[TowerTalk] Dipole resonates lower than expected.

[Robert Chudek]

Thanks for the many replies!

I knew velocity factors of coax changed depending upon the dielectric, but I 
couldn't remember if this was true for a single wire radiator. Everyone 
agrees it does. Gene, AD3F, stated the 5% lowering of resonance is typical 
for PVC insulated wire. Thanks everyone for the confirmations!

Also, this is just a temporary dipole to get something in the air for right 
now. I have hard drawn wire on the spool but didn't want to wrestle with it 
the other evening. (There's nothing more irritating that having something 
come out of the dark of the night and whack you on the back of the head!)

So I'll trim my existing dipole about 25 KHz high. Then the resonant point 
can QSY down the band over the next couple of weeks as the wire stretches. 
Actually, it's getting colder here in Minnesota. The temperature drop might 
cancel the stretch. I might be able to get a couple of months out of this 
deal! :-)

73 de Bob - K0RC

----- Original Message ----- 
From: "W0UN -- John Brosnahan" <shr at swtexas.net>
To: "Robert Chudek" <k0rc at pclink.com>; <towertalk at contesting.com>
Sent: Sunday, October 16, 2005 11:13 AM
Subject: Re: [TowerTalk] Dipole resonates lower than expected.


> Hi, Bob--
>
> Height may be an issue to some extent but I can confirm that plastic 
> jacket
> house wiring will lower the frequency as well due to its effect on the 
> velocity
> factor on the wire -- just as coax cable dielectric defines the VF.  In my 
> college
> days I had a good friend who built a 4L quad for 20M and used house 
> wiring.
> It had a very bad pattern and VSWR.  I told him to strip off the plastic 
> jacket
> and he did, and all was well from that time on.
>
> The other issue with house wiring is that it is soft-drawn copper wire and 
> will
> stretch some under tension.
>
> So you have three things that want to lower the resonant frequency.  The 
> low
> height, the jacket, and wire stretch.  I would be surprised if it DID 
> match the
> SIMPLISTIC theory but it seems to be doing just what it should under the
> circumstances of a more complete theory.
>
> One way to make the soft copper not stretch any more is to pre-stretch it
> to make it hard drawn.  This used to be down (in MY good old days) by 
> tieing
> it to the bumper of a car and and a tree pulling on it.  With plastic 
> bumpers
> this is probably not a good idea!    ;-)
>
> 73--John  W0UN _____________________________________________________
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