[time-nuts] GPS Antenna Feed Line Decision
kb8tq at n1k.org
Sat Sep 2 19:31:46 EDT 2017
Modern distribution amp chips are likely to be sub 2 db NF. Gain is generally just a bit more than
the loss through any post filtering and the passive power splitter after it.
> On Sep 2, 2017, at 7:02 PM, Wes <wes at triconet.org> wrote:
> 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!
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