Lopsided dipole

[Rick K5IAR]

I know I'm not the brightest bulb on the tree so I would like to ask the brain trust, what would be the ramifications of a dipole with one leg about ten feet shorter (or one leg longer by ten feet) using ladder line to feed it?  I think I would use the 1KW Matchbox for tuning.  Would the performance be drastically reduced or would it simply affect the pattern?

Thanks,
Rick/K5IAR

[KA1ZGC]

Like everything else in the hobby, "it depends".

What you're describing is an off-center fed dipole. If you surf around for things such as the off-center-fed Zepp, you'll get some idea of the end result.

I would be wary of running such an ant if your balanced line is coming into the shack, since your feedline is bound to go out-of-balance, causing it to radiate. The feedline may also exhibit a diminished common-mode rejection, meaning it may more readily couple EMI from nearby sources, but this is far less of a concern.

I've never really run the numbers on it, but those are the caveats that spring to my mind first. Others have far more experience than me.

--Thom
Killer Agony One Zipper Got Caught

[WA1GFZ]

Matchbox does not like unbalanced loads

[Rick K5IAR]

Hmm.. thanks guys, both great observations.  I guess maybe I'm barking up the wrong tree. 

How about an inverted vee that is not aligned horizontally, but has equal sides?  I can find enough real estate (maybe) if I run one leg north and south and the other leg northwest and southeast. 

Rick

[WA1GFZ]

do the best you can to put up a balanced antenna as high as possible even if it is a bit short. You can hang  vertical conductors off the end to make it longer if you have to adjust the way the match box tunes. fc

[K1JJ]

How about an inverted vee that is not aligned horizontally, but has equal sides?  I can find enough real estate (maybe) if I run one leg north and south and the other leg northwest and southeast. 
Rick

Agreed on all comments.

The pattern will remain basically the same with an off-center or center fed dipole - but stay center fed if you can.

An off-center or zepp fed antenna's openwire feedline will actually cancel, from what I understand. Evidently with an end fed wire, one feeder leg induces the current into the other even though they are not connected to equal size legs.  Maybe Don will chime in - I've seen his posts about it before. I'd have to read more on it. However, I prefer the balanced feed with equal legs, even if shorter than optimum.


As for the inverted V with the legs not spread out straight...  First, any time the legs of a dipole are pulled in on themselves in either the vertical or horizontal plane, some cancelation takes place. It's a matter of degree. Picture an extreme example where the legs are pulled in so far that they are only 1' apart at the ends. This is now a transmission line and fully cancels any radiation. Even Zirconium encrusted tweezers wouldn't help here.

So, it's a matter of degree. The best dipole config is totally flat and straight. The rule of thumb for both planes is for the legs not to exceed a 90 degree angle between themselves. (in either plane) Even 90 degrees  is pushing it to the edge of wetness.  So, hopefully the ends are high enough above ground to satisfy the vertical plane. Your description of the legs in the horizontal plane  meets this requirement fine too

Contrary to popular belief, there is no "V beam" effect by pulling the ends of a 1/2 wave dipole in a particular direction. You need at least a coupla wavelengths to see this skewed V beam pattern gain.

As Frank said, definately go for a balanced antenna and try your best to get the ends out as flat (vert height)  and straight (horiz plane) as you can.  End loading is FB too.

Good luck -

Tom, K1JJ

[KA1ZGC]

If your choices are:

A - Run both legs along the same straight line and fold the ends to fit the property

- or -

B - Run the legs at different angles to one another to allow both legs to stay straight and still fit the property

...my vote would be for option "B". I'm honestly basing this on intuition more than anything else, but that's what my gut tells me.

If you wind up having to use option "A", I would recommend folding the ends down towards the ground rather than back on themselves (actually, folding them up toward the sky would be best, but not physically feasible). IIRC, K1KBW has an ant whose ends are folded downwards, but of course, he has a rig that could weld crowbars together.

Option "B", says my gut, will give you a somewhat skewed pattern, with somewhat less gain toward the "inside" of the chevron, and probably put a bit more of your energy towards high angles than a perfectly straight dipole. It should still work FB.

Alas, I really have to defer to the Honorable Mr. Vu on this one. His gut is smaller than mine, but his Antenna Kung Fu is strong indeed.

There are many ways to squeeze a radiator into a space that is slightly too small that are quite effective. You might find the Handbook helpful, they've historically had a treatise on shoehorning big ants into small areas.

--Thom
Kraft Advertisement One Zesty Grated Cheese

[WA1GFZ]

Worse case that also works is KB3AHE approach. Fold the ends back like a folded dipole and don't attach the ends. It plays. Franks antenna is only about 70 feet long but gets out well on 75.
I think the inverted Vee close to the ground is about the most useless antennas.
Cebic and I had a few emails after his last QEX article just before he went SK and he said an open loop is also a usable antenna. He did a number of tests on 2 meters to confirm his simulation.

[Bacon, WA3WDR]

Direct coax feed about 6% off-center, on a self-resonant dipole, with the shield on the longer leg, can give substantially zero RF on the shield.  On 75 meters this is about 7' or 8' off-center.  I haven't tried it on HF, but I did it on UHF in the lab.  But nobody understood it, so we opted for a ferrite sleeve balun instead.  (That worked fine.)

[Pete, WA2CWA]

My 160 M antenna fed with coax: One leg is 12 gauge wire and runs straight out South from the center insulator to a tree. The other leg is made of an old length of RG58C/U (with ends shorted). From the center insulator, it runs 25 feet West, then 35 feet North, and then the remaining length runs East. Loads well, low SWR around 1885, and seems to work anything I can hear. Radiation pattern Don't have a clue and probably would be unhappy if I really knew I never sweat the small stuff; I'm from Jersey

[Rick K5IAR]

Thanks Pete and all.  This is exactly the kind of information I was hoping to get.  Pete, I tend to agree, if it works I really don't care what the pattern or how it runs.  I just want to be sure it has a chance before I string it.  At my age it's a pretty good chore.  No matter how I run it I will have a long run of feed line (100' approximately)  and that's why I opted for the ladder line.  I am thinking about the W7FG line right now so I will need some kind of support close to where it connects to the wire to help with the weight.  I may have room for a 135' flat top, but it won't be straight and I'll have to put a support pole close to the middle.  I guess it would be okay to loop the ladder line from the center connection over to the support pole then on to the shack.

Thanks,
Rick

[N3DRB The Derb]

if you were going to try such a thang with the flashbox, you'd want to feed it with the coax connector out poot, not the balanced wire feeders.

and the left side balanced bolt wud be grounded I thinks. Everyone forgets to ground or unground that as needed, a lot of times all that there is is a empty hole right above that terminal.

my new antenna will be a W7FG dipole. I was thinking maybe I just might have enough room to also use the old antenna somehow as a lazy H pointing NE-SW.

[Steve - WB3HUZ]

10 feet on what band? On  160 meters it  probably won't  make much difference. On the higher bands, you may have some "issues."

I'd just put up the antenna you planned on, but make the other leg 10 feet shorter to match, or fold back or let dangle down the extra 10 feet that won't currently fit.

[Rick K5IAR]

Sounds like a winner, Steve.  I've never used real ladder line, how much does the stuff weigh?  I have a pretty long run to make, about 100', so I will need some kind of support for the line, correct?  I just took another long look at the area and I think I can get a 125' to 130' flat top up if I can get a line over the tree out back.

Rick

[Steve - WB3HUZ]

I don't know the weight per foot or 100 foot is for ladder line,  but it's pretty light - far less than RG-8/213 or even 8x.

You can probably just tied it off at the two ends (the shack end and the end where the run goes vertical) with bungie cords or similar. You want something that will keep sag to a minimum but still allow for movement when the wind blows.

[W4EWH]

Even Zirconium encrusted tweezers wouldn't help here.

They work on skyhooks made of dental floss, but only in Montana.

W1AC

[K1JJ]

  I've never used real ladder line, how much does the stuff weigh?  I have a pretty long run to make, about 100', so I will need some kind of support for the line, correct?  I just took another long look at the area and I think I can get a 125' to 130' flat top up if I can get a line over the tree out back.
Rick

Rick,

125' will work FB, OM.

Try this: Open wire with no spreaders!  You could actually use two strong posts spaced 100' apart.... one at the antenna and one at the house.   Run two parallel #12 or #10 wires spaced one foot apart as the feeders. Use standard antenna insulators on the posts to hold the feeder wires. No spacers are required if they are pulled reasonably tight.  This is great since there's no spacers for rain or ice to form on and change swr. Also, the loss is even lower.  I used this method to pull openwire lines from the top of the tower to the ground. A small sway of the feeders in the wind is OK.  One foot spacing is good up to at least 20M and probably higher for full cancellation..

For the openwire run from the ground to the antenna, use 4"-6" long spreaders spaced  apart every 3' made of Lexan, PVC, Teflon or whatever. Call in the Yellow pages to the plastic houses around and ask them if they have scrap. You can usually get everything you need real cheap. Or just buy some 3/8" Lexan rod and cut/drill your own.

I like Steve's idea above. Yes, get whatever antenna length you can fit into the area. Get it as high and flat as possible. It's better to have a smaller antenna flat and high rather than a longer antenna zig-zagged and lower. Less noise pick up, house absorbsion, RFI  and better pattern. BTW, if an ant is at least 3/8 wavelength long, you are OK loss-wise, as long as you use #12 or larger wire and a good quality ant tuner.  Do a search for the "K1JJ tuner" on this site for pics and samples of an easy to build tuner.

T

[N3DRB The Derb]

once you try balanced open wire, you will never go back. Coax sux. 

[ka3zlr]

I always liked this one if yer short on Room:

http://www.amwindow.org/tech/htm/160smallants.htm