[TowerTalk] Dipole resonates lower than expected.

[Robert Chudek]

Thanks Tom,

You know... I ordered the new Handbook & Antenna Book and need to start 
reading... It's been too long since I've played with this stuff! You are 
right, of course, the measurement problem you stated reminds me of my dog 
chasing his own tail!

I should have said I was looking for my minimum VSWR to end up at 3.775 MHz, 
not necessarily the antenna "resonance". So maybe it is resonant at 3.775! 
The VSWR doesn't drop lower than 1.6:1 in the best case scenario, but that 
was 200 KHz low, as I questioned. This evening I whacked off 6 feet (3 on 
each side) and the minimum VSWR moved up to around 3.760 MHz.

I can't compare before and after exactly because Murphy visited this 
afternoon, right after I trimmed the wires. When I turned on my AEA HF 
Analyst, it went berserk! It worked just fine this AM, but it didn't this 
PM. And it was just sitting on the desk, turned off, not attached to 
anything! Just my luck.

73 de Bob - K0RC

----- Original Message ----- 
From: "Tom Rauch" <w8ji at contesting.com>
To: "Robert Chudek" <k0rc at pclink.com>; <towertalk at contesting.com>
Sent: Sunday, October 16, 2005 7:12 PM
Subject: Re: [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.
>>
>> My target frequency was 3.775 MHz. That's 124 feet on my
> calculator.
>
> Hi Bob,
>
> We cannot measure resonant frequency in the shack unless
> resonance happens to occur with the same resistance as the
> feedline.
>
> In other words if your dipole had 75 ohm pure resistance at
> resonance,  the point where you measured 75 J0 would be the
> same at the analyzer end of the cable as at the antenna.
>
> If you cut the cable to a multiple of 1/4 wl you can still
> be misled because it is only 1/4 wave or a multiple thereof
> at one frequency. (Any cable a multiple of 1/4w long would
> be J zero at the analyzer end when it was J zero at the
> antenna end regardless of SWR on the line.)
>
> If the line has standing waves you have to either normalize
> the line out of the picture (network analyzers do that, your
> antenna analyzer cannot) or do a lot of trimming and
> testing.
>
> 73 Tom
>
>
>
>
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