Relation of radiation resistance and terminal resistance
 

On 5/31/2011 3:40 PM, John S wrote:
On 5/31/2011 5:04 PM, K7ITM wrote:
On May 31, 1:35 pm, John wrote:
On 5/31/2011 1:52 PM, Jeff Liebermann wrote:



On Sat, 28 May 2011 14:25:47 -0700 (PDT),
wrote:

How many amateur radio operators use this kind of academic preening
when they are putting up a dipole.

Me, me, me. Even the simplest antenna is influenced by nearby
structures, towers, poles, elevation, guy wires, position of coax
feed, chain link fences, and grounding system. That makes a simple
dipole not very simple. I've helped a few local hams model their
houses allowing prediction of takeoff angles, mysterious nulls,
optimum height, and cut length. While modeling (I use 4NEC2) does
take some learning and understanding, it does offer an improvment over
the tradition ham radio cut-n-try.

Too much hand-waving here to be useful to most folks.

Speak for yourself please. I like postings that are over my knowledge
level so that I learn something new. It's also nice to know *WHY*
things work, or don't. Learn by Destroying(tm).

I agree, Jeff.

I like antennas that are naturally short-circuited by design and can be
grounded, making the feed point essentially grounded for DC and lower
frequencies. One such antenna is the folded unipole. Its only problem is
that the feedpoint resistance is about 120 or so ohms.

So, I had this idea. The usual monopole (or ground plane) has about
30-35 ohms resistance. To get 50 ohms it is common practice to droop the
radials about 45 degrees. Since that raises the feedpoint resistance,
would raising the radials lower the feedpoint resistance of the folded
unipole and, if so, what effect would it have on the pattern?

EZNEC said to raise the radials of the folded unipole about 23 or so
degrees to get 50 ohms and the pattern would not be affected.

So I built one and it works swimmingly. I had to make some minor
adjustments in element lengths but that was fairly easy with the vector
voltmeter. Hooray for modeling.

Cheers,
John - KD5YI

Hooray also for using your head, John, and realizing that raising the
radials would _probably_ have that effect -- then having that verified
by a model, and then by an antenna that works well for you in
practice.

As a matter of fact, I can see where just jumping into it without the
benefit of modeling would probably have resulted in giving up on it. I
had to adjust many things (such as radial tilt) before I learned about
how things were going to be affected. Modeling is like having an antenna
breadboard but a whole lot less work.

Another way that should work: make the two parallel conductors
different diameters, with the correct spacing. You might also try
making a self-supporting grounded quarter wave, resonant with its
radials, and fed with a parallel conductor that doesn't go all the way
to the top of the quarter wave...

I looked at those (EZNEC) years a go and was never satisfied. I have
learned much more now, so I might benefit from another look.

So there are three different arrangements, perhaps with pretty similar
electrical characteristics, and you can then pick among them for the
one that suits your construction practices the best. Perhaps there
are some more "grounded" monopole designs you throw into the mix.

Yes, if you're "just throwing up a dipole," maybe you don't worry
about things like this, but there are those of us who like to think a
bit deeper about things. I can only hope I remain infinitely tolerant
of those who like to think much deeper than I about many things.

Cheers,
Tom

C'mon, Tom. You're a very knowledgeable person and I value your input.
That's plenty deep.

By the way, I used Walt's inherent balun to make a diamond-shaped
antenna which needs no additional balun. It is 50 ohms at the feedpoint.
It, too is inherently short-circuited and, with the inherent balun,
probably groundable.

It was an interesting exercise that went like this:

1. I like a loop for the inherent short-circuit.
2. It has about 100-120 ohms terminal resistance. I want 50 ohms.
3. It needs a balun. But, I don't really want one.

A folded dipole is about 300 ohms. A half-wave shorted transmission line
is about 0 ohms. So, if you take a shorted half-wave transmission line
and spread the wires apart at the 1/4W point all the way to where it
becomes a folded dipole, it seems to me that the terminal resistance
will go from zero to 300 ohms and 50 ohms is in there somewhere.

I tried it in EZNEC and found that to be the case. I found that, if the
acute angle of the rhombus is about 51.5 degrees, then the terminal
resistance is about 50 ohms (adjust perimeter along with angle to get
50+j0).

Ok, fine. That takes care of everything but the balun. In Walt's
Reflections III, he discusses the half-turn bifilar loop (page 22-10).
But what was intriguing was the inherent balun. Aha!

So, after modeling as well as I knew how, I constructed a rhombus
(diamond-shaped) antenna with the right half of the diamond being coax
(inherent balun) and the left half of the diamond being 14 ga wire.




Are you feeding this with coax? Why not use a big choke at the
feedpoint on the outside of the coax (i.e. a bunch of suitable ferrite
toroids)?

It's not like antenna itself cares which side is connected to shield or
center conductor (as long as no current is being carried on the outside
of the shield?)

It should work basically identically as a scheme with a coax balun, so
you could choose which ever is mechanically or cost-wise more
convenient. (I happen to have a box full of 31 mix toroids for such
things, you might happen to have extra coax or a clever way to support it..)

Relation of radiation resistance and terminal resistance
 

On 5/31/2011 5:59 PM, Jim Lux wrote:
On 5/31/2011 3:40 PM, John S wrote:
On 5/31/2011 5:04 PM, K7ITM wrote:
On May 31, 1:35 pm, John wrote:
On 5/31/2011 1:52 PM, Jeff Liebermann wrote:



On Sat, 28 May 2011 14:25:47 -0700 (PDT),
wrote:

How many amateur radio operators use this kind of academic preening
when they are putting up a dipole.

Me, me, me. Even the simplest antenna is influenced by nearby
structures, towers, poles, elevation, guy wires, position of coax
feed, chain link fences, and grounding system. That makes a simple
dipole not very simple. I've helped a few local hams model their
houses allowing prediction of takeoff angles, mysterious nulls,
optimum height, and cut length. While modeling (I use 4NEC2) does
take some learning and understanding, it does offer an improvment over
the tradition ham radio cut-n-try.

Too much hand-waving here to be useful to most folks.

Speak for yourself please. I like postings that are over my knowledge
level so that I learn something new. It's also nice to know *WHY*
things work, or don't. Learn by Destroying(tm).

I agree, Jeff.

I like antennas that are naturally short-circuited by design and can be
grounded, making the feed point essentially grounded for DC and lower
frequencies. One such antenna is the folded unipole. Its only
problem is
that the feedpoint resistance is about 120 or so ohms.

So, I had this idea. The usual monopole (or ground plane) has about
30-35 ohms resistance. To get 50 ohms it is common practice to droop
the
radials about 45 degrees. Since that raises the feedpoint resistance,
would raising the radials lower the feedpoint resistance of the folded
unipole and, if so, what effect would it have on the pattern?

EZNEC said to raise the radials of the folded unipole about 23 or so
degrees to get 50 ohms and the pattern would not be affected.

So I built one and it works swimmingly. I had to make some minor
adjustments in element lengths but that was fairly easy with the vector
voltmeter. Hooray for modeling.

Cheers,
John - KD5YI

Hooray also for using your head, John, and realizing that raising the
radials would _probably_ have that effect -- then having that verified
by a model, and then by an antenna that works well for you in
practice.

As a matter of fact, I can see where just jumping into it without the
benefit of modeling would probably have resulted in giving up on it. I
had to adjust many things (such as radial tilt) before I learned about
how things were going to be affected. Modeling is like having an antenna
breadboard but a whole lot less work.

Another way that should work: make the two parallel conductors
different diameters, with the correct spacing. You might also try
making a self-supporting grounded quarter wave, resonant with its
radials, and fed with a parallel conductor that doesn't go all the way
to the top of the quarter wave...

I looked at those (EZNEC) years a go and was never satisfied. I have
learned much more now, so I might benefit from another look.

So there are three different arrangements, perhaps with pretty similar
electrical characteristics, and you can then pick among them for the
one that suits your construction practices the best. Perhaps there
are some more "grounded" monopole designs you throw into the mix.

Yes, if you're "just throwing up a dipole," maybe you don't worry
about things like this, but there are those of us who like to think a
bit deeper about things. I can only hope I remain infinitely tolerant
of those who like to think much deeper than I about many things.

Cheers,
Tom

C'mon, Tom. You're a very knowledgeable person and I value your input.
That's plenty deep.

By the way, I used Walt's inherent balun to make a diamond-shaped
antenna which needs no additional balun. It is 50 ohms at the feedpoint.
It, too is inherently short-circuited and, with the inherent balun,
probably groundable.

It was an interesting exercise that went like this:

1. I like a loop for the inherent short-circuit.
2. It has about 100-120 ohms terminal resistance. I want 50 ohms.
3. It needs a balun. But, I don't really want one.

A folded dipole is about 300 ohms. A half-wave shorted transmission line
is about 0 ohms. So, if you take a shorted half-wave transmission line
and spread the wires apart at the 1/4W point all the way to where it
becomes a folded dipole, it seems to me that the terminal resistance
will go from zero to 300 ohms and 50 ohms is in there somewhere.

I tried it in EZNEC and found that to be the case. I found that, if the
acute angle of the rhombus is about 51.5 degrees, then the terminal
resistance is about 50 ohms (adjust perimeter along with angle to get
50+j0).

Ok, fine. That takes care of everything but the balun. In Walt's
Reflections III, he discusses the half-turn bifilar loop (page 22-10).
But what was intriguing was the inherent balun. Aha!

So, after modeling as well as I knew how, I constructed a rhombus
(diamond-shaped) antenna with the right half of the diamond being coax
(inherent balun) and the left half of the diamond being 14 ga wire.




Are you feeding this with coax? Why not use a big choke at the feedpoint
on the outside of the coax (i.e. a bunch of suitable ferrite toroids)?

I don't understand. The built-in inherent balun needs no choke.

John

Relation of radiation resistance and terminal resistance
 

On 5/31/2011 4:12 PM, John S wrote:

Are you feeding this with coax? Why not use a big choke at the feedpoint
on the outside of the coax (i.e. a bunch of suitable ferrite toroids)?

I don't understand. The built-in inherent balun needs no choke.


But doesn't it need an extra length of coax on one of the sides?

Relation of radiation resistance and terminal resistance
 

On 5/31/2011 6:30 PM, Jim Lux wrote:
On 5/31/2011 4:12 PM, John S wrote:

Are you feeding this with coax? Why not use a big choke at the feedpoint
on the outside of the coax (i.e. a bunch of suitable ferrite toroids)?

I don't understand. The built-in inherent balun needs no choke.


But doesn't it need an extra length of coax on one of the sides?


No. The left-hand side is #14 wire while the right-hand side is coax
(obviously left and right can be reversed). A BNC female is at the
bottom, bayonets downward, terminals upward.

o
/ \
/ \
/ \
/ \
+ +
\ /
\ /
\ /
\ /
oo

The coax (right side) is connected as usual to the BNC and bent as
shown. However, the coax's center conductor *only* is connected at the
top to the #14 wire on the left of the diamond. The bottom end of the
#14 wire is connected to the BNC connector shell at the bottom.

The piece of coax on the right forms the balun. It is close to 1/4W
internally considering the velocity factor of the coax, but about 1/2W
externally.

As I said, it isn't exact. It measures very well here. I measured VSWR
of 1.02 but that was not the important part of all this. I have yet to
measure current on the transmission line from the BNC to the source to
my satisfaction.

If I can supply clarifying info, let me know.

73,
John

Relation of radiation resistance and terminal resistance
 

On 5/31/2011 7:48 PM, John S wrote:


The coax (right side) is connected as usual to the BNC and bent as
shown. However, the coax's center conductor *only* is connected at the
top to the #14 wire on the left of the diamond. The bottom end of the
#14 wire is connected to the BNC connector shell at the bottom.

The piece of coax on the right forms the balun. It is close to 1/4W
internally considering the velocity factor of the coax, but about 1/2W
externally.

As I said, it isn't exact. It measures very well here. I measured VSWR
of 1.02 but that was not the important part of all this. I have yet to
measure current on the transmission line from the BNC to the source to
my satisfaction.

If I can supply clarifying info, let me know.

73,
John


I meant to give Walt (W2DU and Reflections III) credit for the inherent
balun as before. My only contribution is my discovery (or re-discovery)
that the terminal impedance of the antenna can be adjusted within the
limits mentioned in my earlier post.

One other post script: As the quad departs from a square, it seems that
the antenna gets a bit more touchy with regards to dimensions. It is
difficult to tell for sure, though, since I'm doing this at 434 MHz and
*everything* is touchy there.

Cheers,
John

Relation of radiation resistance and terminal resistance
 

On May 31, 3:46*pm, John S wrote:
On 5/31/2011 5:40 PM, John S wrote:

A folded dipole is about 300 ohms. A half-wave shorted transmission line
is about 0 ohms. So, if you take a shorted half-wave transmission line
and spread the wires apart at the 1/4W point all the way to where it
becomes a folded dipole, it seems to me that the terminal resistance
will go from zero to 300 ohms and 50 ohms is in there somewhere.

I tried it in EZNEC and found that to be the case. I found that, if the
acute angle of the rhombus is about 51.5 degrees, then the terminal
resistance is about 50 ohms (adjust perimeter along with angle to get
50+j0).

As an aside, I found it time consuming to adjust angles and repeat the
source impedance test in EZNEC. So, I created an Excel spreadsheet where
I could simply input the perimeter, the acute angle, height above
ground, wire gauges, and number of segments and wrote a short VBA to
gather the spreadsheet results and create an EZNEC importable file.

Man, what a time saver.

73,
John

That sounds like something that would be valuable to others, too,
John. You might think about making it available...

BTW, I do think pretty deeply about a lot of things, but there are far
more I don't bother with. Still, people who do think deeply about all
those other things hold my respect for what they do, whether it's
topics I have any interest in or not. Along with that is an
understanding of how important it is that we can freely share what we
learn with others.

Cheers,
Tom

Relation of radiation resistance and terminal resistance
 

On 5/31/2011 11:59 PM, K7ITM wrote:
On May 31, 3:46 pm, John wrote:
On 5/31/2011 5:40 PM, John S wrote:
As an aside, I found it time consuming to adjust angles and repeat the
source impedance test in EZNEC. So, I created an Excel spreadsheet where
I could simply input the perimeter, the acute angle, height above
ground, wire gauges, and number of segments and wrote a short VBA to
gather the spreadsheet results and create an EZNEC importable file.

Man, what a time saver.

73,
John

That sounds like something that would be valuable to others, too,
John. You might think about making it available...


It is so narrowly focused on the rhombus that I doubt it would be useful
to anybody else. But, it is available to anybody who wants it.

John