What is a Balun and why do I need one?
Within this section we will look at the various methods and requirements of a balun in the hope that you will be better placed to make the right choice for your antenna. There are many different types of balun and some incorporate impedance transformation too and it is this areas which leads to some confusion.
For 50Ohm direct feed antennas, just a 1:1 balanced to unbalance transformer is needed. However, other antennas need transforming from 12.5 to 50Ohm up to (and beyond) 200Ohm to 50Ohm. These types need the impedance to be transformed in addition to the feedpoint balancing effect. So why do we need any form of balancing at the feedpoint in any case?
Coax Cable
Coax is a wonderful thing. It's invention has allowed the feeding of antennas through a manner of different substances, conditions and places the a balanced feed line could not. However, there are issues that need to be over come when feeding an antenna with coax.
It has become commonplace that the Radio Ham or even commercial entities use coax to feed antennas due to the convenience of so doing. So much so, commercial modern day transceivers have an unbalanced output designed to feed and receive directly from coax. Coax can be feed through walls, under-ground, up the side of our towers or metal poles without any drastic affect on the Antenna or it's tuning - or does it?
The best way to feed most antennas is with a balanced feeder. The reason being, most antennas (including Yagis which are of interest to us here) are balanced antennas. If we feed a balanced antenna with an unbalanced feed line, issues occur at the feed point; namely common mode currents which appear as a result of this balanced/unbalanced miss-match. These currents travel back down the coax and radiate and thus, the coax becomes apart of our antenna system and radiates.
In instances where we are feeding a multi-band antenna, it may not matter too much to the operator as long as he is radiating a signal from his antenna (despite that there is more likelihood that he will cause interference and have RF in the radio shack due to the coax being apart of the radiating antenna). However, when we are feeding a Yagi, we need to ensure only the antenna itself radiates. Having the coax feeding our Yagi radiating will act to distort the radiation pattern of our antenna. No, this problem will not cause a high SWR. In fact, it may even reduce the SWR seen in the shack giving the ham a false sense of security that things are OK when they are not.
Balanced line feeders
The most common balanced line feeder in Ham Radio today is the 300 Ohm ribbon used within the very common G5RV antenna. Any twin line can be used as a balanced line feeder; speaker wire, bell wire, even mains flex. They will all have different characteristics but will all provide a balanced feed to an antenna if used that way. The twin feeder will not radiate (or very little anyway) as a result of the phase of the RF in each feed line which is 180 degrees out of phase in each leg. This means, one side of the feed line cancels the other out so no radiation occurs.
Disadvantages of a balanced line
Firstly, we will need a balun to use a balanced line feeder in any case as our radios today are not presenting a balanced output. The next point is the effect that any close by objects have on the feed line, walls, buildings in general, towers, all metal objects, ground, everything! We need the feed line to be in as much open space as possible in order to ensure the balance feed line can perform as it should. Beginning to understand why a balanced feed is not in mainstream use for the Ham?
I will not go any further into balanced line theory as we are not going to use one. We are going to feed our Yagi with coax and minimize the losses and likelihood of common mode currents on our feed line at the same time.
The Dipole Centre
The dipole is an important part of the Yagi, anything that goes on (or wrong) here is reflected throughout the rest of the Yagi. Below are 2 images of a Dipole removed from a Yagi. The cross section at the top representes the dipole itself while the down-wire is the coax. The pink lines indicate current distribution within these 'elements'. Let's first take a look at fig 1.
Fig1. A dipole with a balanced feed
Within Fig1 we can see a nice, clean and balanced distribution of current through the dipole itself with no radiation within the coax cable. This represents a scenario where a perfect balun is placed at the feedpoint. The downward line (from the dipole) marked '4' represents the coax cable feeding the dipole.
Fig2. A different story if the feedpoint is not balanced
Fig2 shows the distortion within the dipole if the coax is allowed to radiate. This may not impact vertical or mono-pole antennas so much but for a directional antenna such as a Yagi, this is disasterous! Ensure you have a good balun at the feedpoint of your Yagi to ensure your antenna pattern stays clean and symmetrical.
The balun - What does it do?
A balun is a matching device which takes an unbalanced input from your coax line and provides a balanced out for an antenna or balanced feedline. Have you wondered where the name balun comes from? Balanced to Unbalance, that's it.
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The 1:1 balun
Fully Symmetrical Coaxial Balun
We need a 1:1 balun at the feed point of our Yagi. This means the balun will connect to a 50 Ohm unbalanced line and present a balanced 50 Ohm output. There are a number of ways to do this in my opinion. The first is a coaxial 1:1 balun as described by I0QM at this link: http://www.iw5edi.com/ham-radio/files/I0QM_BALUN.PDF and for which there is a photo below.
This uses 2 pieces of additional coax cable at the feed point of the antenna. A full explanation is given with the document created by I0QM. This is the best method of producing a fully symmetrical balance at the feed point (using coax cable) without the losses seen in a torroid wound equivalent. However, it does have a number of draw backs. The first is it is relatively narrow in bandwidth. This is course one of the major benefits of G0KSC Yagis (being wide-band) so do we really want to inhibit performance? The next is the additional connections and coax we are introducing which means additional losses in our feed system in addition to another point where weather (water mainly) could gain access to our antenna and feed line and as a result, de-tune our antenna or make it completely defective.
Finally, in addition to being narrow bandwidth, this type of balun can only be used on one band rendering this balun useless for our multi-band Yagis.
A Fully Symmetrical Coaxial Balun
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Choke balun
My preferred balun for wide and multi-band use is not really a balun as such, it is a choke. Basically, this is a coil in the coax feeding the antenna as close to the feed point as possible. This coil acts as an RF choke and prevents the common mode currents returning back down the feed line. However, Current may not be symmetrically distributed through the radiating element using this method. While it does work, it is not an 'ideal' method and certainly if a mono-band Yagi/antenna is going to be used, then a fully symmetrical balun should be first choice.
The above said, this method of feed point choke has a number of benefits listed below:
- Non frequency specific - the choke balun is not limited to a particular frequency or bandwidth so it can be used effectively with both OWA mono-band and multi-band Yagis.
- The choke balun is easy to make and implement - with the previous version, coax velocity factor needs to be known in order to calculate the lengths of coax and these need to be exact. If not, the balun could do more harm than good. The coax choke balun is not affected in the same way
- No additional connections required - for me this is one of the most important benefits, especially at VHF. I use one single piece of coax from the back of the rig, through the coaxial balun and right up to the feed point. This minimizes losses and connections and therefore, any potential issues that can happen at a later stage in the life of the antenna and it's feed line
- The best part of this choke balun is it is extremely easy to make! Lets look now at how this is done
See bottom of this page for Choke build instructions
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Pawsey Stub
19/11/2010 Many Hams have seen inconsistencies with the Pawsey stub, largely believed to be due to the varied velocity factor seen in most coax cables. Steve, G3TXQ contacted me with a very interesting viewpoint and provided some seemingly valid information explaining why only coax of the same type should be used for the Pawsey stub, not a separate, dissimilar wire. I therefore would no longer recommend this style of Pawsey stub but will keep the information here. My antenna balun with a feedpoint choke may give the best, all round performance: http://www.hamradio.me/antennas/coax-velocity-factor-part-2.html#more-1528
This is a very interesting method of balancing the feeder at the feed point of the antenna and I have modified this balun from it's original make-up. All we need to do here is connect the coax to the antenna in the traditional way. However, in addition to the coax, we add a piece of wire to the point of the antenna that the centre core of the coax connects to, run the wire back along the coax and connect this wire to the braid or outer core of the coax at 1/4 wave back along the COAX. As this is a 1/4 wave length of coax, velocity factor needs to be taken into account too. This provides a fully symmetrical balun at the feedpoint of the antenna. In early versions I coiled the Pawsey stub. However, I now recommend keeping this in a straight line and connecting to the boom where the wire connects to the outer braid of the coax. This provides two additional features. The first, it provides a DC ground to the loop or driven element, further reducing potential noise pick up. Second, As it is in a straight line and running along the boom towards the mast, it is taking place that additional feedline would have to and therefore, reducing additional losses.
The G0KSC Pawsey as built by EI2GLB. Feeding the coax directly through the boom helps reduce potential pattern distortion
PA0WRS installed this impedance transforming version of the G0KSC Pawsey stub on his 50Mhz 5el
The 'Pawsey wire' can be clearly seen in the above PA0WRS example. This ensures a fully symmetrical balanced match at the dipole centre. Coax bending around the boom near the feedpoint can distort the radiation pattern. Arranging the coaxial transformer/balun in the above way moves the point at which your feedline has to go around the boom much further along the boom and away from the feedpoint. Any potential issue is drastically reduced the further you move away from the feedpoint so arranging the transformer/balun long the centre of the boom for it's whole length is ideal. Do not be tempted to coil the balun, this servers no purpose other than to reduce the power handling capability!
As discussed above, the point at which the wire connects to the outer braid of the coax is 1/4 wave length along the COAX rather than a 1/4 wave of wire. Therefore, when calculating our length we need to take into account the velocity factor of the coax we are using. The calculation is as follows:
300/Frequency x coax velocity factor x.25 The above is a Pawsey stub for my 70MHz LFA Yagi. The coax I used was Westflex 103 which has a velocity factor of .85 (RG213 is .66)so the calculation was as follows:
300/70.2 x.85 x.25 so my wire length had to be long enough to connect 940 mm back long the coax from the point where the coax was split in two. NOT the end of the centre core of the coax.
The Pawsey stub is an excellent method of producing a true balanced input at the antenna at minimal cost. Again, like the coaxial balun above, this is only good for mono-band antennas due to the relatively small bandwidth.
Please follow this link here for more information on coax cable velocity factors. Don't be put off of an OWL Yagi, if you chose a 12.5 Ohm version, you can swap the split dipole for a folded dipole and the antenna becones a 50 Ohm antenna and required just a 1:1 balun!
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Creating a Choke Balun
Note: although called a choke balun, this DOES NOT match the unbalanced feedline to the balanced antenna. It simply stops (or helps reduce) common mode currents flowing on the coax as a result of the miss-match at the feedpoint. This will also result in an imbalance in the radiation of the pattern too AND reduce performance. Ultimately when feeding a Beam of any kind, I real 1:1 balun should be used such as the coaxial and Pawsey mentioned above.
I always make mine when the antenna it is intended to be installed upon is complete and ready to be installed on the mast. The reason for this is I am able to accurately install the balun in place by measuring the coax out along the boom and in most case, keep with one length of coax with no joins from the antenna to the radio.
First, we should prepare the coax for connection to the antenna feed point. Assuming we are going to install one single piece of coax from the antenna, through the balun and to the transceiver, cut the coax at one end and prepare it in order that you can see around 5 to 10mm of inner core with the same amount of braid. Keep in mind that the antenna dipole starts at the point that the inner and outer core of the coax split and therefore, this 5-10mm of each should finish where the coax is whole again (see picture).
Slide one ring end over each piece of the coax showing and test the connection on the antenna. the last thing you want to do is solder the rings to the coax and they are not long enough! Once you know where they need to be, solder the joints and ensure there is sufficient heat to allow the solder to flow deep in the joint. This will ensure a good connection and limit the chance of any water ingress.
Next we need to ensure our joint is sealed well reducing the chance of water being able to seep into the coax. One way is to install an insulator box at the feed point. However, from my experiments, it is much more difficult to keep the elements aligned this way. Generally, the dipole ends up a little higher than the rest of the Yagi elements which does have an impact on the pattern. Furthermore, if water were to gain access to this isolation box it could sit inside the box and cause the same issues as it would if it gained access to the coax.
I purchase good quality self amalgamating and simply wrap the feed point of the coax directly. Make sure to use plenty and ensure that you have covered every possible access hole to water. Once this is complete, you are ready to connect the rings to the antenna feed point.
Having connected the coax, decide where you want to place the balun (example in the picture at the top of this page) and mark this point on the coax with a ring of low-tack tape, perhaps insulation tape. Now take off the coax once again for a moment.
Winding the balun
Now we need a few extra tools in order to create our balun. Do you have a spray-can somewhere in the house? This could be furniture polish or a can of WD40. Any standard size household spray can will do. next, we need 4 to 6 strong cable ties that are long enough to go around 4 to 5 lengths of the coax we are using. Place each one of these cable ties face up on the side of the spray-can and place the low-tack tape over one side of the cable tie to hold it in place. now wrap the tape a 1/4 of the way around the can and place another cable tie. Do this until we have 4 to 6 of these cable ties in a ring around the can. Then place another line of tape around the over end of the can to fully secure the cable ties onto the can.
Now locate the mark you made earlier on your feed coax. Hold this point on one end of the can so as it can not move, making sure that the tape holding the cable ties on the can is outside of the point where you are holding the coax.
Now roll the can until you have 4 to 5 turns (50/70Mhz 2 is OK for 144MHz and 1 tight turn on 430Mhz. This should be tested with clip on ferrite chokes after the balun. If the SWR changes, you have too many or too few turns. Change and try again) of coax on the can. At this point, we need to remove the tape holding the cable ties in place on the can. the rolled coax will not be holding them to the can so they should not move. Loop back the cable ties and slot the ends together and tighten as appropriate.
You are now done with you balun! You may now remove the can and install the coax/balun onto your boom, you are ready to go!
