On 5/28/2011 2:25 PM, Frank wrote:


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

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

People who are running NEC models, particularly NEC2 as opposed to NEC4,
tend to be interested in this kind of thing.

While building antennas and running them up the flagpole/tower/tree to
test is fun and enjoyable, you can save a whole bunch of time with some
modeling ahead of time (and besides, sometimes the weather isn't good
for antenna building/testing)

On 5/30/2011 4:25 PM, Joel Koltner wrote:
"Frank" wrote in message
...
How many amateur radio operators use this kind of academic preening
when they are putting up a dipole.

Not many, certainly... but I would offer that those who do are able to
enjoy the hobby more thoroughly than those who don't. Whether or not
that additional enjoyment makes up for the extra time needed for
learning, well, that's up to each individuals.

I personally care about these things, and even for hams who think they
don't... most all of them have probably thoughts to themselves, at one
point or another, "Hey, what happens if you feed a dipole off-center?,"
and it's nice that someone else has already gone through the effort to
figure it out such that the answers are readily Google-able! :-)


And there's a whole raft of "off center fed" dipoles of one sort or
another out there, particularly for multiband applications, so it's nice
to understand that what's really going on is that the feedpoint
impedance you see is basically the same as feeding at the center, but
run through a "transformer".

A bit of theory helps one evaluate all the "secret recipe worked 1000
countries on top band with 1 Watt and a 3 foot long antenna" stories too.

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

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On May 31, 9:47*am, Jim Lux wrote:
On 5/30/2011 4:25 PM, Joel Koltner wrote:



"Frank" wrote in message
....
How many amateur radio operators use this kind of academic preening
when they are putting up a dipole.

Not many, certainly... but I would offer that those who do are able to
enjoy the hobby more thoroughly than those who don't. Whether or not
that additional enjoyment makes up for the extra time needed for
learning, well, that's up to each individuals.

I personally care about these things, and even for hams who think they
don't... most all of them have probably thoughts to themselves, at one
point or another, "Hey, what happens if you feed a dipole off-center?,"
and it's nice that someone else has already gone through the effort to
figure it out such that the answers are readily Google-able! :-)

And there's a whole raft of "off center fed" dipoles of one sort or
another out there, particularly for multiband applications, so it's nice
to understand that what's really going on is that the feedpoint
impedance you see is basically the same as feeding at the center, but
run through a "transformer".

Or another way to think about it (perhaps closer to what's going on)
is that it's like changing the position of a tap on a resonant tank
circuit. If you get into building RF filters using coupled
resonators, you'll appreciate that changing the tap position on the
input and output resonators changes the filter's operating impedance,
as seen at the input and output, though just as in the off-center fed
antenna, the current distribution in the resonator (or antenna)
changes relatively little. For the case of the antenna, of course,
you have to decouple the transmission line very carefully if you feed
it off-center -- or else just allow for the fact that there _will_ be
antenna currents on the transmission line e.g. an end-fed half-wave).

A bit of theory helps one evaluate all the "secret recipe worked 1000
countries on top band with 1 Watt and a 3 foot long antenna" stories too.

;-)

Cheers,
Tom

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

In article , John S wrote:

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.

Slick... and I bet that the appearance of it raises the occasional
question and/or eyebrow!

The matching approach I've usually seen for folded monopoles, is to
use a quarter-wave transformer made out of (e.g.) RG-6 or another
75-ohm coax... this brings the impedance down to something not too far
from 50 ohms. Your method avoids the need for this.

If I were to build one I think I'd stick a fat cap over the end of
each raised radial... just to reassure myself that I wasn't setting up
an "automated pigeon-skewering device" of sorts :-)

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

On May 31, 1:35*pm, John S 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.

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...

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

On 5/31/2011 4:26 PM, Dave Platt wrote:
In , John wrote:

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.

Slick... and I bet that the appearance of it raises the occasional
question and/or eyebrow!

I use it at home. Nobody in the area knows enough to even blink an eye.

The matching approach I've usually seen for folded monopoles, is to
use a quarter-wave transformer made out of (e.g.) RG-6 or another
75-ohm coax... this brings the impedance down to something not too far
from 50 ohms. Your method avoids the need for this.

I am aware of that method. I think the challenge for me was to have it
all inherent.

If I were to build one I think I'd stick a fat cap over the end of
each raised radial... just to reassure myself that I wasn't setting up
an "automated pigeon-skewering device" of sorts :-)


Hmmmm. I didn't think of that.

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.

The velocity factor of the coax means that its electrical length is
about .6 of a half wave (or .3 lambda) where I wanted 1/4W, but it is
close enough. I have not been able to measure current on the outside of
the coax, but it may be because I have not yet created a sufficiently
sensitive probing method.

I've not yet installed it. Maybe in the next few months I can get to it.

Cheers,
John - KD5YI

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