hy to all ! where can i find a link with the plan how to make a folded dipol
for 80 meter band my problem is not to much place on my roof, thank you 73
9a6nfg JN65TC Luca

"gianluca" wrote in message
...
hy to all ! where can i find a link with the plan how to make a folded
dipol for 80 meter band my problem is not to much place on my roof, thank
you 73 9a6nfg JN65TC Luca

A folded dipole is the same length as a classic wire dipole. The folded
term relates to the two parallel conductors not the overall length..

Dale W4OP

Dale, W4OP wrote:
"A folded dipole is the same length as a classic wire dipole. The folded
term relates to two parallel conductors not the overall length."

true. A 1/2-wave folded dipole has the same radiation pattern as a
straight dipole. Its feedpoint resistance is higher, depending on the
number of conductors in the dipole and their relative sizes. Another
difference is more significant. The folded dipole is also resonant when
it has two wires and it is only 1/4-wave long. It is resonant because
its circumference is then 1/2 wavelength.

The feedpoint resistance of the resonant 1/4-wave long center-fed folded
dipole made from the same diameter wire all the way around is 6000 ohms.
Special matching may be needed, However, gain is only 1/2-dB less than a
1/2-wave dipole and its radiation pattern is almost the same. Its
bandwidth is only 5% of its center frequency while the bandwidth of the
1/2-wave folded dipole is 45% of its center frequency. Bandwidth of the
thin single-wire center-fed dipole is 34%.

Arnold B. Bailey is the source of this information in "TV and Other
Receiving Antennas".

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
The folded dipole is also resonant when
it has two wires and it is only 1/4-wave long. It is resonant because
its circumference is then 1/2 wavelength.
The feedpoint resistance of the resonant 1/4-wave long center-fed folded
dipole made from the same diameter wire all the way around is 6000 ohms.

Richard, that doesn't make sense to me. Such an antenna could be fed
with 1/4WL of 600 ohm open-wire line and achieve an impedance of 60
ohms looking into the matching section. That's just too good to be true.

I just ran an 80m folded dipole on 160m using EZNEC. It says the
feedpoint impedance is 32+j1900 ohms. That's a long way from
resonance and 6000 ohms.
--
73, Cecil http://www.qsl.net/w5dxp

Cecil, W5DXP wrote:
"Such an antenna could be fed with 1/4 WL of 600 ohm open wire line and
achieve an impedance of 60 ohms looking into the matching section."

Exactly.

The actual impedance of the 1/4-wave folded dipole is an inverse
function of the wire diameter. It can be chosen to present 6000 ohms.
Arnold B. Bailey has studied the behavior, developed the formulas, and
published graphs for the user to choose wire sizes for folded dipoles of
either 1/4-wave or 1/2-wave overall length. He does a sample calculation
and showes his work for one particular 1/4-wave folded dipole.

On page 414 of "TV and Other Receiving Antennas" Bailey writes:
"At first resonance (where the total wire length is 1/2-wave and overall
length is 1/4-wave), the current in the continuous rod (entire wire
length) approaches a constant value, varying from a maximum in the
center (farthest from the drivepoint) to about 0.7 of maximum at the
ends. For this reason the radiation resistance RR approaches the value
for a constant-current rod. The practical value is approximately
RR=570(l squared), where l is the nominal over-all length and equals
0,25 wavelength, thus resulting in a value of 35.6 ohms."

The 1/4-wave folded dipole shares some of the characteristics of a
short-circuited stub. It is a resonant circuit element which converts
the low impedance at its short-circuit into a high impedance at its
feedpoint.

If the 1/4-wave folded dipole is a little short of 1/2-wave around its
perimeter, it presents an inductive reactance. If a little longer
electrically than 1/2-wave around, it presents a capacitive reactance.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
The actual impedance of the 1/4-wave folded dipole is an inverse
function of the wire diameter. It can be chosen to present 6000 ohms.
If the 1/4-wave folded dipole is a little short of 1/2-wave around its
perimeter, it presents an inductive reactance. If a little longer
electrically than 1/2-wave around, it presents a capacitive reactance.

Using EZNEC, I just did a frequency sweep on a #14 folded dipole
around half its design frequency. There were no apparent resonant
effects. How big does the wire have to be for a 130ft folded dipole
to exhibit a feedpoint impedance of 6000 + j0 ohms on 160m? Why is
no one using such an antenna on 160m?
--
73, Cecil http://www.qsl.net/w5dxp

You should realize that this antenna is much narrower banded than a half
wave dipole. Using a transmission line matching section with such a high
transformation would probably narrow it further.

The folded dipole described by Bailey (his data sheet 10-10) can be
easily modeled with EZNEC or a similar program. A transmission line
matching section can be included in the model if desired.

Roy Lewallen, W7EL

Richard Harrison wrote:
Cecil, W5DXP wrote:
"Such an antenna could be fed with 1/4 WL of 600 ohm open wire line and
achieve an impedance of 60 ohms looking into the matching section."

Exactly.

The actual impedance of the 1/4-wave folded dipole is an inverse
function of the wire diameter. It can be chosen to present 6000 ohms.
Arnold B. Bailey has studied the behavior, developed the formulas, and
published graphs for the user to choose wire sizes for folded dipoles of
either 1/4-wave or 1/2-wave overall length. He does a sample calculation
and showes his work for one particular 1/4-wave folded dipole.

On page 414 of "TV and Other Receiving Antennas" Bailey writes:
"At first resonance (where the total wire length is 1/2-wave and overall
length is 1/4-wave), the current in the continuous rod (entire wire
length) approaches a constant value, varying from a maximum in the
center (farthest from the drivepoint) to about 0.7 of maximum at the
ends. For this reason the radiation resistance RR approaches the value
for a constant-current rod. The practical value is approximately
RR=570(l squared), where l is the nominal over-all length and equals
0,25 wavelength, thus resulting in a value of 35.6 ohms."

The 1/4-wave folded dipole shares some of the characteristics of a
short-circuited stub. It is a resonant circuit element which converts
the low impedance at its short-circuit into a high impedance at its
feedpoint.

If the 1/4-wave folded dipole is a little short of 1/2-wave around its
perimeter, it presents an inductive reactance. If a little longer
electrically than 1/2-wave around, it presents a capacitive reactance.

Best regards, Richard Harrison, KB5WZI