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I want to match a 50-ohm coax feed wire to a 40-meter NVIS half-wave dipole. NVIS antenna examples I've found are usually based on a regular dipole (72 ohms in free air). However the impedance drops by a factor of four (to 12 ohms) when it is lowered to 7 feet (https://www.w9xt.com/page_radio_gadgets_nvis_antenna.html). (Note: the author's numbers seem not to correlate with his factor, but at any rate the impedance drops very low).

Just for reference, a folded dipole in free air is between 200 and 300 ohms (https://www.w8ji.com/folded_dipole.htm). Therefore, its impedance would drop by a factor of four to 50-75 ohms, making it closely match 50 ohm coax.

My idea was for a regular dipole (not a folded dipole) and use a 4:1 balun to raise the antenna impedance by a factor of four from 12.5 ohms to 50) to match the coax and transmitter impedance.

Problem: Maybe some baluns are not designed to be reversed because the side designed to be high impedance balanced antenna would then be on the unbalanced coax, and vice versa with the low impedance side. Furthermore, transferring power the side driving the low impedance antenna might draw more current than the balun wire was designed for.

I have not found answers in any Radio Amateur's Handbook (I have six ranging from 1956 through 2012), or on the Internet.

So what is the proper way to match 50-ohm unbalanced coax to 12.5 ohm balanced dipole?

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    $\begingroup$ A folded dipole has 4x the impedance of a non-folded equivalent, if the conductor sizes are equal. So how do you figure a folded dipole has an impedance of 200 ohms? $\endgroup$ – Phil Frost - W8II Oct 6 at 18:48
  • $\begingroup$ @PhilFrost-W8II From the same W9XT webpage that the other information in his question came from. Please look at the reflector under the dipole there and comment on it if you wish. Maybe W9XT is implying that that reflector is lowering the impedance. $\endgroup$ – Mike Waters Oct 6 at 19:30
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    $\begingroup$ The W9XT page writes, "The impedance of a folded dipole [in free space] is about 200 ohms. The impedance of the regular dipole dropped by a factor of 4 when it was lowered to 7’. Starting at 200 ohms, and reducing it by a factor of 4 gives you 50 ohms, a perfect match!" It seems the author is wrong about the free-space impedance of a folded dipole (it's more like 300 ohms), but in any case the point he's trying to make is a folded dipole multiplies this 12.5 by 4, yielding 50. Not 200. $\endgroup$ – Phil Frost - W8II Oct 6 at 23:04
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    $\begingroup$ @MikeWaters I added reference links to the question and also edited it heavily to make it clearer. $\endgroup$ – Peter Buxton Oct 7 at 2:34
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    $\begingroup$ @PhilFrost-W8II The author seems to have switched dipole types in his sentence. He mentioned a "regular dipole dropped by a factor of 4..." but his next sentence "Starting at 200 ohms..." referred to the folded dipole as I understood. I was interested in the regular dipole and only added the folded dipole info for reference (see my edited question). $\endgroup$ – Peter Buxton Oct 7 at 2:40
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Where did you hear that a 40m center-fed dipole λ/4 high has a feedpoint impedance of only 12.5Ω? It will be 75Ω or more. A center-fed λ/2 dipole is only 50Ω at one height.

Here are some graphs. As you can see, you don't need a 4:1 balun.

Ignore the top graph in this first image.


Dipole impedance vs height

The graph below is based on theoretical values

Dipole impedance vs height


75Ω coax will be a better match, and the worst case VSWR mismatch to your rig will be only 1.5:1.

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  • $\begingroup$ I found these here. They correspond to the graphs in many antenna books. (Sorry about the mess, I'll crop them when I can get to my desktop computer.) $\endgroup$ – Mike Waters Oct 5 at 19:24
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    $\begingroup$ One site where I found 12 ohms was in [link] (w9xt.com/page_radio_gadgets_nvis_antenna.html) I do see what you're saying, based on the charts. I read several places that NVIS is best between 10 and 15 feet. For 40 meters, 10 feet height would be near 16 ohms (not 12 as I thought). I see that it rapidly rises with increase in height, so it could be 50 ohms if the height were approximately 23 feet. However, then wouldn't the NVIS affect be reduced? $\endgroup$ – Peter Buxton Oct 5 at 22:12
  • $\begingroup$ @PeterBuxton Since that is a separate question from your original one here, I suggest that you ask a new question. But you may find the answer in one of these previous questions here about NVIS coverage. Also, there is a lot of misinformation out there, specifying a height that did not to be that low for NVIS radiation. Having said that, W9XT is using reflectors and I don't fully understand their effect here. $\endgroup$ – Mike Waters Oct 5 at 22:36
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    $\begingroup$ I didn't reallymean to ask a new question. Raising the height to attain 50 or 75 ohms would still be less than λ/4 high, therefore still NVIS by definition. Originally I was concerned about stressing the transmitter from mismatch but now I feel confident not to use a balun and just to trim for SWR and resonance and call it good for experimenting. $\endgroup$ – Peter Buxton Oct 6 at 1:19
  • $\begingroup$ It is considered good practice to have a 1:1 choke balun at the feedpoint to prevent radiation and noise pickup caused by common-mode current on the outside of the shield. Pretty sure that there is already questions and answers about that here. The search feature here is your friend. :-) $\endgroup$ – Mike Waters Oct 6 at 2:58
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Many baluns will work just fine in either direction, though there isn't just one kind of "4:1 balun".

This kind is wound on two cores, and works as a common-mode choke:

schematic

simulate this circuit – Schematic created using CircuitLab

A common-mode choke works in either direction, so it matters not which end is balanced and which is unbalanced (or if both ends are unbalanced, or both balanced, for that matter.)

However this kind is wound on a single core and relies on ground potential being at the midpoint of the balanced terminals:

schematic

simulate this circuit

This can work OK given a well-balanced load on the right, and an unbalanced load on the left. But flipped the other way (with the balanced load on the left) it will try driving each terminal of the balanced load to different voltages relative to ground, so you'll get quite a lot of common-mode current.

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  • $\begingroup$ Excellent explanation, Phil. That really answered the problem I noted in my question. Do you think that reversing one and driving 12 ohms would be too much current for the balun? $\endgroup$ – Peter Buxton Oct 7 at 15:51
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    $\begingroup$ @PeterBuxton Depends on the power. You can use Ohm's law to figure out what the current will be. $\endgroup$ – Phil Frost - W8II Oct 7 at 16:11
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How to match a 12.5 ohm balanced antenna to a 50 ohm unbalanced line... I think that has been well answered but one point worth mentioning, and a relevant anecdote...

There are 2 books by Jerry Sevick directly addressing how to do this: - Sevick's Transmission Line Transformers, Theory and Practice, 5th Edition (2014), by Mack and Sevick - Understanding, Building and Using Baluns and Ununs (2002), by Sevick

The first book is very theoretical, the second --which I have not yet read-- is said to be more hands-on. Both are worth reading.

I happen to have a 12.5 ohm 40m balanced dipole (short dipole, not NVIS). A lot of work went into designing and building, including the 4:1 balun (wound using 25 ohm coax; not easy to find). When I finished and took measurements, I found the impedance was closer to 50 than 12.5 ohms. It works better without the 4:1 balun! Of course it certainly has a strong 1:1 common mode choke.

My point is that you might consider building the antenna first. Measure the actual impedance, then build the balun. You might find that a matching network may work better for you, as mentioned already by @alexanderalekseev-r2auk

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  • $\begingroup$ I didn't understand your first statement that matching has been well answered. Did you mean in answers here or on the Internet? It is interesting about your 4:1 balun experience. Today I did build the antenna and posted it as an answer here. Actually I think I have the same results that you have with your dipole. $\endgroup$ – Peter Buxton Oct 8 at 2:59
  • $\begingroup$ I meant the answers here. I did not read all of the answers carefully but certainly Phil's answer (how to make a balun) and Alexander's answer (Use a matching network) were spot on. Mike's answer was a bit over my head :-) I tend to not have the time to post real answers here, but the two points-in-progress (still have some editing to do) seemed relevant and useful enough to mention. $\endgroup$ – Chris K8NVH Oct 8 at 12:09
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From personal experience, a good 1:4 balun (50 to 200 Ohm) doesn't necessary work as a good 4:1 balun (50 Ohm to 12.5 Ohm). You can wind just a 4:1 transformer and use it with a 1:1 balun though.

Another simple way to match any impedance is to use an LC-network. There are a lot of online calculators. Personally I particularly like this one. Using this approach I matched my 40m delta loop antenna to all bands from 10m to 80m using a separate LC-network for each band.

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    $\begingroup$ Why the down votes? This is both pertinent and useful. $\endgroup$ – Chris K8NVH Oct 7 at 17:12
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    $\begingroup$ Probably because "1:4 balun" and "4:1 balun" are not well defined. There are many such devices, some of them are symmetrical. $\endgroup$ – Phil Frost - W8II Oct 7 at 21:04
  • $\begingroup$ @Aleksander Alekseev - R2AUK Interesting to hear of your experience with baluns as you mentioned. The calculator you mentioned was over my head for my present level of knowledge. $\endgroup$ – Peter Buxton Oct 8 at 2:49
  • $\begingroup$ @PeterBuxton you can find the description of the balun I mentioned here ham.stackexchange.com/questions/15115/… $\endgroup$ – Aleksander Alekseev - R2AUK Oct 9 at 8:16
  • $\begingroup$ @PeterBuxton regarding the calculator - it's not that difficult. You just need to wind L of given inductance and buy a 3kV NP0 capacitors that when connected in parallel will have the given capacitance, then connect L and C according to the schematic. Zs and Zl are source and load impedance accordingly, i.e. you already have them. If you are interested in a little more theory you can start from en.wikipedia.org/wiki/… $\endgroup$ – Aleksander Alekseev - R2AUK Oct 9 at 8:41
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Today I made the antenna and included some data, for what it's worth, although this isn't really an answer because the other answers helped me to get to this.

It is a dipole with 33 feet per leg (untrimmed) of insulated #14 stranded wire, with a ceramic insulator and about 10 feet of polyester rope at each end. The height is 10 feet. There are three 8-foot metal T-posts with 3 feet of 1" PVC pipe clamped to the top for RF isolation. I laid 70 feet of extension cord on the ground below the antenna to act as one reflector (more are recommended). The center post holds a plastic box with a current choke made of 13 turns bifilar on an FT140-61 torroid. The feed cable is 66 feet long.

I tested it for SWR in the 40-meter band at 10 watts and was able to get SWR of 4:1 at 7.280 MHz, 2:1 at 7.150, and 1.3 at 7.000 MHz. That indicates to me that the resonance is slightly low so it needs to be trimmed. So it looks like it will tune with good SWR as soon as I can trim it. I believe the height of 10 feet is approximately 0.073 wavelength, so the graph of impedance vs. height indicates that it might be about 12 ohms. I'm concerned that my SWR readings are not right if that's the case. More experimenting is in store.

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  • $\begingroup$ Welcome to the wonderful world of antenna construction! If you get a chance, measure the antenna impedance with am appropriate analyzer. I suspect you will find it to be intriguing. $\endgroup$ – Chris K8NVH Oct 8 at 12:16
  • $\begingroup$ I do hope to actually measure the impedance. My only tool available right now was an American Electronics 95-128 SWR meter, I think made for CB. I'm considering setting up a QRP level RF source and oscilloscope with bridge to measure phase difference (as a primitive analyzer). $\endgroup$ – Peter Buxton Oct 8 at 19:27

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