[amsat-bb] Re: Mobile Coax?

Edward Cole kl7uw at acsalaska.net
Sat May 30 09:15:21 PDT 2009

At 06:48 AM 5/30/2009, Bob Bruninga wrote:
> > Other Car Tips:
> > Convert from RG-58 to LMR-240-UF
> > or RG-213.
>I'd take that with a grain of salt.  The length of coax in a car is 
>about say 10 feet.  The loss of 100 feet of RG-58 is say 5 dB? and 
>the loss of fancy stuff might be 3 db?  But the diffrence for only a 
>10 foot run is only .5 dB versus .3 dB or only 0.2 dB.  Nothing at 
>all to even consider compared to all the work it will take, and the 
>lack of flexibility and trying to run something almost like pipe 
>through places where a simple wire (RG-58) fits.
>My lesson was learned 40 years ago when I go my first 100 lb UHF 
>mobile rig (tubes) just after highschool.  The boat anchor filled 
>the entire trunk of my MGB.  But the first thing I did when we go 
>the lot of them in my club was spend a day replacing the 8" internal 
>piece of RG-58 in mine from the Transmitter output over to the 
>chassis connector with a 8" run of RG-8.  It was hard work getting 
>that 8" piece of RG-8 coax inside the radio and routed all around 
>the internal chassis.
>The elmer at the time laughed.  He said you just wasted a day and 
>all that work to save 0.01% of loss.  So now your radio works at 
>100% where as before it worked at 99.99%.  Losing 3 dB of course is 
>one thing (50%), but trying to worry about that last 1% when the 
>effort is tremendous is just not effective.
>Anyway, just my 2 cents...
>Bob, Wb4APR

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.

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.

73, Ed - KL7UW

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