Towertalk
[Top] [All Lists]

Re: [TowerTalk] Coax

To: "[TowerTalk]" <towertalk@contesting.com>
Subject: Re: [TowerTalk] Coax
From: "Joe Subich, W4TV" <lists@subich.com>
Date: Mon, 09 Apr 2012 16:15:22 -0400
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
> ## 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
>    _______________________________________________
>
>
>
>    _______________________________________________
>    TowerTalk mailing list
>    TowerTalk@contesting.com
>    http://lists.contesting.com/mailman/listinfo/towertalk
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk

<Prev in Thread] Current Thread [Next in Thread>