> ## and its even easier with any digital wattmeter. It gets easier if
> the max power is 100w on the 1st meter, and less than 100w on the 2nd
> meter.
This assumes that the two meters are identical, calibrated identically
and with identical deviations with frequency. At the very least, I
would take measurements "both ways" - once with meter 1 on the input
and again with meter 2 on the input - and average the readings. That
should reduce the calibration errors.
> Once below 100w, then I’m down to reading power in .1 watt
> increments.
The Telepost (N8LP) LP-100 meters can provide readings to 0.01 watt
below 150 (I think) watts. However, again, one needs to be concerned
about calibration accuracy *and* significance. Just because one can
read a meter to 10 or 100 mW, is 1 that even significant since the
devices are rated for 3% accuracy (3 watts at 100 watts)?
73,
... Joe, W4TV
On 4/9/2012 10:28 AM, Jim Thomson wrote:
>
>
> From: TexasRF@aol.com
> Sent: Monday, April 09, 2012 6:22 AM
> To: jim.thom@telus.net ; towertalk@contesting.com
> Subject: Re: [TowerTalk] Coax
>
> 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
>
> ## and its even easier with any digital wattmeter. It gets easier if the
> max power is 100w on the 1st meter,
> and less than 100w on the 2nd meter. Once below 100w, then I’m down to
> reading power in .1 watt increments.
> I own 4 of these power master watt meters. They are superb. The bird and
> also my CD meter are long gone obsolete,
> along with the myriad of slugs.
>
> Jim VE7RF
>
>
>
> 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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