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I got myself a TV "Rabbit Ears" module originally for a TV. It came with a small length of coax, permanently attached to the antenna foot, and a Belling-Lee connector on the other end.

My idea was to use an RG-58 coax with an SMA plug with this antenna, because it is easy to make a handle for it, and it's easy to store.

Inside the mounting point for the two telescopic antennas, there is a ferrite core, with a couple loops around it, going off to the two telescopic antennas:

Balun schematic

In the schematic, (a) is four turns, (c) and (d) are two turns, each. As I said it's a dual-ferrite core, so two holes in one block of ferrite (ignore the center tap on the (a) side, I couldn't find a transformer symbol with one inductor on one side, and two disjunct inductors on the other, with a core).

I also made a couple of pictures:

Balun

They are rather shoddily soldered together, and while handling the center core wire broke off from the solder point:

Balun, other side (ignore the broken connection between core and balun)

As you can see in the pictures above, then it's one ferrite block with two holes in it. A wire running from core to shield of the coax goes through both holes, looping four times. Then, each from the core and the shield a wire loops through either hole, twice and then connects to either of the two telescopic antennas, which I have removed, so handling is a bit easier.

Since this is a TV UHF antenna, this has to be 75 Ω, but I'm wondering how I should now modify the balun, such that the impedance is 50 Ω.

My first approach was to remove the balun altogether, and directly solder a length of 50 Ω RG-58 to either of the antenna solder points and that'll be it. But I'm not entirely sure that would be a good approach, since I believe the balun is here for a reason...

Instead of coming up with just a number of turns, it would be nice, if someone could provide a formula how to calculate these, or a place where I can find further information.

UPDATE: I think I've found the device that was wired up in my antenna in blog post Make a wideband antenna matching transformer.

So, it's a 4:1 balun, with 75 Ω on the coax-side. This means the antenna has an impedance of 300 Ω. 300 Ω for a dipole? It sounds strange to me, but I don't know.

Assuming this is a 300 Ω antenna, I'd need a 6:1 transformer / balun to match it up with 50 Ω. The closest I can practically wire up is a 2 and 5 turns transformer, resulting in a 6.25:1 transformer. I'm not sure that would be even usable. The closest match without going into crazy figures is 49 and 20 turns, resulting in 6.0025:1, which I think would be close enough.

I don't think I have a way of finding out the exact impedance of my rabbit ears antenna, so I guess I just have to assume it's 300Ω going by the fact how the balun / transformer was wired up, etc.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Mike Waters ♦
    Apr 17, 2018 at 1:14

4 Answers 4

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The feedpoint impedance of the rabbit ears antenna will be 72 ohms or less. It is basically a dipole antenna with shortened legs and potentially orientated in a Vee configuration. So with 50 ohm coax, attempting to exactly match impedance is a frivolous exercise.

Any notion of the balun correcting SWR is ill conceived. In a receive configuration, the SWR is established by the relationship of the receiver input impedance and the Zo of the feedline. The antenna is the signal source. Any mismatch between the receiver and the feedline will cause the impedance of the transmission line at the antenna end to be transformed. If this transformed impedance is not a match for the feedpoint impedance of the antenna, the antenna will not transfer its maximum received power to the feedline causing it to re-radiate some of the power. Given the potentially wide variation of the antenna impedance, matching the two impedances through a matching device is not warranted. A more practical approach is realized by simply adjusting the antenna length and configuration for the best received signal.

A properly constructed balun will minimize common mode currents on the coax but with a compromise antenna such as rabbit ears, concern about common mode currents is not warranted.

My advice would be to simply connect the 50 ohm coax directly to the rabbit ears and make the best of it.

The balun as pictured appears to be a transmission line transformer, probably a 4:1 configuration. Such devices do not work on the basis of turns ratios like more typical transformers.

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  • $\begingroup$ I've found very similar values while reading through antenna theory books and checking out an engineering boot about RF. It seems the typical dipole impedance is 73 + j42.5Ω. The reactance can be eliminated by adjusting the length of the antenna, while the resistance kinda stays at around 70Ω. As for the device: In case it is a transmission line transformer (it would be consistent with other material I've linked), I'll read up about what this does. I'll finish making the antenna now, and check back. Thanks for the insight. $\endgroup$
    – polemon
    Feb 22, 2018 at 21:44
  • $\begingroup$ The mismatch between the antenna, and the impedance seen at the end of the cable (receiver+cable), is real and causes a loss of signal. As antennas are reciprocal devices, it's easier to analyse SWR in the transmitting case, but the same effect occurs on receive. A 4:1 SWR results in a ~2 dB loss, transmit or receive. $\endgroup$
    – tomnexus
    Apr 24, 2018 at 13:57
  • $\begingroup$ @tomnexus The 2 dB loss for a 4:1 SWR statement should be qualified. This is true only if when transmitting, the Zo of the transmitter matches the Zo of the transmission line (TL) or when receiving, if the Zo of the antenna matches the Zo of the TL. In all other cases the losses will likely be less. $\endgroup$
    – Glenn W9IQ
    Apr 25, 2018 at 4:21
  • $\begingroup$ @tomnexus ...continued. Since the Zo of the transmitter rarely matches the Zo of the TL in amateur radio applications, the 2 dB generalization is not a good reference for hams not fully aware of all of the factors. $\endgroup$
    – Glenn W9IQ
    Apr 25, 2018 at 4:55
  • $\begingroup$ True, if the transmitter impedance is far from 50, which is likely the case. But using it as a receive antenna, the radio probably is near 50 Ohms, so the mismatch at the antenna will attenuate the signal. Re-reading, I think this is what you said, anyway! $\endgroup$
    – tomnexus
    Apr 25, 2018 at 20:06
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A little impedance mismatch on the receiver is not a big deal. 75 to 50 ohms is an SWR of 1.5:1. For most applications, anything under 1.6:1 is considered a "good match". For a receive-only application, the match is even less critical.

Besides, the impedance of the antenna is going to vary with its surroundings and how the ears are oriented.

And, you've said you don't have an antenna analyzer to accurately measure the impedance.

Given all this, little is to be gained by improving the match, and between environmental variation and the lack of equipment to measure it, any attempt you would make at improving the match would lack sufficient precision to make the difference between 50 and 75 ohms relevant in comparison.

If I were in your situation, I'd just repair the solder joint and perhaps put a different connector on it. It will work fine.

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  • $\begingroup$ Great answer, Phil. I don't even think he needs a balun, unless his rabbit "ears" is a folded dipole. Assuming it's not, what the heck is that balun for? $\endgroup$
    – Mike Waters ♦
    Feb 20, 2018 at 19:39
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    $\begingroup$ Honestly I'm not sure. It may not even be a balun. $\endgroup$
    – Phil Frost - W8II
    Feb 20, 2018 at 19:54
  • $\begingroup$ From your reference: "SWR is a measure of the impedance mismatch between your antenna and transceiver." For a receiver, is not the SWR by definition a measure of the impedance mismatch between the receiver and the transmission line? That is to say that the antenna is the source and the receiver is the load. $\endgroup$
    – Glenn W9IQ
    Feb 20, 2018 at 21:10
  • $\begingroup$ Well, then why was in in there in the first place? It is pretty much exactly the thing that I've linked to in the "UPDATE" section of my post. $\endgroup$
    – polemon
    Feb 20, 2018 at 21:10
  • $\begingroup$ Btw. I know it's a moot thing this, but I also want to have a learning experience with a rather simple thing like this. Once I've learned these basics, etc. I can move on, etc. $\endgroup$
    – polemon
    Feb 20, 2018 at 21:29
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The impedance ratio is the square of the turns ratio.

That multi-aperture core is known as a binocular core. There's some info about it here.

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  • $\begingroup$ OK, the page you've linked has a slightly different wiring, as the primary is never connected to the secondary pair, as it was in my case. The wiring is much simpler as I understand it: five turns on the high-impedance side (coax) and two on the low-impedance side (antenna). Correct? $\endgroup$
    – polemon
    Feb 18, 2018 at 1:27
  • $\begingroup$ I thought I'm looking at this: onetransistor.blogspot.de/2015/10/… $\endgroup$
    – polemon
    Feb 18, 2018 at 1:36
  • $\begingroup$ @polemon For 50 ohms, use 2t and 6t. For 75 ohms, 2t and 5t. $\endgroup$
    – Mike Waters ♦
    Feb 18, 2018 at 1:37
  • $\begingroup$ Ah right, that's what I meant. five turns is for 75Ω. Still, though, what sort of wiring is what's being used originally? And does the 9:1 transfomer work in VHF and UHF (as the antenna is designed for)? $\endgroup$
    – polemon
    Feb 18, 2018 at 2:13
  • $\begingroup$ The core material in your antenna is almost certain a ferrite mix that's designed for VHF and above, unlike the 73 mix on that Beverage antenna page. $\endgroup$
    – Mike Waters ♦
    Feb 18, 2018 at 2:34
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I might be misinterpreting the second photograph, but that looks a lot like a 4:1 Guanella balun (see http://www.hbphoto.com/Radio/Baluns_101.pdf , slide 18). If you are in the United States, you can buy one of these in any hardware store for very little money. The link you provided is also a 4:1 Guanella balun.

I suspect the main purpose of this is not to provide a 4:1 impedance transformation, though. I suspect the main purpose is to reduce common mode current and, like every other contributor on this page has already said, you probably do not need it.

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