[amsat-bb] Re: Mobile Coax?
ka1rrw at yahoo.com
Sat May 30 09:00:15 PDT 2009
You comments are valid for 2-meters. You are not going to see much difference between 15 feet of cheap coax and good coax. Howver, as you go higher in frequency the losses increase.
On the 440 band the Coax Loss difference between 15 feet of RG-58 and LMR-240 is approximately 1 dB.
For Raw power, switching from 15 feet of RG-58 (~5 millimeter cable) and LMR-240 (-6 Millimeter cable) will give you 15-20% more RF at the antenna.
So for people that are counting dB’s in mobile setup, this is just one more option to squeeze out a little more performance.
And for those of you what want to run amplifiers on 440, you will find that RG-58 has approximately a 105-watt limit. The LMR-240 (-6 Millimeter cable) will support 380 watts.
Here in Boston, Hams are starting to used 900 Mhz repeaters. The stock RG-58 coax just sucks up your TX and RX power on 900 mhz.
Low Loss Coax for Home and Mobile usage
Most of the mobile antennas mounting systems that come with coax use the
cheapest and highest loss coax available. If you read the fine print
on these coaxes you will be surprised to find that your power loss may
be as high as 40% and the coax will melt above 110 watts (on 440 mc).
More and more of us are using 440 and higher frequencies. By selecting
the correct coax you can save 1 or more DB of loss, even for a short
length as 15 feet.
Here are some number I put together on coax loss for the typical mobile
dB Loss (440 mc) dB Loss (900 mc)
Length FT TX Freq/Length
15' 100' Pwr 15' 100' Pwr
RG-58/U 1.5 9.9 105w 2.2 14.5 60w .195"
RG-58/A 1.5 9.9 105w 2.2 14.5 60w .195"
RG-8X 1.2 8.1 250w 1.8 12.1 150w .240"
LMR-240UF 0.5 3.2 380w 0.7 4.7 260w .240" .059
The coax RG-58 comes in many flavors, No letter, A, C and U.
The letter "U" is sometimes a solid 20 AWG copper center.
The letters "A" and "C" are stranded, the center size would be similar
to solid, and will have a little more loss than Solid center.
Most mobile installations will need 15 to 17 feet of coax.
RG-8X and LMR-240 are the same size, however LMR has foil shield and
lower published loss values. The UF version of 240 is the Ultra Flex
flavor and can bend easier with a only a slight change in loss ratings.
>From this list of small car style coax, the LMR beats them all; in
lowest loss for the size and highest transmit power loads. I was
planning on installing a 440 AMP in my car, but after looking at the
coax, I realized that I would fry the stock RG-58 coax.
I was looking for an NMO Mag mount that has LMR-240 installed. I was
not able to find any, so I made my own. I took a Round style Larsen NMO
mount and replaced the existing RG-58A coax with LMR-240. Use two screw
drivers to pop the magnet off the metal mount. Remove the two Phillips
screws hidden under the magnet. Remove the metal frame and now you will
have access to the coax mounting point. Replace coax and glue on the
magnet with epoxy cement.
45 watts, in, 15 feet of LMR-240, = 40 watts out
45 watts, in, 15 feet of RG-58, = 37 watts out (~10% power loss difference)
20 watts, in, 15 feet of LMR-240, = 17 watts out
20 watts, in, 15 feet of RG-58, = 14 watts out (15-20% power loss difference)
10 watts, in, 15 feet of LMR-240, = 7.7 watts out
10 watts, in, 15 feet of RG-58, = 5.8 watts out
--- On Sat, 5/30/09, Bob Bruninga <bruninga at usna.edu> wrote:
> From: Bob Bruninga <bruninga at usna.edu>
> Subject: [amsat-bb] Mobile Coax?
> To: "Amsat BB" <amsat-bb at amsat.org>
> Date: Saturday, May 30, 2009, 10:48 AM
> > 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
> Sent via AMSAT-BB at amsat.org.
> Opinions expressed are those of the author.
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