Richard Clark wrote:

On Wed, 21 Nov 2007 12:09:31 -0800, Robert Smts wrote:

I'd like to know whether that is because the
effective mean height of the antenna varies according to the shape, or
because of where the antenna is fed.

Hi Robert,

You can model this, and all variations easily with EZNEC available
online (and is within the limitations of its free use).

I'll look at it and see. I tried the free version but it wouldn't install
with Crossover Pro (I don't use Windows) and I may have to install VMWare
first.

As for feeding it at the top, as you illustrate. That would tend to
be unbalanced with the transmission line dropping in close proximity.
Without special attention to choking this line, it will become a
radiator for some of the bands (it'll be a guess). This may not be
all that bad, but you might find the antenna tunes oddly.

Well, I can pull the antenna coax over about 20 feet from the edge of the
corner of the loop before it goes down the tree. What concerns me is that I
need to add an extra 120 feet of coax to feed it this way. The shack
conveniently is quite near where the antenna cable would come down if I fed
it at the bottom corner. That's why I was curious about the radiation angle
effect of feeding it from the bottom, as long as I had the loop up high
enough.


You could as easily put up a dipole and skip the complexity (hint,
loops are NOT low noise).

Hmmm. I had an 80 metre loop some years ago, in a noisy urban environment,
and had the opposite experience, which is why i was interested in trying
it. Oh well, wire's cheap. I do already have a dipole at 100 feet.

Thanks, Richard.


Bob, VE7HS