OCFD fed with Twin Feeder instead of 4:1 Balun and Coax

[HB9PJT]

In your antenna model, you have forgotten to add the coaxial cable. The feed remains where you have it now. But you must connect the coaxial matel to either the left or right leg. If the line is exactly lambda/4, if the line does not end earthed. Or lambda/2 if the line ends earthed and the feed is low impedance. Then you will see a strong common mode current.

This can of course be explained with the Kirchhof. But if you don’t understand the sheath waves, you have no chance…

I would never use an antenna without BALUN/common mode rejection.

73, Peter - HB9PJT

[VK3AFW]

Hi Rob,
Thanks for the graphics. However we have an entrenched offset think group amongst us.

If the feeder and the antenna are at right angles all is good.

Now people say waddabout the common mode current. Ah well that is self inflicted. If you use an angled feed line one side of it gets a bit more induction and if left alone you have a three wire feeder. Two wires carrying current as before but now there is a third wire - the outside of the coax or both wires of the twin feeder - strangely we call this common mode current.

An inverted Vee ocf may be subject to common mode effected but that is something else.

Kirchoff must still be satisfied.

The old timers knew all about this and twisted the twin feeder about once every 400 mm. This effectively cancelled out the common mode current. Strangely it still works.

So without any common mode current to blur the picture, we have two feed line currents in and two equal antenna currents out.

And yes Brian, the feeder supplies power just like a hose supplies water but it’s not a good analogy.

73
Ron
VK3AFW

[HB9BCB]

OCF dipoles from Owen Duffy’s perspective - as a possible introduction to this topic …

https://owenduffy.net/blog/?p=1663

[DM1CM]

Heinz, thanks for that link to the article by Owen Duffy - I will add it to my constantly-growing series of references on the subject. The more, the merrier!

I’ve also found this link quite helpful - it’s an article by Roy Lewallen W7EL from 1985. It helps me to better understand what Peter HB9PJT was trying to say wrt to coaxial cable lengths. Of course, as a licensed ham, I should know all this, but I find myself to be very forgetful these days - having one’s mind refreshed in this way by re-visiting such topics is a great help. Peter is of course perfectly correct when he says “if you don’t understand the sheath waves, you have no chance…”

For me, the most important take-away from the Lewallen article is the sentence

The proportion of current which flows each way [i.e current flowing into the antenna half connected to the coax sheath, or current flowing along the sheath outer] is determined by the relative impedances of the two paths.

This is such an instructive statement - obvious when one thinks about it - and links directly into what Peter was saying.

I’ve recently built a 40-meter principal band coax-fed OCF dipole for SOTA use (pace Brian) and trimmed/tested it in the field, and its’ performance on 40m, 20m, 15m and 10m - at least in terms of VSWR - is very close to that predicted by a NEC program. Never having constructed any kind of wound-ferrite balun/unun, I opted for a commercial 4:1 unun and I use that at the feed-point. Having read a couple of articles on related subjects, I think I’ll also opt for a current balun (coax+ferrite beads) close to the rig.

[HB9BCB]

In the vast majority of cases of published VSWR values, the antenna is seen as a two terminal device - in this simplified model the influence of the feed line is ignored (e.g. if due to common mode current on the feed line the antenna receives a 3rd leg → influence on the radiation properties).

Based on a three terminal model, the complex impedances of the two antenna legs and common mode impedance to ground can be measured/checked using a simple but meaningful measuring method proposed by Kevin Schmidt, W9CF.

Note: In the case of a coax feeder, the presence and magnitude of common mode current can be measured/checked using a current clamp across the coax cable.

Owen Duffy, Circuit of an antenna system
https://owenduffy.net/blog/?p=14267

Owen Duffy, Find three terminal equivalent circuit for an antenna system
https://www.owenduffy.net/calc/pbtt.htm

Owen Duffy, Common mode current and coaxial feedlines
https://owenduffy.net/blog/?p=428

Owen Duffy, Measuring common mode current on coax
https://owenduffy.net/blog/?p=458

[DM1CM]

@HB9BCB

Based on a three terminal model, the complex impedances of the two antenna legs and common mode impedance to ground can be measured/checked using a simple but meaningful measuring method proposed by Kevin Schmidt, W9CF.

This is all true, and well and good - but when activating a SOTA summit, using 10 or 20 watts output power into a less-than-perfect wire antenna/coax/rig setup, nobody really cares about the theoretical considerations, or measuring the last piece of reactance in the system. If the rig accepts the antenna/feedline conditions, and enough QSOs can be made, then the system is a “good” one.

As far as I, and many others, are concerned - that’s good enough on the day. There are plenty enough activators who don’t even bother to use a 1:1 balun when feeding a symmetric dipole with coax. Home to tea and cake…

To the topic at hand - several sources say that an OCF dipole ought not to be fed with ladder line/twin feeder. What do I know, I’m just quoting what I’ve read online…

[MM0FMF]

@DM1CM

nobody really cares

You might not care but many people do.

@DM1CM

I’m just quoting what I’ve read online

The problem is if it’s online it must be right. Seen when a J-pole design for 2m with incorrect dimensions was copied and reprinted by many. If you followed the design it would not work. A bit of maths showed the dimensions to be incorrect and not surprisingly fixing the dimensions was needed. The subject of transmission lines and that of antennas are conceptually difficult ( DC short circuits becoming RF open circuits etc. ) and if you don’t study things carefully it’s easy to fall into the incorrect reasoning behind some of the “truths” out there.

I don’t do this kind of engineering day in day out, I did a lot of it 40+years ago an University but I need to work at it to remember what I was taught so as to understand what should happen and what can happen. It’s perfectly understandable for people who didn’t study this and just dabble to get confused and re-quote what cannot possibly work.

[DM1CM]

@MM0FMF

@DM1CM

nobody really cares

You might not care but many people do.

I actually had you in mind Andy when you said, somewhere fairly recently in the SOTA Reflector, that you had not used/did not bother to use a balun when feeding a linked dipole with coax. Perhaps that was a one-off occurrence?

Most of the rest of your message is on-point patronization. Duly noted, thanks a bunch - no, I did not study electrical theory at college, I’m happy for those who did and who followed the linear path to find jobs in which they were able to use exactly what they had studied.

[G4OBK]

@DM1CM

To the topic at hand - several sources say that an OCF dipole ought not to be fed with ladder line/twin feeder.

Forgetting the technicalities of this rather pointless argument… If you did feed that OCFD aerial with ladder line or 300 ohm twin, I doubt if your transceiver with the built in ATU would match it. If you tried to match it with the built in then you would need another balun at the feedpoint to the radio. That would likely get warm if the impedance on the input to it was sufficiently high or low, as it is likely to be with that feeder on most if not all bands. Use coax though with your OCFD and most built in ATUs would have sufficient range to provide a low SWR on most bands. My experience with an 80m size coax fed OCFD is that it will match with a built in ATU on all bands 80m-10m, except on 60m, when an external ATU with a broader matching capability must be used.

73 Phil G4OBK

[DM1CM]

Hi Phil,

Those considerations were probably in the back of the mind of those people who wrote that ladder-line/twin feeder ought not to be used when feeding an OCF dipole. But, what do I know - I believe everything I read online…

[G4AZS]

According to the interesting Own Duffy article linked higher up this thread, and to the ARRL Antenna book, the OCF dipole was designed to give a reasonable match at its fundamental and even harmonics, typically 3.5, 7.0 and 14MHz, when fitted with a 4:1 balun at the feed point and fed with 50 ohm coax (no ATU required).
As with a centre fed dipole, though, this assumes “free space” eg dipole top at least half a wavelength up in the clear, and a bit of careful trimming.

I enjoy threads like this because they challenge what I thought I knew, and prompt me to follow up references and exercise the little grey cells!

Many radios now have built in ATUs, which may extend the frequency range in practice, depending on feeder length, balun choice and careful trimming.

Using OWL feeder makes it into a different antenna system, so I would expect to have to make adjustments to the element lengths and feeder length and, as Phil says above, pay attention to losses in any balun and matching unit.

[DM1CM]

@G4AZS

According to the interesting Own Duffy article linked higher up this thread, and to the ARRL Antenna book, the OCF dipole was designed to give a reasonable match at its fundamental and even harmonics, typically 3.5, 7.0 and 14MHz, when fitted with a 4:1 balun at the feed point and fed with 50 ohm coax (no ATU required).

We are advised by many to be aware that what we read online may not necessarily be the truth, the whole truth and nothing but the truth. A few observers go so far as to point out that the oft-quoted ~200Ω radiation resistance of an OCF dipole (and hence the need for a 4:1 matching unit) is a myth, and that its’ actual value depends very much on the OCF dipole legs length ratio, as well as the usual culprits like frequency, height of antenna above ground, ground type, etc.

For instance, the radiation resistance of a 33/67 ratio 40-meter OCFD operating on 40 meters is actually around 80Ω, on 20 meters it’s around 110Ω, and on 10 meters about 160Ω (numbers from a NEC program), so a 4:1 unun would be useful in certain situations only - perhaps a 2:1 might be better. The 4:1 one I bought, and am currently using, has an “in-built current choke”, but who knows how good that might be in practice?

Again, it’s a matter of building something reasonably “to specs”, try it out and if it works well enough, use it, but always with an eye on what would be ruled out by purely theoretical considerations. We’re all of us technicians of sorts, but we know that even the most unlikely setups can sometimes work in extreme circumstances, i.e. the dipole that just fell down in a strong wind but which still manages to push out a signal. On the hill, you use what you’ve got.

[G4AZS]

@DM1CM

A few observers go so far as to point out that the oft-quoted 200Ω radiation resistance of an OCF dipole (and hence the need for a 4:1 matching unit) is a myth, and that its’ actual value depends very much on the OCF dipole legs length ratio, as well as the usual culprits like frequency, height of antenna above ground, ground type, etc.

Yes, it seems that the term “OCF” was originally coined for a very specific antenna system design, but is now often used generically to refer to any piece of wire fed away from its centre. This can lead to confusion.

[ZL1MJL]

Why all this talk about OCFD fed? If using a dipole of a length that doesn’t resonate on any of the bands then the dipole can be fed with open wire in the middle without any imbalance and simply be tuned with an ATU (and either have a balanced ATU or use a choke between ATU and transceiver).

73
John
ZL1MJL

[DM1CM]

@ZL1MJL

Why all this talk about OCFD fed? If using a dipole of a length that doesn’t resonate on any of the bands then the dipole can be fed with open wire in the middle without any imbalance and simply be tuned with an ATU

It all depends what one wants from an antenna. The OCFD style of antenna has a hundred-year old history, and with good reason: it can deliver more than “just” a center-fed dipole, by being naturally resonant on more than just one band or frequency range.

To illustrate this point, here’s a VSWR scan of a center-fed dipole cut for the 40-meter band, with arms sloping at 30° to the horizontal, and center-height at 15 meters AGL:

The center-fed dipole is naturally resonant on 40 meters and also on 15 meters, due to the harmonic relationship between the two bands. All well and good … but there’s more.

Feeding this same antenna off-center, one can achieve resonance on more than two bands, if one chooses an appropriate ratio of the lengths of the two arms of the antenna.

For instance, here’s the VSWR curve of the same antenna, now fed off-center, with arms of 33% and 67% respectively (the most-often quoted or chosen ratio):

Things look a little more interesting now: this antenna is resonant on three bands 40m, 20m and 10m, although we’ve now lost the 15-meter band.

Here’s a further example, a 18% to 82% ratio split OCFD - my particular favourite:

We now have an antenna which is naturally, in and of itself, resonant on four bands: 40m, 20m, 15m and 10m (it’s also resonant on 6m, but who ever uses 6m, right? )

So, the OCFD can be resonant on more than one band, no tuner required on those bands - one just has to decide how to arrange the two arms of the antenna to achieve resonance on the bands of choice.

EDIT: just to be clear, these curves show VSWR at the antenna feed-point - if using a lossy coax feed, the values at the TRX end will be (even) lower than those displayed here.

Cheers, Rob

NEW EDIT: Graphics updated, without losing the import of the argument.

[ZL1MJL]

When using a full size OCDF antenna then one has to accept that the radiation pattern has lobes on the higher bands whereas if one was to use a 66 or 44 foot antenna (as mentioned by e.g. Cebik) then it is a normal dipole pattern. I know what I prefer (I do not see using an ATU as a problem).

73
John

[MM0FMF]

@DM1CM

is naturally resonant on 40 meters and also on 15 meters,

Not really. The resonance around 21MHz falls sufficiently out of the amateur band that the match between 21-21.45 results in a high enough SWR that many if not most solid state radios will enter some kind of SWR protection by reducing power or give a high SWR warning and not transmitting.

[DM1CM]

@MM0FMF

Not really. The resonance around 21MHz falls sufficiently out of the amateur band that the match between 21-21.45 results in a high enough SWR that many if not most solid state radios will enter some kind of SWR protection by reducing power or give a high SWR warning and not transmitting.

Well, I used a NEC (Numerical Electromagnetics Code) simulation to produce the curves above! But, of course, everybody knows NEC, as used by countless RF professionals worldwide, is pretty useless when measured against the personal beliefs of the odd radio amateur. One lives and learns, and bows to ones’ true masters.

But joking and joshing aside, I regularly use a home-brew 18% to 82% ratio-split OCFD (as in the 3rd example above), and activate with it. I still take a RigExpert AA-55 Zoom with me on each activation, and each time test the antenna’s VSWR curve, both across multiple bands 40m through 10m, as well on each individual band, and the RigExpert shows precisely the same VSWR values, adjusted for the run of coax, as the NEC engine generates. Seems that NEC and RigExpert know a thing or two … but what would I know, I’m just an amateur, and proud of it.

YMMV, as always…

[DM1CM]

@ZL1MJL

When using a full size OCDF antenna then one has to accept that the radiation pattern has lobes on the higher bands whereas if one was to use a 66 or 44 foot antenna (as mentioned by e.g. Cebik) then it is a normal dipole pattern.

Yes, you’d see the classic figure-of-eight dipole pattern all right, agreed - but only if your antenna were situated in free space. Otherwise, when situated at a “normal” height, the pattern will be much affected by the ground, as well as the height of the antenna above the ground, and practically anything else around the antenna (trees, buildings, fences, you, etc.)

And if you drive your 66-ft or 44ft center-fed dipole on other bands, whether you use an ATU or not, then you’ll still get multiple lobes - you can’t beat physics, there’s no such thing as a free lunch. The extra lobes are the product of the radio waves themselves interacting with each other, and with their surroundings - the lobes owe much less to the type of dipole antenna being used.

Just to illustrate this point, here are 3D representations of radiation patterns produced by a perfectly symmetrical 40-meter dipole, fed at the center (so 50% : 50%), with the center at 12m AGL, the arms sloping downwards at 20°, and on various HF bands :

There are lobes where lobes are to be expected. It has to be said that this particular antenna is resonant (or is close to resonant, pace Andy) on only two bands, namely 40m and 15m. Using an ATU may help with getting a signal out on the other bands, but the ATU will have no effect on the radiation pattern lobes. They is what they is.

[MM0FMF]

@DM1CM

Seems that NEC and RigExper

Yes but we’re talking about the centre fed case where your own graph shows the SWR at 21.0MHz to be about 3.0 : 1. In practical tests, 2 different 40m centre fed dipoles cut for 7.07MHz (so usable on CW and the extended SSB section above 7.1MHz) both showed unacceptable SWR when used on 15m. Resonance in this case was around 21.6 to 21.7MHz and the SWR was around 2.2 : 1 at 21.425, so high up 15m that it seldom gets used. The only way to get acceptable match was to add short extensions of about 15-20cm to ends of the 40m dipole.

This is what your own image shows. I don’t know if NEC models the end effect capacitance of wires.