feed line for vertical wire dipole

[DaveJacobsen]

i have a wire dipole that was used in an attic. I wish to use it as a vertical antenna hung from a tall tree. The previous owner said I would have no problems as long as I lead the feed line away from the lower antenna element. (so it runs at an angle to it rather than letting gravity take it parallel to the lower antenna element)

At first this made sense -- but thinking about it -- doesn't that apply to ladder line and not coax? I mean, when using a wire dipole in an inverted V, it's not at a 90 degree angle...
If I need to have it 'away' from the antenna, how far away? Pulling it at 90 degrees for any distance will be hard.

At this location, it is not practical to use a traditional antenna, but we have lots of very tall trees I can get the antenna up higher than i could with a antenna tower (that I could afford).

 

[jonwienke]

A CB dipole antenna needs to be vertical, and the feedline needs to be horizontal for at least 1/4-wavelength for best performance.

 

[K7MEM]

At first this made sense -- but thinking about it -- doesn't that apply to ladder line and not coax? I mean, when using a wire dipole in an inverted V, it's not at a 90 degree angle...
If I need to have it 'away' from the antenna, how far away? Pulling it at 90 degrees for any distance will be hard.

The feed line needs to be perpendicular to the antenna elements for ladder line or coax.A good distance would be as "jonwienke" specified, 1/4 wavelength. This type of antenna is common on 2 Meters and 70 CM.

An Inverted-V is a different antenna, but the feed line is "mostly" perpendicular. When the feed line stops being "mostly" perpendicular (small included angle), the Inverted-V antenna doesn't work very well any more. If you increase the included angle enough, it becomes a dipole and the feed line is perpendicular.

I have a old Radio Shack CB Vertical Dipole. It's often referred to as a "Sleeve" dipole. The bottom half of the dipole is a hollow tube covered in fiberglass. The feed line (coax) runs up the center of the hollow tube and the ground terminates at the other end. The center conductor is then connected to the upper whip. Overall, it is about 18 feet tall. It works very nice and is simple to mount on a mast.

I have run some tests with a "Sleeve" Dipole made completely out of coax. Run horizontally, it is actually a End-Fed Dipole. But as you go lower in frequency (20 or 40 Meters) the weight of the coax doesn't make it very practical. The coax holds up fine, but the weight tends to pull the PL-239s apart unless its supported in many places.

Martin - K7MEM

 

[PrimeNumber]

Hanging a dipole vertically from trees and getting the feed line in perpendicularly is always a pain. An easier way is to make a j-pole out of wire and 450 ohm line. Here's a video if you want to DIY it:
https://www.youtube.com/watch?v=Xyh18GyniTo
Under $10, some assembly required.

If you want things pre-assembled, LNR Precision makes a pretty nice half-wave wire that uses a transformer instead of that 8' of 450 ohm twin lead. Here's the link:
Store | LNR Precision Inc
About $58, but it is plug-and-play.

Either is very easy to hang, and the coax can hang straight down with no hassle.

 

[DaveJacobsen]

Thanks for the j-pole idea; the youtube video sure make it simple.
Just one question -- he gives a length for the radiating element; is it total length of the physical wire, or distance the wire covers? (there's about 6-8" that folds back on itself at the insulator.)

I'll find some ladder line & make one up -- that really looks totally simple & perfect for hanging from a tree. way easier than what I was thinking.

 

[Rred]

"I have run some tests with a "Sleeve" Dipole made completely out of coax. Run horizontally, it is actually a End-Fed Dipole. "
No, if it is end-fed or center-fed (and a "sleeve" dipole is center fed) that never changes. With a sleeve dipole, the feed point is always at the center. The outer jacket of the coax (or another "sleeve") is applied OVER the coax cable, surrounding it, from the center feed point, back for 1/4 wavelength, while the inner conductor is extending 1/4 wavelength "up" from that same center feed point. The feed point is still in the center, no matter how you turn the antenna around.
Other than needing a good choke or other RF prevention on the coax where it enters the end of the sleeve (not the feedpoint) they're really cheap and simple. Tuning the "sleeve" to the correct length usually requires some tinkering and measuring, but will also reduce any RF problems on the cable.
They're not miracle antennas, but they do a solid job when you need an 'end fed' without the extra counterpoise or ground issues.

 

[PrimeNumber]

Thanks for the j-pole idea; the youtube video sure make it simple.
Just one question -- he gives a length for the radiating element; is it total length of the physical wire, or distance the wire covers? (there's about 6-8" that folds back on itself at the insulator.)

It's the distance the wire covers, i.e. add another 6" to 8" to fold over.

I'll find some ladder line & make one up -- that really looks totally simple & perfect for hanging from a tree. way easier than what I was thinking.

When you do, we want to hear how it works. Good luck!

 

[DaveJacobsen]

The video shows 450 ohm ladder line -- I have 300 ohm here; think that'll make much of a difference? I've searched & read other pages that referenced 300 ohm -- is one better than the other for this?

 

[PrimeNumber]

300 ohm line will likely move the proper feed point around a bit, maybe a few inches. Finding the right feed point is the hard part, and I built mine before that video came out -- it took a lot of hunting and re-soldering, and I ended up replacing the entire chewed-up original section of ladder line. (That's OK though, I recycled part of it into a 2m slim jim antenna.)

Also, the 300 ohm line I've used in other projects is a lot more delicate than 450 ohm line, with much finer wires along each leg. It's all smaller, and a lot harder to work with. Also, some of the "good" 300 ohm stuff has a tough outer jacket. It's hard to cut through the jacket to make the taps at the feed point without slicing through the wires.

So yes, 300 ohm line can no doubt be made to work, but it takes a lot of the "easy and fun" out of the project. Unless you feel like experimenting (which can be a very good thing), I'd just go with 450 ohm. If there's a nearby hamfest coming up you can pick some up there, or find it online. If you know any hams, there's a good chance they'll have some spare laying around. Here's a link that may help:
JSC 450 OHM-#1318 Cables Ladder Line

 

[JayMojave]

Hello DJ: Installing the Dipole vertically and having the coax leaving the dipole at a ninety degree angle or broadside is well worth the effort, as these dipole antennas work very well. Don't forget to use a 1 to 1 current Balun, there all over the net.

Jay in the Great Mojave Desert

 

[DaveJacobsen]

Why a 1:1 current balun over an air choke?
While I can find lots of information on what a balun is, how to construct it, but nothing on why I should use one over not ...

 

[prcguy]

What your trying to do by inserting a balun at the feedpoint of a dipole is to isolate the feedline from the antenna and attempt to bridge the unbalanced coax to the balanced antenna and reduce common mode RF currents from flowing on the coax shield.

A ferrite based 1:1 current choke can do this effectively because it can have a high value of resistive impedance, which turns the currents into heat. A ferrite loaded choke is also a very broad band and can be effective over most if not all the HF band.

An air wound choke made of coax can have a high value of reactive impedance but that will simply reflect the common mode current back toward the antenna to be reflected back to the choke in an endless cycle and will not absorb the currents or turn it into heat. Air wound chokes are also very narrow band and usually only useful for one HF band. When you see someone selling some coax wound over a length of PVC pipe and claiming it covers the entire HF band, just mark them down as uninformed or a snake oil salesman.
prcguy

Why a 1:1 current balun over an air choke?
While I can find lots of information on what a balun is, how to construct it, but nothing on why I should use one over not ...

 

[DaveJacobsen]

Thank you! that makes sense. I really appreciate the explanation.

 

[prcguy]

I find when using an inverted V dipole hanging in the clear which has a good match to 50 ohm coax, I get no perceivable RF on the outside of the coax. Sometimes you need a choke, sometimes you don't.

If you need a common mode choke, this guy has some of the best I've seen: Common Mode Choke

His long tubular versions are not just some ferrite beads slipped over coax, they have many separate chokes inside with various winding's of small Teflon coax around each one and they all add up to a massive amount of resistive choking impedance.

You might also click on the link to GM3SEKs presentation on common mode chokes from the MyAntennas link, its got some good information.
prcguy

Thank you! that makes sense. I really appreciate the explanation.