Probably a stupid question, but...
 

Dave wrote:

Quck question, Roy,

Would it matter if I "separated" the whips electrically with, say, 55 feet
of coax? (That's approx the length I get for RG-174 coax, which is
something like 1/8" in diameter, with a velocity vactor of .66 and working
with 9 MHz.) Just a thought, but I don't know whether it has any merit or
not. And I am thinking I could adjust that "length" with an RLC circuit
through which I sort of "tune" it. What say you? Is this line of thought
worth persuing? Or would wiggling he whips still throw everything off?

And I do seem to have a fair abount of patience with this sort of thing.
Been working on the current project for about 2 years, had it working on and
off, taking it apart occasionally to implement some new retrofit or
engineering change. I am on disability, and have nothing but time on my
hands.

Thanks for any feedback...

You can answer most of your questions by modeling it with EZNEC. The
free demo program available from http://eznec.com is perfectly adequate
for the job. After going through the "Test Drive" tutorial in the
manual, I suggest that you take a look at the d_Cardioid.ez example file
to see how you can model an array with an ideal feed system. What you
should do is model your array in a similar manner, with two perfect
current sources. With the sources equal in magnitude and 180 degrees out
of phase, you'll get a bidirectional pattern, and it won't be sensitive
to frequency or element spacing. But then see what happens when you
change the phase and/or magnitude of one of the sources just slightly,
to simulate what any real phasing network would do. What happens to the
pattern? Look at the feedpoint impedances at various frequencies, and
see if you can figure out how you'll make a network to deliver the
correctly phased currents into those impedances. Another thing you can
do is try phasing them for a unidirectional pattern by giving the
sources a relative phase angle of 180 degrees minus the electrical
spacing of the elements (which of course will be different at each
frequency). You can get a nice looking pattern, but you'll find it
extremely sensitive to frequency and element spacing.

EZNEC will give you the opportunity to turn some of that available time
into an educational experience. I guarantee you'll learn a lot in the
process. If you want to learn even more about phased arrays, see Chapter
8 of the _ARRL Antenna Book_. The phased array section was completely
rewritten and updated for the latest (21st) edition.

Roy Lewallen, W7EL

Probably a stupid question, but...
 

"Roy Lewallen" wrote in message
...
Dave wrote:

Quck question, Roy,

Would it matter if I "separated" the whips electrically with, say, 55
feet of coax? (That's approx the length I get for RG-174 coax, which is
something like 1/8" in diameter, with a velocity vactor of .66 and
working with 9 MHz.) Just a thought, but I don't know whether it has any
merit or not. And I am thinking I could adjust that "length" with an RLC
circuit through which I sort of "tune" it. What say you? Is this line
of thought worth persuing? Or would wiggling he whips still throw
everything off?

And I do seem to have a fair abount of patience with this sort of thing.
Been working on the current project for about 2 years, had it working on
and off, taking it apart occasionally to implement some new retrofit or
engineering change. I am on disability, and have nothing but time on my
hands.

Thanks for any feedback...

You can answer most of your questions by modeling it with EZNEC. The free
demo program available from http://eznec.com is perfectly adequate for the
job. After going through the "Test Drive" tutorial in the manual, I
suggest that you take a look at the d_Cardioid.ez example file to see how
you can model an array with an ideal feed system. What you should do is
model your array in a similar manner, with two perfect current sources.
With the sources equal in magnitude and 180 degrees out of phase, you'll
get a bidirectional pattern, and it won't be sensitive to frequency or
element spacing. But then see what happens when you change the phase
and/or magnitude of one of the sources just slightly, to simulate what any
real phasing network would do. What happens to the pattern? Look at the
feedpoint impedances at various frequencies, and see if you can figure out
how you'll make a network to deliver the correctly phased currents into
those impedances. Another thing you can do is try phasing them for a
unidirectional pattern by giving the sources a relative phase angle of 180
degrees minus the electrical spacing of the elements (which of course will
be different at each frequency). You can get a nice looking pattern, but
you'll find it extremely sensitive to frequency and element spacing.

EZNEC will give you the opportunity to turn some of that available time
into an educational experience. I guarantee you'll learn a lot in the
process. If you want to learn even more about phased arrays, see Chapter 8
of the _ARRL Antenna Book_. The phased array section was completely
rewritten and updated for the latest (21st) edition.

Roy Lewallen, W7EL

Thank you, Roy. Seriously. I've been putting off trying EZNEC out, but
guess it's been long enough. Like you say, time to put some of my spare
time into a serious learning experience. Thank you for that link, too. Now
I have no excuses. :)

I do appreciate your feedback, and encouragement. Sorry if I was being
lazy. I really don't know why I've been putting it off, but I realize now
that I have.

'preciate it.

Dave

PS: Thanks too for the tip on the latest edition of the ARRL Antenna
Handbook. Now I know I need to get that edition, and not an older one.

Probably a stupid question, but...
 

"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

Probably a stupid question, but...
 

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

Probably a stupid question, but...
 

"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

Probably a stupid question, but...
 

"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

Probably a stupid question, but...
 

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

Probably a stupid question, but...
 

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

Probably a stupid question, but...
 

"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 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.

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.

Thanks again for your help...

Dave

Probably a stupid question, but...
 

"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