Mythbusting J-Pole Antennas
For an antenna to work correctly, it must do two things well 1) Accept power from the feed line impedance match, SWR (ideally) 1:1 2) Radiate power in a pattern that is useful for the task at hand
Typical vertical dipole. Center fed. Impedance will match 1:1 at one frequency (usually center of band). Feed Line Each leg approx 1/4 λ (Wavelength) Note 1/4 + 1/4 = 1/2 Approximately 1/2 λ overall A dipole is essentially 2 1/4 λ end-fed antennas back-to-back
Feed Line Fields cancel out here Antenna Fields do not cancel here Q: What's the difference between an antenna and a feed line? A: They both carry a signal, but a feed line does not RADIATE a signal because the electric and magnetic fields are canceled out by the other side of the feed line. At the antenna, we don't have the canceling effect so the signal will radiate.
X L Smith Chart for matching impedance. The center is the SWR 1:1 bulls-eye where we want the antenna's impedance to go. X X C 2:1 SWR circle SWR goes up logarithmically. The edge is (in theory) infinite SWR.
X L X 148 X 146 X 144 A real antenna used on more than one frequency (such as a regular 2 meter antenna) will plot out as a curve (in green). X C
Let's double the dipole instead of two 1/4 λ legs, let's make each leg 1/2 λ long. Feed Line Each leg approx 1/2 λ (Wavelength) Approximately 1 λ overall Q: We know two 1/4 λ antennas back to back (a dipole) will match impedance. Will two 1/2 λ antennas back to back match impedance?
X A: NO! End-fed half wave antenna impedance shows up here on a Smith Chart about infinite impedance, virtually 100% of power reflected back at radio.. 2:1 SWR circle
Long Side Short Side Facts: The 440 band is the third harmonic of the 2 meter band (146 * 3 = 438). In other words, 2 meters has three times the wavelength of 440. The Long Side of a J-Pole is three times longer than the Short Side
Two Meter Side J-Pole Myth #1 A 2 meter J-Pole has two sides because 440 Side The long vertical is the the 2 meter side and the short vertical is the 440 side. Is this myth Confirmed? Busted?
Two Meter Side 440 Side J-Poles J-Pole Myth #1 A 2 meter J-Pole has two sides because The long vertical is the the 2 meter side and the short vertical is the 440 side. BUSTED Is this myth Confirmed? Busted?
This part of a J-Pole is the actual antenna. It is a 1/2 λ end-fed antenna.
WAIT A MINUTE! Didn't we just say that a 1/2 wavelength end-fed antenna doesn't work? X 1/2 wavelength end-fed antenna = infinite SWR!
This part of a J-Pole is the actual antenna. It is a 1/2 λ end-fed antenna. The lower part of the J-Pole is not an antenna. It does NOT radiate energy. It is a 1/4 λ rectifier that matches impedance with the 1/2 λ endfed antenna, allowing it to accept the signal from the feed line.
1/2 λ long 1/4 λ long. The two sides are fed so that each side has an opposite signal to the other side.
Center of coax feed line to long element. Braid of coax feed line to shorter element A C B ¼ w a v e l e n g t h Rectifier portion: 1/4 λ long. The two sides are fed so that each side has an opposite signal to the other side. Location A has infinite resistance (it's an open circuit.) Location B has 0 resistance (it's a short circuit). Location C is the sweet spot with 50 ohms resistance which matches our radio and feed line.
X X The rectifier takes the signal coming on at 50 ohms and 0 reactance and changes it to several thousand ohms and 0 reactance. Signal now matches the antenna.
J-Pole Myth #2 A 2 meter J-Pole will also match impedance with a 440 signal. At the feed line: 2 meter or 440 signal A-OK Is this myth Confirmed? Busted?
At the feed line: 2 meter or 440 signal A-OK J-Poles J-Pole Myth #2 A 2 meter J-Pole will also match impedance with a 440 signal. Is this myth Confirmed? CONFIRMED Busted?
X X Two meters is 3 times longer than 440 (440 is the third harmonic of 2 meters). The 440 signal simply makes an extra loop around the Smith chart and winds up the same place as the 2 meter signal, matching the antenna impedance.
J-Pole Myth #3 Since a 2 meter J-Pole matches impedance with a 440 signal, a 2 meter J-Pole is an excellent antenna for use on 440. Is this myth Confirmed? Busted?
J-Pole Myth #3 Since a 2 meter J-Pole matches impedance with a 440 signal, a 2 meter J-Pole is an excellent antenna for use on 440. Is this myth BUSTED Confirmed? Busted?
Remember, antennas not only must accept a signal (match impedance), but must RADIATE the signal in a useful pattern. 2 Meter J-Pole with 2 meter signal.
2 Meter J-Pole with 440 signal.
Q: Okay, a 2 meter dipole doesn't work right on 440. Is there any way to make a dual band J-pole that will work well on both bands? A: Yes! You can do this by adding a 440 trap.
1/2 wavelength for 2 meters uses the full length of the antenna. TRAP. Stops 440 (at the bottom of trap). Allows 2 meters to pass unhindered (blue). 1/2 wavelength for 440 (green). 1/4 wave rectifier. Matches impedance between 1/2 wave end-fed antenna and 50 ohms for both 2 meters AND 440 (red).
Antenna (green) Short braid to center of coax at top. Outer braid (black) Center of coax (red) The trap is just a piece of coax that is ¼ electrical wavelength long on 440 MHz. A piece of coax this length has interesting properties. A current flowing from the bottom will see the OPPOSITE of what is happening with the wires (the braid and the center conductor) at the top. If we SHORT the braid to the center conductor at the top (creating a short circuit), a 440 signal will see an OPEN circuit (the opposite of what is happening on the other side of the coax). Antenna (green)
Antenna (green) Short braid to center of coax at top. Outer braid (black) Center of coax (red) Therefore, a 440 signal STOPS at the trap because it sees an open circuit. On 2 meters, the trap is much shorter than ¼ wavelength, so a 2 meter signal pass right through the trap because it sees nothing but a bit of wire there. Antenna (green)
Antenna (green) Short braid to center of coax at top. Outer braid (black) Center of coax (red) To compute a trap, take the basic equation for computing wavelength: 300 =λ (meters) f ( Mhz) Then divide the answer by 4 to get ¼ wavelength. Then you need to multiply that by the velocity factor of the coax. Let's say we're using coax with a 75% velocity factor and want a ¼ trap on 440... Antenna (green)
Antenna (green) Short braid to center of coax at top. Outer braid (black) Center of coax (red) To compute this trap, take the basic equation for computing wavelength: 300 440 =.68108(wavelength/meters) Now, divide the answer by 4 and multiply by the VF to get the answer in meters.68108 4.75(VF )=0.12784(meters) To convert to feet, multiply meters by 3.28, then to get inches multiply by 12. 0.12784 3.28 12=5.03 inches Antenna (green)
Real, practical J-Poles
Cheap as Chips J-Pole Item Quantity Price ¾ inch EMT 1 10' section $ 3.94 #12 hose clamp (½ to 1 ¼ inch) 3/8 x 2 foot long threaded steel rod 1 bag of 10 8.57 1 1.97 3/8 nuts 2 0.24 3/8 lock washers 2 0.40 3/8 flat washers 2 0.28 SO-239 1 1.79 Total $17.19
Cheap as Chips J-Pole Other items not in the price list 9 inches of 1 PVC pipe 3 inches of 16 ga wire 2 inch piece of aluminum flat bar, machined to hold SO-239 2 #6 bolts, nuts & lockwashers 440 coax trap
Cheap as Chips J-Pole Cheap as Chips J-Pole antenna build at Fred and Bonnie's place Later this year maybe August or September Interested?