John

Since I'm a poor reader, I'm not confidant I fully understand your
objective. But, a folded monopole (1/4 wave) with a very fat "fed" element
and a very thin "grounded" element, with very close spacing will have an
input impedance lower that 150 ohms at *resonance*. It occurrs to me that
you already know that.

Jerry.



"The other John Smith" wrote in message
nk.net...
Good evening, Gentlemen.

A thought experiment:

Start with a regular 1/4-wave monopole ground plane. The literature says
it
looks like half the value of a dipole, about 35 Ohms, when resonant. It
would be nice to have the resistance at the terminals be a bit higher, and
I
very much value a grounded element anyway, so let's let it evolve into a
folded monopole. The literature says it should now have about 4 times the
terminal resistance of the original 1/4-wave we started with (about 140
Ohms). Huh. Now it's a bit high.

They tell me that shortening the antenna below resonance will lower the
resistance and introduce capacitance. But I think I have also seen in the
literature that the antenna can be viewed as a transmission line. A
shorted
portion of parallel conductor transmission line (the folded monopole) less
than 1/4-wave long looks inductive. But wait! Which will win? Will the
shortness of the antenna look capacitive or will the transmission line
dominate and the antenna will look inductive?

Even better, is there some choice of the folded section wire diameters and
spacing that will give an inductance that will exactly offset the
capacitance due to shortness? So, then, is there a folded monopole of such
dimensions that the resistance is 50 Ohms (due to being shorter than 1/4
wave) with no terminal reactance (due to the inductive design of the
"transmission line" cancelled by the shortness of the antenna's
capacitance)?

Brain hurts.

John, KD5YI

"Jerry Martes" wrote in message
...

John

Since I'm a poor reader, I'm not confidant I fully understand your
objective. But, a folded monopole (1/4 wave) with a very fat "fed"
element
and a very thin "grounded" element, with very close spacing will have an
input impedance lower that 150 ohms at *resonance*. It occurrs to me
that
you already know that.

Jerry.


You're very perceptive, Jerry. How did you come to think I would know that?

You are correct, of course. The strange thing is that the graphs depicting
that sort of behavior is not in the 18th edition of the ARRL Antenna
Handbook (unless I somehow missed them) but they are in the 1977 edition of
the Radio Amateur's Handbook. I had hoped to use a single wire formed into a
hairpin for the sake of simplicity, but I am tempted to use the different
sized conductors nevertheless. I have even chosen a ratio of 4:1 for the
diameters.

Hmmmm. I wonder how it would work with, say, 1/2 by 1/16 inch metal bar and
wire. Or maybe orient two bars of the same size 90 degrees (looking down the
long dimension) so that the wide side of the "fat" part sees the narrow edge
of the "thin" part. Like this:


.-.
| |
| | .-------.
| | | |
| | '-------'
| |
'-'

but connected across the top. Actually it could be made from one long bar
with a twist at the top where the hairpin occurs. I don't know how to model
that, so I'll have to build and try it.

The foremost objective is to learn. But, at the end of this particular quest
I hope to wind up with a suitably rugged antenna mounted to the top of a
remote data gathering device transmitting in the 70 cm band. The sensors
will measure temperature, rainfall, soil conductivity, and anything else I
decide might be fun to know about. In fact, I plan to put another one down
by the county road at the site where it floods. That remote sensor will tell
me when the road goes under water so I can take an alternate route rather
than having to back the car 1/4 mile to a turn-around spot.

So much to do, so little time.

Thanks for your reply.

John

John

I do have a book with a graph of Zin as a function of diameters and
spacing of the folded dipole. But, I dont have any equations. I suspect
someone on this news group has knowledge of where to get some math tools for
predicting the antenna impedance. I'd be glad to scan the page 16-6 of the
1990 ARRL Handbook for you if it would help.

Jerry



"The other John Smith" wrote in message
ink.net...

"Jerry Martes" wrote in message
...

John

Since I'm a poor reader, I'm not confidant I fully understand your
objective. But, a folded monopole (1/4 wave) with a very fat "fed"
element
and a very thin "grounded" element, with very close spacing will have an
input impedance lower that 150 ohms at *resonance*. It occurrs to me
that
you already know that.

Jerry.


You're very perceptive, Jerry. How did you come to think I would know
that?

You are correct, of course. The strange thing is that the graphs depicting
that sort of behavior is not in the 18th edition of the ARRL Antenna
Handbook (unless I somehow missed them) but they are in the 1977 edition
of
the Radio Amateur's Handbook. I had hoped to use a single wire formed into
a
hairpin for the sake of simplicity, but I am tempted to use the different
sized conductors nevertheless. I have even chosen a ratio of 4:1 for the
diameters.

Hmmmm. I wonder how it would work with, say, 1/2 by 1/16 inch metal bar
and
wire. Or maybe orient two bars of the same size 90 degrees (looking down
the
long dimension) so that the wide side of the "fat" part sees the narrow
edge
of the "thin" part. Like this:


.-.
| |
| | .-------.
| | | |
| | '-------'
| |
'-'

but connected across the top. Actually it could be made from one long bar
with a twist at the top where the hairpin occurs. I don't know how to
model
that, so I'll have to build and try it.

The foremost objective is to learn. But, at the end of this particular
quest
I hope to wind up with a suitably rugged antenna mounted to the top of a
remote data gathering device transmitting in the 70 cm band. The sensors
will measure temperature, rainfall, soil conductivity, and anything else I
decide might be fun to know about. In fact, I plan to put another one down
by the county road at the site where it floods. That remote sensor will
tell
me when the road goes under water so I can take an alternate route rather
than having to back the car 1/4 mile to a turn-around spot.

So much to do, so little time.

Thanks for your reply.

John

"Jerry Martes" wrote in message
...
John

I do have a book with a graph of Zin as a function of diameters and
spacing of the folded dipole. But, I dont have any equations. I suspect
someone on this news group has knowledge of where to get some math tools
for
predicting the antenna impedance. I'd be glad to scan the page 16-6 of
the
1990 ARRL Handbook for you if it would help.

Jerry

I accept your generous offer and extend my thanks. If anybody else is
interested, you could put it on a.b.s.e for all of us. If you find no
interest from anybody else, you can email it to me at jocjo_john at yahoo
dot com.

If I learn anything out of all this, I'll let you know.

Thanks again.

John