• Balun use and Antenna Updates



    To Balun or Not to Balun That TV antenna? When should it be done?

    In an article of mine published in Popular Communications, January 2013, "An Omnidirectional DTV Antenna - With Gain," I included a little bit about why I was no longer using the 4 to 1 (4:1) balun to convert my twin lead to my TV's 75 Ohm antenna input. I also asked for reader input and posed some questions. I think some of the questions are now answered and that the baluns I have are working correctly.

    My reasoning was that when I did this for the bowtie antenna using twinlead feed line, I noticed my TV got a better signal without the balun than with it.

    I think I now have a better handle on the reason. Recently (September/August 2012), I built two four-element bowtie turnstile TV antennas. One was a dipole design and the other was the full-wave loop design.

    The dipole antenna has a design impendence of about 73 Ohms while the full-way loop has the design impendence of 300 Ohms.

    "Ideally, a half-wave (Lamda/2) dipole should be fed with a balanced line matching the theoretical 73 ohm impedance of the antenna. A folded dipole uses a 300 ohm balanced feeder line." - Dipole antenna - Wikipedia, the free encyclopedia

    With the dipole design, I found that using a balun was not only of no real help, it actually made the signal measurably worse.

    On the other hand, more experience has now shown that for an antenna based on a full-wave loop with a nominal design impendence of 300 Ohms, not using a 4:1 balun actually made matters worse and using the balun made the reception noticeably better.

    When trying to decide when to use a balun, no matter which feed line you're using, base the decision only on the design impedance of the antenna and not on the feed-line being used. So, in the case of using 300 Ohm feed-line to a 72 Ohm Bowtie (dipole) antenna, do not use a 4:1 balun. But, if you're using a folded-dipole (fullwave loop) antenna with a design impedance of 300 Ohms, use the 4:1 balun some time before attaching it to the 75 Ohm TV input.

    This also probably answers the question as to why all bowtie antennas came with 300 Ohm fee line. It is because: 1. The 300-Ohm feed line is very low loss. 2. The TV had 300-Ohm input connections. 3. Even today, no balun is needed, even with the 300-Ohm feed line.

    Of course, it could all be due to the manufacturers not wanting to change the production line to use coax instead of the twin lead.

    Now for the antenna test.

    Inclement Weather Antenna Test results

    Oct 29, 2012: Here are the results of the Hurricane Sandy tests of my Turnstile-Bowtie antenna. The building article about this antenna will be published in Popular Communications in the January 2013 issue. http://www.popular-communications.com/

    I have a single element Turnstile-Bowtie antenna above the kitchen on a mast with my 144MHz/440MHz J-Pole antenna attached to the chimney. So it's up about 20+ feet above the ground and about 25 feet or more higher than the 4-element Turnstile-Bowtie antenna (TBA4).

    The TBA4 is in the garage and to get Baltimore stations, it is going through the garage wall or perhaps the metal door. Neither the wall nor the door have windows. The neighbor's house or trees may be in the way.

    That's the line-of-sight situation as near as I can figure it.

    The result on Monday, October 29th, during the start of Hurricane Sandy during a heavy rain, was that the TV in the kitchen was flaking out on all Baltimore major network stations NBC, CBS, and ABC, while MPT Ch 67 was solid. Fox 45 wasn't even solid during a light rain. MPT Ch 62 was MIA.

    The TBA4 antenna was receiving all major stations solidly NBC, CBS, and ABC, while MPT Ch 67 and 62 were both solid. Fox 45 wasn't even solid during a light rain.

    What a difference! The TBA4 is working beautifully while in the garage I look forward to seeing what it does once I get it in the air!

    Phil Karras, KE3FL

    All my blog articles are listed at: Karras' Corner
    or
    Karras' Corner Article Links on my KE3FL web site.

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    Comments 11 Comments
    1. VOS's Avatar
      VOS -
      Your results with the use of a balun don't seem that surprising.
      What does seem a bit surprising is why one would build a 73/75 Ohm antenna?
      It's been a long long time, since I was in class, but the engineer instructor explained why 300 Ohm TV antennas were used.
      The impedance of air varies in the mid three hundred ohm impedance range, so 300 ohm antennas were a close match.

      It might be of interest to see the difference between your "dipole antenna with a design impedance of about 73 Ohms" and the same design with a 300 ohm impedance
    1. PhilK's Avatar
      PhilK -
      Quote Originally Posted by VOS View Post
      Your results with the use of a balun don't seem that surprising.
      What does seem a bit surprising is why one would build a 73/75 Ohm antenna?
      It's been a long long time, since I was in class, but the engineer instructor explained why 300 Ohm TV antennas were used.
      The impedance of air varies in the mid three hundred ohm impedance range, so 300 ohm antennas were a close match.

      It might be of interest to see the difference between your "dipole antenna with a design impedance of about 73 Ohms" and the same design with a 300 ohm impedance
      Hi Vos,

      Been there done that. Sorry but the impedance of the air is of no consequence but I did build a 300 ohm turnstile antenna and it has been tested in two locations with the results that it was extremely bad at receiving any UHF station in the Ch 25 - 30 range. I'm not sure why at this point since a single folded dipole turnstile back in 2003 worked just as well as the bowtie turnstile did.

      This could have been caused by my using a bit longer elements than I used for the botwie version but I still expected it to have similar characteristics with perhaps a broader bandwidth, which is typical of the full-wave loop antenna (or also called a folded dipole antenna) over a dipole, its showing was not at all impressive and so I've started dismantling it to use the pieces to make another bowtie turnstile antenna.

      I wonder where you got the impression that all TV antennas were designed for 300 Ohm impedance. As far as I can tell from looking at every TV antenna I've ever seen they are all based on a simple standard dipole, the log-periodic dipole array (LPDA) is, the bowtie is, the "rabbit ears" is.

      a log-periodic antenna (LP, also known as a log-periodic array or log periodic beam antenna/aerial) is a broadband, multi-element, directional, narrow-beam antenna that has impedance and radiation characteristics that are regularly repetitive as a logarithmic function of the excitation frequency. The individual components are often dipoles, as in a log-periodic dipole array (LPDA).
      - Log-periodic antenna - Wikipedia, the free encyclopedia

      I'd like to see what the LPDA impedance plot is vs frequency it obviously is not 73 Ohms everywhere, I may see if i can hunt down a plot, unless you have one?

      In any event, while all TV antennas in my experience have been based on a dipole they have all been "fed" using 300-Ohm twin lead wire. The two main reasons for this, that I know of, are, 1. Much lower loss than 75 Ohm coax, 2. much less expensive than 75 coax.

      I have built two different types of TV loop antennas, one was a full-wave loop for VHF/UHF TV that I wrote about and was published in Popular Communications, "The Stealth Loop Antenna (Throw a little more wire in the air)" February 2004, p 6-7. I used this antenna at a friend's house to allow her to receive TV stations in her townhouse where she was not allowed an outside antenna. This was a square antenna based on the full-wave length of Ch 2, which at 54 MHz is about 4.5 feet on a side. The other type was a single element folded dipole turnstile also published as, "Phil's Easy Way To Improve Your UHF TV Reception", Popular Communications, January 2003, p 6-10. Both did the job and both are 300 Ohm impedance. But I don't remember ever seeing a 300 Ohm impedance commercial TV antenna.

      PhilK
      see: http://www.popular-communications.com
    1. PhilK's Avatar
      PhilK -
      Now that I think about it I have seen log periodic TV antennas based on a folded dipole, at least for the UHF section, so that is a 300 Ohm antenna.

      Do you know of others?

      PhilK
    1. VOS's Avatar
      VOS -
      I haven't seen/used all the antennas out in the market, but everyone I have seen is either a 300 ohm with a twin lead connection, or has the balun as part of it for the coax connection.
    1. PhilK's Avatar
      PhilK -
      Quote Originally Posted by VOS View Post
      I haven't seen/used all the antennas out in the market, but everyone I have seen is either a 300 ohm with a twin lead connection, or has the balun as part of it for the coax connection.
      Yup, That's right, everyone you've seen has a twin lead connection or a balun but that doesn't mean that the antenna is a 300 Ohm antenna it just means they connected a balun or 300 Ohm twin lead to it or put two screws for twin lead feed-line.

      They did this for every antenna and yet almost all of them are based on a dipole and that's 73 Ohms not 300.

      I think at first they used 300 Ohm feed line because coax was not available & twin lead is much less expensive.

      Later they did it because they had always done it that way so why change the production line?

      Now you will find most antennas are coming out with female F-connector ports because VHF is gone, twin lead is no longer being manufactured and all TVs have the 75-Ohm female F-connector.

      Now, this doesn't mean that all antennas are 75 Ohm impedance, it just means this is how things are being done for good or bad, it just is.

      On the positive side we won't have to use a balun any longer for 73 Ohm antennas.

      On the bad side we lose much more signal using even the best of coax when compared to twin lead.

      You cannot assume that TV antenna manufacturers are doing anything that actually follows best antenna design practices, you can only assume that they are doing whatever is least expensive for them, the same reason that twin lead was used on dipole TV antennas like rabbit ears & bowties as well as log periodic antennas.

      PhilK
    1. VOS's Avatar
      VOS -
      I'll only "assume" the engineer teaching the class knew what he was talking about, and that 300 ohms was used because it closely [or sort of] matched the impedance of air.

      Maybe the engineers at Winegard can better explain this.
    1. PhilK's Avatar
      PhilK -
      Quote Originally Posted by VOS View Post
      I'll only "assume" the engineer teaching the class knew what he was talking about, and that 300 ohms was used because it closely [or sort of] matched the impedance of air.

      Maybe the engineers at Winegard can better explain this.
      No, probably not. 300 Ohms was used because the feed-line of the day was 300 Ohms, the radios of the day also could handle almost any antenna impedance so as long as the antenna matched the feed-line there was no impedance bump & reflections, for the frequency of interest. The "impedance" of the air had nothing to do with it.
    1. VOS's Avatar
      VOS -
      It's been thirty years, so I can't go back and ask him.
      Doing a bit of googling returns the impedance of free space [a vacuum] being 377Ω, while the radiating load of an antenna being 73Ω.
      I don't know if this has to do with directionality or what.
      The engineer wasn't an idiot, so "probably not" isn't enough to sway this for me, but getting some feedback from the engineers at Winegard should, but then this also isn't important to me enough to contact them.
    1. PhilK's Avatar
      PhilK -
      Here's another tidbit I've just thought of, I don't know why it took me so long.

      When we design a phased array of antennas the over-all impedance goes down, so for instance if I phase two 73 Ohm antennas the new impedance is now ~ 36.5 Ohms. Well, if we look at the quad phased bowtie array we have four 73 Ohm antennas which should be ~ 18 Ohms, so if anything the balun 4:1 impedance matching transformer should be reversed & the 1 side go to the antenna and the 4 side to the TV to bring 16 Ohms back up to ~ 73 Ohms to match the TV!
      Attachment 692

      Perhaps that's why my experience has shown that it's better to NOT use the 4:1 matching transformer. Using it means we're attaching a 4:1 transformer to a 16 Ohm antenna thus bringing the 16 Ohms down further to about 4 Ohms which then goes to the TV antenna input!

      I've got to try that! I'll have to see if I can "see" which side is which on some of these cores I've taken out & put one back in backwards so that the 1 side goes to the antenna & the 4 side to the TV & test for any improvement.
    1. VOS's Avatar
      VOS -
      Quote Originally Posted by PhilK View Post
      Here's another tidbit I've just thought of, I don't know why it took me so long.

      When we design a phased array of antennas the over-all impedance goes down, so for instance if I phase two 73 Ohm antennas the new impedance is now ~ 36.5 Ohms. Well, if we look at the quad phased bowtie array we have four 73 Ohm antennas which should be ~ 18 Ohms, so if anything the balun 4:1 impedance matching transformer should be reversed & the 1 side go to the antenna and the 4 side to the TV to bring 16 Ohms back up to ~ 73 Ohms to match the TV!
      Attachment 692

      Perhaps that's why my experience has shown that it's better to NOT use the 4:1 matching transformer. Using it means we're attaching a 4:1 transformer to a 16 Ohm antenna thus bringing the 16 Ohms down further to about 4 Ohms which then goes to the TV antenna input!

      I've got to try that! I'll have to see if I can "see" which side is which on some of these cores I've taken out & put one back in backwards so that the 1 side goes to the antenna & the 4 side to the TV & test for any improvement.
      I asked this question in another of your threads, but you seem to ignore questions you either don't have an answer for or just don't like.

      What's the difference between this 4 bowtie and 4 dipoles like this:


      To take this even farther:



      If there was any merit to your "tidbit", the impedance would be in the single digits, and for that matter your bowtie would have the same problem, wouldn't it?
      It has two elements on each side, so "if" each were 73Ω, then at the common point this would be 36.5Ω and if combined into a four bowtie array would be 9.1Ω
    1. PhilK's Avatar
      PhilK -
      Oh, I seem to have missed quite a bit haven't I. Sorry about that Vos. I guess I should set it up so that anything I write & get comments on I get a notice for. I'll have to work on that.

      First, there is a difference between a phased array and a Yagi-Uda type antenna, commonly referred to as a Yagi or Yagi antenna. A phased array of antennas is an array of antennas, meaning each antenna will work as a single antenna if disconnected from the array and connected to the receiver.

      A Yagi antenna, as you've shown in the here, usually has one driven element, a simple dipole (though I have seen actual bow tie elements as the driven element, reflectors, and directors.) At least that's my experience in amateur radio though there may be others with more than one driven element. However, not every element is driven unless it is a log-periodic antenna, at least as far as I am aware.

      Now, again, I'm no antenna expert but what I've read is that the impedance of the Yagi antenna is made to match what we want with some type of matching network, different type of driven element, different length, something, so I have no idea what the basic impedance would be for a Yagi of different numbers of reflectors (the slightly longer elements behind the driven element) and the directors (the slightly shorter elements in front of the driven element).

      Both the pictures above look similar to a Yagi design but I'm not sure. My understanding is that the driven element is length X, then all directors are a little smaller and placed a given fraction of a wavelength in front of the driven element and each additional element is the same distance then away from the element behind it. The distance I believe is always the same but as each director gets further away it becomes even smaller than the one behind.

      This all works in a reversed manner for the reflectors. As they get further away from the driven element they get a little longer. They too are spaced a given fraction of a wavelength away from the element in front of it.

      NOTE: A Yagi antenna, in amateur radio at least, is designed for a rather narrow portion of the RF frequency range when compared to a TV antenna so I imagine when used as a TV antenna the design changes a bit? I'm not sure, but perhaps the directors and reflectors having shorter and longer lengths is what widens the frequency response of the antenna.

      The next important antenna is the Log-periodic antenna. Here again, from my reading, the impedance of this antenna can be adjusted between 300 and 500 Ohms (or something like that but I'm pretty sure of the 300 Ohm lower limit, I'm not as sure that 500 Ohms is the upper limit) but in any event this antenna, while it looks similar to a Yagi design, is rather different. In this design every element is a driven element but they are all driven opposite of each other and they work as if they were a collection of single three-element Yagi antennas. So, every three elements acts as a Yagi with one reflector, a driven element, and one director. The reflector and director elements are reverse connected to the feed line when compared to the center/driven element.
      Simply look up "Log-periodic antenna" on the web or "Yagi Antenna" here's a good one on the Yagi-Uda antenna: http://www.yagiantenna.net/

      On Yagi antennas with more than 4 elements, the impedance at the feed point becomes low (like 20 Ohms).
      For the log-periodic antenna Wiki has a good article at: https://en.wikipedia.org/wiki/Log-periodic_antenna

      So, while I did NOT , and still don't, know for sure about the impedance of a quad element bow tie antenna, I suspected that it would have reduced impedance from reading these kinds of articles. This may not be the case, I admit that, this was a gedanken (German for "thought") experiment and can certainly be wrong but I thought it was a good place to start.

      I have run some experiments and I will be publishing the article about using or not using a 300 to 75 Ohm transformer with the single element bow tie as well as the quad element bow tie. I will admit right here that my thoughts on the matter were not in 100% agreement with the experimental results. That's what makes this fun in my opinion.