----- Original Message -----
From: "Michael Tope" <W4EF@dellroy.com>
To: <jsb@digistar.com>; <towertalk@contesting.com>; "Jim Lux"
<jimlux@earthlink.net>
Sent: Wednesday, September 22, 2004 7:55 PM
Subject: Re: [TowerTalk] CobWebb antennas - success stories?
> Hey Jim,
>
> It would be interesting to look at near field plot from
> the Cobb vs. a regular dipole. On his website, the
> inventor claims that it doesn't couple very strongly to
> nearby objects and is therefore good from an EMC/
> TVI perspective. I am a little skeptical of this claim.
I too am skeptical of this claim. Certainly, it's small physical size makes
it easier to keep it clear of other stuff, and since near fields drop off as
1/r^3, that helps a bunch. The high E field is going to be between the
ends (although not to the degree it is in the cap in a compact loop), which
also helps keep it away from other things.
On one of his pages he provides some theoretical basis for low EMI/EMC, and
it's not so much that it's the loop, but that it's H polarized, and high
enough above the ground that it's in a null for the victim receiver's
"antenna" (which is like a dipole close to the ground)
If
> it radiates as efficiently as he claims, I suspect it will
> couple to nearby objects just as readily as other
> similarly efficient antennas (at least to the same
> order of magnitude). In fact, he is claiming you can't
> use it in a parasitic array because it won't have enough
> mutual coupling to interact with a parasitic element.
Well... given that it's 1/4 wavelength on a side, the closest you could get
another one is 1/2 lambda away, a lot farther than the spacing of a typical
parasitic element in a Yagi. If you spaced it, say, 1/4 lambda apart, (3/4
lambda center to center), then the mutual coupling would be quite low.
There are some other stacking arrangements though.. you could have them
partially overlap, for instance (like dipoles in echelon).
On the other hand, I don't agree with his statement that you're better off
just lifting it higher, rather than building a phased array (where both
would be driven). Directivity is a "good thing" in that it can suppress
interfering signals, over and above providing the 10 log10(N) gain possible
from a widely spaced array.
Something else he didn't contemplate is vertically stacking two of these,
much like is done on 2m with those halo antennas. That would squeeze the
vertical pattern a lot.
> That sounds like pure baloney.
>
> 73 de Mike, W4EF..........................................
Jim, W6RMK
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