I had an idea that seems to work.

I like inherently DC-shorted antennas because of an experience I had
about 30 years ago with thunderstorms. Using a home-brew quarter-wave
vertical on 2 meters, my receiver got completely blocked for about 10
seconds by a lightning stroke a few miles from my location. I was
terrified that I had lost the front end of the IC-2AT. Fortunately, I
did not, but that experience apparently imprinted on my so-called brain.
I know that I could add a shunt inductor across the terminals to prevent
this, but I prefer a natural or inherent solution.

The folded vertical unipole is inherently "grounded". (I use quotes here
because what I really mean is that the antenna input is DC-connected to
the common.) So, I like that antenna. However, its terminal impedance is
about 4 times that of a simple ground plane.

Well, I noticed that the terminal impedance of a simple quarter-wave
unipole was about 30 or so ohms until the radials are sloped downward at
about 45 degrees which raises the terminal impedance to about 50 ohms.

In my case, I needed to lower the terminal impedance (about 120 to 140
ohms) of the folded antenna. Well, what if I sloped the radials *upward*
approximately 45 degrees to lower the terminal impedance to about 50 ohms?

It worked, and EZNEC says that the radiation pattern is pretty much like
the usual ground plane vertical.

I just thought I would share this info with the group. I also have a
configuration for a full-wave loop (inherently at DC common) on 70 cm
that has an infinite balun thanks to Walter Maxwell's book Reflections
II, page 22-10. How I manage to get the terminal impedance to 50 ohms is
interesting, but I will save that for another post.

Cheers,
John KD5YI

On Feb 2, 5:48*pm, John - KD5YI wrote:
I had an idea that seems to work.

I like inherently DC-shorted antennas because of an experience I had
about 30 years ago with thunderstorms. Using a home-brew quarter-wave
vertical on 2 meters, my receiver got completely blocked for about 10
seconds by a lightning stroke a few miles from my location. I was
terrified that I had lost the front end of the IC-2AT. Fortunately, I
did not, but that experience apparently imprinted on my so-called brain.
I know that I could add a shunt inductor across the terminals to prevent
this, but I prefer a natural or inherent solution.

The folded vertical unipole is inherently "grounded". (I use quotes here
because what I really mean is that the antenna input is DC-connected to
the common.) So, I like that antenna. However, its terminal impedance is
about 4 times that of a simple ground plane.

Well, I noticed that the terminal impedance of a simple quarter-wave
unipole was about 30 or so ohms until the radials are sloped downward at
about 45 degrees which raises the terminal impedance to about 50 ohms.

In my case, I needed to lower the terminal impedance (about 120 to 140
ohms) of the folded antenna. Well, what if I sloped the radials *upward*
approximately 45 degrees to lower the terminal impedance to about 50 ohms?

It worked, and EZNEC says that the radiation pattern is pretty much like
the usual ground plane vertical.

I just thought I would share this info with the group. I also have a
configuration for a full-wave loop (inherently at DC common) on 70 cm
that has an infinite balun thanks to Walter Maxwell's book Reflections
II, page 22-10. How I manage to get the terminal impedance to 50 ohms is
interesting, but I will save that for another post.

Cheers,
John KD5YI

Interesting design, as it is shorter than a standard ground plane
vertical. Have you built and tested one?

Wayne W5GIE

On 2/6/2011 10:32 AM, Wayne wrote:
On Feb 2, 5:48 pm, John - wrote:
I had an idea that seems to work.

I like inherently DC-shorted antennas because of an experience I had
about 30 years ago with thunderstorms. Using a home-brew quarter-wave
vertical on 2 meters, my receiver got completely blocked for about 10
seconds by a lightning stroke a few miles from my location. I was
terrified that I had lost the front end of the IC-2AT. Fortunately, I
did not, but that experience apparently imprinted on my so-called brain.
I know that I could add a shunt inductor across the terminals to prevent
this, but I prefer a natural or inherent solution.

The folded vertical unipole is inherently "grounded". (I use quotes here
because what I really mean is that the antenna input is DC-connected to
the common.) So, I like that antenna. However, its terminal impedance is
about 4 times that of a simple ground plane.

Well, I noticed that the terminal impedance of a simple quarter-wave
unipole was about 30 or so ohms until the radials are sloped downward at
about 45 degrees which raises the terminal impedance to about 50 ohms.

In my case, I needed to lower the terminal impedance (about 120 to 140
ohms) of the folded antenna. Well, what if I sloped the radials *upward*
approximately 45 degrees to lower the terminal impedance to about 50 ohms?

It worked, and EZNEC says that the radiation pattern is pretty much like
the usual ground plane vertical.

I just thought I would share this info with the group. I also have a
configuration for a full-wave loop (inherently at DC common) on 70 cm
that has an infinite balun thanks to Walter Maxwell's book Reflections
II, page 22-10. How I manage to get the terminal impedance to 50 ohms is
interesting, but I will save that for another post.

Cheers,
John KD5YI

Interesting design, as it is shorter than a standard ground plane
vertical. Have you built and tested one?

Wayne W5GIE

Yes, I have. The one I constructed was for the 70 cm band. I have
misplaced the frequency/impedance data I took, but I remember that it
compared closely with EZNEC. I did not try to measure the pattern, but I
did not notice any dead spots when receiving.

The only thing I don't like is that it is more time-consuming to tune
because you can't just chop off the end like you can with the unfolded one.

John KD5YI