[TowerTalk] Coax Losses on 160 and 75?

[Jeff AC0C]

The fellow originally asked what coax to run.  I have a comment there.  RG11 
is fine.

If you say it's a budget consideration, and it's 80/160, and it does not 
need to last 100 years, then lay that RG11 out there and rock and roll.  You 
may not even need to treat the 50/75 ohm bump, and if you do, it's simple as 
well.

I've got about 300 foot of it running off to the north end of my property 
here to feed the 80/160 transmit vertical.  Been laying on the ground for 
the last 4 years - oh, on a "temporary" basis.  There is another strip of it 
for a 30m vertical dipole.  And a bunch more feeding a 40m dipole in the 
trees.  Oh, and the 4-square has RG11 as well.

The one issue that is worth mentioning is making a solid contact with the 
rather lousy shield that RG11 has, at least the stuff I have.  It's the 
essence-of-braid wrapped around some foil.  I used crimp connectors (the 
braid is aluminum) and that allows you to make a solid electrical bond.  You 
can't wiggle the coax around like you can with a copper braid though so make 
sure you weatherproof and strain-relief it.

73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie

-----Original Message----- 
From: Jim Brown
Sent: Sunday, August 07, 2016 1:35 AM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Coax Losses on 160 and 75?

On Sat,8/6/2016 4:35 AM, Jim Thomson wrote:
> My dummy load does not have a mismatch, coax is 50 ohms, swr is  1:1
> at each end of the coax, and there is no reflected power..at either end. 
> T

Jim,

The actual Zo of coax is NOT constant with frequency, and it is not a
REAL number -- it is complex, of the form R +/- jX. The value of Zo
CONVERGES to its nominal value at VHF, but at lower frequencies, it's a
different value. You can see computed numbers for many common cables in
N6BV's TLW software, which comes free on the CD that is packed with the
ARRL Antenna Book. The accuracy of Dean's data has been questioned, but
the concept is quite correct.

SO -- coax that we CALL 50 ohm coax can have a Zo of 52 + j 2, so a 50
ohm resistor is NOT a perfect match. This variation from the "nominal"
value is greatest at lower frequencies.

73, Jim

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