The elements of the antennas were made from coat hanger wire with the varnish sanded off one end of each element to ensure good electrical contact with the connectors mounted on the SO-239.
The element lengths for the antennas as I built them were as follows:
2 meters (VHF) 19 5/8"
70 centimeters (UHF) 6 1/4"
My suggestion, in part because my locale may be unique, is to cut the elements somewhat long and trim for best SWR match for your particular location.
N1GY- The simple Approach to Ham Radio
and My Model Railroad Hobby
Finally, an improvement on the SO-239 1/4 wave Ground Plane Antenna!The completed SO-239 Hub. As you can see in this photo, the fittings from the tie strip have been both bolted and soldered onto the flange of the SO-239.
This project came about after I saw an item in the November 2011 issue of QST. A ham from Spain, Pedro Motilla, EA5BFT, wrote an item in the “Hints and Kinks” column about a new way to assemble that old standby, the ¼ wave groundplane antenna. I take no credit for this design. Pedro has made a real improvement on the old design and he deserves all the credit. His design changes make this an easy antenna for the new ham to build and also make it easily stowable in a "GO KIT" for ARES or other event.
Now, I have built this design of antenna several times before, but they always had the same problem. When they are completed, there is no way to store them efficiently without bending the radials or taking up a lot of mostly empty space. Pedro’s solution is elegant and simple. He suggests using the fittings and screws from a European style tie strip to bolt the radials and radiating element on, rather than soldering them. That way the elements can be unfastened and stored along with the SO-239 hub in a very small space.
Since I use dual-band radios in most of my ham radio adventures, I decided to build two antennas this way, one for 2 meters and one for 70 centimeters. The only snag in the assembing of the hubs was that the fitting on the center conductor of the SO-239 did not want to snug up as I wanted it to. I solved the problem by compressing the assembly between the jaws of a "Panavice" while I tightened the bolt. This technique worked very well. You might have to file the center conductor of the SO-239 a bit as well, particularly if it has been soldered before.
To hold them up I fashioned a “Tee” out of ¾” PVC pipe and tee fittings as shown in the diagram and the photos on this page. Both antennas were trimmed to present a good SWR and a diplexer that I had on hand was added to allow just one coax run down the mast to the radio. Actually, the unit I had on hand is a "triplexer" with a third port for an HF antenna. I checked with the manufacturer and was assured the third port would not cause any problems. I capped the HF port with a plastic cap so it would not accidentally get hooked up and their advice has proven correct. I realize that there are plenty of dual-band antenna designs available, but since I had all the parts on hand this arrangement would cost me virtually nothing.
After I had assembled the entire project, I painted the flange of the 440 antenna red and added a strip of red tape to the coax jumpers that connected to that antenna. This was done to hopefully avoid assembling the 2 meter antenna to the 70 centimeter side of the diplexer and vice-versa.
The project has lived up to all of my expectations. While not a gain antenna design, placing the assembly on top of a 22 foot mast gives me all the range I need for my assignments for ARES or public service type events. When the event is over, all the parts store in or on the PVC frame and make a small package that stores easily. I will add a photo of the assembly in use just as soon as the weather improves. My thanks to Pedro, EA5BFT for an excellent update to an old design.