[time-nuts] RG6 or LMR400 for GPS Antenna (Symmetricom 58532A and T-bolt)

Pete Stephenson pete at heypete.com
Wed Apr 20 22:48:20 UTC 2016


On Wed, Apr 20, 2016 at 10:41 PM, Ryan Stasel <rstasel at uoregon.edu> wrote:
> All,
>
> I’m going to be installing a “permanent” antenna at home, and will need a run of about 100ft to get from my workstation, to the mast I’ll be mounting the antenna on (Symmetricom 58532A). I’ve seen some indication that both the antenna and the Trimble Thunderbolt won’t have any issues with running over 75ohm cable, but thought I’d ask the “experts” whether I’d be better off with some RG6 Quad-shield, or LMR400 (I’ve got a local source that doesn’t know what LMR400 is, or what it’s worth)?
>
> Obviously I’d prefer to run and crimp RG6, but if I’d be better off with LMR400, I’d rather run that now than go back into the crawlspace again. =)

I'm hardly an expert, but according to the Times Microwave calculator
at <http://www.timesmicrowave.com/calculator/>, RG6 (of unspecified
type, presumably double not quad-shielded) at 1542MHz will have a loss
of 12dB over a 100ft length. LMR400 will have a loss of 5.2dB over
that same length.

The datasheet for your specific cable should show the loss figures.

The 58532A has an amplifier with a gain of >30dB, so it should work
well even at moderate cable lengths.

Keep in mind that the Thunderbolt Starter kit came with a Trimble
Bullet antenna (similar gain to the 58532A) and 75 feet of RG6; it'll
almost certainly work fine with 100ft of cable: the manual for the
Thunderbolt recommends RG-59 cable (presumably because it's cheap and
common) and states "The maximum practical cable run is just over 100
feet." A graph in the manual shows RG-59 losing 15dB over 100ft,

To be safe, you could always test it by connecting the 100ft of cable
to the antenna and putting it outside in a more convenient location
that has a similar view as your mast and seeing how the Thunderbolt
likes it.

Also, keep in mind that the 58516A splitter can have between +3 and -3
dB of gain depending on your luck as to how it was made. The manual
says that for relatively lossy RG-213 cable and the worst case
performance of the 58516A, you should be fine with up to 174 feet of
cable with no line amplifier.

> Also, if it helps, I’ll probably have a Symmetricom/HP 58516A at/near the T-bolt so I can experiment with other GPS(DO)s as well (especially one of the JRMiller boards I bought and built (but never finished) ages ago). Which brings the question, will the T-bolt provide the oomph needed to power that splitter and the antenna over that length of cable?

Short answer: Yes.

Longer answer: The Thunderbolt manual says it can supply 5V at up to
45mA. The 58532A antenna draws a max of 27mA (with 20mA being
typical). The 58516A splitter manual draws it uses 10mA. Worst case
usage is 37mA, which is within the limits for the Thunderbolt.

Considering both conductors, 100ft of LMR400 has a DC resistance of
0.304 ohms, so the voltage drop would only be 0.01V over that length.
That's well within specs for the antenna (5V +/- 0.5V) and the
splitter (4.5 to 30V).

The specs for Belden 1189AP quad-shield RG6/U cable with a copper-clad
steel center conductor and aluminum braid lists the total resistance
for both conductors to be 3.28 ohms over 100 feet. That's a worst-case
voltage drop of 0.12V over that distance, again within spec for both
devices.

In short, LMR400 would be a better choice in terms of both signal
attenuation and DC resistance, but the difference is more or less
academic and either cable should work fine.

Cheers!
-Pete

-- 
Pete Stephenson



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