Re-boxing the Balun Designs 1113s

I repaired and reinstalled the common mode choke (balun) on the TH6. As readers may recall, the PVC enclosure for the Balun Designs 1113s shattered in several places. It was a valuable lesson on the limits of PVC and, of course, became blog fodder. In this article I'll describe how I went about the repair. The same balun on the TH7 was in good condition when I sold the pair this summer.

My first decision was how to mount the balun. The PVC enclosure is not mandatory and, indeed, many hams leave their ferrite toroid baluns unboxed so that they are well ventilated. No balun is perfectly efficient and there is heat dissipation, though small for well designed devices. Without an enclosure the balun is exposed to the elements, and that entails other risks.

My first thought was to mount the balun directly to resin backing plate. Mounted below the yagi boom it is shielded from most precipitation and is fully ventilated. 

I reconsidered when I inspected the balun and found that it does not match the device depicted on the manufacturer's web site. A picture of the currently marketed balun is on the right.

The ferrite is wound with small diameter coax. This is teflon dielectric coax that can handle far higher power than you might guess from its size. It's expensive but you need very little of it to wind baluns. Unlike RG213 and similarly sized coax, many turns can fit on a 2.4" toroid and the turns can be tightly wound (small minimum bend radius). This keeps the size of the balun small and able to fit in a standard 4" × 4" × 2" PVC electrical box. The downside is that the loss is high compared to larger coax, but that is not typically a problem at HF if the choking resistance is high (thousands of Ω).

You can see that the 1113s I have is not wound with coax. At some point the manufacturer switched from a wire-wound transmission line balun to coax without changing the product number. In my opinion the change in design is more than enough to require a product number change. Both designs can be perfectly fine but they are not the same. For example, their behaviour when subjected to high SWR or a highly unbalanced antenna (e.g. end fed wire). But let's move on.

As can be seen, instead of leaving the balun unboxed I opted for a replacement PVC electrical box. I had a spare on the shelf so it was a convenient choice that also eliminated my concerns about the weather implications of leaving it exposed. The non-coax design weighed on my decision because water, snow and ice on the bare wires is a concern.

To my surprise, the new box and the old were identical, right down to the manufacturer (Carlon) details embossed on the insides. I measured the positions of the several holes and drilled them in the same places on the new box. Not even a wire had to be bent for a perfect fit. The black cable ties with screw flanges provide support for the ferrite toroid to keep it suspended within the box.

The new box did not easily mount on the resin backing plate. The 4 mounting screws had to be forced through because they were ⅛" farther apart than the enclosure's screw tabs can accommodate. To compare the boxes I positioned the broken off tabs and old enclosure on the plate to check alignment. They were also not aligned to the backing plate holes. 

That made me wonder whether the lateral tension of the forced mounting screws played a role in the breakage. There's no good way to test that possibility. I reamed the holes on the backing plate so that the mounting screws dropped in without resistance.

Before taking it up the tower I did a quick bench test. I swept the SWR from 1 to 29 MHz with two 30 Ω resistors in series across the balun binding posts. The impedance is almost exactly 60 + j0 Ω at the lowest frequency (SWR 1.2) and it degrades, as expected, with increasing frequency. The inductance of the balun output pig tails and resistor leads is to blame.

The test was successful and also demonstrates an important lesson. Where does the balun end and the antenna begin? It's worth a few moments of thought.

The answer should be obvious: the antenna begins where the wires diverge on exiting the toroid (top centre). That must be the case since there is reduced field cancellation when the wires are not parallel, and approaches zero cancellation when they diverge as in a dipole. That is, the wires no longer form a transmission line.

The leads to the studs and from the studs to the physical antenna element are part of the radiating structure. Antenna manufacturers like Hy-Gain (I'm using this balun on a TH6) specify an exact length for the leads from the driven element clamps to the balun terminals for this very reason -- in this case the length is 6". Depending on the internal wiring of the balun the length of the driven element may need to be adjusted. Usually the effect is minor when the leads are short. For this balun that is 2" per side since the box width is 4". That's a small enough effect that I can ignore.

To reduce the risk of PVC breakage, this time I mounted the balun above the boom. It's more exposed but if the mounting tabs break there is less risk of catastrophic failure. I anticipate no weather-related trouble since the seal on the box is quite good. Sealing the coax connector is easier when it is on top so that should also do well despite being more exposed to the weather.

An accurate SWR measurement is difficult because my body was close to the driven element. On 10 meters the SWR was high with my body less than 1 meter from the driven element. The distance was limited by the short coax jumper to the analyzer and how far away I could stand without adjusting my harness. I figured it was good enough under the circumstances, and indeed the SWR measured in the shack was as it should be. Mission accomplished.

It'll be interesting to see how well the new PVC enclosure withstands the weather. It's an exact replacement for the original and we saw how that did after years of exposure in my station and, before that, in the station of its original owner.

I'll make one last observation before I close. Notice how the frequency range of the balun is specified by the manufacturer. The power rating is for 160 through 10 meters while its effective range is less. This is not surprising but can mislead if not read carefully. Also, the usable frequency in the text is 40 to 10 meters, not 40 to 6 meters as shown in the table. Is it carelessness or something more?

I have another of these baluns on the 80 meters inverted vee. I did not read the fine print before installing it or I would have reconsidered. However, although it may not be very effective on 80 that does not mean it is ineffective. For a non-directive antenna far from the house, even a common mode impedance of less than 1000 Ω can be sufficient. I haven't experienced any interaction problems when operating SO2R.

Common mode baluns have a frequency range determined by the ferrite mix, the transmission line and how it's wound. Unfortunately, measuring common mode rejection (impedance) is very difficult and experts often fail to agree on how to measure it and how to interpret the measurements. Be very careful whether you build or buy a common mode choke. The ferrite mix used by Balun Designs is supposedly custom which makes it difficult to judge without an independent measurement.

If you'd like to build your own common mode choke there are many resources. Perhaps one of the best places to learn is provided by K9YC.