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2 Meter Choke Baluns



 
 
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  #1  
Old May 11th 06, 10:20 PM posted to rec.radio.amateur.antenna
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Default 2 Meter Choke Baluns

From what I have read there seem to be two common types of choke baluns
employed at 146 MHz, the coiled coax balun and the W2DU type with
ferrite beads over the coax. Looks like other types of baluns such as
the sleeve balun or a Guanella toroidal balun could also be used.

Other than the obvious restrictions imposed by physical mounting
arrangements, what are the advantages and disadvantages of each type.

There seems to be considerable variation in the design of coiled coax
baluns, with most designers advocating a few turns of coax with a
diameter of 3 to 8 inches. One designer was convinced that anything
over 3/4 of an inch would not be functional at VHF frequencies. Others
maintain that the coil of coax should be wound on a core keeping the
turns aligned, while others simply coil the coax up and tape it
together. Some recommend one coax width of air between turns.

With regard to ferrite beads on the coax type of balun, how many
ferrite beads of what type are most effective? Some advocate that
another set of ferrite beads be placed at a point below the feedpoint.

Seems there is lots of latitude in construction of these types of
baluns. Are they really this forgiving, or are there a lot of less than
full effective balun designs?

I'm trying to decide what is the best option for me, and am confused at
all the variations.

Darrell
VA7TO

  #2  
Old May 12th 06, 03:15 PM posted to rec.radio.amateur.antenna
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Default 2 Meter Choke Baluns

"Darrell Bellerive" wrote:
There seems to be considerable variation in the design of coiled coax
baluns, with most designers advocating a few turns of coax with a
diameter of 3 to 8 inches.


Well, one sure wouldn't want the circumference to be one wavelength. :-)
--
73, Cecil, W5DXP


  #3  
Old May 12th 06, 07:51 PM posted to rec.radio.amateur.antenna
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Default 2 Meter Choke Baluns

Cecil Moore wrote:
"Darrell Bellerive" wrote:
There seems to be considerable variation in the design of coiled coax
baluns, with most designers advocating a few turns of coax with a
diameter of 3 to 8 inches.


Well, one sure wouldn't want the circumference to be one wavelength. :-)


Tie a knot in the coax, and pull tight until the loop inductance
resonates with the capacitance.

Always wondered if that one would work...


--
73 from Ian GM3SEK
  #4  
Old May 12th 06, 08:17 PM posted to rec.radio.amateur.antenna
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Default 2 Meter Choke Baluns


"Darrell Bellerive" wrote in message
oups.com...
From what I have read there seem to be two common types of choke

baluns
employed at 146 MHz, the coiled coax balun and the W2DU type with
ferrite beads over the coax. Looks like other types of baluns such

as
the sleeve balun or a Guanella toroidal balun could also be used.

Other than the obvious restrictions imposed by physical mounting
arrangements, what are the advantages and disadvantages of each

type.

There seems to be considerable variation in the design of coiled

coax
baluns, with most designers advocating a few turns of coax with a
diameter of 3 to 8 inches. One designer was convinced that anything
over 3/4 of an inch would not be functional at VHF frequencies.

Others
maintain that the coil of coax should be wound on a core keeping the
turns aligned, while others simply coil the coax up and tape it
together. Some recommend one coax width of air between turns.

With regard to ferrite beads on the coax type of balun, how many
ferrite beads of what type are most effective? Some advocate that
another set of ferrite beads be placed at a point below the

feedpoint.

Seems there is lots of latitude in construction of these types of
baluns. Are they really this forgiving, or are there a lot of less

than
full effective balun designs?

I'm trying to decide what is the best option for me, and am confused

at
all the variations.

Darrell
VA7TO

======================================

The first thing to do is define precisely what you want the choke
balun to do and why. Then you might be getting somewhere.

If you don't know, you don't need one. Then you can forget and stop
worrying about it.


  #5  
Old May 12th 06, 09:57 PM posted to rec.radio.amateur.antenna
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Posts: n/a
Default 2 Meter Choke Baluns


"Darrell Bellerive" wrote in message
oups.com...
From what I have read there seem to be two common types of choke

baluns
employed at 146 MHz, the coiled coax balun and the W2DU type with
ferrite beads over the coax. Looks like other types of baluns such

as
the sleeve balun or a Guanella toroidal balun could also be used.

Other than the obvious restrictions imposed by physical mounting
arrangements, what are the advantages and disadvantages of each

type.

There seems to be considerable variation in the design of coiled

coax
baluns, with most designers advocating a few turns of coax with a
diameter of 3 to 8 inches. One designer was convinced that anything
over 3/4 of an inch would not be functional at VHF frequencies.

Others
maintain that the coil of coax should be wound on a core keeping the
turns aligned, while others simply coil the coax up and tape it
together. Some recommend one coax width of air between turns.

With regard to ferrite beads on the coax type of balun, how many
ferrite beads of what type are most effective? Some advocate that
another set of ferrite beads be placed at a point below the

feedpoint.

Seems there is lots of latitude in construction of these types of
baluns. Are they really this forgiving, or are there a lot of less

than
full effective balun designs?

I'm trying to decide what is the best option for me, and am confused

at
all the variations.

Darrell
VA7TO

=======================================

A choke balun made from RG-58 coax cable, centred on 145 MHz and
having a high impedance over the 2 meter band has the following
dimensions -

6 turns of RG-58
Length = 30 mm = 1.2"
Diameter = 37.5 mm = 1.5"

Measurements are between centers of turns.

Make it neatly.
----
Reg, G4FGQ


  #6  
Old May 12th 06, 10:04 PM posted to rec.radio.amateur.antenna
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Default 2 Meter Choke Baluns

And for more impedance, put one every odd 1/4 wavelength... it's the
macrame choke!

And then we'll start r.r.a.a. debating the effectiveness of overhand
vs. sheet bend and....

  #7  
Old May 12th 06, 10:28 PM posted to rec.radio.amateur.antenna
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Default 2 Meter Choke Baluns

43 material ferrite beads are quite effective at VHF. I would (and do)
take this route at VHF because they're more compact than a coax choke,
and you don't need too many of them to get substantial choking
impedances.

I was pointed to this PDF (big) for impedance vs. frequency curves.

http://www.fair-rite.com/newfair/pdf...log.pdf#page=1

A 2643012702 bead of the sort that fits (most of the time!) over RG-58
has a typical 1 turn impedance of 38 ohms at 100MHz according to the
catalog. You can decide on an appropriate choking impedance and stack
beads in the number you need.

Don't worry about chokes further down the coax unless you're really
having feedline radiation problems. A choke at the feedpoint will take
care of RF current conducted onto the feedline. If you're getting
induced currents on the feedline, get it away from the antenna! It's
VHF! :-)

I'm using bead baluns on my 6m moxon, my 2m and 432m yagis... I might
not notice the difference without them, but I had a box of, I dunno,
800 of those 43 material beads lying around, so I'm using bead baluns
for everything.

I've hung around r.r.a.a. and read many posts about cracking at high
power if the choking impedance is low enough that more than a few watts
are being dissapated in the choke.

I've also read quite a bit about conducted RF from all the junk
electronics in everyone's house being kept OFF THE ANTENNA by an
effective common mode choke, and this is why I'm going to keep using
them whether I feel that I need a balun or not for antenna pattern.

Back when my very understanding parents let me bury all my feedlines
for 40 feet under the lawn to build my high-school antenna farm, I
didn't worry about choking off shield currents, but now that I'm in an
apartment, everything I can do to keep antenna and noise separated is
going to get done.

73,
Dan
N3OX
www.n3ox.net

  #8  
Old May 12th 06, 10:44 PM posted to rec.radio.amateur.antenna
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Default 2 Meter Choke Baluns

wrote:

And for more impedance, put one every odd 1/4 wavelength... it's the
macrame choke!


Is that anything like macrame headaches?
--
73, Cecil
http://www.qsl.net/w5dxp
  #9  
Old May 13th 06, 06:27 PM posted to rec.radio.amateur.antenna
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Default 2 Meter Choke Baluns

If you put 100 FT-43 beads around your head, you're sure to get a
migraine choke headache, I think.

It'll keep the conducted common mode RF off your torso though.

Still have to worry about the radiated.

 




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