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

"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