J-Pole Antennas

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).

A

B

D

C

½” Copper Pipe Cap

½” Copper Pipe Elbow

½” Copper Pipe Tee

½” Copper Pipe Cap

Coax

½” Copper Pipe

½” Copper Pipe

6 X 32Brass Bolt

(for adjustment)

6 X 32Brass Bolt

(for adjustment)

¼ λ¼ λ

½ λ

A

B

D

C

½” Copper Pipe Cap

½” Copper Pipe Elbow

½” Copper Pipe Tee

½” Copper Pipe Cap

½” Copper Pipe

½” Copper Pipe

A 8460/fB 2808/fC 276/fD 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.

Frequency (Mhz) 146 Mhz 220 Mhz

"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

A

B

D

C

½” Copper Pipe Cap

½” Copper Pipe Elbow

½” Copper Pipe Tee

½” Copper Pipe Cap

½” Copper Pipe

½” Copper Pipe

A

B

D

C

½” Copper Pipe Cap

½” Copper Pipe Elbow

½” Copper Pipe Tee

½” Copper Pipe Cap

½” Copper Pipe

½” Copper Pipe

70cm (440) Copper Cactus

A 19 ¼”B 6 ¼”C 2 ¼”D ¾”

Adding Adjustment Bolts

• Some of the articles add bolts to the J-Poles. 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.

52.375"

110.75"

2.5"

¼ λ RG-17436" Long – coiled on 1" PVC Pipe

Coax Center Conductor

Terminated on Long side of J-Pole Coax Shield

Terminated on short side of J-Pole

~ 2 ½”

PL-259

From: www.mydarc.de/dk7zb/start1.htm

#14 Insulated wire

450 ohmLadder

Line

6M J-Pole450 ohm Ladder Line

Matching Details

18 ½”

39 ½”

1.25"

Coax Shield Terminated on short

side of J-Pole

~ 1 ¼”

450 ohmLadder

Line

2M J-Pole450 ohm Ladder Line

Matching Details

Coax Center Terminated on long

side of J-Pole

300 ohm flat twinleadJ-Pole

1 ¼”

15 ¼”

¼”

37 ¼”

1”

10”

24¾”

½”

5”

12”

¼” ¼”

144 Mhz

220 Mhz

440 Mhz

Cut