Creating A Dual Band 144 - 440 Vertical
In the middle of trying out different antennas I found out the antenna switch I had was faulty...soo much for my excelent mono banders. Soo still in the mood to make antennas I went out and searched for a few dual banders. Still looking for a apartment friendly and minimal space design I tried the following.
It is essentially a 2m dipole. Using RG58 coax, you strip 457mm off leaving only the dieletric and center conductor. At the point where the center conductor starts come back 447 mm and perform a choke balun (9 turns of coax). The shield in the RG58 acts as the other 1/2 of the dipole. The choke balun prevents the shield from acting as a radiator, effectively cutting off the rf.
You can use any easy to mold metal (I used tin foil), cut a piece 235mm wide and long enough to go around your pvc pipe that you are going to put the antenna in. Place the foil approximately 345mm from the top of the antenna. Slide the foil up and down to adjust for best SWR on both bands....although I could not get a perfect 1:1 I did get a 1.4 SWR on 144 and a 1.3 on 440. This is indoors and a multitude of structures near by could be affecting the feedback.
I found putting washers down the pipe kept the coax centered. I used my handy and very newbie 3d modeling skills to make a couple of washers. You can download the 3d model stl file here