Natural balun/Antenna
 

Hi, Gurus -

Shown in "Antennas for All Applications" on page 820, figure 23-17 (a) is a
folded dipole which looks as if it could be easily constructed from RG58.
This antenna would be appealing because of the natural balun. However, they
make no mention of the required coax velocity factor nor of the terminal
impedance. I tried constructing one and measuring with a vector voltmeter
but the data were inconclusive. That is, I don't think I know how to make
measurements like this at 440 MHz yet. It is much trickier than I thought.

In any case, can someone tell me:

- Can RG58 be used, or will the velocity factor mess up the antenna?
- What will be the terminal impedance?
- Will the terminal impedance change as the gap between the elements vary?

(I consider the "terminal" at the midpoint of the bottom element)

Thanks.

John (KD5YI)


BTW, I can post a picture tomorrow or the next day, if necessary, when I can
lay hands on a computer with a scanner.

J

On Sun, 26 Sep 2004 15:08:39 -0500, "John Smith"
wrote:

Hi, Gurus -

Shown in "Antennas for All Applications" on page 820, figure 23-17 (a) is a

Hi John,

Are you sure of the title? Author?


73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Sun, 26 Sep 2004 15:08:39 -0500, "John Smith"
wrote:

Hi, Gurus -

Shown in "Antennas for All Applications" on page 820, figure 23-17 (a) is
a

Hi John,

Are you sure of the title? Author?


73's
Richard Clark, KB7QHC

Yes, Richard. I am holding the book in my hands. It is the third edition by
John D. Krause and Ronald J. Marhefka.

73
John

Yes, Richard. I am holding the book in my hands. It is the third edition by
John D. Krause and Ronald J. Marhefka.

Kraus. John Krause writes about Opera.

Hi John,

The sub-title threw me. Don't have that one. I will await your scan
of that page.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
Yes, Richard. I am holding the book in my hands. It is the third edition
by
John D. Krause and Ronald J. Marhefka.

Kraus. John Krause writes about Opera.


Yes, of course. It's embarassing to misspell a noted author's name. My
apologies.


The sub-title threw me. Don't have that one. I will await your scan
of that page.

73's
Richard Clark, KB7QHC

I may be able to get it posted late today. I will put it on a.b.s.e., if
that's acceptable.

John

On Mon, 27 Sep 2004 09:10:59 -0500, "John Smith"
wrote:

I may be able to get it posted late today. I will put it on a.b.s.e., if
that's acceptable.

Hi John,

I am unfamiliar with the acronym. Do you have a website?

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Mon, 27 Sep 2004 09:10:59 -0500, "John Smith"
wrote:

I may be able to get it posted late today. I will put it on a.b.s.e., if
that's acceptable.

Hi John,

I am unfamiliar with the acronym. Do you have a website?

73's
Richard Clark, KB7QHC

The acronym is for the news group alt.binaries.schematics.electronics. But,
I can make it available from a Web site, no problem.

Thanks,
John

"John Smith" wrote in message
...
Hi, Gurus -

Shown in "Antennas for All Applications" on page 820, figure 23-17 (a) is
a folded dipole which looks as if it could be easily constructed from
RG58. This antenna would be appealing because of the natural balun.
However, they make no mention of the required coax velocity factor nor of
the terminal impedance. I tried constructing one and measuring with a
vector voltmeter but the data were inconclusive. That is, I don't think I
know how to make measurements like this at 440 MHz yet. It is much
trickier than I thought.

In any case, can someone tell me:

- Can RG58 be used, or will the velocity factor mess up the antenna?
- What will be the terminal impedance?
- Will the terminal impedance change as the gap between the elements vary?

(I consider the "terminal" at the midpoint of the bottom element)

Thanks.

John (KD5YI)


The picture of this antenna can be found at

http://www.sophisticatedsolutions.us...d%20Dipole.jpg

(assuming I did everything right).

On Mon, 27 Sep 2004 21:18:45 GMT, "John Smith"
wrote:
Shown in "Antennas for All Applications" on page 820, figure 23-17 (a) is
a folded dipole which looks as if it could be easily constructed from
RG58.

Um, actually it quite explicitly states it uses "Cool Cable." You
might try some Hip Cable as a substitute.

This antenna would be appealing because of the natural balun.

The "natural" of the BalUn is probably in the balance and the
orthogonal geometry.

However, they make no mention of the required coax velocity factor nor of
the terminal impedance.

Reckon on the outer surface velocity factor (it is really only a thick
radiator) which, if you are using solid coax is 1 (lower if it has
insulation).

I tried constructing one and measuring with a
vector voltmeter but the data were inconclusive. That is, I don't think I
know how to make measurements like this at 440 MHz yet. It is much
trickier than I thought.

Please share your data.


In any case, can someone tell me:

- Can RG58 be used, or will the velocity factor mess up the antenna?

already answered.

- What will be the terminal impedance?

Depends on dimension, folded dipoles multiply the load on the basis of
separation and ratios of thickness of each side of the fold (in this
case I presume it is identical - so the relation, if I recall
correctly will be roughly 4 fold for halfwave dimension or 300 Ohms).

I concentrate mostly on HF design, so someone will probably (now why
would I say probably?) correct any mistakes offered here.

- Will the terminal impedance change as the gap between the elements vary?

Only if it is a remarkably variation. Bench top precision in
construction at 440 MHz is not going to upset anything radically.

(I consider the "terminal" at the midpoint of the bottom element)

It is actually at the gap. The midpoint of the bottom element is
electrically neutral which makes mounting simpler.

The picture of this antenna can be found at

http://www.sophisticatedsolutions.us...d%20Dipole.jpg

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Mon, 27 Sep 2004 21:18:45 GMT, "John Smith"
wrote:
Shown in "Antennas for All Applications" on page 820, figure 23-17 (a)
is
a folded dipole which looks as if it could be easily constructed from
RG58.

Um, actually it quite explicitly states it uses "Cool Cable." You
might try some Hip Cable as a substitute.

I put some RG58 on ice.

I tried constructing one and measuring with a
vector voltmeter but the data were inconclusive. That is, I don't think
I
know how to make measurements like this at 440 MHz yet. It is much
trickier than I thought.

Please share your data.


I no longer have the data. I thought it was worthless since it made no sense
and I discarded it.



In any case, can someone tell me:

- Can RG58 be used, or will the velocity factor mess up the antenna?

already answered.

- What will be the terminal impedance?

Depends on dimension, folded dipoles multiply the load on the basis of
separation and ratios of thickness of each side of the fold (in this
case I presume it is identical - so the relation, if I recall
correctly will be roughly 4 fold for halfwave dimension or 300 Ohms).

So, I should use 300 Ohm coax?

- Will the terminal impedance change as the gap between the elements
vary?

Only if it is a remarkably variation. Bench top precision in
construction at 440 MHz is not going to upset anything radically.

(I consider the "terminal" at the midpoint of the bottom element)

It is actually at the gap. The midpoint of the bottom element is
electrically neutral which makes mounting simpler.

Suppose I construct the antenna as shown except put a Tee at the bottom for
convenience. What will be the impedance at that point? After all, that would
now be the feed point, yes?

The picture of this antenna can be found at

http://www.sophisticatedsolutions.us...d%20Dipole.jpg

73's
Richard Clark, KB7QHC

Thanks,
John

On Tue, 28 Sep 2004 13:52:05 GMT, "John Smith"
wrote:

So, I should use 300 Ohm coax?

Hi John,

No such thing, or it would be quite expensive (amounting to no such
thing).

Suppose I construct the antenna as shown except put a Tee at the bottom for
convenience. What will be the impedance at that point? After all, that would
now be the feed point, yes?

No. The feed point is at the inner conductor connection to the outer
conductor. The bend in the line (where you would put the tee) is
merely a bend in the line. You could choose any point in the line,
would that qualify it as a feed point? No. Your eye is merely drawn
to it through the magic of an illustration's symmetry. As a practical
matter, yes you could insert a tee there (provided you open one leg,
you see? even here we have to maintain the continuity out to the true
feed point).

I must add that my analysis on the Z transformation may be at fault.
Again, my focus is in larger antennas where these kind of machinations
would be prohibitive. Others should have chimed in by now, but they
seem transfixed with my postings on politics or bored altogether. It
seems to me that you already have the text description before you.
Certainly in a 800+ page tome Kraus isn't mute on the subject?

On the other hand, you have the instrumentation and the scale of
construction is not so demanding that you couldn't resolve this
yourself at the bench in an evening. We have too many dream weavers
here already and I have pleaded with them to perform simple tests that
apparently befuddle them. Imagine me posing the task of taking
several SWR measurements being responded to with 100 posting threads
of confusion as to how!

I would use solid coax simply because I have several hundred feet of
precision material. It was what we used for 900 MHz spread spectrum
so 400 MHz is no great shakes.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Tue, 28 Sep 2004 13:52:05 GMT, "John Smith"
wrote:

Suppose I construct the antenna as shown except put a Tee at the bottom
for
convenience. What will be the impedance at that point? After all, that
would
now be the feed point, yes?

No. The feed point is at the inner conductor connection to the outer
conductor. The bend in the line (where you would put the tee) is
merely a bend in the line. You could choose any point in the line,
would that qualify it as a feed point? No. Your eye is merely drawn
to it through the magic of an illustration's symmetry. As a practical
matter, yes you could insert a tee there (provided you open one leg,
you see? even here we have to maintain the continuity out to the true
feed point).


I think you misunderstand, but that is most likely because I have a hard
time conveying what's in my mind. Especially in print and expecially at my
age. I didn't used to have that problem...

I take a half-wave (at 1.0 velocity factor) length of RG58 with a BNC
connector on one end and make a hairpin loop. This is the left side of the
folded dipole. The BNC connector is at the bottom center. I take another
half-wave piece of RG58 with a BNC connector connected to the shield only,
and make a hairpin loop for the right side of the folded dipole. Now I
install a T at the bottom between the left and right sides. As for the upper
element, I attach the center wire of the left half to the shield of the
right half. I repeat, the center conductor of the coax on the right side is
not used.

Now I have an antenna completely constructed with a feedpoint at the bottom
center via the T. What is the impedance at the T?

I must add that my analysis on the Z transformation may be at fault.
Again, my focus is in larger antennas where these kind of machinations
would be prohibitive. Others should have chimed in by now, but they
seem transfixed with my postings on politics or bored altogether. It
seems to me that you already have the text description before you.
Certainly in a 800+ page tome Kraus isn't mute on the subject?

Almost mute, yes. The only discussion for this particular arrangement is
beneath the figure where it says "(a) A folded dipole has zero potential at
the midpoint of the lower dipole half at all frequencies. Thus, this point
is ideal to attache a coaxial cable."

On the other hand, you have the instrumentation and the scale of
construction is not so demanding that you couldn't resolve this
yourself at the bench in an evening. We have too many dream weavers
here already and I have pleaded with them to perform simple tests that
apparently befuddle them. Imagine me posing the task of taking
several SWR measurements being responded to with 100 posting threads
of confusion as to how!

I recently got some of the instruments I now have. Owning them, however,
does not mean I know what I'm doing with them. This is my first foray into
UHF and the instruments and techniques are new to me. Yes, I can handle
complex arithmetic, but that doesn't mean I know how to set up a test
properly at 440 MHz.

Resolve this in an evening? I first have to learn how to make the
measurements with some accuracy and repeatability before I can get
meaningful results. At this frequency, I've learned, even getting a true
short circuit takes care.

Also, I have nobody to work with me as I try all this new stuff. This group
is the only resource I have with people who are knowledgeable enough to
answer such questions.

I'm doing the best I can.

73,
John

On Tue, 28 Sep 2004 22:54:22 GMT, "John Smith"
wrote:
I think you misunderstand, but that is most likely because I have a hard
time conveying what's in my mind. Especially in print and expecially at my
age. I didn't used to have that problem...

Hi John,

I understood you, the illustration is sufficient.

I take a half-wave (at 1.0 velocity factor) length of RG58 with a BNC
connector on one end and make a hairpin loop. This is the left side of the
folded dipole. The BNC connector is at the bottom center. I take another
half-wave piece of RG58 with a BNC connector connected to the shield only,
and make a hairpin loop for the right side of the folded dipole. Now I
install a T at the bottom between the left and right sides. As for the upper
element, I attach the center wire of the left half to the shield of the
right half. I repeat, the center conductor of the coax on the right side is
not used.

This is as it should be - for a folded halfwave dipole. However,
there is some question that this is intention of the design expressed
with great paucity of facts by Kraus. More on this below.

Now I have an antenna completely constructed with a feedpoint at the bottom
center via the T. What is the impedance at the T?

It will be the transform of that load found at the opposite side where
the signal is developed by the connection of the inner conductor to
the outer shield. More on this below.

Almost mute, yes. The only discussion for this particular arrangement is
beneath the figure where it says "(a) A folded dipole has zero potential at
the midpoint of the lower dipole half at all frequencies. Thus, this point
is ideal to attache a coaxial cable."

More useful is it can be attached to a metal support without
interfering.

I recently got some of the instruments I now have. Owning them, however,
does not mean I know what I'm doing with them. This is my first foray into
UHF and the instruments and techniques are new to me. Yes, I can handle
complex arithmetic, but that doesn't mean I know how to set up a test
properly at 440 MHz.

You need to add a directional coupler to separate out the forward and
reverse components is all.

Resolve this in an evening? I first have to learn how to make the
measurements with some accuracy and repeatability before I can get
meaningful results. At this frequency, I've learned, even getting a true
short circuit takes care.

Do you have a manual? I've been trying to find a link to HP manuals
that was offered in this group in the last couple of months. The "App
Notes" written during the 60s offer a complete education without
requiring a double-E to understand.

Also, I have nobody to work with me as I try all this new stuff. This group
is the only resource I have with people who are knowledgeable enough to
answer such questions.

I'm doing the best I can.

You'll get there, by and by.

73's
Richard Clark, KB7QHC

On Wed, 29 Sep 2004 04:15:57 GMT, Richard Clark
wrote:

More on this below.

It seems in my effort to find the page(s) associated with HP and your
equipment, I forgot to expand on the issues promised.

The antenna is generally known as a "shielded, balanced dipole." This
is often applied to receive applications. For the dimensions you
suggest (halfwave overall length), the Z multiplication would indeed
present a 6:1 mismatch - on average.

To answer your question about what Z would be present at the "feed
point": it would fall along the circumference of a constant SWR 6:1
circle (speaking of a Smith Chart solution) depending upon the length
of the electrical distance to the true feed point. This will be
greater than the halfwave of the antenna structure given that its
electrical length is not confounded by insulation properties.

I have seen this design used for repeaters, but in a 4 bay
configuration. Undoubtedly the harness feeding it was composed of
halfwave sections placed in parallel to draw down the mismatch (at
least it acted like this, the wiring was hidden within the greater
supporting structure).

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Wed, 29 Sep 2004 04:15:57 GMT, Richard Clark
wrote:

More on this below.

It seems in my effort to find the page(s) associated with HP and your
equipment, I forgot to expand on the issues promised.

The antenna is generally known as a "shielded, balanced dipole." This
is often applied to receive applications. For the dimensions you
suggest (halfwave overall length), the Z multiplication would indeed
present a 6:1 mismatch - on average.

To answer your question about what Z would be present at the "feed
point": it would fall along the circumference of a constant SWR 6:1
circle (speaking of a Smith Chart solution) depending upon the length
of the electrical distance to the true feed point. This will be
greater than the halfwave of the antenna structure given that its
electrical length is not confounded by insulation properties.

I have seen this design used for repeaters, but in a 4 bay
configuration. Undoubtedly the harness feeding it was composed of
halfwave sections placed in parallel to draw down the mismatch (at
least it acted like this, the wiring was hidden within the greater
supporting structure).

73's
Richard Clark, KB7QHC



Thanks, Richard.

John