> 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
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