1. Emergency Communications Portable J-Pole Construction
August 7, 2012
Heather Schulz, KC9QXD
Larry Noldan, NZ9I
Waukesha County ARES/RACES
Waukesha County, WI
Some material “borrowed” from:
Dan Murray, W9VE, “Building a Dual-Band Mentorfest, 10/23/2004
Steve Yates, AA5TB, “The End Fed Half Wave Antenna”
Jim Ambercrombie, N4JA, “Understanding Antennas For The Non-Technical Ham”

2. Rubber Ducks are Kinda Like Rubber Chickens
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
Better HT antennas – Less loss, but still limited
¼ wave antenna on cookie sheet – Now we’re getting somewhere
5/8 wave antenna on cookie sheet – and a bit further, BUT
WHO WANTS TO CARRY A COOKIE SHEET & MOBILE ANTENNA WITH THEIR HT?

3. Lecher Lines (No – Not THAT kind of Lecher!)
A Lecher line is a pair of parallel wires or rods that were used to measure the wavelength of radio waves. They form a short length of balanced transmission line. When attached to a source of radio frequency power such as a radio transmitter, the radio waves form standing waves along their length. By sliding a conductive bar that bridges the two wires along their length, the length of the waves can be physically measured. Austrian physicist Ernst Lecher, improving on techniques used by Oliver Lodge and Heinrich Hertz, developed this method of measuring wavelength around However, better methods of frequency measurement are available today, and Lecher lines are now more often used as components in radio frequency equipment, serving as resonant circuits, narrow band filters, and impedance matching devices.
Source: Wikipedia

4. Half Wave Antenna
When an antenna that is ½ wave length long is end-fed, a standing wave develops on it.
The standing wave consists of both current and voltage that are 90 degrees out of phase.

5. End-Fed Half Wave Antenna
When fed at the center (most common), voltage at feedpoint is low and impedance is about 72 ohms
As feedpoint moves away from center, voltage and impedance increase
If fed from end, impedance on order of – 5000 ohms

6. Matching 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. `
High Impedance here
Current
Voltage
¼ λ
50 ohms about here
Zero Impedance here

7. More Stubs A half-wave stub acts like two quarter-wave 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)

8. Impedances along J-Pole Matching Stub
“Thunder is good, thunder is impressive, but
it is lightning that does the work.” - Mark Twain
Operation of the J-pole depends on two different functions: matching and radiation
Radiation gets all the credit, but will not happen without proper matching.
Voltage-fed dipole
(This part does the work of radiating)
¼ wave matching stub
(This part does the work of
matching the line to the dipole
so that it can do its work.)

9. Components Required

10. Here’s What to Build

11. The Game Plan for Lots of Bands

12. Strip the Shorted End

13. Short the Stub

14. Solder the Short

15. Measure for the Notch

16. Notch the Cable

17. Expose the Matching Point

18. Remove Coax Outer Insulation
First step in creating the feedpoint connection

19. Separate Braid from Center Conductor

20. Coax Soldered at Antenna Feedpoint

21. Wrap Feedpoint and Short

22. Strip Coax for PL-259

23. Attach PL-259 Connector

24. Do QC Check – SWR Match, etc.