]

 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-----
From: K7LXC at aol.com
To: towertalk at contesting.com; jim at audiosystemsgroup.com
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,  
towertalk-request at contesting.com 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 
less 
>>loss on the received  signals. If  you can't hear them - you can't work 
them, 
>>no  matter how much power you're  running. 

>  Yes, BUT --  coax loss on RX only matters if the limitation is noise in 
your 
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, etc.is just as good  on receive as your 
multi-element yagi if it was all fed with the same  coax? Isn't gain gain and 
loss 
loss? 
 
Cheers,
Steve    K7LXC




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