I like the coax standards because you can use them to check frequency,
too... The length of the coax and it's velocity factor don't change all that
much with temperature (unless you're using fancy Gore cables with PTFE
dielectric). You just put a tag on the coax with where the dips are
supposed to be (frequency wise), and what frequencies it should generate a
particular Z at.
Rod's resistive loads are a good check on the SWR bridge accuracy, but don't
check the reactive part or the frequency, while a TL standard does. One can
cross check frequency easily (zerobeat it against a known standard (WWV),
feed a 10 MHz standard reference (from your trusty HP Z3801.. $250 for ppb
accuracies) into the counter input, listen for the birdies with a
(presumably) more stable receiver)...
The reactive part is a pain with discrete components, although you can
select components and characterize them. The coax, while bulky, is
convenient.
Jim, W6RMK
----- Original Message -----
From: "Rod Elliott" <ve3irf@sympatico.ca>
To: <towertalk@contesting.com>
Sent: Saturday, August 30, 2003 7:35 AM
Subject: Re: [TowerTalk] MFJ259B
> I agree with Jim re the older MFJ259. I've had mine about 5 years. I use
it
> daily for tuning a transmatch on an antenna fed with ladder line. It has
> seen lots of action including several Field Day operations. It's SWR and
> frequency calibration have remained perfect, although over the years the R
> reading has slipped from 50 to 40 ohms at SWR=1:1. But I don't care... the
> R value is not needed for practical antenna tuning.
>
> Re calibration standards, I believe an easier way is to solder two 100-ohm
>
> I've made these in 0-ohm, 50-ohm, 25-ohm, 100-ohm sizes. The 0-ohm is made
> with a very short piece of 12-AWG copper wire instead of resistors. Using
> the 25 and 100-ohm sizes should give a SWR reading of 2:1, so along with
> the 50-ohm unit, you get a good idea of the linearity of SWR readings
below
> that point.
>
> These standards tell me that my MFJ259 has stayed in excellent SWR
> calibration over the years, regardless of the aberrations of he R reading.
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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