[TowerTalk] Dipole resonates lower than expected.

[ersmar@comcast.net]

Bob:

     There have been postings here on TT about the correction factor required for dipoles when constructed with insulated wire.  Most sources showed a 3-5% shortening when PVC insulated wire was used.  That is in the ballpark of your cited figure, below.  (The 'Skins are playing right now, otherwise I'd spend a bit more time looking for the prior postings.) 

73 de
Gene Smar  AD3F
<The difference between theory and practice in theory is less than the difference between theory and practice in practice.>  


> I slapped together an 80m dipole using the L=468/F formula and find that it 
> resonates about 200 KHz lower than expected.
> 
> My target frequency was 3.775 MHz. That's 124 feet on my calculator.
> 
> I built it using some left-over solid copper 12 gauge plastic jacketed house 
> wire. I center feed it with 100' of RG-59/a coax. One end is up 60 feet, the 
> other end about 40 feet. Using my AEA HF Analyst, it dips at 3.585 MHz with a 
> VSWR of 1.6:1. This is a 5.3% difference in length from the formula.
> 
> I can easily trim the length to meet my design goal, but I am curious why theory 
> doesn't match reality a little closer. Does the plastic jacket affect the 
> length? I would expect the resonant point to drop somewhat if the antenna was 
> "dragging on the ground". Or is 50' of height not adequate?
> 
> Thanks for any insight!
> 
> 73 de Bob - K0RC
> 
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