02-03-2013, 12:35 PM
How to Build a Dual-Band Antenna for 2M/70cm
[attachment=52401]
Introduction
• Rubber duck nearly an isotropic source, but not as good
– Victim’s head absorbs some of the RF (maybe that explains a
few things…)
– Capacitance to body makes a “sortaground” that supplies half of
the antenna plus a decent dummy load.
– Directionality based on loss, not gain
• Alternatives commonly used
– 5/8 antenna on cookie sheet
– ¼ wave antenna on cookie sheet
Stubs
• An open or shorted quarter-wave stub inverts the
impedance at the input end.
– If the far end is shorted, there is a very high impedance looking
into the stub
– If the far end is open, there is a short looking into the stub.
– If you start at the shorted end and move toward the input, you
will notice that the impedance is zero at first (high current, very
low voltage). As you move away from the short, you’ll see the
impedance rise (less current, more voltage) until you reach a
voltage max at the input to the stub.
– This gradual change in impedance can be used to match a
feedline to the stub. The higher the impedance of the feedline,
the farther from the short the feedpoint must be.
Stubs Again
• A half-wave stub acts like two quarterwave
stubs.
– If you put 2 quarter wave stubs together, you
invert the impedance twice!
– Since the impedance is inverted twice, looking
into a half wave stub you see exactly the
same impedance that exists at the far end.
• But a three-quarter wave stub acts
precisely a quarter-wave stub (in a
lossless line)
J-pole construction
– Required Materials
• ½” EMT conduit, 10’ section (Home Despot $2.89)
• 3/8-24 x 36” all-thread rod (Elliot’s $2.59)
• 3/8-24 locking nuts, 3 each (Elliot’s $0.20 each)
• Number 12 hose clamps, 3 each (Elliot’s $0.59 ea)
• 5” Stanley L-bracket (Despot $2.59)
• UHF to 3/8” antenna mount (Texas Towers $4.89)
– Optional Materials
• ¾” EMT conduit, 6” or 8” hose clamps, qty 3,
An Even Better J-Pole - Electrically
The gain is about 7.4 dBi.
This value is about 2.3-2.4 dB
higher than the average gain
of a single-radiator J-pole.
Indeed, although many folks
like to bandy the gain
advantage of a collinear
arrangement as 3 dB greater
than a single section, we
rarely obtain in real antennas
more than about a 2.0-2.5 dB
increase in gain.
• Compared to many vertical
antennas, the collinear J-pole
shows a remarkable reduction
in high-angle radiation. For
any vertical collinear array,
the only place from which to
obtain energy for increased
gain at lower elevation angles
is from the high-angle energy
of a single section.
[attachment=52401]
Introduction
• Rubber duck nearly an isotropic source, but not as good
– Victim’s head absorbs some of the RF (maybe that explains a
few things…)
– Capacitance to body makes a “sortaground” that supplies half of
the antenna plus a decent dummy load.
– Directionality based on loss, not gain
• Alternatives commonly used
– 5/8 antenna on cookie sheet
– ¼ wave antenna on cookie sheet
Stubs
• An open or shorted quarter-wave stub inverts the
impedance at the input end.
– If the far end is shorted, there is a very high impedance looking
into the stub
– If the far end is open, there is a short looking into the stub.
– If you start at the shorted end and move toward the input, you
will notice that the impedance is zero at first (high current, very
low voltage). As you move away from the short, you’ll see the
impedance rise (less current, more voltage) until you reach a
voltage max at the input to the stub.
– This gradual change in impedance can be used to match a
feedline to the stub. The higher the impedance of the feedline,
the farther from the short the feedpoint must be.
Stubs Again
• A half-wave stub acts like two quarterwave
stubs.
– If you put 2 quarter wave stubs together, you
invert the impedance twice!
– Since the impedance is inverted twice, looking
into a half wave stub you see exactly the
same impedance that exists at the far end.
• But a three-quarter wave stub acts
precisely a quarter-wave stub (in a
lossless line)
J-pole construction
– Required Materials
• ½” EMT conduit, 10’ section (Home Despot $2.89)
• 3/8-24 x 36” all-thread rod (Elliot’s $2.59)
• 3/8-24 locking nuts, 3 each (Elliot’s $0.20 each)
• Number 12 hose clamps, 3 each (Elliot’s $0.59 ea)
• 5” Stanley L-bracket (Despot $2.59)
• UHF to 3/8” antenna mount (Texas Towers $4.89)
– Optional Materials
• ¾” EMT conduit, 6” or 8” hose clamps, qty 3,
An Even Better J-Pole - Electrically
The gain is about 7.4 dBi.
This value is about 2.3-2.4 dB
higher than the average gain
of a single-radiator J-pole.
Indeed, although many folks
like to bandy the gain
advantage of a collinear
arrangement as 3 dB greater
than a single section, we
rarely obtain in real antennas
more than about a 2.0-2.5 dB
increase in gain.
• Compared to many vertical
antennas, the collinear J-pole
shows a remarkable reduction
in high-angle radiation. For
any vertical collinear array,
the only place from which to
obtain energy for increased
gain at lower elevation angles
is from the high-angle energy
of a single section.