KB6NU's Ham Radio Blog

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What is the feedpoint impedance of a bent dipole?

Posted: 20 Aug 2020 12:18 PM PDT
http://feedproxy.google.com/~r/kb6nu...m_medium=email






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.Â*

I have inductors wound for 40 meters and will try them later today. Once
the antenna is working on both 20 metersÂ* 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 1.5Â* 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 couldnt get the SWR any lower than 1.5:1, so I guess something else is
going on there. Perhaps its too close to the condo.Â* 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 PortableÂ*Vertical by WB3GCK


These are all great resources. For example, I wish I had known about K7MEMs
article when I was pondering how to get on 80m from my small(ish) city lot.Â*

We discussed this project over lunch today (via Zoom, of course), and he
said,

I tested with my KX2 and gotÂ* matches of better that 2:1Â* on 20m and 40m.
Â*
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.


Â*
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