So what do we need, and how do we put it together?
The parts required are:
- Approx 13 feet of wire
- A couple of insulators
- A short length of nylon or other non conducting cord
- A jubilee clip
- A ground (tent) peg
The first thing we need to do is calculate the length of wire required for 17m. You can also do this mod for 12m with recalculation for the wire length. For the 17m band the center frequency is 18.118Mhz. If we divide 234 by the center frequency we get a result of 12.915, which is the length in feet for the wire add-on.
For practical purposes, cut the wire to 13 feet. It most probably won’t be necessary to trim it to 12.9 feet in order to obtain a good SWR. When I made the add-on, 13 feet worked perfectly. I used Flexweave, but any stranded wire may be used, just remember to use a heavy enough gauge for the power level you are using.
The next calculation is for the height of the spider. If you have a tilt over plate then simply lay the antenna into the horizontal position and measure base to spider.
If you don’t have the tilt plate like myself, the easiest method is to remove the antenna from the bottom support and measure from the spider to the end of the tube. You then need to add another 12 inches to this measurement to give you the height of the spider.
Subtract 12′ 4″ (148 inches) from the height you’ve just calculated. This will give you the rough distance below the spider where the top end of the wire will need to be. Using this dimension, secure a suitable length of nylon or other non-conducting (and weatherproof) cord form one of the spider tubes about 6 or 7 inches from the vertical element. Attach one end of your piece of wire to the other end of the cord with an insulator.
Reassemble (or tilt back up) the antenna with your wire dangling down. Drive a tent peg or similar into the ground approximately 6 inches away from the antenna, leaving around 4 or 5 inches sticking up. This peg will be used to secure the bottom end of the wire. The arrangement can be seen in the picture.
Attach an insulator to the peg, or whatever you’ve used, and thread the wire through it. Gently pull the wire to keep it taut enough so that it doesn’t move too much in the wind. Secure the wire with a cable tie to prevent it slipping. If all the calculations have gone to plan, you should end up with about 7 or 8 inches of free wire. If not, you’ll need to make adjustments wherever you feel it is easiest until you obtain the 7 or 8 inches.
Completely undo the jubilee clip and place it around the main antenna support tube, do it up, but leave it very loose. Prepare the end of the wire and insert it between the jubilee clip and the main support. Holding the jubilee clip about an inch above the bottom of the tube, tighten fully thereby clamping the wire in place. Make sure there are no stray strands shorting to the mounting plate or ground connections anywhere.
That’s it! Check the SWR, make relevant adjustments if required.
In my case the SWR on 20m went up by a very small amount, but nothing worth worrying about. The SWR for 17m is 1.2:1 across the whole band (and beyond, top and bottom)