One of the biggest problems when dealing with foam filled heliax, is how to make a reliable termination, without having to apply a soldering iron to the copper shield, or inner conductor. Foam and heat is definitely not compatible! The cable manufacturers can supply specific couplers to suit the various cables, but these are expensive and alternative methods are often sought. The most reliable, and certainly the most inexpensive method that I have devised for LDF4-50 is described below.

Requirements of cable terminations are that they should be inexpensive, water resistant, maintain the line impedance and, above all, be mechanically sound. This device meets all these criteria and I have yet to experience a failure.

You will require an PL-258 female coupler, (sometimes referred to as a VHF female coupler or barrel nipple), and two nuts from PL259 plugs. Most of us will have plenty spare nuts from PL259 plugs kicking about the shack, so the total outlay will probably be no more than the cost of the female coupler. We start by soldering the two nuts back to back. This is easily done if you first ensure that the back of the nuts are flat and the exposed surface is tinned. The way I join the nuts is to place one tinned surface on top of the other and apply heat from a small blow torch. It would be possible to use a large soldering iron, but the blow torch provides a more reliable method.

The next step is to prepare the heliax. Starting from the end, I count back ten convolutions and then score around the outer cover with a Stanley knife. Do this at the trough of the convolution as this makes it easier to get the nut started when it comes to screwing the nut on. Take off the outer cover by running the knife down the cover and peeling it off. The next step is to cut through the copper shield at the fifth convolution from the outer cover. I use a junior hack saw for this. Just be careful not to cut too deep. Finish off cutting through the foam to the inner conductor with the knife. Repeat this step again at the second convolution from the outer cover, then remove both the loose pieces. Discard the longer piece, but retain the section with the three convolutions as this will be required to maintain the impedance.

Reduce the diameter of the inner conductor such that it is a good fit into one end of the the female coupler. You can do this with emery cloth or with a flat file if you feel competent to do it that way. We are now ready for the final assembly. Cut the inner conductor such that it is 30 mm long (1 1/8 inch) Screw the joined nuts onto the cable outer sheath until the copper shield butts against the shoulder inside the nut. You will have to use pliers or some form of wrench to get the required leverage , as it is important that the nut shoulder butts firmly against the copper shield. Slip the impedance retaining section inside the outer nut and follow this with the PL-258 section. Tighten this firmly, compressing the complete assembly.

That's it. You will now have a mechanically sound connector that will have no dramatic effect on the impedance of the line and it will be water resistant. If you wish to use the connector outside, slip a piece of 3/4 inch heat shrink over the completed assembly. If you are careful when applying heat, you should be able to shrink onto the completed assembly, giving a totally waterproof connector from LDF-4 to UR67 and RG213, or indeed to join sections of LDF-4 together..