October 22, 2012
New 10 Meter Dipole
About a month ago I started working on an End-fed half wave dipole for 10 meters. The results were quite unsatisfactory (for reasons yet to be explored) so I decided to go a slightly different route. Some research and a look at the book More Wire Antenna Classics
I started out by measuring out 251 inches of wire for one leg of the antenna. Then as I was measuring out the other leg my spool of wire came to its end. I was now left wondering if I was going to be able to build the antenna I wanted as I was looking to build something that would work mostly in the Novice subband of 10 meters as that tends to be where most of the activity is. So I created a quick model of a dipole with 248 inch legs and was left with a reasonable result. The SWR was a bit on the high side with an impedance of 293.8-j1000 ohms at 28.4 MHz.
I plugged this load value into my Smith Chart program (LinSmith) and added a series 450 ohm transmission line. Much to my surprise, 180 cm (roughly 70 inches) of line would bring the impedance down to 46.5+j1.2. I updated the antenna model with a 70 inch transmission line and while the low SWR point was much higher in the band than I hoped I was happy to try it to see what happened in real life. Thus I began assembling the antenna.
I put the antenna up in my front yard with the center in one tree, one end in another tree on the other side of my driveway and the opposite end tied to a flagpole. The height is around 12 feet and SWR around 28.4 MHz is surprisingly lower than predicted by the model (around 1.5:1).
Time will tell how well this antenna works as I was too preoccupied to attempt to work anyone during the nice opening on 10 meters today. However I did hear a W7 and several VEs working an EC8 this afternoon which was quite encouraging.
Update 24-Oct-2012: Today I worked Les, VP9/N1SV, in Hamilton, Bermuda and received a 59 report using this antenna and 100 watts on my IC-706.
October 20, 2012
6 Meter Rigid Dipole
Some years ago, I laid my hands on an old 49 MHz cordless telephone "extender" antenna. The kit acutally included two fiberglass whips 55 inches long along with some coax, mounting hardware, and some additional components necessary for retro-fitting the unit to a cordless telephone base. When I came across it I thought that it may be possible to use as a 6 meter antenna. Fast forward about 10 years, I still had the thing in storage and decided that it was finally time to give it a try.
The two identical antenna elements that came included with the range extender are intended to be used with the included mounting bracket in a vertical dipole configuration. They have the standard 3/8-24 threaded base. Also included was an antenna mounting stud with an SO-239 connector just like those used for mounting mobile antennas. I had another stud like it (minus the SO-239) and went to a local building supply store to find some additional hardware to put the antenna into service.
I used a pair of 3/8-24 hex bolts one inch long to mount the two threaded couplers (one from the antenna kit, and the other that I had in my junk box) to a 3/4 inch PVC conduit body. The conduit body, I thought, would be a perfect center insulator. It has holes in it to be attached to a piece of PVC conduit to get the thing up in the air for testing and is actually quite inexpensive. It also has plenty of room inside to add a toroid as a balun if it turns out to be necessary. Finally, it has a nice rubber gasket on the cover to help seal out the elements.
The feedline is connected to a chassis mount style SO-239 connector mounted to the back of the conduit body via an extra hole I drilled. Some 24 AWG insulated wire is then connected to the antenna mounting studs using some crimp-on ring terminals (which I soldered as well for good measure). Finally, I capped off one of the conduit holes as it will not be used. The "back" conduit hole will be used to attach a length of PVC conduit to use as a mast.
The antenna is now assembled and clamped to a chair in my garage. Of course the next step is to get the antenna outside and up in the air to perform some measurements. That will be a task for another day.
