"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

"Roy Lewallen" wrote in message
...
The short answer to your question is no, with practical impedances. It
turns out you'd need a very low impedance transmission line (on the
order of a few ohms, if I recall correctly) to track very well.

(snip excellent explanation)

Not only that, but it also appears that the effort results in an _increase_
in the real part as well. Just the opposite of what I wanted.

Thanks again for explaining.


John

The other John Smith wrote:
"They tell me that shortening the antenna (folded monopole) below
resonance will lower the resistance and introduce capacitance."

I believe that is only half right. A too-short (less than
1/4-wavelength) antenna worked against a ground plane will have a lower
resistance than a 1/4-wave antenna. But, unlike the open-circuit less
than 1/4-wave whip, which has a series capacitive reactance, folding the
too-short element not only transforms its resistance to a higher value,
it reverses the sign of the input reactance. The too-short folded
monopole has a series inductive reactance.

Just like the shunt-fed grounded tower antenna, the inductance can be
tuned out with only a simple series variable capacitor, not an inductor
as is required with a too-short whip. See Fig 19, page 6-10 of the ARRL
Antenna Book, 19th edition for an example of a shunt-fed tower capacitor
tuning arrangement. The shunt-feed arrangement makes a too-small loop
just as a short folded monopole does.

Q of the folded monopole is lower than a whip because the folded antenna
is fatter. This gives more bandwidth.

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote in message
...
The other John Smith wrote:
"They tell me that shortening the antenna (folded monopole) below
resonance will lower the resistance and introduce capacitance."

I believe that is only half right. A too-short (less than
1/4-wavelength) antenna worked against a ground plane will have a lower
resistance than a 1/4-wave antenna. But, unlike the open-circuit less
than 1/4-wave whip, which has a series capacitive reactance, folding the
too-short element not only transforms its resistance to a higher value,
it reverses the sign of the input reactance. The too-short folded
monopole has a series inductive reactance.

Just like the shunt-fed grounded tower antenna, the inductance can be
tuned out with only a simple series variable capacitor, not an inductor
as is required with a too-short whip. See Fig 19, page 6-10 of the ARRL
Antenna Book, 19th edition for an example of a shunt-fed tower capacitor
tuning arrangement. The shunt-feed arrangement makes a too-small loop
just as a short folded monopole does.

Q of the folded monopole is lower than a whip because the folded antenna
is fatter. This gives more bandwidth.

Best regards, Richard Harrison, KB5WZI



Yes, I now understand. I mistakenly said shortening the length of a
*non-folded* monopole lowers resistance and raises capacitance. I have done
some modeling in EZNEC and with a Smith chart as Roy Lewallen suggested and
learned a great deal. I would have done this earlier, but I didn't know I
could do it the way Roy said.

Thanks to all respondents, and to you two in particular.

John