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