The other day, a friend of mine emailed me:
I am putting up a multiband, right-angle dipole. That is to say that one element is vertical, the other horizontal. The vertical element is a 17-ft. telescoping whip, and the horizontal element is about 10% longer than the vertical element.
I have inductors wound for 40 meters and will try them later today. Once the antenna is working on both 20 meters and 40 meters, I will tap the inductors and get the antenna to work on 30 meters. I need inductors in both legs, obviously.
When I connect my antenna analyzer to the antenna without the inductors, I find that I have an SWR of 1.5:1 on 20 meters. Because the feedpoint impedance of a dipole is 75 Ω, my 1.5 SWR indicates resonance to me.
QUESTION: Does my SWR of 1.5 really indicate that the antenna impedance is 75 ohms?
My initial reaction was that he was right, but then I got thinking about it. When you make an inverted vee out of a dipole antenna, the impedance drops, so it really should be closer to 50 ohms than 75 ohms. He noted that he couldn’t get the SWR any lower than 1.5:1, so I guess something else is going on there. Perhaps it’s too close to the condo. If any of you have had experience with this kind of antenna, please weigh in below.
Here are some resources that he used for concocting this antenna:
- Bent Dipoles by KK4OBI. As noted in the article, a simulation of a bent dipole yields an SWR of about 1.2:1.
- Electrically Shortened Center-Fed Half-Wave Dipole by K7MEM
- Toroid Coil Winding Calculator
- A Lightweight Portable Vertical by WB3GCK
These are all great resources. For example, I wish I had known about K7MEM’s article when I was pondering how to get on 80m from my small(ish) city lot.
We discussed this project over lunch today (via Zoom, of course), and he said,
I tested with my KX2 and got matches of better that 2:1 on 20m and 40m.At that point, it occurred to me that I had a remote antenna tuner in the garage. I replace the matching network I built with the tuner and got better than 1.5:1 on all bands 80m-10m.
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