[amsat-bb] Re: Mobile Coax?

Nate Duehr nate at natetech.com
Sun May 31 14:44:57 PDT 2009

On May 30, 2009, at 10:15 AM, Edward Cole wrote:

> Yep.  Commercial NMO mount mobile VHF/UHF antennas typically come
> with 17-feet of RG-58 and a connector to install when coax is trimmed
> for the particular installation.  I have way too many 100w mobile
> installs in my past ;-)  Of course, FM repeater design is for
> overkill on signal margins so no one sweats coax loss for mobiles.

While I agree with the comments that typical mobile installations   
have no need for low-loss coax, I've started using the slightly more  
expensive NMO mounts with low-loss coax in mobile installations for a  
different reason... the mounts are typically installed once and left  
alone, but someday I might want to hook up the 927 MHz FM rig and put  
an antenna on one of those mounts... and at 927 (or higher... say an  
Icom ID-1 for D-STAR use at 1.2 GHz), that lossy coax on the typical  
Larsen NMO mount becomes a problem.  Larsen (and others) make the  
mounts with better coax on them for only a few more dollars, and  
amortizing that cost out over the 8 years I've owned the current  
vehicle (and counting), I think mobile installations fall under the  
same old Elmer's line "buy the best coax you can afford and you'll  
only have to install it once!".

Of course, if all you're ever planning on doing in the mobile station  
is VHF/UHF... it definitely doesn't matter.  I want the flexibility to  
swap the antennas and run higher bands.

> The repeater sites may see 100-150 foot hardline runs, though.  I
> have one 120-ffot tower with 17 antennas and the coax are 1/2 or 7/8
> inch Heliax with the longest run 180-feet.

Good repeater builders/designers treat the installation almost exactly  
the same as good weak-signal terrestrial or satellite folk's  
installations, for sure... we're trying not to lose any of that 100 mW  
pip-squeak signal from an HT in some guy's basement 20 miles away who  
is too lazy to do the math to figure out his path loss, when he calls  
complaining that the repeater is "deaf".  LOL!  Hardline on  
everything, commercial quality high gain antennas, appropriate levels  
of band-pass or window filtering and pre-amps... a good repeater site  
looks a heck of a lot like a station set up to receive satellites or  
terrestrial weak-signal SSB... other than the antennas are fixed, and  
they're vertically polarized...

Repeater installations also mean avoiding cabling with foil-over-braid  
and other things that don't duplex well, and various other things that  
satellite and weak-signal enthusiasts DON'T usually have to worry  
about at home stations... I have no qualms with using LMR-400 here at  
the house... using it at the repeater site can quickly become a  
disaster that eats up multiple weekends.

A good repeater, done right, isn't cheap... nor is it as simple as  
slapping up a cheap Diamond antenna on a tower in a good location and  
calling it "done"... and the money isn't in the radios.  It's (just  
like weak-signal terrestrial and satellite work) in the antennas!  :-)

Nate Duehr, WY0X
nate at natetech.com

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