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LFA - baluns, is it self balancing?

18 January, 2018 - Reading time: 7 minutes

Further to my previous post about the LFA, I have now completed construction on my "hybrid" version. The next step is to prepare a balun for the feed. But what balun (if any) is needed? In my previous yagi, the only "decoupling" I ever had was three clip on ferrite clamps of unknown impedance at the feedpoint. To make matters worse, I don't think my previous weatherproofing was that good. Nevertheless, I've learnt a bit in that time, and intend to make improvements on this antenna.

In researching baluns, I discovered there have been many different types of variations used on the LFA in particular. So many in fact, I don't think most designers/builders of these antennas ever research how effective their balun really is. Is it doing the proper job? I found too this article to be of interest. The author states that the full wave loop on these antennas have a natural balance, effectively eliminating a majority of current flow on nearby things such as the coax, or boom. To verify this in MMANA, I decided to add an extra wire, drooping down, but connected to the feedpoint, a 1/4 length long. Below is the YU7EF EF0211 (my old antenna).

Notice the current distribution. Vertical current components are shown in Red and horizontal in Blue. There is a significant amount of current flowing on the vertical wire down from our feedpoint. This would effectively be common mode current flowing on our coax (or mast), disrupting the pattern, and causing SWR issues. This wire (coax, mast) then becomes part of the antenna. Also notice how badly the current distribution is in the dipole. Below is the radiation pattern.

 As can be seen, the antenna looks nothing like it used to. Gain is down, F/B is shot to pieces. Now lets compare with the hybrid LFA.

As is clearly seen, there is hardly any current flowing on the vertical wire now. The current distribution in the dipole is also more symmetrical.  

There is a small amount of pattern distortion, but it is leaps and bounds better than the split dipole. In MMANA you can zoom in on the current. It takes a lot of zoom to see the smallest current flowing on the vertical wire.

Here is a comparison of current flowing on the boom.

As can be seen, significant current flows on the boom with the split dipole. 

Whereas the split dipole has a lot of current flow on the boom, the full wave loop has very little. This seems to correlate with the linked article earlier. I'd call this a more symmetrical dipole feed. Interestingly here, Justin states that "the loop is not self-balancing, not in all real-world conditions in any case". Interesting.

So does that mean that a balun is not required with these antennas? This perhaps explains why so many hams who build these antennas see easy to obtain results and performance by just directly feeding with coax. However, for purists out there, a balun is not necessarily a bad thing. 

The most common form of balun (and what is preferred on G0KSC's site) is a choke balun. This comes in many forms, the most common being the "Ugly balun". This consists of a few turns of the coax just after the feedpoint. The idea being that the inductance formed in the coil creates a RF choke. The problem with this type of balun is there are not many published test results of exactly "how many turns" one needs. If you get it right, an extraordinarily high impedance of typically unknown value can be obtained at the centre frequency of the antenna. In this case, it works really well, if you're lucky.

Another common method is sliding ferrites cores over the coax. The impedance at the centre frequency can be known using the ferrite manufacturer's datasheets. Sliding multiple ferrites increases the sum of the impedance, however a large number of ferrites is usually required.

Two other interesting "baluns" I found builders were using were the G0KSC hybrid, consisting of a bent piece of tubing, or flat bar connected to the centre conductor of the feedpoint and back 1/4 wavelength then attached to the boom. A small variation on this was also a piece of 1/4 wavelength stub piece coax (calculated for velocity factor) attached to the feedpoint, then the far end shorted to the boom. Both of these "baluns" are not really baluns at all. All these create is a "DC grounded" antenna, i.e. the centre conductor is connected to the shield. In my opinion all this does is limit the bandwidth of the antenna (could be a good thing?) and creates the DC path. Alternatively, a much easier method, at least with the LFA, is to directly connect the centre of the non feed end of the driven element directly to the boom. This effectively does the same thing as the two "baluns" mentioned above, albeit without the possibility for measurement errors.

So what will I do on my antenna? I think I will slide several ferrite cores of known impedance over the feedline to stop any small amount of common mode current that shows in the models. I'll also attach the centre of the far end of the driven element to the boom for a DC ground. 

The amount of current radiated down the coax and on the boom of the LFA antenna doesn't concern me enough to warranty building a "proper" balun, such as the I0QM balun. This would only increase coax losses, and allow more places for water to get in. In the next post, I'll detail my results.