[Elecraft] folded dipole of topic

[Earl W Cunningham]

Guy, K2AV wrote:

"I am very perplexed about your statement on the bandwidth of a folded
dipole..... An 80 meter dipole at 75' shows a 1.5:1 bandwidth of 145 kHz,
while a folded dipole (double wire all the way) shows a 1.5:1 bandwidth
of 205 kHz.  Approaching a 50% increase."
==========
I apologize for my incorrect statement that a folded dipole does not have
a significant increase in bandwidth compared to a simple dipole.  I was
thinking of shunt-fed towers, where the popular belief is that if the
shunt wire diameter is increased significantly by constructing a "cage"
of wires for use as the shunt wire, the bandwidth is signicantly
increased (which it isn't).

I modeled Guy's 80-meter dipole and folded dipole using EZNEC 3 to see
what the difference would be.  I used 3600 kHz as the design frequency
and used the high accuracy model over real "average" ground at 75' and in
the clear with the following results (#14 AWG copper wire was used for
both antennas):

DIPOLE:

Length for resonance = 132.254'

Equation = 476.1144/f(MHz)

Feedpoint resistance at resonance = 89.35 ohms

SWR at resonace = 1.19:1 (75-ohm feedline)

1.5:1 SWR points = 3535 & 3660 kHz (125 kHz BW)

FOLDED DIPOLE WITH 1" SPACING BETWEEN WIRES:

Length for resonance = 131.1226'

Equation = 472.04136/f(MHz)

Feedpoint resistance at resonance = 350.5 ohms

SWR at resonance (75-ohm line thru a 4:1 balun) = 1.17:1

1.5:1 SWR points = 3520 & 3675 kHz (155 kHz BW, 24% better than the
dipole)

Modeled in free space, the required lengths were a bit different for
resonance, of course.  The feedpoint resistances in free space were quite
close to the theoretical 72 ohms and 288 ohms respectively.

73, HNY, de Earl, K6SE