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OCF Dipoles Fed with Ladder-Line

from Cecil Moore, W5DXP on December 30, 2017
View comments about this article!

Off Center Fed Dipoles Fed With Ladder-Line
By Cecil Moore, W5DXP, Rev. 1.0, 10/01/17

With the availability of balanced antenna tuners, more hams may be tempted to feed their Off Center Fed Dipoles (hereafter called OCFDs) with ladder-line or open-wire feedline to lower feedline losses. Unlike a coax-fed OCFD, the inability to install a common-mode choke at the antenna feedpoint may result in extremes of common-mode current depending on the length of the feedline. This article presents some data on such systems, compares them to Center Fed Dipoles (CFDs), and discusses the potential problems of feedline radiation and RF-in-the-shack. We will not worry about the radiation patterns of the antennas and will concentrate on the common-mode characteristics.

The examples chosen for this article are a typical 80m (test frequency equals 3.69 MHz) OCFD compared to the same length CFD fed with the same length of ladder-line. The following convention written 44/88/65 means an OCFD with one 44-foot leg, one 88-foot leg, and 65 feet of ladder-line for the feedline. A similar CFD would be written 66/66/65. These are typical values for an 80m ladder-line fed OCFD or CFD. Let's look at the current envelope distribution of the two antennas according to EZNEC.

The CFD exhibits a familiar antenna current envelope where the out-of-phase transmission line currents result in in-phase radiating currents on the antenna wire, just as it should be. However, the OCFD exhibits phasing problems. We do not want out-of-phase currents on the antenna at the feedpoint because the two antenna elements are fighting each other and reducing the radiation magnitude in the far field. The only thing that can cause the antenna currents to be out of phase is for the transmission line currents to be in phase. That's the definition of common mode currents, i.e. currents that are in phase in the transmission line. The thing that causes the antenna radiation to be reduced is the same thing that causes the feedline to radiate, i.e. common mode currents are a double whammy taking radiation away from the antenna and giving it to the feedline. Fig. 2 shows the magnitudes of the common mode versus transmission line mode currents for the above OCFD and the CFD.

The reason for the out-of-phase currents on the OCFD antenna become clear. The (bad) common-mode currents on the transmission line at the OCFD feedpoint are almost five times the magnitude of the (good) transmission line currents. Yet the CFD has close to zero common mode current at its feedpoint simply because the impedance looking into each leg of an ideal CFD is the same value whereas the OCFD impedances are wildly different. It seems obvious that the OCFD needs some changes if it is going to be a trouble-free performer on 80m.

One thing we can do is change the length of the ladder-line. 65 feet was a convenient length for us but is a worst-case length for common mode problems because it is 1/4 wavelength on 80m. A high impedance at the source is seen by the common mode signals as a low impedance at the feedpoint. We can reduce the common mode problems by making the feedline 1/2 wavelength instead of 1/4 wavelength as seen in Fig. 3.

Now we are getting somewhere. The common mode current is still there on the OCFD but now its amplitude is 1/10 of the transmission line current amplitude at the antenna feedpoint. That will put the antenna currents in phase as seen in Fig. 4 and result in acceptable radiation from the antenna. In fact, it is now hard to tell the difference between the OCFD and the CFD antenna current distribution. There will still be feedline radiation because of the common-mode standing wave but its radiation level may be tolerable. It may even improve the low angle radiation ala Carolina Windom style.

The fact that the common mode current on the feedline of the 44/88/130 OCFD is low at both the source and the feedpoint is a distinct advantage in keeping RF out of the shack with a good common-mode choke-balun. The 1/2 wavelength of feedline keeps both the OCFD and OCF impedances at the source at a reasonable value.

Conclusion: If one feeds an 80m OCFD with ladder-line, the results are best when the ladder-line is an integer number of 1/2 wavelengths long. With any luck, the feedline will be one wavelength long on 40m, two wavelengths long on 20m, and four wavelengths long on 10m.

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OCF Dipoles Fed with Ladder-Line  
by W9YW on December 30, 2017 Mail this to a friend!
Thanks for the research. I wonder the part of elevation in terms of reflectivity and forcing take-off angle geometry. Guess I need to model it ;)

73 W9YW Tom
 
OCF Dipoles Fed with Ladder-Line  
by WA3SKN on December 30, 2017 Mail this to a friend!
I would recommend a transformer at the feed point. Otherwise the feed line will radiate due to the inbalance and distort the antenna pattern big time.
73s.

-Mike.
 
OCF Dipoles Fed with Ladder-Line  
by K7NSW on December 30, 2017 Mail this to a friend!
TWO COMMENTS: (1) W5DXP combines technical expertise and the ability to teach those do not have that expertise. This is a valuable article. (2) a shame he does not have a bio on QRZ. I would like to know a bit about him. The usual stuff - education, history in the hobby, description of his station, his primary interests in the hobby. No need for any personal info beyond that.
 
RE: OCF Dipoles Fed with Ladder-Line  
by KE2TR on December 30, 2017 Mail this to a friend!
Cicil has a real simple web page, he has some excellent info on those pages and like most hams who like to experiment on antennas he models first then try's out different configurations. I don't really care about a bio which for most on these forums is a form of chest beating but Cicils experiences do work and that my friends is a good bottom line. I had taken from his web page about ZS6BKW center fed doublet used with a modified 1:1 balun with adding 700Pf of capacitance on the input side of the balun were the coax comes in and the results on 75m was simply excellent. Thanks to Cicil I now have an antennas that covers 75,40,20,17,12,10 and 6 with more than acceptable swr and it really does perform well on 75 now without terrible losses in the feed line. Having antennas that for most that cannot install a tower and even dont have the room for multiple wire antennas is a growing concern and Cicil is always on the move to design and build a better simple antenna system which to me is the true spirit of ham radio.
 
OCF Dipoles Fed with Ladder-Line  
by W8WZ on December 30, 2017 Mail this to a friend!
Thank you for this well researched article.

I do not see the benefit to feeding an OFC dipole with open wire feeders.

If one wishes to use open wire feeders, one can simply feed a traditional centerfed balanced dipole and use that antenna on all bands without worrying about loss, coupling RF out of the Xmtr with an impedance matching network. This is one of my favorite antennas and I presently have two of them in the air.

The benefit to feeding a dipole off center, is that it allows one to couple RF into it (an unbalanced antenna) using an unbalanced feedline (coax) and have a decent impedance match on multiple frequenices thereby allowing coax to be used on a multi band wire without loss issues. This removes the need for the impedance matching network required by the balanced line antenna described in the paragraph above. In short, the only reason to use an OCF dipole is to be able to use coax on a single wire multi band dipole without a tuner.

Putting ladder line into the equation removes that single benefit and adds issues of balanced/unbalanced interaction and common mode current issues that could be minimized and/or avoided by simply using the traditional balanced Center fed dipole fed with low loss. balanced feeders.

That said, both OCF coax fed dipoles and regular dipoles fed with ladder line are very good ways to get multiband performance with a single wire antenna. 73, Carl W8WZ
 
OCF Dipoles Fed with Ladder-Line  
by K5UJ on December 31, 2017 Mail this to a friend!
Balanced feedlines work best with balanced loads. If the load is unbalanced (monopole, unbalanced dipole, antennas of odd geometry) the feedline should be unbalanced. A possible exception are beams and quads that rotate. They are balanced loads but often fed with coax, although years ago hams fed them with balanced parallel wire line.

The Off Center Fed dipole, will always cause a balanced feedline to radiate because the currents in the line at any instant are not equal. It is much better to balanced the dipole by putting the feedpoint in the center, and employing well constructed balanced line and a balanced link coupled matching network to transition to unbalanced modern transmitter output networks.

Most operators choose a "balun" and unbalanced line (coax) for their OCF dipoles but in many cases this is inverior because the transformer at the feedpoint will be reactive, and heat up losing power. The balanced line balanced network method is much more efficient even at higher vswr on the line.

Happy New Year
 
OCF Dipoles Fed with Ladder-Line  
by KU3X on December 31, 2017 Mail this to a friend!
Feeding an OCF with balanced feeders is like buying a pair of shoes that are too big and then stuffing paper in the front of the shoes to make them fit.
Never feed an unbalanced antenna with balanced feeders. “It does not work well” just the pair of shoe scenario. Don’t try to make something work that will not work properly by design.

The concept of the OCF is a multi band antenna, without traps, that can be fed with coaxial cable.

No matter how you slice it, if you feed an OCF with balanced line, “the balanced line WILL radiate.” So if you really want RFI issues, use balanced line on an OCF !”

Barry G. Kery, KU3X
 
RE: OCF Dipoles Fed with Ladder-Line  
by WC4R on December 31, 2017 Mail this to a friend!
Great article, well written, super documentation, educational. Thank you for the time and effort!

K7NSW- (and OT) Many hams are anti-QRZ due to Fred's policy. Don't assume that is the go-to place for all hams. See qrzcq.com or callbook.us
 
RE: OCF Dipoles Fed with Ladder-Line  
by KC2GUY on December 31, 2017 Mail this to a friend!
Cecil presents some compelling examples here...and it's something to think long and hard about. Most people's responses are knee-jerk. I have been to Cecil's page, which is full of very very helpful information. He is a very knowledgeable ham, and has helped many in this hobby....including me. It is people like him, who contribute to this hobby in a constructive way, that make it great.....and make us think outside the box ..like feeding an OCF dipole with ladder line.....
 
RE: OCF Dipoles Fed with Ladder-Line  
by K6AER on January 1, 2018 Mail this to a friend!
Most RFI issues are not as a result of feed line radiation but from radiation of the antenna itself. I am amused at some one complaining about RF getting into their microphone and the antenna is ten feet away from the shack.

A good balun is the best way to prevent feed line common mode. With out a feed line current meter the next best way to check for common current is old school and to run an amplifier and see if you get bit on the microphone.
 
RE: OCF Dipoles Fed with Ladder-Line  
by W5DXP on January 1, 2018 Mail this to a friend!
Maybe something can be done about common-mode on ladder-line. Take a look at G019.pdf from this web page:

https://www.pulseelectronics.com/library/white_papers/

Section 6 covers a transformer that has little effect on differential signals but tends to short common mode signals to ground.

Thanks for the opinions and kind remarks.
--
73, Cecil, w5dxp.com
 
OCF Dipoles Fed with Ladder-Line  
by DJ0IP on January 3, 2018 Mail this to a friend!
The advantages of feeding the dipole in the center are well known. The disadvantages of feeding off-center are thought to be well known but in fact I have not seen much real life test data on real antennas.

Further, every time I read something about OCFD, it uses the example of one-third, two-third split. The Windom is nearly 100 years old now and the thinking described above is nearly as old. Time to update the thinking.

The downside of feeding the dipole in the center is that you have extremely high impedance (SWR) on the even harmonic bands. The disadvantages of feeding the OCF dipole at the classical 1/3 - 2/3 split with openwire are, first as presented earlier, Common Mode Current (CMC) on the feedline, and second, extremely high impedance on 15m.

The farther we move the feedpoint away from the center, the worse the CMC gets. There is no law requiring us to move far off center. By moving to a much lesser split, say 40 to 43%, the impedance on all of the classical ham bands including 15m is very reasonable and easily matchable with just about any matchbox, provided we address the CMC issues. The CMC level is lower than with the classical split. We address the issue with a 1:1 Guanella choke.

The result is an all band OCFD that is easily matchable with just about any matchbox, unless you choose an unlucky feedline length.

I hope some of you will give this idea a try before trashing it without trying.

HNY everyone!

73
Rick, DJ0IP
 
OCF Dipoles Fed with Ladder-Line  
by WA3DQS on January 6, 2018 Mail this to a friend!
Thanks for a well-written article. I am currently using an OCFD with a 20/80 ratio, based on the W8JI design. It is fed with 1/2 wave of 300 ohm twinlead (approx 120 ft but that depends on the VF) to a Comtek 4:1 balun rated for 5kw. It's not a perfect antenna on every band - the OCFD is always a compromise regardless of where the balun is located. But the key take-away from this article is that you can't use a random length of twinlead for this antenna configuration - it should be a half wave on the lowest band. Short the top end of the twinlead and hoist it up it the trees (no antenna yet - just the feedline.) Trim the feedline length for resonance at 3.5 MHz (or 7.0 MHz if your lowest band is 40m.) Now add the antenna elements - using a 20/80, 33/67, 41/59 or whatever flavor of OCFD you have decided on after running a few test cases in EZNEC (each ratio has its own good and bad points.) Trim the antenna elements to get the best dip on a higher band - in my case it was 12m. I have had good results using an LDG AT-1000 autotuner, but a good manual tuner will do a better job (especially on 80m.) As always, the antenna/tuner system is only as good as the ground system to which it's connected.
 
OCF Dipoles Fed with Ladder-Line  
by K6BRN on January 24, 2018 Mail this to a friend!
Cecil:

Thank you for a very well written article with a concise explanation that makes the interactions between feed-line and antenna seem intuitively obvious. Well done!

Brian - K6BRN
 
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