[TowerTalk] Dipole resonates lower than expected.

[Dale L Martin]

> Subject: [TowerTalk] Dipole resonates lower than expected.
> 
> I slapped together an 80m dipole using the L=468/F formula and find 
>that it
> resonates about 200 KHz lower than expected.
> 
There's the biggest part of the problem, Bob.  You "slapped" it 
together.  Everyone knows that antenna building must be done with 
reverence and deliberation.

> 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?
>

All the responses have addressed the effects of insulation on the 
wire, Bob.  But, no one has commented on what the analyzer result was 
in a 75 ohm system.  Does your analyzer evaluate 75 ohm systems or 
52/50 ohm systems?

While the results (determination of resonance and/or minimal SWR) may 
be useful, are they accurate and effective.

That is, are you tuning the system (radiator and feedline) and not the 
antenna (the radiator) because you're using 75-ohm coax feedline?  Or 
is the coax impedance irrelevant?

73,
dale, kg5u