vertical dipole for 2m

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DY2_TTY

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I decided to build a vertical dipole for 2m.

I am going to use 1/4" copper tube.

Is the distance of the elements (gap in the middle) important?

I also read about this antenna needs a BalUn... how do I know the length of coax I need to use? I plan to put a pl259 connector at the end of the balun.

Thank you
 

ChrisABQ

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I built one from aluminum tube, works great on the 151 mhz public safety band that I'm targeting. I have about 1 inch in between my elements and my coax connections are 1/4 inch from the end of the elements (closer to the end is best). I'm using an feedline choke (lots of RF around here) that is set to 145 to 150 mhz (depends on the wrap), at about 53 inches in length wrapped about a 1.5 inch air cylinder (I'm using a fishing line spool for mine). Works very good, 80 to 100 miles range and the choke has eliminated 80% or more of the RF interference.
 

prcguy

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The gap is kind of fuzzy because the antenna starts where the feedline splits and heads to each dipole element. If you make the gap 1/4" then the copper elements are just about all of the antenna. if you make the gap 3" then about 3" of your bare feedline is part of the antenna and the copper elements will be that much shorter.

Coiling up coax is a lousy choke and unless you have test equipment to tune it you might be doing more harm then good. If the coiled coax choke is the right value it reflects some of the RF currents on the outside of the coax back toward its source but that can be reflected back and forth in an endless cycle.

Its much better to use Ferrite, which will absorb the RF currents and turn it into heat. For VHF you can use 3 or 4 snap on Ferrite beads with a #43 mix. Or if the hole in the bead is big enough you can wrap 3 to 4 turns around a single bead and get the same or better choking result.

Or if you don't use a feedline choke at all you probably won't notice any difference in most cases.
prcguy
 

nd5y

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Is the distance of the elements (gap in the middle) important?
Yes. The gap should be as short as possible. The wires connecting the ends of the tubing to the feedline or balun are electrically part of the antenna.

I also read about this antenna needs a BalUn... how do I know the length of coax I need to use? I plan to put a pl259 connector at the end of the balun.
If you are planning to make a 1/2 wave balun out of coax you would use an electrical 1/2 wavelength of coax. A 1/2 coax balun has a 4:1 ratio that will transform 50 ohms from the transmitter and feedline to 200 ohms. You don't use that type of balun with a dipole. A dipole has about 72 ohm impedance. You can probably feed it directly without a balun with no problems. If you still think you need a balun you might try putting ferrite cores on the coax at the feed point or make a sleeve balun.
 

DY2_TTY

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Its much better to use Ferrite, which will absorb the RF currents and turn it into heat. For VHF you can use 3 or 4 snap on Ferrite beads with a #43 mix. Or if the hole in the bead is big enough you can wrap 3 to 4 turns around a single bead and get the same or better choking result.
prcguy
This will make my life easier...

Do you mind pointing me in the right direction, I am searching on amazon and there are a few. What is a #43 ?

Thanks
 

teufler

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I have made several tape measure beams that are basically a dipole with a director and reflector. Connecting the two driven elements, I have used a wire connecting the two., j Used an analyzer and adjusted the two elements further or closer as need. About 1/2 inch. Do a search with Google for tape measure beams, you will find photos and construction details.
 

prcguy

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This is more of what I was referring to: Ring Core Ferrite Bead Choke Coil Clamp RFI Cable Clip 10-Pack Snap On 5mm | eBay

A 5mm version should just fit over RG-58 coax and a 13mm will fit over RG-8. Most of these are a #43 mix or similar ferrite and good for the 2m band if you use 3 or 4 in series on the coax right at the antenna. Or if you use a 13mm size you can wrap 3 turns of RG-58 around it and that is more effective than several in series.
prcguy
 

TheSpaceMann

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You may want to simplify things and just build a simple 2 meter ground plane! You could make it out of wire.
 

cmdrwill

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SpugEddy

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I have already mad one last week and it works well, but the radials are easily bent and cumbersome and I think the antenna wont last long if I put it outside permanently because of the strong winds and dust storms we have in Az.
Use stainless steel rod. (4mm or 1/8") Much sturdier and harder to bend out of
your 45 degree bend. I made one on a metal 4x4 electrical box
cover using an So-239 connector and stainless steel rod for the
ground plane. Outside for 2 years now and works just fine.
 
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