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 Antenna” @ Mentorfest, 10/23/2004 Steve Yates, AA5TB, “The End Fed Half Wave Antenna” Jim Ambercrombie, N4JA, “Understanding Antennas For The Non-Technical Ham”

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?

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 1888. 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

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.

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 1800 – 5000 ohms

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

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)

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.)

Components Required

Here’s What to Build

The Game Plan for Lots of Bands

Strip the Shorted End

Short the Stub

Solder the Short

Measure for the Notch

Notch the Cable

Expose the Matching Point

Remove Coax Outer Insulation First step in creating the feedpoint connection

Separate Braid from Center Conductor

Coax Soldered at Antenna Feedpoint

Wrap Feedpoint and Short

Strip Coax for PL-259

Attach PL-259 Connector

Do QC Check – SWR Match, etc.