[amsat-bb] Re: Coaxial line question

Edward Cole kl7uw at acsalaska.net
Sun Jan 24 09:09:40 PST 2010

At 01:04 AM 1/24/2010, Luc Leblanc wrote:
>Yes again a returning question "preamp at the tower or near the shack"
>I will have to do soon some maintenance work on my satellite 
>antennas tower. I planned to use 2 heliax 1/78" 45'  individual line. Each
>line will be terminated by high quality 2psp coaxial microwave 
>switch feeding UHF and VHF antennas. I don't know if i can move my VHF and
>UHF preamp at the other end near the shack where each line will 
>enter the house on short LMR 400 run 10 feet max?
>I know the theory says the best place to put a preamp is at the 
>antenna but with the very low loss on the Heliax coaxial line,  is in the
>real world there will be any significant degradation in the noise 
>figures who will be really noticeable?
>Note: If the relays are introducing too much loss i can feed the 
>antenna directly from them on the 8' run on LMR-400 directly on the female
>N type heliax connector.
>P.S. One commercial microwave technician is telling me that i will 
>be able to see difference only on labs spectrum/signal generator in
>short i will be unable to tell any difference due to the short 
>length of my coaxial line. He told me he's playing with 300 to 600 feets of
>lines in his day to day work and 45 feet heliax run are nearly 
>consider as jumper line in his world...
>Luc Leblanc VE2DWE
>Skype VE2DWE
>DSTAR urcall VE2DWE
>Sent via AMSAT-BB at amsat.org. Opinions expressed are those of the author.
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OK, Luc.  To make this decision you need to do the math.  But you can 
just try comparing performance by installing the preamps at the 
antenna and at the shack and make your own evaluation.  I'm guessing 
that is not convenient as antennas are on a tower.

You mention running two coax lines "1/78".  I do not know what that 
is but it sounds like ridiculously small stuff like RG-174.  Or did 
you intend to say 7/8-inch?  7/8-inch Heliax is low loss (0.44 
dB/100-ft and 0.83 dB/100-foot respectively) so that certainly helps 
when installing a preamp at the shack.  But without numbers it is all 
hand waving and no decision is possible.  How long is the 
Heliax?  Also, not familiar with "psp" relays.  What is their 
specification for insertion loss?  If more than 0.2 dB they are not 
very good.  I use relays rated at 0.05 dB loss.

What is the NF and gain of the preamps?  What is the NF of the radios 
that will be fed?  Get all that and coax loss figures and input them 
into my spreadsheet program to find out what the  overall NF will be.

Add sky noise temp, Antenna noise temp, and radio bandwidth to 
determine minimum signal power that produces a zero SNR.
Then you will have the figures to determine the effect.

My overall experience is you will not have the best receiving system 
unless the preamps are at the antennas.  You may be "satisfied" with 
preamps in the shack if you never compare with having them at the 
antenna.  That is sort of saying driving a Ford Pinto is "just as 
good" as driving a Ferrari.  Both will get you to a destination, but 
the ride is definitely different ;-)

Regarding the opinion of the commercial mw engineer is not 
necessarily valid.  Does he work with weak-signal 
detection?  Probably not.  My qualifications are ten years as mw 
engineer with NASA detecting spacecraft near the edge of the solar 
system.  That is definitely "weak-signal".  The typical receiving 
system at Goldstone had a noise temp of 16K at 2115 MHz and a minimum 
detectable sensitivity of -185 dBm (the best system had -198 
dBm).  You can achieve -152 dBm with amateur equipment at 
432-MHz.  Typical 70cm ham station is -122 dBm without low noise preamps.

73, Ed - KL7UW, WD2XSH/45
  BP40IQ   500 KHz - 10-GHz   www.kl7uw.com
500-KHz/CW, 144-MHz EME, 1296-MHz EME
DUBUS Magazine USA Rep dubususa at hotmail.com

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