Changing dipole resonance by changing one leg?
 

How effective is it to cut a one-band dipole for the highest frequency
desired and clipping an extension on one leg to get low SWR on lower
frequencies?

Ken KC2JDY

Changing dipole resonance by changing one leg?
 

Chevy454 wrote:
How effective is it to cut a one-band dipole for the highest frequency
desired and clipping an extension on one leg to get low SWR on lower
frequencies?

Would probably work with ladder-line, but not with coax.
You would be turning your antenna into an off-center-fed
dipole with a relatively high feedpoint impedance.
--
73, Cecil http://www.w5dxp.com
"According to the general theory of relativity,
space without ether is unthinkable." Albert Einstein

Changing dipole resonance by changing one leg?
 

On Oct 7, 6:02*am, Chevy454 wrote:
How effective is it to cut a one-band dipole for the highest frequency
desired and clipping an extension on one leg to get low SWR on lower
frequencies?

Ken KC2JDY

Still the same band, right? Try it; I think you'll find it works
well. Though it unbalances the leg length slightly, you probably
won't notice much difference between lengthening one leg or equally
lengthening both. It may be possible for you to try equally
lengthening both just to see, if you care.

Cheers,
Tom

Changing dipole resonance by changing one leg?
 

On Tue, 07 Oct 2008 08:29:14 -0500, Cecil Moore
wrote:
Would probably work with ladder-line, but not with coax.
You would be turning your antenna into an off-center-fed
dipole with a relatively high feedpoint impedance.

Guess I need a tuner, then. A small piece of wire is more convenient,
but c'est la vie.

I'll cut the legs for the center of the range I will be using and keep
SWR low with a tuner.

Ken KC2JDY

Changing dipole resonance by changing one leg?
 

Chevy454 wrote:
On Tue, 07 Oct 2008 08:29:14 -0500, Cecil Moore
wrote:
Would probably work with ladder-line, but not with coax.
You would be turning your antenna into an off-center-fed
dipole with a relatively high feedpoint impedance.

Guess I need a tuner, then.

I may have been mistaken about your intent. Are you using
the above idea to extend your frequency range within a
single band? I earlier assumed you were going for multi-
band operation.

If you are expanding frequency coverage within a single
band, your idea should work just fine.
--
73, Cecil http://www.w5dxp.com
"According to the general theory of relativity,
space without ether is unthinkable." Albert Einstein

Changing dipole resonance by changing one leg?
 

On Tue, 07 Oct 2008 09:02:52 -0400, Chevy454 wrote:

How effective is it to cut a one-band dipole for the highest frequency
desired and clipping an extension on one leg to get low SWR on lower
frequencies?
Ken KC2JDY

What band? What frequency range? How much space do you have? Numbers
are always helpful.

I dunno about an asymmetric dipole. I've never seen it done, which
makes me suspicious. It should be very easy to model to see what
happens. Numbers?

If you want bandwidth from an HF dipole, look into a cage or birdcage
antenna. It's especially effective at 160 and 75/80 meters. There's
no increase in gain over a dipole, but the usable bandwidth is much
better. Even better, is a biconical, which is a cage antenna where
all the wires come together at the feed point, and are spread into a
cone shape at the ends. You can get several decades of usable
bandwidth out of a biconical with a fairly constant gain. That's why
it's used for EMC/RFI testing:
http://www.smeter.net/antennas/wire-cage-dipole.php
http://www.arrl.org/news/stories/2001/05/03/2/
http://personal.ee.surrey.ac.uk/Personal/D.Jefferies/antennexarticles/cage.html
http://jproc.ca/rrp/whitehorse.html (bottom of page)
http://images.google.com/images?hl=en&q=cage+dipole
http://www.smc-comms.com/cage_dipole.html


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

Changing dipole resonance by changing one leg?
 

On Oct 7, 8:02*am, Chevy454 wrote:
How effective is it to cut a one-band dipole for the highest frequency
desired and clipping an extension on one leg to get low SWR on lower
frequencies?

Ken KC2JDY

It works. You are not shifting the center point of the dipole
enough to really cause any problems as far as balance, etc.
I do it all the time with portable dipoles.

Changing dipole resonance by changing one leg?
 

Ken,

If you design the antenna for a high frequency, and then start clipping, it
will resonate at a higher frequency. No amount of clipping is going to give
you low SWR at a lower frequency.
--
-larry
K8UT
"Chevy454" wrote in message
...
How effective is it to cut a one-band dipole for the highest frequency
desired and clipping an extension on one leg to get low SWR on lower
frequencies?

Ken KC2JDY

Changing dipole resonance by changing one leg?
 

"Larry Gauthier (K8UT)" wrote in message
...
Ken,

If you design the antenna for a high frequency, and then start clipping,
it will resonate at a higher frequency. No amount of clipping is going to
give you low SWR at a lower frequency.
--
-larry
K8UT

I think he is talking about clipping extra wire to one end, as with a clip
lead, not clipping off a piece of existing wire. That would not be
repeatable anyhow.

Tam/WB2TT

"Chevy454" wrote in message
...
How effective is it to cut a one-band dipole for the highest frequency
desired and clipping an extension on one leg to get low SWR on lower
frequencies?

Ken KC2JDY

Changing dipole resonance by changing one leg?
 

Jeff Liebermann wrote:
Chevy454 wrote:

How effective is it to cut a one-band dipole for the highest frequency
desired and clipping an extension on one leg to get low SWR on lower
frequencies?
Ken KC2JDY

What band? What frequency range? How much space do you have? Numbers
are always helpful.

I dunno about an asymmetric dipole. I've never seen it done, which
makes me suspicious. It should be very easy to model to see what
happens. Numbers?

If you want bandwidth from an HF dipole, look into a cage or birdcage
antenna. It's especially effective at 160 and 75/80 meters. There's
no increase in gain over a dipole, but the usable bandwidth is much
better.
Even better, is a biconical, which is a cage antenna where
all the wires come together at the feed point, and are spread into a
cone shape at the ends. You can get several decades of usable
bandwidth out of a biconical with a fairly constant gain. That's why
it's used for EMC/RFI testing:
http://www.smeter.net/antennas/wire-cage-dipole.php
http://www.arrl.org/news/stories/2001/05/03/2/
http://personal.ee.surrey.ac.uk/Personal/D.Jefferies/antennexarticles/cage.html
http://jproc.ca/rrp/whitehorse.html (bottom of page)
http://images.google.com/images?hl=en&q=cage+dipole
http://www.smc-comms.com/cage_dipole.html


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

I use a 4-wire cage inverted vee on 80m. With it cut a bit long (1:1 at
around 3650 KHz), it's barely 2:1 at 4000KHz. Spreaders are 1/2" thinwall
PVC... 24"x24". I used the handy design program at http://www.smeter.net/.

73,
Bryan WA7PRC