This is just a cascaded noise figure situation.
The first stage is the antenna (preamp) which has 26 dB gain (assumed) and an
unknown noise figure. Assume it's a dB or so. Let the second stage gain be a
negative value equal to the cable loss and the second stage noise figure be
equal to the cable loss in dB. Assume a lousy 10 dB NF and 10 dB gain for the
distribution amp.
If the cable loss is 5 dB then the cascaded noise figure and gain are: 1.26 dB,
31 dB gain.
If the cable loss is 2 dB then the cascaded noise figure and gain are: 1.13 dB,
33 dB gain.
I would use RG-6.
See: https://www.pasternack.com/t-calculator-noise-figure.aspx
The secret here is the 26 dB gain.
Wes N7WS
On 9/2/2017 11:57 AM, Clay Autery wrote:
Having decision-making problems for the materials for my GPS main
feedline. Going to use a TM LMR stock, just can't decide how big to go
with it...
26 dB 5vdc antenna on top of a 38 foot mast. Feed will come down the
inside/center of mast and exit near the bottom, thence routed through a
window and to the GPS distro amp. Antenna will feed GPSDO, NTP Server,
Blitzortung System Blue station, and one other device TBD.
Just cannot decide how big to go with the antenna to distro amp feed...
Assuming 50 feet total (38' mast + 12 feet to amp in shack) @ 1800 MHz
(closest to 1725 MHz), here are the losses from just this piece
(ignoring the amp to device jumpers):
-240 = 5.45 dB XXX - too much loss?
-400 = 2.85 dB
-500 = 2.30 dB XXX - too hard to find
-600 = 1.85 dB
-900 = 1.25 dB
Money not necessarily a consideration as this is a short run for a
permanent installation. Don't anticipate ever moving the GPS antenna to
the tower.
For 900 and likely 600, likely would not be able to do it in one piece
as routing it out of the mast and into the shack would get complicated.
Would likely bring it out of the mast at the bottom with a right angle
connector, and then use a smaller diameter jumper for the last 12 feet.
500 is pretty uncommon stock wise and it and connectors are harder to find.
I already have the tooling for both 240 and 400... but I definitely
don't want to challenge ANY of the devices for signal gain.
So it mostly boils down to easy vs. more effort ($$ aside).... Is it
worth the additional trouble to move from -400 to -600 or -900? To NOT
lose the 1-1.6 dB additional?
I'd appreciate your recommendation and reasoning. Thanks in advance!
73,
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
