[TowerTalk] Dipole resonates lower than expected.

[Jim Lux]

At 03:59 PM 10/16/2005, Robert Chudek wrote:
>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! :-)


Dipoles are pretty broad band.. a 5% change in frequency isn't going to 
make the feedpoint impedance all that weird, so if you have a tuner, you 
can just go for it (unless you're feeding it with 200 ft of RG-174 or 
something like that).

[I just put up an antenna this morning that resonates at 6.85 (2:1 BW is 
6.74-6.97) instead of the planned 7.2, and after spending a bit of time 
calculating the loss, it looks I lose a big 0.3 dB from the reactive power 
circulating back and forth, on top of the 0.7 I already lose in the coax 
run.. I'm not going to worry much for now]

Jim, W6RMK