[SEDXC] [SECC] Question about 30 meter halfwave vertical for the antennagurus

Bill Coleman aa4lr at arrl.net
Sat Oct 23 19:32:33 PDT 2010

On Oct 23, 2010, at 12:05 PM, Tom W8JI wrote:
> I'd bet money your 50 foot tower is not a 1 or 2 inch dameter pole with 
> nothing on the top.
> More than likely you have some big antennas on the top, and the tower is 
> pretty thick.

Rohn 25, so you code model it like an 10" diameter tube and that would be pretty close.

The A3S on top isn't that big, but it does provide some loading. It's likely electrically close to a 1/4 wave on 80m.

> The difference between that and your tower is length, top loading by 
> antennas, thickness, and feed system. They are not remotely similar in 
> anything but length.

Good to know!

> <<<But I agree about the 43 foot thing. It seems to be a cult-like 
> condition, like the 88 foot dipole in QRP circles. Of course, LB modelled 
> the 88 foot dipole at 100 feet and 70 feet. Most of the guys who put these 
> up rarely get them over 35 feet.>>>
> LB's error was not considering feedline loss and matching systems, not in 
> height.

The whole idea of that particular modeling study was to find a "backup" antenna that would still have a usable pattern. At 100 feet, even such a short doublet is OK on 80m, feed and matching losses aside. Even at 70 feet, it still has a clean pattern.

At 35 feet, the pattern breaks down on 80m, even 40m. No more magic.

> Any dipole, even one a dozen feet long, will model with a good pattern and 
> good gain if we ignore losses. The problem is getting power into it.
> Same as the problem with a thin vertical stick with no loading.

Tom, it seems there always something to learn from your posts. Thanks!

Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr at arrl.net
Web: http://boringhamradiopart.blogspot.com
Quote: "Not within a thousand years will man ever fly!"
            -- Wilbur Wright, 1901

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