3/2 wave dipole
 

Probably my first HF antenna as a novice was a 40/15 meter dipole
where 40 meters was 1/2 wave and 15 was 3/2 wave. I have never had a
problem dropping SWR on 15 meters to 1:1.1 with this antenna center
fed. However, as I understand antenna radiation on a wire, the
optimum point should be somewhere else along the wire to match the
impedance properly. I suspect it should be close to 1/3 of the way
from one end.

Any thoughts about this?


--
73 for now
Buck
N4PGW

Buck wrote:
Probably my first HF antenna as a novice was a 40/15 meter dipole
where 40 meters was 1/2 wave and 15 was 3/2 wave. I have never had a
problem dropping SWR on 15 meters to 1:1.1 with this antenna center
fed. However, as I understand antenna radiation on a wire, the
optimum point should be somewhere else along the wire to match the
impedance properly. I suspect it should be close to 1/3 of the way
from one end.

Any thoughts about this?

Think about it. There would be three current maximum points
on a 3/2WL wire. These three current maximum points correspond
to the low impedance feedpoints on the antenna. They would be
at 1/6, 1/2, & 5/6 from the ends. 1/3 from the end of a 3/2WL
would be a high impedance point, i.e. not a good feedpoint.
--
73, Cecil, W5DXP

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Cecil, W5DXP wrote:
"There would be three current maximum points on a 3/2WL wire."

Yes, and I believe they occur 1/4-weave back from the open-circuit ends
of the antenna and at the very center of the antenna.

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote in message
...
Cecil, W5DXP wrote:
"There would be three current maximum points on a 3/2WL wire."

Yes, and I believe they occur 1/4-weave back from the open-circuit ends
of the antenna and at the very center of the antenna.

Best regards, Richard Harrison, KB5WZI

That's how my math works too

Dale W4OP

On Wed, 20 Apr 2005 23:21:29 -0500, (Richard
Harrison) wrote:

Cecil, W5DXP wrote:
"There would be three current maximum points on a 3/2WL wire."

Yes, and I believe they occur 1/4-weave back from the open-circuit ends
of the antenna and at the very center of the antenna.

Best regards, Richard Harrison, KB5WZI

Thanks, That's what I was looking for. so I can use the antenna with
1/4 - 3/4 or center loaded.

Would the off-center setting create another resonant frequency for the
antenna?

Assuming the antenna were cut for 1/2 wave at 7 MHz, it would operate
on 21 MHz, but if off-center fed, would it also resonate on 20 MHz
with the 1/4 wave element?

Thanks.
--
73 for now
Buck
N4PGW

Dale Parfitt wrote:

"Richard Harrison" wrote in message
...

Cecil, W5DXP wrote:
"There would be three current maximum points on a 3/2WL wire."

Yes, and I believe they occur 1/4-weave back from the open-circuit ends
of the antenna and at the very center of the antenna.

Best regards, Richard Harrison, KB5WZI


That's how my math works too

Even less generally, one could think of the feedpoint in terms of
1/12-wave, 1/4-wave, or 1/2-wave from the end, depending on frequency.
But I'd be inclined to use 1/6 or 1/2 of the total length for fear of
brain fartage.

ac6xg

Some years ago, John Belrose VE2CV published an article in QST about
off-center fed antennas, based on a combination of modeling and
building. While they look attractive on a simulation program (where the
feedline wasn't also modeled), he found that in practice it was very
hard to keep the feedline common mode current down to a low value. The
consequence of common mode current is that the feedline becomes part of
the antenna, and that makes it very difficult to duplicate or to realize
the idealized performance you might expect if you hadn't taken feedline
radiation into account. Multiple current baluns should be able to reduce
the common mode current to a reasonable value, but they'd have to be
used unless you want a lot of feedline radiation and performance that's
difficult to predict.

Roy Lewallen, W7EL

Buck wrote:
On Wed, 20 Apr 2005 23:21:29 -0500, (Richard
Harrison) wrote:


Cecil, W5DXP wrote:
"There would be three current maximum points on a 3/2WL wire."

Yes, and I believe they occur 1/4-weave back from the open-circuit ends
of the antenna and at the very center of the antenna.

Best regards, Richard Harrison, KB5WZI


Thanks, That's what I was looking for. so I can use the antenna with
1/4 - 3/4 or center loaded.

Would the off-center setting create another resonant frequency for the
antenna?

Assuming the antenna were cut for 1/2 wave at 7 MHz, it would operate
on 21 MHz, but if off-center fed, would it also resonate on 20 MHz
with the 1/4 wave element?

Thanks.

A bit more on the baluns Roy mentioned...

If the feedpoint is a high impedance (an even number of half-waves back
from both ends of the antenna), it's difficult to get enough choke
impedance to do much good if you put a single balun right at the
feedpoint. It may end up being overkill, but if you don't have a good
way to measure your line for antenna currents, you'll do well to put
chokes about a quarter wave apart at the highest operating frequency.
You want to avoid having a length of line between two chokes being a
resonant half-wave long without anything in the middle to break up the
resonance. So for operation on multiple bands covering a wide range,
you should think about putting in three or more chokes.

Of course, if you put it up without any chokes and it does what you
want, that's fine...just don't expect it will match the model of an
isolated wire.

Cheers,
Tom

Dale Parfitt wrote:
"Richard Harrison" wrote:

Cecil, W5DXP wrote:
"There would be three current maximum points on a 3/2WL wire."

Yes, and I believe they occur 1/4-weave back from the open-circuit ends
of the antenna and at the very center of the antenna.

That's how my math works too

Remember I said the current maximums would occur at 1/6,
1/2 (3/6), and 5/6 points? In a 3/2WL antenna, 1/6 is
1/4WL from the end.
--
73, Cecil, W5DXP


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Buck wrote:
Thanks, That's what I was looking for. so I can use the antenna with
1/4 - 3/4 or center loaded.

Note that is 1/4WL which is 1/6 distance (16.67%) from
the ends. It is *NOT* 1/4 of the antenna. Hint: There are
six 1/4WLs in a 3/2WL antenna.

Would the off-center setting create another resonant frequency for the
antenna?

Nothing changes much. The radiation pattern will favor the
long side of the antenna. Since the currents are unbalanced
there may be an abundance of feedline radiation.
--
73, Cecil, W5DXP


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On Thu, 21 Apr 2005 10:56:46 -0700, Roy Lewallen
wrote:

Some years ago, John Belrose VE2CV published an article in QST about
off-center fed antennas, based on a combination of modeling and
building. While they look attractive on a simulation program (where the
feedline wasn't also modeled), he found that in practice it was very
hard to keep the feedline common mode current down to a low value. The
consequence of common mode current is that the feedline becomes part of
the antenna, and that makes it very difficult to duplicate or to realize
the idealized performance you might expect if you hadn't taken feedline
radiation into account. Multiple current baluns should be able to reduce
the common mode current to a reasonable value, but they'd have to be
used unless you want a lot of feedline radiation and performance that's
difficult to predict.

Roy Lewallen, W7EL

A few years back I put up an OCFD. Feed it with open line. Seemed to
work pretty well and behaved itself running less than about a
100-watts. However, when I increased my power to above 500-watts I was
able to work every electrical and electronic appliance in the house!
The common mode on the transmission can be a killer!

Danny, K6MHE

Nothing changes much. The radiation pattern will favor the
long side of the antenna. Since the currents are unbalanced
there may be an abundance of feedline radiation.

Then, would the center feed be the best way to go?


--
73 for now
Buck
N4PGW

Then, would the center feed be the best way to go?

For sure. I'd rather center feed, and have to use a
matching device, than use OCF, and not have to use
a matching device. The matching device can be as
simple as a series transformer made from 75 ohm, or
whatever coax. The feed of a 3/2 wave dipole is quite
close to that of a full wave loop. Usually higher than 50
ohms, if clear of the ground. The same series transformer
matching scheme will usually work for either one.
Say if you were on 21 mhz, and the SWR was a bit high.
Say 100 ohms feedpoint impedance, "more common with
antennas high off the ground", you could add 7.6 ft of 75
ohm coax, from the end of the normal 52 ohm feedline,
to the feedpoint of the antenna. That would give you a good
match. Might mess you up on 40m a bit though....I'm not sure
the exact effect it would have on the lower "1/2 wave" frequency.
Maybe not a whole lot...I guess you could switch it in and out
with a relay...BTW....Here, I don't have a very good match on
15 using my 40 dipole, but it's probably cuz I have 160/80/40
on one feedline...I just slap the tuner inline if I want to work 15..
I have used a series transformer on a 3/2 dipole on 20m....It
worked...It was one way I could run a EDZ type antenna, but
easily feed with coax. But...I used .75 wave per side, instead
of the .64 wave per side of the normal EDZ. That tunes out
the reactance, and all you need is the transformer. Sometimes,
you don't even need that as many 40/15 dipoles users know..
MK

A 3/2 wave fed at 1/6 or 16.67 % off one end show low fairly Low SWR's
at the follow frequencies if the wire is cut to 3/2 W @ 7 MHz

7MHz , 9.25 mhz, 11.75mhz , 21.25 MHz (about 7:1 into 50 ohms) and 26
MHz .. These are if fed with 50 ohm line. .
Cheers 73 Dave kc1di

In article ,
says...


On Thu, 21 Apr 2005 10:56:46 -0700, Roy Lewallen
wrote:

Some years ago, John Belrose VE2CV published an article in QST about
off-center fed antennas, based on a combination of modeling and
building. While they look attractive on a simulation program (where the
feedline wasn't also modeled), he found that in practice it was very
hard to keep the feedline common mode current down to a low value. The
consequence of common mode current is that the feedline becomes part of
the antenna, and that makes it very difficult to duplicate or to realize
the idealized performance you might expect if you hadn't taken feedline
radiation into account. Multiple current baluns should be able to reduce
the common mode current to a reasonable value, but they'd have to be
used unless you want a lot of feedline radiation and performance that's
difficult to predict.

Roy Lewallen, W7EL

A few years back I put up an OCFD. Feed it with open line. Seemed to
work pretty well and behaved itself running less than about a
100-watts. However, when I increased my power to above 500-watts I was
able to work every electrical and electronic appliance in the house!
The common mode on the transmission can be a killer!

Danny, K6MHE


If I was going to run open line, I would not go with
an off-center feed. I don't think it buys you much.
On the other hand, off-center feed with coaxial feedline
can buy bandwidth over center feed. Our 80/75 dipole
is off-center fed with a 4:1 voltage balun followed
by a bead balun on the feedline. The SWR is quite low
from 3.5 to at least 3.8 MHz. Solid-state finals and
Alpha 87A finals like it very much. No trouble with
getting into appliances at 1500 watts.

73,
Bob
K3PH

--
+----------------------------------------------+
| Bob Schreibmaier K3PH | E-mail: |
| Kresgeville, PA 18333 | http://www.dxis.org |
+----------------------------------------------+

Buck wrote:
Nothing changes much. The radiation pattern will favor the
long side of the antenna. Since the currents are unbalanced
there may be an abundance of feedline radiation.

Then, would the center feed be the best way to go?

Balanced center-feeding a balanced antenna will tend to
minimize the common-mode currents but some antennas, like
the Carolina Windom depend upon feedline radiation for
their radiation patterns. What are you trying to
accomplish?
--
73, Cecil, W5DXP

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On Fri, 22 Apr 2005 10:17:55 -0500, Cecil Moore
wrote:

Buck wrote:
Nothing changes much. The radiation pattern will favor the
long side of the antenna. Since the currents are unbalanced
there may be an abundance of feedline radiation.

Then, would the center feed be the best way to go?

Balanced center-feeding a balanced antenna will tend to
minimize the common-mode currents but some antennas, like
the Carolina Windom depend upon feedline radiation for
their radiation patterns. What are you trying to
accomplish?

When trimmed to SWR, I get less than 1.5:1 on fifteen meters. My
plans are to build a multiband antenna, parallel elements, where each
band is 3/2 wave dipoles and fed with 50 or 75 ohm coax. I haven't
tried this yet, but I would hope I can have a few gain dipole antennas
in a multi-band format.


--
73 for now
Buck
N4PGW

I presume you realize that a 3/2 wave dipole has multiple lobes, with
nulls between. At many or most angles, a half wavelength dipole has
greater gain.

You can get just two narrow lobes from a 3/2 wave dipole by rotating the
wires 30 degrees to form a horizontal "V" having a 120 degree included
angle. I've used this trick with a 40 meter dipole being used on 15
meters -- it doesn't have much effect on the 40 meter pattern. (You'll
see some TV antennas made this way for the same reason -- the high TV
channels are about 3 times the frequency of the low channels.) But
again, if a station isn't in the right direction, you'll do better with
a half wave dipole than a 3/2 wave one.

A "gain" antenna isn't of much use if the gain is in the wrong
directions. In fact, it's worse than a lower gain one, since the higher
gain it gets in a few directions comes at the expense of lower gain in
the remainder.

Roy Lewallen, W7EL

Buck wrote:

When trimmed to SWR, I get less than 1.5:1 on fifteen meters. My
plans are to build a multiband antenna, parallel elements, where each
band is 3/2 wave dipoles and fed with 50 or 75 ohm coax. I haven't
tried this yet, but I would hope I can have a few gain dipole antennas
in a multi-band format.

Interesting comments Roy
But it does utelize the use of dipoles that are not straight but angled. It
is well known that pointed /angled dipoles does have advantages such as
reducing rotational area, reduction of
reactances that can lead to as much extra gain as 1 Db together with a
broader
banded antenna.
In this particular case he has achieved an extra 3 db by stacking.
Your point regarding direction of main lobs is quite important but lessens
in importance if the antenna is rotatable. Unfortunately I searched the web
page of this new antenna but was unable to find any antenna patterns,
which puts one in a "believe it or not" situation regarding its capability
versus claims.
Art

"Roy Lewallen" wrote in message
...
I presume you realize that a 3/2 wave dipole has multiple lobes, with nulls
between. At many or most angles, a half wavelength dipole has greater gain.

You can get just two narrow lobes from a 3/2 wave dipole by rotating the
wires 30 degrees to form a horizontal "V" having a 120 degree included
angle. I've used this trick with a 40 meter dipole being used on 15
meters -- it doesn't have much effect on the 40 meter pattern. (You'll see
some TV antennas made this way for the same reason -- the high TV channels
are about 3 times the frequency of the low channels.) But again, if a
station isn't in the right direction, you'll do better with a half wave
dipole than a 3/2 wave one.

A "gain" antenna isn't of much use if the gain is in the wrong directions.
In fact, it's worse than a lower gain one, since the higher gain it gets
in a few directions comes at the expense of lower gain in the remainder.

Roy Lewallen, W7EL

Buck wrote:

When trimmed to SWR, I get less than 1.5:1 on fifteen meters. My
plans are to build a multiband antenna, parallel elements, where each
band is 3/2 wave dipoles and fed with 50 or 75 ohm coax. I haven't
tried this yet, but I would hope I can have a few gain dipole antennas
in a multi-band format.

On Sun, 24 Apr 2005 15:12:26 -0700, Roy Lewallen
wrote:

I presume you realize that a 3/2 wave dipole has multiple lobes, with
nulls between. At many or most angles, a half wavelength dipole has
greater gain.

Yes, I believe it has four major lobes and a few minor lobes as well.
None are exactly perpendicular to the wire (IIRC).


You can get just two narrow lobes from a 3/2 wave dipole by rotating the
wires 30 degrees to form a horizontal "V" having a 120 degree included
angle. I've used this trick with a 40 meter dipole being used on 15
meters -- it doesn't have much effect on the 40 meter pattern. (You'll
see some TV antennas made this way for the same reason -- the high TV
channels are about 3 times the frequency of the low channels.)

You got me on this one. I haven't looked into the 'V-Beams'. I was
thinking they were 1/2 wave dipoles. TV antennas are also a form or
Log-Periodic aren't they?

But
again, if a station isn't in the right direction, you'll do better with
a half wave dipole than a 3/2 wave one.
A "gain" antenna isn't of much use if the gain is in the wrong
directions. In fact, it's worse than a lower gain one, since the higher
gain it gets in a few directions comes at the expense of lower gain in
the remainder.


Yes, this is a form of compromise antenna. I guess if I were looking
for the perfect (omni) antenna, it would be a loop.

Many hams use G5RVs or tuned dipoles cut for their lowest desired
band. While they all work, they all have their lobes on certain
bands, probably each band with different lobes.

I realize that the antenna will have nulls and lobes. If possible, I
would want to arrange the antenna in such a way as to use the lobes to
my benefit. Primarily, it is an experimental antenna as much for the
fun of building it as for using it.

I also plan to build a multi-band beam, but not right away.

Thanks for the input. I'll check into the 'V' antenna.


Roy Lewallen, W7EL


--
73 for now
Buck
N4PGW

Roy Lewallen wrote:
A "gain" antenna isn't of much use if the gain is in the wrong
directions.

Buck could get a good idea of the radiation pattern using
EZNEC (even the free downloadable version from www.eznec.com).
--
73, Cecil, http://www.qsl.net/w5dxp


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Please disregard the posting below as I misunderstood Roy's posting was in
reference to a different thread.
Sorry about that
Art
" wrote in message
news:unWae.17625$NU4.2343@attbi_s22...
Interesting comments Roy
But it does utelize the use of dipoles that are not straight but angled.
It is well known that pointed /angled dipoles does have advantages such as
reducing rotational area, reduction of
reactances that can lead to as much extra gain as 1 Db together with a
broader
banded antenna.
In this particular case he has achieved an extra 3 db by stacking.
Your point regarding direction of main lobs is quite important but lessens
in importance if the antenna is rotatable. Unfortunately I searched the
web page of this new antenna but was unable to find any antenna patterns,
which puts one in a "believe it or not" situation regarding its capability
versus claims.
Art

"Roy Lewallen" wrote in message
...
I presume you realize that a 3/2 wave dipole has multiple lobes, with
nulls between. At many or most angles, a half wavelength dipole has
greater gain.

You can get just two narrow lobes from a 3/2 wave dipole by rotating the
wires 30 degrees to form a horizontal "V" having a 120 degree included
angle. I've used this trick with a 40 meter dipole being used on 15
meters -- it doesn't have much effect on the 40 meter pattern. (You'll
see some TV antennas made this way for the same reason -- the high TV
channels are about 3 times the frequency of the low channels.) But again,
if a station isn't in the right direction, you'll do better with a half
wave dipole than a 3/2 wave one.

A "gain" antenna isn't of much use if the gain is in the wrong
directions. In fact, it's worse than a lower gain one, since the higher
gain it gets in a few directions comes at the expense of lower gain in
the remainder.

Roy Lewallen, W7EL

Buck wrote:

When trimmed to SWR, I get less than 1.5:1 on fifteen meters. My
plans are to build a multiband antenna, parallel elements, where each
band is 3/2 wave dipoles and fed with 50 or 75 ohm coax. I haven't
tried this yet, but I would hope I can have a few gain dipole antennas
in a multi-band format.

On Mon, 25 Apr 2005 00:25:22 GMT, "
wrote:

Please disregard the posting below as I misunderstood Roy's posting was in
reference to a different thread.
Sorry about that
Art


Thanks, I was wondering about that.


--
73 for now
Buck
N4PGW

Buck wrote:

Yes, I believe it has four major lobes and a few minor lobes as well.
None are exactly perpendicular to the wire (IIRC).

At 3/2 wavelengths, there are lobes perpendicular to the wire, but
they're down around 4 dB relative to the maximum lobes.

You got me on this one. I haven't looked into the 'V-Beams'. I was
thinking they were 1/2 wave dipoles. TV antennas are also a form or
Log-Periodic aren't they?

The 3/2 wave antenna bent into a "V" with 120 degree included angle is a
form of the "V beam", similar to half a rhombic. As the antenna gets
longer, the angle of the "V" has to get more acute to make the lobes
align. You can use the "V" shaped elements to make a Yagi, log periodic,
or other array. TV antennas are often a combination of those two.

. . .
I realize that the antenna will have nulls and lobes. If possible, I
would want to arrange the antenna in such a way as to use the lobes to
my benefit. Primarily, it is an experimental antenna as much for the
fun of building it as for using it.

Nothing at all wrong with that. It'll be both fun and educational. You'd
also have fun with an antenna modeling program, which would let you see
the kinds of patterns you'll get.

. . .

Roy Lewallen, W7EL

Roy Lewallen wrote:
Absolutely. Most of the comments I made aren't relevant if the antenna
can be rotated.

Now all that needs to be done is figure out how to
rotate a 3/2WL antenna for 160m. :-)
--
73, Cecil, http://www.qsl.net/w5dxp

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Cecil,
That is the exact problem I have with my antenna which comprises of
reflectors only( no directors) that are manipulated in shape to reduce
reactance swings. Expect to have another shot at it sometime this week
It was easier to put up a long boom with 13 elements or a duplicate antenna
where the reflector elements were not manipulated in shape than this present
experimental antenna.
But even a fraction of a dB gain increase becomes an magnet
for a experimentor.
As for your 180 metre form could one not borrow a Tee shaped
crane from a building site and place elements along its boom ?
I'm sure that would be strong enough to rotate in safety.
Regards
Art


"Cecil Moore" wrote in message
...
Roy Lewallen wrote:
Absolutely. Most of the comments I made aren't relevant if the antenna
can be rotated.

Now all that needs to be done is figure out how to
rotate a 3/2WL antenna for 160m. :-)
--
73, Cecil, http://www.qsl.net/w5dxp

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