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.
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.
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.
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.
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…
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.
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.
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.
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…
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.
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.
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.
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).