Jim, the example compares 200ft of LMR400 to RG213. The stated difference
is .358 dB/100ft for a total of .716 dB.
That is a power ratio of 1.179 or 100w vs 117.9w. That is easily measured
with a bird meter.
73,
Gerald K5GW
In a message dated 4/9/2012 8:04:42 A.M. Central Daylight Time,
jim.thom@telus.net writes:
Date: Sun, 08 Apr 2012 21:11:58 -0700
From: Jim Brown <jim@audiosystemsgroup.com>
Subject: Re: [TowerTalk] Coax
On 4/8/2012 9:01 PM, Jim Thomson wrote:
> LMR-400db .666 db
> LMR-400UF .799 db
> RG-213U 1.024 db
RG213 is not a spec, it's a very broad generic description. There are
RG213s built with thin copper braid and others with heavy copper braid.
The loss in cable is a direct function of how much copper they use to
build it. Based on resistance data, Davis's 213 is directly equivalent
to LMR400 for use on the HF bands.
## say what. Run 200 feet of 213-U into a dummy load at the far end, with
a wattmeter 1 foot before the dummy load.
You'll need far more precise instrumentation than a Bird to measure a
difference of 0.1 dB Rather, you'll need a scope, RF voltmeter, or
spectrum analyzer that can resolve 0.01dB, you'll need to measure by
substitution, and you'd better make a bunch of measurements and compare
them.
73, Jim K9YC
## Andrew .5 inch heliax uses a copper clad solid aluminum center
conductor. Andrew is the not the only game in town.
Other makers will offer .5 inch heliax in both copper clad Al and also
solid CU. Are you trying to tell us that the solid Cu
variety will result in lower loss. .875 inch heliax uses a hollow copper
tube. LMR-600 uses 5.5 gauge copper (7 x strands)
for the center conductor. Which one do you think will have the lowest
dc resistance for the center conductor.
## If Davis RF’s version of 213 has way lower dc resistance than belden
213, then the strands would have to be a LOT bigger. If that was
true,
the OD of the cable would have to be a lot bigger, and all those 213
cables appear to be .405 inch OD. You can’t just use heavier gauge strands
for the braid and be able to use the same pl-259. Well maybe you can,
if you made the sheath from thinner material to maintain the .405 inch
OD. t
## LMR cables use a tinned outer braid...on top of a 360 deg AL wrap of
foil. Most eng notes will tell you that 90% of losses in coax cable
are
dielectric losses..and not dc resistance loss’s.
## Bird products have limitations to them. Any array solutions power
master wattmeters will read in .1 watt increments, but only below 100w.
Above 100w, they read in 1 watt increments. Pretty easy to see the
difference between 1500w and 1498 w. (.00579 db) or the diff
between 1500 and 1499 w (.002896 db) Pretty easy to measure the diff
between 1500w and 1467w (.00966 db)
## It would not be rocket science to measure the difference between
davis 213, Belden 213..and LMR-400. Use 200-300 feet of each
of the 3 x cables, as long as they are all identical length, and use the
same freq to test em.... like 29.0 mhz
## The problem with using any scope is... if the scope is off by say 5%,
your results will be off by twice that. Voltage squared divided by 50
ohms
= power. The bottom line is..even a bird will show the big diff between
300 ft of belden 213 and 300 ft of .875 inch heliax on 29 mhz. Its
like
apples and oranges.
## If you are implying that dc resistance is the predominant factor in
coax cable loss, then all these various formulae + online loss
calculators must be out to lunch, and I find that hard to believe.
## Bigger coax is cheaper than a bigger amplifier. I want a bare min of
2.5 kw at the feed point of the ant on any band. With a crank up tower
I am limited to flexible coax up the side of the tower. The best I can
come up with is LMR-1200DB to the base of the tower..and RG-393
up the side of the tower.
Jim VE7RF
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