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[TowerTalk] Coax Set up losses/259B Accuracy?

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Subject: [TowerTalk] Coax Set up losses/259B Accuracy?
From: (Jim Reid)
Date: Fri, 26 Jun 1998 11:25:17 -0900

To the group,

Have now had my new MFJ 259B for a couple of days.  It is pretty
tough to set an exact frequency;  just a touch to the knob,  and
the frequency jumps lots  of kHz above 10 mHz,  or so.

Anyway,  was anxious to do something with it,  hi.  So first test was
to see what length it would read out when attached to my vertical
antenna "farm" coax run.  The true length is 220 feet of LMR 400
coax,  which goes into a Top-Ten six way antenna switch.  When
the Top-Ten has no control voltage to it,  the antennas are all
put  to ground,  and the coax feed is open,  or floating.  The 259B
manual says length can be measured so long as the cable is 
not terminated in resistance;  so open should suffice.  Per
The Wireman's Wirebook,  LMR400 ought to have a vF of
0.85,  and have losses of just under 0.3 dB per 100 feet at
7mHz;  a bit over 0.4 dB at 14mHz;  and about 0.5 dB at 21 mHz.

With the 259B,  my piece of LMR400 measures to be 223 feet 
long,  assuming it has a VF of 0.85.  As actual physical length
is 220 feet,  the true VF then is about 0.84.  Actually,  the
measurement is probably about as accurate as is the MFJ
meter,  I would suppose.

Also,  using the 259B to measure the loss of my length of
LMR,  I measured the following:

1.830.....0.5 dB, or about 0.23 dB per 100 feet.  LMR400 should 
                                   be about 0.14 dB at 1.8mHz

3.520.....0.7 dB,  or 0.32 per hundred,  should be 0.2,

7.020.....1.0 dB,  or 0,45 per hundred, should be 0.29,

21.020...1.6 dB,  or 0.73 per hundred,  should be 0.5.

and this same slope continues all the way up the frequency scale;
almost perfectly linear increase in loss with frequency,  and
always higher per 100 feet than what one would
expect from LMR400.  In fact,  the loss "measures" higher than
would be typically expected from 9913 and 4XLSuper8 coax.

So,  the question becomes:  which is not  quite as expected?
The 259B or the 220 feet of LMR400?  I have no idea what
the manufacturer's tolerances of either might be.  Maybe
I am just seeing an overlap of tolerances,  both meter and
coax are a bit low from typical,  and they are summing to
my disappointing  measured numbers.  If the 259B is
"accurate"   my cable  losses are disappointing,
for what I paid for it;  it's only about 12 weeks or so old.

I know when I laid out the cable,  it was tough to handle!
Wants to stay coiled,  and I managed to get a pretty
tight "kink" a couple of times while trying to pull
the cable into the running space around the yard,  and
back into the "jungle" out back.  Perhaps the kink caused
some damage that contributes this extra loss.  Cable is,
of course,  quite straight,  or just gentle bends now.

Or, maybe I should have gone out in the rain and disconnected
the coax from the Top-Ten switch,  though per the circuit,
ought to not be necessary,  right?

Anyway,  wanted a good guess of the losses in my
feed line run when I get around to doing my vertical/added
elevated radial experiments later on (when the blooming
rain out here stops for awhile!).

73,  Jim,  KH7M

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