J-Pole Antennas
WB5CXC
J-Pole Antenna
• J-Pole antennas are shorted end feed
antennas.
• They consist of two elements, that are
shorted together at the bottom. The shorter
element is ¼ λ and the longer is ¾ λ.
• It is feed with 50 ohm coax, a little off the
bottom of the shorted portion.
• The most common J-Pole is known as the
‘Copper Cactus’ because it is made from
copper water pipe (and looks like a cactus).
6 X 32
Brass Bolt
(for adjustment)
½” Copper
Pipe Cap
½” Copper
Pipe
½λ
6 X 32
Brass Bolt
(for adjustment)
½” Copper
Pipe Cap
A
D
¼λ
B
C
½” Copper
Pipe Tee
½” Copper
Pipe Elbow
½” Copper
Pipe
¼λ
Coax
½” Copper
Pipe Cap
½” Copper
Pipe
½” Copper
Pipe Cap
A
D
A
B
C
D
B
C
½” Copper
Pipe Tee
½” Copper
Pipe Elbow
½” Copper
Pipe
8460/f
2808/f
276/f
264/f
Dimension are in “Inches”
f is in Mhz
Copper Cactus
• Copper Cactus can be made for 10M, 6M,
2M, 220, and 440.
• The 10M J-Pole is very large and is
probably impractical.
• They are very popular on 6M and above.
½” Copper
Pipe Cap
½” Copper
Pipe
½” Copper
Pipe Cap
A
D
B
Frequency (Mhz)
146
"A"
57.95
inches
38.45
inches
"B"
19.23
inches
12.76
inches
"C"
1.89
inches
1.25
inches
"D"
1.81
inches
1.20
inches
C
½” Copper
Pipe Tee
½” Copper
Pipe Elbow
½” Copper
Pipe
Mhz
220
Mhz
½” Copper
Pipe Cap
70cm (440) Copper Cactus
½” Copper
Pipe
½” Copper
Pipe Cap
A
D
B
C
½” Copper
Pipe Tee
½” Copper
Pipe Elbow
½” Copper
Pipe
A
B
C
D
19 ¼”
6 ¼”
2 ¼”
¾”
Adding Adjustment Bolts
• Some of the articles add bolts to the JPoles. These provide adjustments to the
lengths of the elements.
• I used the standard calculation and did a
model of a 2M Copper Cactus. The SWR
was probably within acceptable limits.
• I then added some bolts and added some
length (antenna too short). The first attempt
proved the bolts are a good addition.
SWR – built to formula
SWR – used adjustment bolts
(first try)
Wire J-Poles
• You can also make J-Poles out of wire.
• Most are made from twinlead (flat TV
cable) or ladder line.
• Common Ladder Line comes in 300 or 450
Ohm.
• You can use your standard formula for
ladder line, but you have to use the
velocity factor to get the final calculations.
#14
Insulate
d wire
6M J-Pole
450 ohm Ladder Line
From: www.mydarc.de/dk7zb/start1.htm
110.75"
Coax Center
Conductor
Terminated on
Long side of J-Pole
Coax Shield
Terminated on short
side of J-Pole
450 ohm
Ladder
Line
¼ λ RG-174
36" Long – coiled
on 1" PVC Pipe
~2
½”
52.375"
PL-259
Matching Details
2.5"
2M J-Pole
450 ohm Ladder Line
39 ½”
450 ohm
Ladder
Line
Coax Center
Terminated on long
side of J-Pole
Coax Shield
Terminated on short
side of J-Pole
18 ½”
~ 1 ¼”
1.25"
Matching Details
37 ¼”
24¾”
12”
300 ohm flat twinlead
J-Pole
Cut
¼”
¼”
¼”
15 ¼”
10”
5”
1 ¼”
1”
½”
144
Mhz
220
Mhz
440
Mhz