[NLRS] 2M Stck help

[Jon Platt]

Hi Kirk.
 
I am not currently an EMEer, so take all of my following comments with a grain of salt.  

There is no doubt that in a perfect World that from a system noise perspective that the best place to put a preamp is at the antenna.  This is because of feedline loss.  However, the less feedline loss that you have the less the benefit of placing the preamp at the antenna.  Additional system design factors that one has to consider is the complexity of remote sequencing, outside environment, maintenance, and cost.  I probably forgot a few.

For your situation, 40 feet of LD4-50A (1/2" hardline) has a loss of 0.32 dB at 144 MHz and the ARR SP144VDG preamp has a stated noise figure of 0.55dB with a gain of 24dB.  There are a lot of online tools that can help with this analysis.  One website that has a cascade systems noise figure calculator is http://www.everythingrf.com/rf-calculators/cascaded-noise-figure-gain-calculator .   Using this tool you can use the feedline (0.32dB noise figure and 0 dB gain) as "Stage 1" and the preamp (0.55dB noise figure and 24 dB gain) as "Stage 2".   This would represent the preamp in the shack.  Then just reverse Stage 1 and Stage 2; this would represent the preamp at the antennas.   With the preamp in the shack I get a system noise figure of 0.85 dB while moving the preamp to the antennas shows a system noise figure of 0.57, or about 0.3 dB improvement.   I will defer to the EMEers in the group as to what a 0.3 dB improvement may mean and to if its meaningful from a receiving performance perspective on 2m.  Myself, I am doubtful as to if this is meaning for 2m, but I defer.

Another thing to consider is that the above analysis does not take into account the relays.  The analysis above should really be re-run taking into account how many relays the receive signal is running through before it gets to the radio.  Plug in the spec'ed relay loss(es) ... maybe 0.2 dB per relay ?   Moving the preamp towards the antenna to reduce the number of relays while using a single high isolation T/R relay may be beneficial.  In any case the cascade system noise figure calculator can be pretty interesting to use.

73, Jon
W0ZQ