Modeling a 17 foot folded dipole made from copper #18 wire spaced at 2
inches at 14.2 MHz with EZNEC shows a feedpoint impedance of 46.1 +
j1893 ohms. This can be resonated, as Richard Harrison recently pointed
out, with a series capacitor. There's no free lunch, though -- at 1 kW,
the voltage across the capacitor is almost 9000 V RMS (about 12,000
volts peak), and even at 10 watts, it's almost 900 volts RMS. Besides
concerns about arcing, you'd have to make sure the insulation across the
capacitor is very good, since even a very small leakage current will
cause significant loss. And you end up with a fairly narrow-banded
antenna, with the 2:1 SWR bandwidth of about 130 kHz. The loss due to
finite wire conductivity is 1.9 dB, which might or might not be
acceptable, depending on the particular use. Increasing the wire size
will reduce the loss, but also the bandwidth -- introducing loss nearly
always improves bandwidth, so reducing it narrows the bandwidth. Without
wire loss, and assuming the resulting 29 ohm feedpoint impedance is
transformed to 50 ohms, the 2:1 SWR bandwidth becomes 80 kHz. Like a
great number of variations, this antenna would surely be useful to some
people in some situations, and might well be better than some other
alternatives. But here's an antenna rule you can take to the bank:
Small--broad band--efficient, choose any two. Any time either a modeling
program or an antenna inventor or seller tell you any different, you
should be very, very skeptical.

Roy Lewallen, W7EL

alhearn wrote:
I've recently done some NEC-2 (MultiNEC) modeling of folded dipoles
which might help answer some of your questions. Translating the
results to folded monoploes should be fairly straightforward.

The model is a half-wave folded dipole for 14.2 MHz in freespace,
resonant at 33.15 feet using #18 wire with 2 inch spacing. The
center-fed input impedance is 289 - j0.01, which is 4 times the
resonant impedance of 72 ohms for a conventional dipole. A folded
1/4-wavelength monopole would have half that impedance, or about 144
ohms.

Examining the R-X curves for this dipole shows that it has
characteristics very similar to a 3/2-wavelength dipole, operating at
its third harmonic, and on a relatively low-slope part of the curves,
indicating a low Q and good bandwidth, similar to a fat dipole.

Shortening the antenna increases capacitive reactance, as might be
expected. However, input resistance *increases* as the length
decreases, which is contrary to our experience with common
1/2-wavelength dipoles. This is because we're on the high side of
full-wave resonance, where very high resistance values exist at its
peak. As we shorten the antenna, we're climbing the full-wave
resistance curve, which peaks when the antenna length is 22 feet. If
we further shorten the antenna past full-wave resonance, we now begin
experiencing a "normal" decrease in resistance as we "slide" back down
the low side of the full-wave resistance spike. However, capacitive
reactance has now quickly changed to inductive reactance as we crossed
full-wave resonance.

If we continue to shorten the folded antenna length, we come to a
length of about 17 feet where the input impedance is 50 + j2000 ohms.
Notice that the impedance is *inductive*, not capacitive as we are
accustomed to seeing with ordinary short dipoles. The inductive 2000
ohms can be cancelled with a series capacitor (or other suitable
matching network). Q has increased (because we're on a relatively
steep part of the R-X curves) and bandwidth has narrowed considerably
from the resonance at 33.15 feet.

So, by reducing the length of the 1/2-wavelength folded dipole from
33.15 feet to 17 feet, we have a 50 ohm resistive impedance by
matching the inductive reactance with a capacitor (or split capacitor)
instead of the usual lossy, low-Q loading coils. Gain and patterns
appear to be the same as a conventional dipole.

Translated to a monopole, the length would be a little more than half
the dipole's 17 feet, to boost feed point resistance from 25 ohms to
50 ohms. My guess is (I haven't modeled it) that this antenna
functions much like a 3/8-wavelength monopole, although much shorter.
Actually building this antenna and placing it the real world will
obviously change the above values.

Unfortunately, it doesn't appear that any combination of element size
and spacing will offset the need for impedance matching with the
shortened folded dipole or monopole.

I hope this makes sense. I'm sure Roy, Cecil, Tom, and others might
have comments/corrections that will be helpful to me and others who
are relative neophytes in the wonderful world of antennas.

Al WA4GKQ