Perils of PVC

PVC products are plentiful and inexpensive in hardware stores. There is pipe, conduit, weather-tight electrical boxes and much more. It is very tempting to this ham. I have used it in endless places throughout my station. The following images taken from my blog will give you an idea just how pervasive it is.

You can see enclosures for baluns, matching networks and switching systems, coil forms, open-wire spacers, wire antenna spreaders, gamma match supports, supports for Beverages and other antennas, and there are more that I haven't included. Flexible PVC is abundant in our stations as wire insulation and coax jackets. I would be surprised if you could not find many examples in your own station.

Despite its utility, there are important considerations before choosing to use PVC. There are many types of PVC and you can't easily know the characteristics of the material you're looking at. There are questions of rigidity (flexibility), dielectric constant, insulation qualities, UV resistance, thermal stability and fatigue life.

Flexible PVC conduit pipe can support my Beverages only because of the wire tension. The pipes would bend if they had to support all the wire weight. The pipes hold the wires in their correct positions and harmlessly bob back and forth in the wind. When the pipe is under load, as it is, for example, at the top of the stinger for my 80 meter yagi driven element (which must not be conductive), a wood dowel is inserted to give it the strength to withstand vertical and horizontal forces.

Depending on additives, PVC can be a very poor dielectric at RF. I use it for coil forms, but only for 80 and 160 meter antennas. As the frequency rises, loss in the PVC coil form can be substantial. 

UV resistance can be difficult to predict for PVC material bought at your local hardware store. I have seen PVC that has weakened in the Sun and I have seen PVC that endured many years outdoors. For example, the PVC jacket of RG213 has additives that give it excellent UV protection. 

Rigid PVC electrical boxes may be less UV resistant but can last a long time. It depends on the manufacturer, of which there are many and I couldn't tell you which are the best. I have seen discolouration and breakage that are due to a combination of UV and thermal cycling in our extreme seasonal cycle. They are also not immune to animals since some like chewing on them, much to my dismay.

With so many variables and uncertainties, many hams eschew PVC entirely while others believe that it's a miracle plastic. The truth is more nuanced. You can avoid PVC, and the convenience and economy it brings, in favour of less available and more expensivematerials. It is difficult to provide good guidance, so I won't.

All of this brings me to the point of this article: PVC structural failure. While doing work at the top of the 150' tower supporting the 3-element 40 meter yagi, I passed the side mount TH6 about halfway up the tower. I wasn't paying it any attention other than to climb past its tower support struts on my way up and then down. It was only on about the third trip down that I noticed something was amiss.

For those of you who don't recognize it, that is a ferrite balun made by Balun Designs. I used these to replace the ineffective Hy-Gain BN86 balun on both my TH6 and TH7. The TH7 (and balun) have been sold so I have two of these products left in my station -- the other is on the 80 meter inverted vee.

I can't say for certain that the discolouration is due to UV damage, but it is suspicious. The other two baluns have a similar appearance -- check the pictures in the link above for the 80 meter antenna -- as does an outdoor electrical junction box. My assessment of what transpired is that the 4 tabs of the standard PVC electrical box snapped off, and later the enclosure shattered at the coax connector due to the cable tension. I have had the mounting tabs snap on other PVC boxes mounted outdoors, and those were of different manufacture. The PVC must have been weakened by UV or thermal stress to break so easily.

I am amused that the reason I hadn't noticed the problem until then is that the antenna continues to work perfectly well. The wire windings on the ferrite core that are connected to the UHF jack and wire studs are holding it together. A corner of the ferrite toroid is visible through the crack in the enclosure. 

I'll have to replace the box, at the very least. It won't be difficult to transfer the ferrite core and hardware to another PVC electrical box. The question is whether I should do that. Won't it just happen again in a few years, or sooner?

Many hams keep their ferrite toroid baluns open-air since they are not perfectly efficient. With a kilowatt there can be 10 watts or more of heat in a choke in which small diameter coax is wound around the toroid or wound with wire as a transmission line transformer. Weather-tight is also heat-tight. Heat build up is unlikely the cause of PVC failure in this case because the TH6 only sees intermittent use, even in contests -- it's my multiplier antenna, and is rarely used for more than a few QSOs in a row.

I may forgo the PVC entirely and mount the balun directly to the underside of the resin plate on which the PVC box was mounted. That provides weather protection and ventilation of the balun without the risk of using PVC. The back plates of all the Balun Design baluns appear to be in good condition. When I come up with a suitable solution I'll write it up for the blog.

Accessibility for repair

Go ahead and use PVC in your station, but do so with an understanding of its merits and demerits. PVC can be more fragile than you may believe. This is particularly true when the PVC is deployed outdoors where it is difficult to repair. 

An example is my use of PVC conduit pipe in gamma matches and coax chokes on the driven elements of long boom yagis. Repair would require lowering the antennas to the ground. I hope that won't be necessary but it is a possibility.

In contrast, the busted balun enclosure on the TH6 is close to the tower and therefore convenient to remove and repair. May all your PVC repairs be as easy as that.