"Dave" wrote in
:


"Owen Duffy" wrote in message
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"Dave" wrote in
:

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Hello Owen,

Sorry I am just now getting back to you on this, but I have been
reading and studying your article. I constructed a loop somewhat
like the one you discuss, but have one question: you speak of a
"feed tee" from which the feedline extends. Should I be able to buy
such a T-shaped device at my local parts outlet? I asked about such
there one time, and was practically laughed at. But it seems such
would be very handy for antenna builders of all types. If my local
parts distributer doesn't have this item, where can I get it and
what should I call it ("feed tee"?)

Dave, the 'feed tee' I refer to is the tee at the bottom of Fig 1. It
is not a standard component that you would buy off the shelf. I have
used it as a descriptive term, sorry if it has confused you. The
important detail is the electrical detail. In commercial loop
constructions, the thing is usually a box, the the loop coax enters
opposite sides of the box with effective circumfrential shielding.
The box is a convenient mounting and good location for an amplifier
if used.


The loop I constructed is extremely primitive, but still functions
nearly as well as the 110' longwire antenna I have used for years.
And it is *directional*, though sometimes noisy (probably because
the feedline does *not* exit and travel symmetrically away from the
loop, I am guessing.)

I am not suggesting that loops aren't directional. Shielding a loop
is one (and only one of several) of maximising the pattern nulls.

Symmetry helps to ensure that the feedline is not effectively part of
the system radiator.

The noise issue may be related to the above.


The main way in which my loop differs from your design (other than
the feedline asymmetry) is the fact that the shield is not cut away
from the center conductor opposite the feed input. Can you tell me
what function this feature serves?

I don't understand just what you mean. Perhaps your construction is
like Fig 3 (from the ARRL Antenna Handbook), but as stated, it
doesn't do what they say it does. That is not to say it doesn't
'work', or that it isn't directional. The stuff about shielding
against electric and not magnetic fields is a flawed explanation.

The real radiator is the outside of the outer conductor, the
feedpoint is the gap, and the construction is a clever way of
achieving maximum symmetry by placing the feedpoint at the top and
routing the coax to the feedpoint in a way that is symmetrical with
respect to the outside of the outer conductor of the loop. If you
don't route the coax away from the tee in a very symmetrical way,
don't waste your time on the complicated construction.

Owen

Hey Owen,

I am unclear on the use of the term "feedpoint" to describe the
exposed center conductor of the coax. Can you give me a little more
detail as to what this means? Sorry, I just don't understand why it
is called the feedpoint. I somehow thought that was where the coax
connecting the antenna to the receiver/transmitter was attached. And

I have used the term feedpoint to denote the point that delimits the role
of the transmission line and the radiator.

just for the record, I am only going to be receiving with whatever
type of loop I end up with. What I am actually seeking is a small

Ok, but broady speaking, the effects that apply to considering the antenna
with a tranmsmitter also apply to using it as a receiver.

If you want to think in receive terms, your coax feed line may have RF
currents induced on the outside of it from local and distant sources, and
if at the tee at the bottom of the loop, that current divides equally into
both halves of the loop, it will not result in a voltage difference at the
gap. The current will only divide to equally if each side of the loop is
symmetrical to the feed line and everything else near to it.

loop that I can attach to my tunable RF amplifier for feeding enhanced
signal to my Sony 7600GR shortwave radio, making a portable unit to
pair with the radio, allowing me to set them both up wherever I wish.

Oh, and the primitive loop I currently have is a piece of coax forming
an (approx) 18" loop, with the center conductor connected to the outer
shield and none of the shield cut away.

Again, I think you are describing the loop shown in Fig 3 of my article.
You don't need to use coax for the loop itself, it is not a balanced loop
as described and coax doesn't help with balance. If it was you intention
that the loop was not susceptible to pickup on the feedline, the antenna
you describe does nothing to prevent that. The shielding explanation for
that type of loop is bunk.


Owen