Where did that 12 - 15 ft above a structure come from? - m
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of jimlux
Sent: Tuesday, June 02, 2009 9:44 AM
To: Dan Zimmerman N3OX
Cc: towertalk@contesting.com
Subject: Re: [TowerTalk] optimum vertical length
Dan Zimmerman N3OX wrote:
>> If one uses the old CB antenna rules of 12-15 ft max height above
>> structure as a constraint, are you better off with some sort of short
>> vertical dipole, up 30 ft on the roof, or with a 45 ft vertical, on the
>> ground?
>
>
> When you start talking about 12 foot high vertical dipoles, you really
need
> to revisit your assumption of a lossless matching network.
<snip>
>
> Anyway, this is straying off the original topic, but I think that the best
> all-HF-bands approach even in a situation where unobtrusiveness is very
> important may be more than one type of antenna...
>
And, also Joe's comments about multiple antennas.
So.. given one fixed length element, the trade is to make it long enough
that the efficiency isn't wretched on the lowest band of operation, but
short enough that you don't get funky high band patterns.. which is tough.
So, you can make a single "variable length" radiator; either by the
SteppIR approach, or with traps. (Leading to an interesting question of
whether, given that you've got a tuner, you could use just one trap..
can one make a "low pass" trap? you'd have one length for, say,
10,12,15,17 and another for 20,30,40,80)
Or, as seems to be a reasonably non-obtrusive solution, a long ground
mounted vertical (for lower bands) and a shorter roof mounted vertical
(for higher bands)
One might also be able to make a fairly inconspicuous multiband vertical
of sorts.. base driven for low bands, center fed for higher bands (coax
up the center of bottom half) with some switching scheme to short the
upper feedpoint remotely.
Leading to yet another interesting question. Given, say, a 40 ft total
vertical length, for low bands, are you better off feeding it as a short
vertical dipole (in the middle) or as a base fed vertical (where you
add losses from the ground)..
I suspect the base fed is better, because you can always put lots of
radials out and make the loss no worse than the bottom half of the
vertical dipole, and the IR losses in a short radiator are always higher
than in a longer radiator.
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