• Matching Transformers, Baluns, and Chokes



    This article originally appeared in 2013 but has been extensively updated.

    I will try to give some basic information about what each of these things is, when to use them, and when not to use them.

    Balun is a contraction of two words, Balanced and Unbalanced, and it refers to the type of antenna to the type of feed-line. An example of a balanced antenna is the full-wave loop, the dipole, and the bowtie antenna. An example of an unbalanced antenna is the long wire antenna, or even a non-center fed dipole antenna. An example of a balanced feed-line is the old TV feed-line standard called twin-lead, usually made of a brown plastic insulator surrounding two parallel wires. An example of the unbalanced feed-line is the now very common TV 75 Ohm coax. There are advantages and disadvantages to both types of feed line, but since it is rather difficult to find twin lead except for amateur radio use, and since most TV stations have moved up into either the upper VHF or the UHF frequencies, we'll stick to using the unbalanced 75 Ohm coax for this discussion.

    A quick note here, the coax we use can be a balanced feed line and the twin lead can be an unbalanced feed line. This can be rather confusing and a good article to read is by Walt Fair, Jr., W5ALT, located at: Antenna Notes

    The use of a balun is to remove the current from the outside of the coax and re-balance the feed line system. This also removes the outside of the coax from being part of the antenna itself so we're only looking at what the antenna brings in as a signal not the antenna and the feed line. There are times when I forget about this when I test without a balun in place but we do need to be aware of the fact that if the system is unbalanced in any way then the feed line will act as part of the antenna.

    This may or may not be a bad thing. For instance, if the system performs better without a balun in place are we really going to complain and put a balun on because otherwise we're using the feed line as part of the antenna? I haven't done so and I don't recommend doing so. (The Balun section was updated on 05/05/2016 -PhilK)

    Chokes are used for choking off RF from being on the outside of the coax shield and thus having the antenna system use this part of the feed-line as part of the antenna. The disadvantage of this is that the antenna pattern may get completely messed up. The advantage of this is that the antenna pattern may just happen to change in a way you like. I will say that any change to the antenna patter is generally a disadvantage. It is usually better to deal with something you know you have rather than trying to deal with a completely unknown situation, which is what you'll have if you end up using the feed-line as part of the antenna system.

    NOTE: RF works differently than DC current in that RF can and does travel both on the inside and the outside of a coax shield and when it does, it is completely independent of the other such that a piece of coax to RF will look like three different wires. In general, as I've said, we do not want the RF on the outside of the shield since this RF will be from the antenna and will change the receiving pattern of the antenna in ways we will be completely unaware of. A very simple choke is simply looping your coax around a small can (as the form) about ten times. Remove the can or use a form such as PVC which will generally not change the RF characteristics of the formed choke.

    The most commonly used item on our TV antennas is the matching transformer. As stated above, it is used to match the antenna to the feed-line and receiver system. Today, all TVs have a 75 Ohm input for the antenna. Also, we can go to almost any store that sells TVs or electronics and find 75 Ohm TV coax. The coax we can find will be anything from a rather poor grade up to a rather good grade, perhaps even to the best RG-6 possible.

    A basic rule here is that if you have a long run to consider, then spend as much as you can afford to pay in order to get the best quality. If, on the other hand, you're only going from the TV to a wall-mounted antenna 10 feet or less away, almost anything will work fine except a real POJ (Piece Of Junk).

    When should these items be used?

    Matching Transformer (MT): Should only be used when the TV antenna is not designed to be 75 Ohm impedance. This means almost always, however, and here's the real kicker: you must know what you are using, both in terms of the antenna impedance and the MT ratio. A case in point: If we're using a log periodic antenna designed with a 300 Ohm impedance, then a 4:1 MT needs to be used to get the best transmission of the signal from the antenna onto the 75 Ohm coax. If, on the other hand, we actually found some 300 Ohm twin-lead, then no MT is needed at the antenna feed point, but it still will be needed at the TV input port.

    If we're using a dipole antenna (bowtie antenna) which is a 75 Ohm antenna, no MT is needed to match the antenna to the feed-line and, if used, it will only make matters worse. Note, since TV frequencies range from 170 to 700 MHz, any single element antenna is going to have impedances vary all over the place as the channels/frequencies change, so while, in general, it will be better not to use an MT for the antenna, there just may be stations for which the MT will help reception. I can only speak of my experience in the Washington D.C. Baltimore area where we have channels 7 ~ 174 MHz, 9, 11, and 13 still at the original VHF frequencies all the way up to UHF Channel 50 frequencies ~ 692 MHz, with a nice representation with stations in between these extremes. So far, I have noticed better reception not using the MT than when using it but, again, your experience may differ.

    The way I figure things is that it costs nothing to test without the MT, so why not test and see if there is a difference for the better?

    When should a Balun be used? The simple answer for TV antennas is never. As far as I know, there are no balanced TV antenna designs. I once designed a full-wave loop TV antenna for a friend since she had antenna restrictions and I was able to staple it to the ceiling. It did rather well for both Baltimore and Washington stations until she changed to cable. This was the only balanced antenna I ever designed for TV use.

    Last, when should we use the choke? This answer is also never. This is because the only time that RF gets on the outside of the coax is when transmitting a signal up to the antenna, and then some of the RF reflects back onto the coax due to an impedance mismatch. Since a TV never transmits, there should never be RF on the outside of the coax and thus we should never need a choke.

    Using a choke will not change the situation if there is no RF on the outside of the coax. The only time it will return the antenna system to its designed state is if there is RF on the outside of the coax shield which would not be the designed situation. If you want to use a choke on your coax, do so. It can't hurt anything. It should be placed near where the coax meets the antenna. Again, this is assuming you are transmitting to the antenna, there is a reflection, and the choke then stops any reflection from getting to the outside of the shield and starting down back to the transmitter. Since we're not transmitting, we should never have RF on the shield anyway, as far as I can see.

    The reverse condition would be that there is a reflection at the point of entry to the TV due to an impedance mismatch. But, here we've already changed any impedance mismatch at the antenna to the coax using an MT so there should be no reflections at the coax to TV connection since we have a matched condition. Again, it can't hurt, so try it if you like.

    I plan to run a few tests of my own to see what happens with and without a choke. When I have some reliable results, I'll post them here. If anyone else does this or knows better than I, please post here as well so we can all benefit.

    Last, matching transformers and baluns are made in the same way, winding wire around a ferrite core, and so MTs are also baluns. This means that our simple TV matching transformer is also a balun. Not only that, but because of this design, a balun also stops current from flowing on the outside of the coax, so it is also a choke. The simple little TV MT is a three-in-one. So, if youíre using an MT you should not need anything else; definitely no choke should be needed.

    If, like me, you find that you donít need an MT then a clip-on choke may be needed. Itís always a good idea to have a few in the TV kit.

    Almost all TV antennas we build or buy are balanced antennas. The most basic balanced antenna is the dipole. On the other hand, coax is unbalanced feed-line. The old standard feed-line (called twin-lead) was balanced feed-line and so matched the balanced antenna just fine and no balun was ever needed. All TVs of old had built-in MTs since most antennas sold were of the log periodic type and the impedance of these antennas was around 300 ohms. This is why 300 ohm twin-lead feed-line was also used. Today, since we use coax, a balun is always needed to convert from the balanced antenna to the unbalanced feed-line and this also keeps RF off the outside of the coax shield so that the coax does not change the antenna pattern by ending up being part of the antenna. What may not be needed is an MT.

    If no balun is used, then a choke will be needed to insure that no RF is on the outside of the coax shield, but again it is very easy to test this out with the simple clip-on chokes available.

    Until next time, Good Watching!
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    Comments 5 Comments
    1. PhilK's Avatar
      PhilK -
      Recently, 08/10/2013 I ran some very quick tests between a number of antennas and using a matching transformer and not using it to see just what the differences would be.

      The bottom line turned out to be that for the one antenna I tested it on the antenna was better without it than with it. This was a 16" element bow tie antenna (not the standard 8" element UHF bow tie) and for this antenna there were three stations weaker by less than 2 dB, two stations equal, two stations better by less than 1 dB, four stations better by between 3 and 6 dB, and four stations better by between 8 and 14 dB, three of these were better by 10dB or more.

      I'll have to test out both the 8, and 12 inch bow tie antennas as well as the quad and a two element bow tie antenna with the Digair analyzer with and without the normal 300 to 75 Ohm matching transformer, but I don't believe things will be much different for any of them, but I won't really know until I test them.

      Till then, good watching!
    1. VOS's Avatar
      VOS -
      This seems to suggest there is more going on.
      The impedance mismatch should only account for Ī2 dB, so anything above/below this are signs of "something else".
    1. PhilK's Avatar
      PhilK -
      Quote Originally Posted by VOS View Post
      This seems to suggest there is more going on.
      The impedance mismatch should only account for Ī2 dB, so anything above/below this are signs of "something else".
      VOS,

      Thanks for that! Can I ask how you come by those numbers? I'd always assumed that the different impedance mismatches would cause different losses.

      I have assumed that the variations I see are because no antenna can be at its designed impedance over such a vast range of frequencies and as such, there will be times when the 4:1 balun will actually help. I'm pretty sure that I'll never get around to trying to calculate all the bow tie antenna impedances for the various TV stations and various size bow tie antennas I'm testing so, I'll probably never know for sure.

      The theory and the curves I've seen indicate that the impedance does change as the frequency changes but it never goes way out like a standard dipole would for a frequency far away from the design frequency of the antenna. The VSWRs were mostly fluctuating from below 6 to above 2 but the two extremes were ~ 7.8 and ~ 1.5. See: Bow Tie Antennas

      Do you know of any other references?
    1. PhilK's Avatar
      PhilK -
      VOS,
      I think you were correct, and that something else may have been causing a problem with the repeatability of this first quick experiment. It may have been way too quick. I'll be publishing an updated version with data for 8, 12, and 16" versions of the bow tie antenna using a more repeatable method of testing.
    1. PhilK's Avatar
      PhilK -
      Please note that I've updated the Balun section today, 05/05/2016.