Probably a stupid question, but...
 

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


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

...
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

Okay, well, all I am trying to do is build something that will pick up weak
signals from a given direction, while ignoring signals and noise from other
directions. Ideally, it would not introduce a great deal of noise from any
source (unlike the loop I currently am experimenting with.) I am not overly
enamored of complex designs, and don't really care what it looks like so
long as it meets the above criteria. Do you know of any fairly simple
designs that would meet this description? If tuning is possible, that would
be a plus.

Thanks,

Dave

Probably a stupid question, but...
 

"Dave" wrote in
:

Okay, well, all I am trying to do is build something that will pick up
weak signals from a given direction, while ignoring signals and noise
from other directions. Ideally, it would not introduce a great deal
of noise from any source (unlike the loop I currently am experimenting
with.) I am not overly enamored of complex designs, and don't really
care what it looks like so long as it meets the above criteria. Do
you know of any fairly simple designs that would meet this
description? If tuning is possible, that would be a plus.

For your MW RO application, I would look at a multi turn (unshielded)
loop, untuned or tuned, but with an effective balun isolating the outer
surface of the outer conductor of the coax feed line from the loop.

I responded originally to your question about shielded loops. Shielding
is only one way to improve loop balance, and most explanations of
shielded loops are flawed.

Google for some designs, and be suspicious of purported shielded loops.

BTW, loops have pattern symmetry about the plane of the loop, so they
don't favour signals from only one direction. Think of them more as
having two diametrically opposed narrow reject regions in the patter, the
two accept regions are much broader. If you want maximum rejection,
balance the loop wrt the feed line and everything else near it.

Owen

Probably a stupid question, but...
 

On Sep 27, 7:48 pm, "Dave" wrote:


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

Okay, well, all I am trying to do is build something that will pick up weak
signals from a given direction, while ignoring signals and noise from other
directions. Ideally, it would not introduce a great deal of noise from any
source (unlike the loop I currently am experimenting with.) I am not overly
enamored of complex designs, and don't really care what it looks like so
long as it meets the above criteria. Do you know of any fairly simple
designs that would meet this description? If tuning is possible, that would
be a plus.

Thanks,

Dave

One thing about small loops.. They are great at nulling ground wave
signals,
but not so great at nulling skywave signals.
So how well a small loop would work will depend on the source of the
noise.
If the source is local, IE: a noisy power line, etc, that signal will
arrive via
a space or ground wave, and you can null that noise very well.
But if the interference is via sky wave, you might get a reduction in
strength,
but usually not a total null.
As a general rule, small loops are best suited to the lower
frequencies.
They work ok for the HF bands, but you may not the see all the
benefits
on those bands that you might on the MW bands. But even one used
for HF should get a decent null on a noise signal as long as the
source
is fairly local.
As far as shielded loops, I've carefully compared both unshielded and
shielded loops, and couldn't really tell a lick of difference as long
as
both are balanced. I've also tried using unshielded loops, but with a
shielded coupling loop. Again, no difference, as mine are fairly well
balanced even using a regular unshielded solenoid loop.
I had just as deep nulls unshielded, as I did shielded.. No difference
in perceived noise either. So I consider using shielded loops an
option,
but usually not needed. As many have mentioned , the only advantage
is to help ensure balance, and in most cases, it's not a problem to
worry about.
I do mount everything very symmetrically though. I make mine with
very simple PVC frames.
IE: one example... http://web.wt.net/~nm5k/loop5.jpg
But my big one is even more simple. Just a thick appx 2-3 inch PVC
"mast", and regular 3/4 inch PVC for a cross arm, using PVC "Tees"
at the ends to thread the wires through.
The 3/4 inch PVC crossarm is run through drilled holes in the larger
PVC
mast. I drill them to fit tight, and I don't even have to glue them,
although
thats an option. I use stands to mount the loops on, and they can be
easily turned.
MK

Probably a stupid question, but...
 

Dave wrote:

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 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 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. . .

I might be able to shed a little more light on this.

If you directly feed an unshielded loop, you cut a gap in the loop and
connect a transmission line across it. Current induced in the loop by a
signal flows from the loop to the transmission line via this connection.

In a "shielded" loop, the signal induces a current on the outside of the
"shield". At the gap, the current flows to the inside of the "shield"
where it induces an equal and opposite current into the inner loop
itself. The inside of the "shield" and the inner loop comprise an
ordinary non-radiating coaxial transmission line, so it can be said that
the gap is where the signal-induced current enters the transmission
line, just like the gap in the unshielded loop. And so the gap in the
"shielded" loop is a feedpoint in exactly the same sense as the gap in
an unshielded loop.

Roy Lewallen, W7EL

Probably a stupid question, but...
 

Richard Clark wrote:



The gap, the short, and the shield all lend the aura of "magic" to an
otherwise conventional loop.

One of the first things I learned in my RF education is that a
gap does not necessarily stop the flow of RF, and neither does
a short! The mysteries of shielding I am still learning about.

I thought I had it all nicely sorted out, and then along came
the Fractal antenna! :-)

Irv VE6BP