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Author Topic: 4:1 Balun  (Read 446 times)
KC0FDE
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Posts: 122




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« on: February 20, 2003, 07:32:25 PM »

I have been wondering lately how to create a 4:1 balun for a j pole. I no very little about the construction of baluns. What would I use to create this balun (what type of wire) and where would I connect it to the jpole (below the j or above the j.etc). I tried looking on websites yet their diagrams make no sense to me... I believe they are more directed towards those who know how to create and install baluns rather than for the guy wanting to know how. I was also wondering if this balun can be used to set the swr to make a widerband thus lowering the swr over the band. Thanks to all that reply.
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AC5E
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Posts: 3585




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« Reply #1 on: February 20, 2003, 08:22:38 PM »

HI Kyle: Properly adjusted, a J-pole is very close to 50 ohms, and has an inherently low SWR over it's operating range. And that's generally more than a ham band wide. Or move the tap point a bit and get a good match to "72" ohm coax, if that's your pleasure.

But a 4:1 balun will turn a good match into a very poor match. That is, your 50 ohm coax through the balun would be looking for a 200 ohm; or 12.5 ohm; load. And it would take a very much mis-tuned J-pole to be that far off.

73  Pete Allen  AC5E
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KB9SAY
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Posts: 22




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« Reply #2 on: February 20, 2003, 10:42:35 PM »

The j-pol antenna is a balanced antenna. Many people feed a j-pol directly with an unbalanced coaxial cable. This can produce a good match, but the outer braid of the coaxial cable can become hot, and radiate and cause unwanted results. Using a balun is a good idea. This will prevent the transmission line from becoming part of the antenna and give the best performance. I have had great results using a 4:1 coaxial balun with a j-pol antenna. The key for the best match is moving the feed point of the balun up the lower section of the j-pol until you get the best SWR match. You have to find the 200 ohm spot if you are using 50 ohm cable, 300 ohm spot for 75 ohm cable. After this is done you can also vary the spacing of the lower two-conductor section of the j-pol (increasing it) above the feed point of the balun. This can improve SWR and bandwidth.
 Making a 4:1 balun is not too difficult. Check out the ARRL handbook. It shows how to make a half wave coax section to make a 4:1 balun. This will work very well for a j-pol antenna.
Good luck
73
KB9SAY
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AC5E
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Posts: 3585




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« Reply #3 on: February 21, 2003, 08:04:32 AM »

Actually, the outside of the coax shield is essentially a "fat wire" attached to the feedpoint of the antenna. At certain lengths the impedance of that wire can offer a better match to the inside of the coax than a mismatched antenna.

Coax radiation CAN be a problem if the antenna is severly mismatched; or if the antenna is close to an odd number of quarter waves in length. Particularly with a Yagi or other beam antenna whose inherent feedpoint impedance can be very low.

But coax radiation is not ordinarily a problem with a well matched J-pole. In fact, I have built several dozen of the things with no feedline radiation problems. But it can happen.

Instead of adding complexity and further mismatching the antenna to correct an unusual situation, a couple of ferrite cores around the coax or a two turn coil on a 2 inch form with RG58, 3 inch form with '8X, or 4 inch form with '213 or similar coax, will stop unwanted radiation.

73  Pete Allen  AC5E
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WB2WIK
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Posts: 20595




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« Reply #4 on: February 21, 2003, 11:19:54 AM »

Pete's (AC5E) suggestion is perfect.

Where did the rumor start that the J-Pole is a balanced antenna?  It's not.  Balanced antennas all have one thing in common, that being that the elements connected to both sides of the feedline are identical in resistance, reactance and geometry.  The J-Pole design is not like that.

WB2WIK/6

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WB6BYU
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Posts: 13250




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« Reply #5 on: February 21, 2003, 03:17:05 PM »

I certainly have seen problems with unbalanced J-poles.
If the bottom of the stub is grounded (as is often the
case when they are made from copper pipe) then neither
side of the coax is at ground potential, and current
can flow back down the outside of the coax and/or the
mast.  In one case that I know of, the J-pole had to
be insulated from the mounting mast to hit relatively
local repeaters.

I wouldn't worry about a balun for a twin-lead J-pole,
but I use them for metal ones mounted on a mast.  The
simplest one is an electrical half-wavelength of coax
(about 13 1/2" or 35cm of solid-dielectric RG-58) with
the braids from each end connected together and the
centers tapped onto the matching stub on the J-pole.
The coax braid is connected to the others, and the
center connects to either center conductor of the
balun.

The 200 ohm tap point is about twice as far up the
stub as the 50 ohm tap point is.

Or you can use a 1 : 1 balun by just winding the coax
feedline into a choke coil at the feedpoint - just
don't try to make the diameter of the coil too small,
especially with foam cable.

Of course, a pragmatist would put up the antenna
without a balun, and only add one later if it doesn't
seem to work as well as it should or there is other
evidence of RF flowing where it shouldn't be.
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N6AJR
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Posts: 9908




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« Reply #6 on: February 21, 2003, 10:17:52 PM »

go to buxcommco.com and buy one for a couple bucks.. I cheat  hi hi..I can't even spell trrifilar winding let alone do one... 73  tom N6AJR
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KB9SAY
Member

Posts: 22




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« Reply #7 on: February 24, 2003, 02:20:31 AM »

What is a J-Pol antenna? The section that radiates is the top ½ wave section. It is an end fed ½ wave antenna. No ground plane needed here. End fed antennas have very high impedance. How can an unbalanced low impedance coaxial cable match a high impedance end fed antenna?
 To a low impedance coaxial cable to match a high impedance end fed antenna you have to transform the low impedance to very high impedance. One way to do this is by using a ¼ wave balanced transmission line (parallel line) stub shorted at on end. The shorted end is at zero impedance, maximum current. This ¼ wave section will not radiate for the same reasons a parallel transmission line will not radiate. Each conductor is 180 degrees out of phase from the other at the same point and will prevent radiation. The open end (where the half wave radiator is attached) is at maximum impedance. Just like the end of a ½ wave antenna, making a good match.
 Since this ¼ wave section is made of a ¼ wave balanced transmission line, the best feed would be a balanced feed line. At the short the impedance is low. As you move away from the short the impedance goes up. To get the best match is to find the location on the ¼ wave stub that matches your feed. If the feed was a very low impedance coaxial cable where the connection point was very close to the short, the ¼ wave open section would act as a choke and a balun may not be needed. 50 ohms is close to the short, but may not be close enough to the short to prevent the outer coax from becoming hot. A balun may perform better.  
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