Since my earlier comment has occasioned a wider debate, here's a copy of my
answer to W8JI
To: <towertalk@contesting.com>
>Date: Wed, 08 Jul 1998 11:24:21
>To: W8JI@aol.com
>From: Pete Smith <n4zr@contesting.com>
>Subject: Re: [TowerTalk] Re: resonance
>In-Reply-To: <327c068e.35a25983@aol.com>
>
>At 01:23 PM 7/7/98 EDT, you wrote:
>>
>>In a message dated 98-07-06 19:07:11 EDT, you write:
>>
>><< >
>> Pete; A low point SWR does not indicate the resonance frequency of an
>> antenna. From what you describe, I'd bet on RF on the outside of the
>> feedline.
>> >>
>>
>>I'd bet Pete has a 50 ohm bridge measuring a 75 ohm line. If that is the
case,
>>SWR readings will change when line length is altered.
>>
>>Remember, SWR will remain constant ONLY if the meter and line have the same
>>design Zo.
>>
>>It's unlikely a very long feedline has enough common mode current left, no
>>matter how much common mode current is developed at the antenna, to cause a
>>large impedance shift at the feedpoint when the line length is altered far
>>from the antenna.
>>
>>It's even more difficult (if not impossible) to find a meter sensitive to
>>common mode current.
>
>Here's the situation -- antenna end inward:
>
>C-3 .... 95 feet RG-213 .... +/- 20 feet RG-11 (used to trim the 75-ohm
section to be an even half-wave multiple at 3500 kHz) .... 200 ft +- of
75-ohm CATV ... +/- 50 feet RG-213 to the shack ... amp ... 3 ft RG8X ...
transceiver.
>
>My logic -- I made the 75-ohm section as exact as I could, on the theory
that it will repeat at one end whatever impedance it sees at the other. If
I had a 50-ohm resistor at the antenna end instead of the C-3, in theory I
should see exactly 50 ohms at the shack end. Of course, the C-3 isn't
exactly 50 ohms, so I presume that impedance transformations are occurring
along the 50-ohm sections (presuming neither one of them is an electrical
quarter wave at the operating frequency). That being the case, the R +/- j
values seen by the bridge will change whenever I change feedline lengths.
>
>Any misteaks here? ;^}
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