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[Articles Home]  [Add Article]  

Shortened Folded Dipoles

Cecil A. Moore (W5DXP) on May 16, 2011
View comments about this article!

No-Tuner Shortened Folded Dipoles

Cecil Moore, www.W5DXP.com, Rev. 1.0, 5/1/2011

Folded Dipoles have historically been popular single-band antennas. It's easy to use a 468/f long piece of 450 ohm ladder-line for the folded dipole element. If we feed the antenna with 450 ohm ladder-line, we can install a 4:1 or 6:1 balun at the shack and obtain a pretty good match to our 50 ohm transmitters (sometimes requiring a tuner).

Some hams don't have room for a full-sized folded dipole and a 4:1 or 6:1 balun doesn't always provide a perfect match. What if we could shorten the length of the folded dipole by about 20% and by proper choice of a 450 ohm ladder-line matching section, we could ensure a good match to 50 ohms through a 1:1 choke-balun?

We can do exactly that. The resonant feedpoint impedance of a 1/2 wavelength folded dipole is known to be in the neighborhood of 300 ohms. When fed with 450 ohm ladder-line, the SWR on the twinlead is close to 1.5:1 with very few standing waves on the feedline.

Can the impedance transforming characteristics of 450 ohm ladder-line (with reflections) be used to achieve a perfect match to a 50 ohm transceiver? Yes, if we can cause a 9:1 SWR on 450 ohm ladder-line, the impedance at a current maximum point on the line will be 450/9=50 ohms. So how can we cause a 9:1 SWR on 450 ohm ladder-line when it is driving a folded dipole? The answer is by shortening the length of the folded dipole to less than 1/2 wavelength. The antenna in the above graphic covers the entire 40m band with an SWR less than 2:1.

If we shorten the folded dipole by ~20% to ~3/8 wavelength, the feedpoint impedance becomes capacitively reactive, causing the SWR to rise to ~9:1 such that the impedance looking into a ladder-line matching section is 50 ohms resistive at a current maximum point on the feedline. Here are the formulas for the dipole length and 450 ohm ladder-line length based on EZNEC modeling.

Folded Dipole Length in feet = ~390/f. 450 ohm ladder-line matching section length = ~170/f

Install a 1:1 current-choke-balun at the end of the ladder-line and run coax from there back to the shack. Adjust the length of the folded dipole and ladder-line to achieve a perfect 50 ohm match on one's favorite frequency requiring no antenna tuner. How can one get more efficient than that?

This approach will work for any HF band. Here's an example for 40m from the above graphic:

Length of folded dipole = 390/7.2 = ~55 feet. Length of 450 ohm matching section = 170/7.2 = ~24 feet. Used with a tuner, this antenna will also work well on 30m, 12m, and 10m.

We can also use 300 ohm twinlead and 600 ohm open-wire line but the formulas change somewhat.

For 300 ohm line: Folded Dipole Length = ~415/f and matching section length = ~159/f

For 600 ohm line: Folded Dipole Length = ~368/f and matching section length = ~188/f

Does anyone need a no-tuner shortened single-band folded dipole? Enjoy.

Member Comments:
This article has expired. No more comments may be added.
 
Shortened Folded Dipoles  
by KB3HG on May 16, 2011 Mail this to a friend!
Cecil,
Nice, you out did yourself again. A field day antenna perhaps?

Tom Kb3hg

 
RE: Shortened Folded Dipoles  
by WY3X on May 16, 2011 Mail this to a friend!
If it's shortening an antenna that is the goal, while maintaining some semblance of efficiency, it's hard to beat a linear-loaded dipole. You can make it out of 450 ohm ladder line, feed one side at center, and solder the far ends together. Then snip/prune the other unsoldered end of the ladder line to resonance. I have a shortened triband 160-80-40M antenna that uses a separate 40M element, and the 80M element feeds traps at the outer tips. It's up as an inverted vee, and across the bottom (forming a triangle) I hung 450 ohm ladder line leading from a pair of traps, which adds 160M coverage. It forms a very efficient 160M antenna in the space of an 80M inverted vee. QST wanted to publish it, but I guess they've changed their minds. I'll inquire of them once more, and if they have (changed their minds) I'll submit the article here.

Great article, by the way!

73, -WY3X
 
RE: Shortened Folded Dipoles  
by W5DXP on May 16, 2011 Mail this to a friend!
Thanks for the nice comments. I needed a 40m antenna and the main goal was to get the impedance looking into the ladder-line (through a 1:1 choke) to be exactly 50 ohms at 7.15 MHz with no tuner required over the entire band. The shortening effect is just a benefit. It is also possible to lengthen the 1/2WL folded dipole to ~87 feet to achieve a 9:1 SWR on the feedline with 50 ohms at a current maximum point but I don't see much benefit in doing that.
 
Shortened Folded Dipoles  
by W4ZDI on May 16, 2011 Mail this to a friend!
I agree, it's a neat idea.

However,this seems an awful lot of work just to save 8 feet of antenna. This length equals 4 feet on each end of a dipole. Isn't it true that this extra four feet can be drooped or bent with insignificant effect on the tuning or efficiency of the standard dipole?

I've done this with my 80M dipole in my cramped 100 foot wide lot after reading about it in some article. But I don't have the means to determine what the real loss of efficiency is as a result of bending at 90 degrees the last eight feet on each end. Could you help me here?

Thanks,
Pierce W4ZDI
 
RE: Shortened Folded Dipoles  
by G3TXQ on May 16, 2011 Mail this to a friend!
Cecil,

Nice design!

If you use commercial "450 Ohm" ladderline with a Zo close to 400 Ohms I reckon the dipole would need to be 57ft long and the feedline 23ft. Best avoid 17m if you try to use it multiband with a tuner - very high dipole copper losses :)

73,
Steve G3TXQ
 
Shortened Folded Dipoles  
by KD5VHF on May 16, 2011 Mail this to a friend!
I've never heard of a commercial made 450 ohm ladder line.
What size wire and what did you use for spacers when you made your "450 ohm ladder-line" ?
Or did you use the commercial made 450 ohm window line?
 
RE: Shortened Folded Dipoles  
by G3TXQ on May 16, 2011 Mail this to a friend!
>>I've never heard of a commercial made 450 ohm ladder line.<<

http://www.dxengineering.com/Sections.asp?DeptID=12&ID=93

73,
Steve G3TXQ
 
RE: Shortened Folded Dipoles  
by NF6M on May 16, 2011 Mail this to a friend!
How well does this 'tunerless' antenna radiate? It seems like you've basically got a 'distributed tuner' using the feedline to match a non-resonant antenna.

A tuner is not necessarily a bad thing. A good flexible matching network allows you to choose the values of the components to transform the impedance with the lowest loss (i.e. maximum radiated power), while some combinations will give 'low SWR' with extremely high loss in the tuning network (80% or more).

If you haven't tried it, W9CF has a cool T-Network tuner simulator you can try out at http://fermi.la.asu.edu/w9cf/tuner/tuner.html
 
RE: Shortened Folded Dipoles  
by K3AN on May 16, 2011 Mail this to a friend!
If you know your way around (pun intended) a Smith Chart, you can come up with ways to connective a number of different reactive loads to specific lengths of feed line to achieve a low SWR at the bottom end of that feed line. The limitation is the available characteristic impedances of the feed lines, unless you "roll your own." And most such designs are single-band only.

The author's design will work with a length of ladder line that's any multiple of 1/2 wave longer (adjusted for velocity factor) than the feed line length given. In some situations it might be beneficial to have a longer length of ladder line to bring the end closer to the ground before transitioning to coax via the balun.
 
Shortened Folded Dipoles  
by G0GQK on May 16, 2011 Mail this to a friend!
I was always under the impression that if you used 300 ohm feeder with a 4:1 balun the result was 75 ohms, near enough for an antenna 50 ft over the ground. Maybe the 4:1 balun is outdated these days everybody seems to think the only one in town is a 1:1

G0GQK
 
RE: Shortened Folded Dipoles  
by G3TXQ on May 16, 2011 Mail this to a friend!
>>I was always under the impression that if you used 300 ohm feeder with a 4:1 balun the result was 75 ohms<<

Only if the feeder is matched! In Cecil's antenna the feedline is designed to be mismatched in such a way that its input impedance will be close to 50 Ohms at a particular distance from the antenna; then you need a 1:1 current balun, because no impedance transformation is required.

Steve G3TXQ
 
RE: Shortened Folded Dipoles  
by W5DXP on May 16, 2011 Mail this to a friend!
> However,this seems an awful lot of work just to save 8 feet of antenna.

As I said above, that's not the purpose. The purpose is to achieve a 50 ohm feedpoint impedance. The shortening is just a side-effect. This is akin to the G5RV matching section. And, of course, there are other ways to achieve the 50 ohm feedpoint impedance.
--
73, Cecil, w5dxp.com
 
RE: Shortened Folded Dipoles  
by N4JTE on May 16, 2011 Mail this to a friend!
I've learned more about wire antennas and matching from Cecil than a bookshelf of published experts.
Tnx Cecil, another idea worth saving.
Regards,
Bob
 
RE: Shortened Folded Dipoles  
by N2EY on May 16, 2011 Mail this to a friend!
Once again, Cecil, you have demonstrated true brilliance.

The concept is so simple, yet so effective. Once one has seen it and understood it, it's hard to forget.

One of the compromises of a shortened antenna - even one that is only slightly short - is reduced bandwidth, yet your design broadens the bandwidth. And it saves some coax.

Personally I would go for the 87 foot version if space were available, to get a tiny bit of gain and a little more efficiency. But the concept is the same.

----

I suspect that your idea could be used to produce an 80/75 meter dipole of simple construction that would cover the whole band with less than 2:1 SWR, no tuner and simple construction.

Something like this:

First, build a three-wire folded dipole, with the middle wire cut in the center and fed. (Three wires gives wider bandwidth for a given wire-to-wire spacing, and mechanical strength). Feed with 600 ohm ladder line, adjusting lengths for 12:1 SWR.

With adequate spacing of the dipole wires, it should be possible to cover the whole band with low SWR. The matching section will be short enough that if the antenna is 50 feet up or so the matching section will not reach the ground, yet long enough to save some coax.

Bunch of ferrite beads at the coax-to-ladder-line junction point, no tuner, no muss or fuss.

I'll bet you could simulate this idea and come up with a workable 80/75 design.

Such broadband monoband dipoles could be very useful on a multi-transmitter FD setup where the rigs are assigned by band. Not only is the need for a tuner at each station eliminated, but the monoband nature of the system would offer some selectivity against interference from other rigs in the setup.

Thanks for another great idea

73 de Jim, N2EY



 
RE: Shortened Folded Dipoles  
by G3TXQ on May 17, 2011 Mail this to a friend!
Cecil's feed system doesn't extend the VSWR bandwidth and nor does "folding" the dipole; it's having a "fatter" radiator that extends bandwidth.

You can certainly extend the idea to 80m and a 3-wire folded dipole fed with 600 Ohm line. I modelled a 97ft version with the three wires spaced a total of 6ft. Fed with about 60ft of 600 Ohm line you can find a perfect 50 Ohm match; but the 2:1 VSWR(50) bandwidth is only 120kHz.

On the other hand if you extend the dipole to its "full" 120ft, the 2:1 VSWR(600) bandwidth becomes 420kHz.

One final point: you need to watch the current distribution along these shortened folded dipoles. For example, on the 97ft 3-wire dipole the current in the middle wire is out of phase with currents on the outer wires for much of its length; that means increased copper losses - about 1dB using #14 wire.

73,
Steve G3TXQ
 
Shortened Folded Dipoles  
by K2BEW on May 17, 2011 Mail this to a friend!
This is just what I was looking for!
I have wanted to build a simple 40 meter antenna that I could use without a tuner.
Thanks for the great article. Now to look for the correct wire.

73
Tom
N2BEW
 
RE: Shortened Folded Dipoles  
by W5DXP on May 17, 2011 Mail this to a friend!
EZNEC sez the shortened 54.5' 40m folded dipole has ~0.2 dB less gain than the 66' 1/2WL version. The 87' version has ~0.2 dB more gain than the 1/2WL version - all using #14 copper wire.

I've added a couple of explanatory graphics to the article which I have posted to my web page:

http://www.w5dxp.com/40mFD1.JPG

http://www.w5dxp.com/40mFD2.JPG
 
Shortened Folded Dipoles  
by W4VR on May 18, 2011 Mail this to a friend!
Good concept and I'm sure it would work quite well, but to save 10 feet of wire on a 40 meter antenna due to a small lot I would put up a regular dipole with the last 5 feet on each end bent downward and feed it with coax and a balun. I ran into a situation many years ago where my lot could not accommodate a half wave dipole on 160, so I bent each end 20 feet and it worked like a charm. I remember putting up a 40 meter folded dipole many years ago using window line and every time it would snow the antenna looked like one big icicle.
 
RE: Shortened Folded Dipoles  
by AD4J on May 18, 2011 Mail this to a friend!
I modeled shortened non-folded (single wire) dipole lengths and found that around 60' (using #18 wire up 45') would work with 300 ohm line (twin lead or window line). This length presents a 6:1 SWR for Zo = 300 ohms. For DX Engineering 300 ohm window line, the feed line should be about 78'. The construction might be a little simpler than for a folded dipole.

Thanks for your innovative ideas, Cecil. I'm currently using a pair of balanced line fed perpendicular doublets using your variable line length scheme for matching. I have a micro-controller that reads the band data from my rig and switches relays to give me the appropriate feed line length on 40 - 6 meters. It also reconfigures one of the doublets to a tee vertical for the low bands.

73,
Jim AD4j
 
RE: Shortened Folded Dipoles  
by AD4J on May 18, 2011 Mail this to a friend!
As an addendum to my post above, you could also use 11' of 300 ohm window line.

Jim AD4J
 
RE: Shortened Folded Dipoles  
by AB9NZ on May 18, 2011 Mail this to a friend!
Great article Cecil. Guys Y'all need to check out the program Cecil put on his webpage for free download. It plots the high current points on the transmission line feeding any sized dipole. It makes matching easy as pie. Thanks much Mr. Moore. Best of 73 de Tom AB9NZ
 
RE: Shortened Folded Dipoles  
by W5DXP on May 19, 2011 Mail this to a friend!
That program is at: http://www.w5dxp.com/IMAXGRAF.EXE

It is a DOS (what's that?) program written in QuickBasic. Unfortunately, it won't run directly under Windows 7 but I have heard that DOSBox allows it to run in a Windows 7 window. It runs on my Windows XP just fine.
--
73, Cecil, w5dxp.com
 
Shortened Folded Dipoles  
by K5YZS on May 23, 2011 Mail this to a friend!
Interesting antenna. However it takes a lotsa room for an full sized dipole flattop on 80 meters and for sure on 160 meters; folding the radiating element to shorten just to get resonance, will surely reduce the transmitted RF. Just to get a match with folded dipole reminds me of the B&W folded dipole at around $400 and about a 10 dB signal down on 80 meters as tested and reported years ago in 73 magazine article authored by Gordon West. The cats whisker antenna is good for up to 3 bands with current feed, and as others have suggested a PROPERLY designed matching network get you full band QSY versus a coax feed dipole; this antenna sorta reminds me of the G5RV which was modeled and current distribution figures published in QST that showed G5RV has many resonance spots--but a a price--it tries to radiate vertically and horizontally at the same time and does nether as well as a low loss feed antenna --- balanced line and an EFFICIENT matching network--like the ATR-30. 73 k5yzs
 
RE: Shortened Folded Dipoles  
by W5DXP on May 23, 2011 Mail this to a friend!
> this antenna sorta reminds me of the G5RV --it tries to radiate vertically and horizontally at the same time and does nether as well as a low loss feed antenna ...<

The G5RV does not "try to radiate vertically and horizontally at the same time". You must be thinking of the inverted-V configuration when the poor user cannot afford two supports. The G5RV is supposed to be a horizontal dipole with a non-radiating matching section.

EZNEC sez this shortened folded dipole antenna is about 0.3 s-unit down from a 1/2WL folded dipole. Seems to me to be a small price to pay for a 50 ohm feedpoint impedance.
--
73, Cecil, w5dxp.com
 
Shortened Folded Dipoles  
by W4AEE on June 8, 2011 Mail this to a friend!
Comments on comments:

W5DXP is a knowledgeable contributor, offering good and practical info using sound principles and theory. His article above is no exception. But, some readers have misunderstood what Cecil was aiming for. He again clarifies his objective in the 3rd comment below the article.

Some of you are wanting a simple, shortened but relatively efficient, resonant dipole without loading coils or stub lines. Take a look at the “linear loading” info mentioned by WY3X (2nd comment below article). The linear loading principal has been around a long time but is often overlooked. It can be used in a variety of ways including shortening dipole elements. Check the section on Linear Loading in the ARRL Antenna Book, 21st edition, page 6-34, especially Fig. 60 (B). That antenna can be built entirely with common, “450-ohm” window line for the elements. The overall antenna length is given by L = 0.7(468/f). Note the 0.7 multiplier. That’s a 30% reduction in overall length when compared with that necessary for a conventional dipole using single wire elements! (I’ll have an article on this subject, soon.)

One commenter above said W5DXP’s antenna seemed to have “basically a distributed tuner, using the feedline to match a non-resonant antenna.” That’s a good observation but Cecil’s method is not an unheard of or unproven idea. Matching the antenna to the feedline using series connected sections of feedline with a different impedances has been done in antenna systems since the 1920’s. A better understanding of this is found in the ARRL Antenna Book, the ARRL Handbook, or any basic, reliable text presenting antenna and transmission line fundamentals.

The same OM correctly noted that a misadjusted “antenna tuner” (“feedline matching network” is a better term) can result in very large power losses while yielding a good VSWR at the input to the tuner. Wrong adjustments (i.e, wrong L/C ratios)are especially easy using T-network tuners! Check out the ARRL online archived article “Getting the Most Out of Your T-network Antenna Tuner” by W4ULD, QST, January 1995, p. 44. In many cases, the simpler and easier to adjust L-network will do just fine.

Another OM, apparently confused by terminology in other comments, said he’d never heard of “commercial 300-ohm ladder line.” The term “Ladder line” is often loosely used among Hams to describe heavy duty, plastic insulated, twin-lead type feedline that has rectangular holes stamped through the insulation between conductors. This method is used to replace some of the plastic with air and, thus, reduce dielectric losses in the line vs. that in solid twin lead. Perhaps the term “ladder line” was confused with “open-wire line” which is two parallel conductors (bare or insulated) with only air between them, held apart by “spreader” insulators, spaced along the line every few feet.

73,
Mike, W4AEE
 
Shortened Folded Dipoles  
by NZ5L on June 14, 2011 Mail this to a friend!
Right. And if the proper length 450 matching section is chosen - typically around 6' - a good match may be obtained with a SINGLE wire dipole of 56'. Plus you get the advantage of a 15 meter EDZ besides. SWR on 40 was 1:1, and on 15 was 1.5:1, so no tuner is required. My "balun" is 12 turns or RG-58 wound on a 6" piece of plastic pipe. Performance on 40 is indistinguishable from a full length antenna, but on 15 the extra gain becomes apparent, so wire orientation becomes important. It's a real win-win. Note: Some adjustment of the feedline may be required.
 
RE: Shortened Folded Dipoles  
by G3TXQ on June 14, 2011 Mail this to a friend!
NZ5L: Are you sure about those figures? A 56ft single-wire dipole will have a feedpoint impedance somewhere around 25-j650 Ohms on 40m; the best SWR with any length of 450 Ohm line would then be around 6:1 - the line would need to be 0.15 wavelengths long.

73,
Steve G3TXQ
 
RE: Shortened Folded Dipoles  
by G3TXQ on June 14, 2011 Mail this to a friend!
Apologies - forget that last post. I **do** see the low 40m VSWR with a relatively short length of ladder-line.

73,
Steve G3TXQ
 
RE: Shortened Folded Dipoles  
by AD4J on June 14, 2011 Mail this to a friend!
As I posted May 18 above, a single wire of about 60' and 11 feet of 300 ohm line would also work.

Jim AD4J
 
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