"Roy Lewallen" wrote in message
...
Trying to phase two antennas that close together at that frequency range
will be an educational experience at best, but more likely just an
exercise in frustration unless you have much more patience than average.
Such an array will be hyper-sensitive to everything. You might be able to
fleetingly see a null after a lot of tweaking, but I seriously doubt
you'll even get that. A tiny change in frequency, wiggling of the whips,
or even movement in the vicinity of the whips will have a profound effect
on any null you might see.

If a null from a small antenna is what you want, you'd have much better
luck with a carefully constructed and balanced ("shielded") loop.

Roy Lewallen, W7EL

Found my copy of Joe Carr's Practical Antenna Handbook, and re-read the
section on phasing verticle antennas. I believe you. Back to square one,
which was the thought that a loop was probably my best bet (I had come to
that conclusion a while back, but forgot why.)

Did try a shielded loop once upon a time, but didn't feel it gave me
anything to look forward to. Guess I'll dig it out and try it again. Will
try shielding it with copper "tape" and see what that buys me. I did try a
piece of coax wound in a triple-turn loop to give me 2.5 or 3 uH with which
to tune, with the shield cut away to expose the center conductor for a
couple inches, but didn't feel this offered anything either. Not sure what
I'll do. Poke around and try different things until I find something that
works better than the rest. Any ideas? I'm all ears. I tried the whips
because I had them on hand, and they were easy to install. Seems I read
somewhere that contrary to conventional wisdom, the shield on a shielded
loop doesn't actually shield at all, but becomes the antenna element.
Anyone know anything about that line of thought? I obviously know nothing,
and am trying to learn. Just don't know where to focus my energies.

Thanks,

Dave

Dave wrote:
. . .
. . .Seems I read
somewhere that contrary to conventional wisdom, the shield on a shielded
loop doesn't actually shield at all, but becomes the antenna element.
Anyone know anything about that line of thought? I obviously know nothing,
and am trying to learn. Just don't know where to focus my energies.

That's been discussed on this newsgroup a number of times. You should be
able to find the relevant threads via groups.google.com.

Yes, the "shield" doesn't shield the antenna -- in fact, the outside of
the "shield" *is* the antenna. What it does is aid in balancing the
antenna, reducing common mode pickup which can reduce the null depth.
"Conventional wisdom" that holds otherwise isn't wisdom at all, but a
lack of understanding of some basic electromagnetic principles.

There's undoubtedly a massive amount of information easily available on
the web regarding building and using small loop antennas. All you have
to do is ignore the ubiquitous "conventional wisdom" explanations of how
a "shielded" loop operates.

Roy Lewallen, W7EL

"Dave" wrote in
:


"Roy Lewallen" wrote in message
...
Trying to phase two antennas that close together at that frequency
range will be an educational experience at best, but more likely just
an exercise in frustration unless you have much more patience than
average. Such an array will be hyper-sensitive to everything. You
might be able to fleetingly see a null after a lot of tweaking, but I
seriously doubt you'll even get that. A tiny change in frequency,
wiggling of the whips, or even movement in the vicinity of the whips
will have a profound effect on any null you might see.

If a null from a small antenna is what you want, you'd have much
better luck with a carefully constructed and balanced ("shielded")
loop.

Roy Lewallen, W7EL

Found my copy of Joe Carr's Practical Antenna Handbook, and re-read
the section on phasing verticle antennas. I believe you. Back to
square one, which was the thought that a loop was probably my best bet
(I had come to that conclusion a while back, but forgot why.)

Did try a shielded loop once upon a time, but didn't feel it gave me
anything to look forward to. Guess I'll dig it out and try it again.
Will try shielding it with copper "tape" and see what that buys me. I
did try a piece of coax wound in a triple-turn loop to give me 2.5 or
3 uH with which to tune, with the shield cut away to expose the center
conductor for a couple inches, but didn't feel this offered anything
either. Not sure what I'll do. Poke around and try different things
until I find something that works better than the rest. Any ideas?
I'm all ears. I tried the whips because I had them on hand, and they
were easy to install. Seems I read somewhere that contrary to
conventional wisdom, the shield on a shielded loop doesn't actually
shield at all, but becomes the antenna element. Anyone know anything
about that line of thought? I obviously know nothing, and am trying
to learn. Just don't know where to focus my energies.

I made a perfecly workable little DF once on a ferrite rod, wound with a
few turns. Worked like gangbusters on the local 2 and 6mhz stuff that I
was trying to locate.



--
Dave Oldridge+
ICQ 1800667

"Dave" wrote in
:

....
were easy to install. Seems I read somewhere that contrary to
conventional wisdom, the shield on a shielded loop doesn't actually
shield at all, but becomes the antenna element. Anyone know anything
about that line of thought? I obviously know nothing, and am trying
to learn. Just don't know where to focus my energies.

I have written a simple explanation on the operation of the so called
'shielded loop' at http://www.vk1od.net/shieldedloop/index.htm .

You may find the article of interest.

If you read and understand the content of the article, you will see the
pitfalls in using tape to 'shield' a loop.

Owen

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

...
were easy to install. Seems I read somewhere that contrary to
conventional wisdom, the shield on a shielded loop doesn't actually
shield at all, but becomes the antenna element. Anyone know anything
about that line of thought? I obviously know nothing, and am trying
to learn. Just don't know where to focus my energies.

I have written a simple explanation on the operation of the so called
'shielded loop' at http://www.vk1od.net/shieldedloop/index.htm .

You may find the article of interest.

If you read and understand the content of the article, you will see the
pitfalls in using tape to 'shield' a loop.

Owen

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"?)

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.)

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?

Thanks for your help.

Dave

On Thu, 27 Sep 2007 09:03:47 -0500, "Dave" wrote:

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?

Hi Dave,

It means there's a cognitive slip between you and Owen (and what is
generally constructed as a "shielded loop").

The cut-away is the feedpoint of the antenna. The two semicircular
sides extending away from it are the dipole arms (or the complete
loop, if you prefer), and the join with trailing feedline is just
that.

You would do well to more completely describe your differences as an
open must exist somewhere in the shield (yes, an irony for what is
called a "shielded loop") for it to work as an antenna.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Thu, 27 Sep 2007 09:03:47 -0500, "Dave" wrote:

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?

Hi Dave,

It means there's a cognitive slip between you and Owen (and what is
generally constructed as a "shielded loop").

The cut-away is the feedpoint of the antenna. The two semicircular
sides extending away from it are the dipole arms (or the complete
loop, if you prefer), and the join with trailing feedline is just
that.

You would do well to more completely describe your differences as an
open must exist somewhere in the shield (yes, an irony for what is
called a "shielded loop") for it to work as an antenna.

73's
Richard Clark, KB7QHC

Hey Richard,

Thanks for the quick reply.

When you say that the cutaway is the feedpont of the antenna, does that mean
it is where the received signal enters the antenna? And the feedline is
where it is carried to the receiver?

Many thanks,

Dave

On Thu, 27 Sep 2007 09:52:39 -0500, "Dave" wrote:

When you say that the cutaway is the feedpont of the antenna, does that mean
it is where the received signal enters the antenna? And the feedline is
where it is carried to the receiver?

Hi Dave,

The single turn, shorted loop with an open shield is merely a
convenient construction. It brings nothing new to the table of RF.

Owen's page pretty much describes it all, but there's always the
off-chance it needs to be said again.

The gap is the feedpoint driving your transmission line.

The gap and driveline drop must be at the points shown for symmetry to
insure balance.

Shielding does nothing but describe a balance. You can as easily
remove the shield and obtain identical performance IF you guarantee
balance. This was done for decades before coaxial cable was common.

The gap, the short, and the shield all lend the aura of "magic" to an
otherwise conventional loop. Being "magic" gives rise to ridiculous
claims applied to it. Being "magic" divorces logic from the design.
That loss of logic begins to migrate among the "magic" crowd such that
they come up with useless antennas.

73's
Richard Clark, KB7QHC

Richard Clark wrote in
:

Shielding does nothing but describe a balance. You can as easily
remove the shield and obtain identical performance IF you guarantee
balance. This was done for decades before coaxial cable was common.

Hi Richard,

For Dave's benefit, I might explain that the risk attendent in using a
small loop on a long transmission line is that the outside of the
transmission line becomes a significant radiator. In the limit, the loop
becomes just a means of exciting the outside of the transmission line as
the main element of the antenna system.

That is often undesirable because it spoils the pattern and / or results
in pickup of undesirable signals, especially from sources close to the
transmission line that has become the antenna.

There are other methods of trying to isolate the transmission line (as
Richard noted), the shielded loop construction is not the only way. For
example, a BALUN is a device that is designed to permit transition from
an balance device (the loop) to an unbalanced device (a coaxial
transmission line).

The shielded loop is widely used for instrumentation purposes, where the
Antenna Factor (related to gain) is calibrated and needs to be
independent of feedline length and routing (within reason).

Owen

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