SKCC 5123T----- FISTS 14979----- Flying Pigs 2331----- NAQCC 3610-----QRP ARCI 14176-----Polar Bear 257

Sunday, May 29, 2011

Shortened Dipole with Matching Section

I tried a shortened dipole (inverted V) by using the IMAXMIN.EXE program on the W5DXP no tuner page. The minimum dipole length the program will accept is 107.19 feet for 3.5 MHz. So I went with 108 feet. The Imax or good length of open  wire feeder is 29.7 feet for velocity factor of .9. My chosen feed-line is 300 ohm window line from DX engineering. Note I am using a bead type choke balun at the end of the 300 ohm feed-line and converting to coax.

I tried this out with a portable mast consisting of 4 foot military mast sections. The bottom is a tripod using a tripod adapter I found on EBay. There is a six foot section of PVC conduit at the top. This gives the antenna a support at the center of 23 feet. The ends are going to stakes or a low fence. The ends are only about 3 feet off the ground. The SWR was very good at 3.5 MHz. A pleasant surprise is that 7 MHz the SWR is under 3 to 1. A full size 80 meter dipole will have a very high impedance on 40 meters. But this dipole is usable on 40 meters with a the internal tuner of my IC-703. I had done a similar check out for a 7 Mhz dipole at 54 feet of length and it tuned under 3 to 1 on 20 meters with a 14 foot 8 inch feed-line.

Now back to the 108 foot dipole. I got it to work on 20 meters by reducing the feed-line to about 24 feet. To try out different feed-line lengths I have a lot of 300 ohm sections of different lengths. I am using the European terminal strips ( the larger size) to join the sections and this works well. So you might say the the 108 foot dipole is like the G5RV and I would say you are right. I don't have one but it likely works the same. The 54 foot dipole is close to a G5RV Junior.

Now I am looking to use either the 108 or the 54 foot version at Field Day based on space available. I am not convinced the guys on the mast are needed with the tripod. They may backfire if a car catches one and turns the whole rig over. I have some homemade A-frame masts from a prior Field Day that I could use to get the ends of the dipoles up at 13 feet.

So credit goes to Cecil W5DXP for this no tuner concept and for me it explains what is going on for the G5RV type antennas.