10 meter dipole
I was in need of an antenna for my 10 meter beacon, so I decided to build a design I remembered seeing in an issue of QST several years ago. A friend of mine built one at the time and it worked great. It is a simple rigid dipole made from two lengths of 1/2" electrical conduit that are separated by a 5/8" wooden dowel that is inserted into the ends of the conduit and held together with hose clamps. It is necessary to cut two 2" or so long slits in the end of the conduit that the dowel rod is inserted into. This allows the hose clamps to compress the conduit onto the dowel rod to hold it all together. The dowel rod I used is four feet long, so I had about 2 feet in each leg of the dipole for rigid support.
The dipole can be fed directly with coax, or a balun can be used. It can also be fed with twinlead or ladder line. It is important to seal the insulating dowel with epoxy to keep it from rotting. I used the hose clamps to attach the coax to the antenna.
Caps should be placed on each end of the antenna to keep out moisture. I found that the threaded 1/2" PVC caps work well on 1/2" conduit. They can be just placed on the ends but I fixed them on with epoxy. Make sure that this is done after shortening the antenna for best VSWR match.
The length for each leg of the dipole is found using the formula L=234/F, where L=length in inches and F=frequency antenna is to be used on. If the antenna is too short, it can be lengthened by increasing the space between them on the insulator and lengthening each side of the coax or ladder line feed on the dowel.
This antenna must be mounted so that it is insulated from ground. Mine is mounted on a 4X4 and attached to a 2X4 that is buried two feet into the ground. I used U bolts to secure the antenna to the 2X4. This antenna could also be mounted horizontally off the side of a tower.
This antenna could also be built for the 12 meter band, and possibly even for the 15 meter band. However, most conduit you find in hardware stores is sold in 10-foot lengths, and a 15 meter antenna will need to be around 11 feet long. Some method lf lengthening the antenna will be needed. It could be done by attaching another foot of conduit to each end by using connector pieces for conduit.
It must be noted that copper tubing can also be used in place of conduit and may actually work better due to the improved conductivity of copper over the conduit. Copper tubing is often used in J-pole construction and there's no reason it couldn't be used here. It is, though, softer than conduit so it may not be as tough, but it should be able to last many years.
If copper pipe is used, it is advisable to paint it.
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