Folding a dipole

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W9BU

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The building lot that my house sits on is only 90 feet wide. There are a number factors which have steered me towards erecting a dipole antenna across the width of the lot. Given that a full-size dipole for 80/75m is over 130 feet long, I have a problem fitting that into the width of my lot.

My current antenna is centered on the lot, so the dipole elements run out like a normal dipole for 45 feet in each direction. That leaves me roughly 20 feet of antenna element on each side. Currently, those elements turn 90 degrees thus forming a broad U when viewed from above that is 90 feet across the base and roughly 20 feet on the sides.

I am relatively pleased with the performance of this antenna. I can trim it so that the SWR is reasonable and, since the proof is in the pudding, I can talk to people using this antenna.

However, I'm always thinking of ways to improve my antenna. Would it be better to orient my antenna elements like a broad Z with one short element going one way and the other short element going the other way? I've played around with both the U and the Z arrangements in EZNEC and I don't see much difference. Thoughts?
 

prcguy

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There is probably little difference between a U or Z arrangement with the amount of wire you are needing to shed since most of the radiation is closer to the feedpoint on a half wave dipole because that's where most of the current flows.

But there are other alternatives like shortening the antenna with a capacity hat on each end. This can be as simple as the end of your dipole wires ending in a T shape and I would guess a T end maybe 8 to 10ft long might make up for 20ft of dipole length on each side.

Or a dipole that is simply shortened up to about 75% of its half wave length and fed the entire way with low loss balanced line will radiate within about 1dB of a full size half wave dipole. Feeding the dipole with balanced line also makes it easy to tune on just about any HF freq with good efficiency and it does not have to be resonant anywhere.
prcguy


The building lot that my house sits on is only 90 feet wide. There are a number factors which have steered me towards erecting a dipole antenna across the width of the lot. Given that a full-size dipole for 80/75m is over 130 feet long, I have a problem fitting that into the width of my lot.

My current antenna is centered on the lot, so the dipole elements run out like a normal dipole for 45 feet in each direction. That leaves me roughly 20 feet of antenna element on each side. Currently, those elements turn 90 degrees thus forming a broad U when viewed from above that is 90 feet across the base and roughly 20 feet on the sides.

I am relatively pleased with the performance of this antenna. I can trim it so that the SWR is reasonable and, since the proof is in the pudding, I can talk to people using this antenna.

However, I'm always thinking of ways to improve my antenna. Would it be better to orient my antenna elements like a broad Z with one short element going one way and the other short element going the other way? I've played around with both the U and the Z arrangements in EZNEC and I don't see much difference. Thoughts?
 

W9BU

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But there are other alternatives like shortening the antenna with a capacity hat on each end. This can be as simple as the end of your dipole wires ending in a T shape and I would guess a T end maybe 8 to 10ft long might make up for 20ft of dipole length on each side.
Interesting idea. I'll have to do some research into that option.

Or a dipole that is simply shortened up to about 75% of its half wave length and fed the entire way with low loss balanced line will radiate within about 1dB of a full size half wave dipole.
Yes, I've considered this.

What I didn't say in my original post is that the center point of my dipole is only about 16 feet off the ground. I've looked at other mounting options, but haven't found one that will fly under the homeowners association's radar and that I can manage without a lot of ladder climbing. So, I make do with what I've got and accept that I have a cloud-burner.

That being the case, a balanced line feeder to a dipole would only be able to run vertically for about 16 feet before I'd have to turn it horizontal to the ground. I'm not sure that would work very well.
 

prcguy

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If you think about it most people with dipoles have them fairly close to the ground like under 30ft and yes that makes a good cloud burner for NVIS on the lower bands. However they still can do well for DX, especially on the higher bands like 20m and up.

I've run countless portable setups with very low dipoles in the 6 to maybe 15ft high range checking into my regular evening 40m nets with good to great signals from CA to the east coast and everywhere in between and without fail. I was recently experimenting with an Eyring antenna which lays completely on the ground and it was only about an S unit below a 30ft high inverted V at the same location with everyone on my nets and we tried it on both 40m and 80m with good success coast to coast.

Many years ago I was looking for a better portable DX antenna for 40m and the new breed of 33ft telescoping fiberglass masts had just hit the market (DK9SQ mast) so I went for a full size 33ft 1/4 wave vertical over four ground radials. I thought this would easily trump a low dipole or at that time my usual portable G5RV held up by the same type of mast.

I trimmed the vertical for a perfect match, got signal reports from CA to CO, MO, KY FL and lots of other points considered DX. I then quickly took down the vertical wire and used the mast to hold my G5RV with the apex about 30ft with the ends just a few feet off the ground. I prepared for reports saying I had dropped an S unit but instead the low G5RV outperformed the full size 40m vertical on 40m.

I even switched back to the vertical and back to the G5RV again and the results were the same, the low G5RV dipole was noticeably outperforming a full size vertical on 40m. I now know if there were lots more ground radials and preferably elevated, the vertical would eventually equal and slightly overtake the low dipole but at the expense of much more wire and hassle for a portable antenna. Plus I would loose the NVIS and multiband aspects of the low dipole in the process.

The point of my rambling is you can put up a very effective local and DX HF antenna with 90ft of space and only 16ft of height. It doesn't matter if the balanced feedline exits the antenna horizontally or vertically since it will not radiate (much), just keep it off the ground and away from metal on its way to the tuner.

You might even look at my favorite wire antenna this week which is the 94ft ZS6BKW, which after its critical length of ladder line is coax fed and has a good match on 40, 20, 17, 12, and 6m without a tuner and on 80m its not that far down from a full size half wave dipole. You can make one of these fairly cheap and with some very small cap hats on the ends it will easily fit your 90ft space. Or just let 2ft of the ends hang straight down. You have lots of options.
prcguy






Interesting idea. I'll have to do some research into that option.


Yes, I've considered this.

What I didn't say in my original post is that the center point of my dipole is only about 16 feet off the ground. I've looked at other mounting options, but haven't found one that will fly under the homeowners association's radar and that I can manage without a lot of ladder climbing. So, I make do with what I've got and accept that I have a cloud-burner.

That being the case, a balanced line feeder to a dipole would only be able to run vertically for about 16 feet before I'd have to turn it horizontal to the ground. I'm not sure that would work very well.
 

LtDoc

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Don't make the mistake that distant station signals only arrive at low TOA, they arrive at all TOAs. Those 'cloud warmer' antennas tend to do just dandy in most cases, not all of course, but most. It's still all determined by propagation. There are any number of people who have done a number of WAS and WAC, etc, with lower than 'ideal' antenna heights. Best advice is to try it and see...
- 'Doc
 

prcguy

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When discussing antennas its usually assumed a "full size dipole" means a half wave at whatever frequency is being discussed. A full wavelength dipole would be an unusual design goal unless its a made for a lower frequency and it ends up a full wavelength somewhere else, like a 135ft 80m dipole used as a multiband and fed with high impedance balanced line.
prcguy



With all due respect (uh oh, now you know you're in trouble!), at 130' what you have is a half wave dipole on 80 meters. A "full size" dipole would be 260' +/-.
 

W9BU

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If I had meant full-wave, I would have said full-wave. ;)

I'm with prcguy...when most folks are talking about dipoles, they are talking about half-wave designs.

I started out using a trap dipole that was exactly 90 feet long, thanks to the traps, and got a little shorter when I tuned it for the higher end of the 75m SSB band. I eventually decided to try a non-trap design hence my reference to "full size".

Yeah, I know. I'm one of those OPs who doesn't tell the whole story up front thus leaving the reply posters guessing.
 

hertzian

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Inverted-U's have always worked well for me, even at low heights.

As long as the horizontal section is at least 50 to 70% of the overall calculated half-wavelength, the wires hanging down have much less current than the center, and if you squint at it, they are really end-loading capacity hats with only 1 spoke. :)

The temptation I'd have in your setup if I wanted to play around would be to bring another wire across from the bottom and create a loop. Feed it at the center bottom, a corner, or perhaps one side with the appropriate ratio balun. Maybe even try it as a delta loop bringing the hanging ends together with some extensions...
 

W9BU

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While the center of my dipole is about 16 feet AGL, the ends are tied off to a privacy fence that is only about 6 feet AGL. Therefore, I don't think I could make an inverted-U work since I don't have much elevation above ground.

The loop is an interesting idea, though.
 

LtDoc

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Some day when you're really bored, Try that loop fed with parallel feed line through a tuner. An antenna like that can be made to work on higher frequencies fairly easily. Certainly won't be the best when used away from it's design frequency, but tends to work nicely. A 'closed' antenna such as a loop tends to be slightly quieter than an 'open ended' antenna ('end effect'). Don't expect interference and noise to go away! It doesn't, but it's just a very little bit 'quieter'.
Especially if an antenna isn't at a "decent" height above ground, and if it's used on bands other than what it was 'designed' for, expect some really weird radiation patterns. With a loop, there's another variable added into that 'weirdness' with patterns because of the shape of that loop. The 'best' shape for a loop antenna is one that has the most area inside the loop. That would mean a circular loop would be 'best' electrically, right? Yeah, well, it may be 'best' but it's certainly not as easy to make/string up as some other shapes. Practical is 'better' than 'perfect', you know?
- 'Doc

...can you tell I like loops?
 
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