alhearn wrote:
"That is, when it`s too short for resonance, reactance is capacitive,
and is inductive if too long."

Just look at Terman`s phase diagram for a shorted transmission line and
I think you will agree with Capt. Lee.

Capt. Raul H. Lee, USNR, K6TS on page 31 of "The Amateur Radio Vertical
Antenna Handbook" wrote:
"The folded unipole feed principle may be easily applied to the short,
top loaded vertical radiator. The transformer action of the folded
unipole is used to give a more favorable input resistance than can be
obtained with a series feed. Another thing that the folded unipole feed
does is to reverse the sign of the input reactance. The input reactance
of a series fed tower shorter than 1/4 wavelength is always capacitive.
This means a series loading coil (spoken of in high power as a helix)
must be used to resonate the tower. With folded unipole input the feed
point reactance is always positive. (Consider the tower and feed wire to
be a shorted transmission line less than 1/4 wave long. Its input
reactance is positive.) Thus, the folded unipole may be fed with a
low-loss capacitive feed network.

Best regards, Richard Harrison, KB5WZI

"Roy Lewallen" wrote in message
...
Those impedances seem pretty low for a folded monopole, unless the
conductor diameter is large.

When modeling two parallel wires like a folded monopole or dipole with
any NEC-2 based program, it's essential that the segment junctions be
aligned. For the folded dipole or monopole, simply make the wires the
same lengths and give them the same number of segments.

Roy Lewallen, W7EL


You are correct again. I had different numbers of segments of the parallel
wires. When the segments are about the same number, the terminal impedance
goes up. I am also trying to run as many segments as practical so the
accuracy is greatest.

John

alhearn wriote:
"That is, when it`s too short for resonance, reactance is capacitive and
is inductive if too long."

True that a too-short open-circuit vertical radiator is impeded by a
capacitive reactance and that a slightly too-long open-circuit radiator
is impeded by inductive reactance. We are aiming for a 1/4-wave antenna.
The switch from leading to lagging power factor or vice versa when
passing through the resonance point is abrupt. See Terman`s diagram. The
folded monopole is a short-circuit 1/4-wave transmission line stub and
it behaves like one.

My 1998 ARRL Handbook displays a group of too-short antennas in Fig
20.44 on page 20.22.
Item (E) of a group of 6 would be 1/4-wave antennas is called a tri-wire
unipole. It`s a vertical tower with a top support to suspend a parallel
wire on either side of the tower. The tower itself is #1 wire of the
tri-wire assembly and it is grounded, not insulated at the earth. Wire
#2 connects the top of the tower to the earth through a variable
capacitor. wire #3 drives the top of the tower and it is insulated from
the earth, and is driven against the earth.

The variable capacitor is used to tune out the too-short folded
monopole`s excess inductance and present 50 ohms to the feedline. The
text says:

"This technique will not be suitable for matching to 50 ohm line unless
the tower is less than an electrical quarter wavelength high."

Why? Over 1/4-wavelength high, the folded unipole is capacitive and
adding more capacitive reactance will detune it even more. The unipole
is synonymous with monopole.

It`s a fact that the sign of the reactance in the folded antenna is
inverted in the too-short folded antenna from that in the too-short
open-circuit antenna. That is a powerful advantage in that low-loss
capacitance can be used to match the too-short folded antenna and your
practical choice may have to be a lossy loading coil to match the
too-short open-circuit antenna.

What`s right is right and isn`t decided by a vote, but on the
probabilities from all the examples I`ve presented they are likely right
by the numbers.

I have a comment on the results of WA4GKQ`s modeling results of the
folded monopole. Models can be wrong for many reasons including garbage
in, garbage out. I`ve measured many folded monopoles in service, at
resonance, accepting measured full power and reflecting negligible
measured power and I`m sure these antennas are working well as evidenced
by expected performance every day over decades at many places here and
abroad. Seeing is believing for me.

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote in message
...
Al, WA4GKO wrote:
"---when it`s too short for resonance, reactance is capacitive, and is
inductive if too long."

Thats exactly correct for an open circuit dipole or monopole, but folded
elements are backwards because they operate like loops and shorted
transmission lines.

Kraus says:
"Consider a two-wire folded dipole shown in Fig. 14-27a. The terminal
resistance is approximately 300 ohms. By modifying the dipole to the
general form shown in Fig. 14-27b, a wide range of terminal resistances
can be obtained, depending on the value of D. This arrangement is called
a T-match antenna."


On the page before that, page 417, Kraus also gives the impedance of the
folded dipole. He says it is 4 times the impedance of a two-wire dipole.
This means that, if the 2 wire dipole is capacitive slightly below
resonance, so is the terminal impedance of the folded dipole. Multiplying a
complex number by a real number does not change the sign of the imaginary
part.

John