# Wonderful Wire Antennas

5 February 2014

**A Baker’s Dozen **

**Wire Antennas **

**Forrest B. Snyder, Jr. **

**N4UTY **

**Gooch’s Paradox or **

**Why Antennas Radiate: **

**RF gotta go somewhere! **

o **Some antennas radiate better than **

**others. The explanation is Resonance. **

o **Resonance depends on frequency ** o **The shortest resonant antenna is an **

**electrical ½ wavelength long. **

o **Length of a resonant ½ wave antenna **

**(in feet) = 468/Frequency in MHz. **

**The Basic Antenna: **

**The ½ Wave Dipole **

• **Length = 438/frequency (Mhz) **

• **At resonance **

– **Current (I) is maximum at feedpoint, zero at ends **

– **Voltage (E) is zero at feedpoint, maximum at ends **

– **At resonance, the antenna looks like a resistor. **

– **Feed point impedance for a resonant dipole = E/I is about 72 ohms **

• **If transmitter output impedance = 50 ohms, and a resonant dipole **

**is fed with 50 ohm coax, the feedline will show a theoretical SWR **

**= 72/50 = 1.44/1. **

• **Actual feed point impedance varies with **

– **Antenna height **

– **Ratio of element diameter to wavelength **

– **Presence of nearby objects. **

• **Resonant on fundamental and odd harmonics **

• **Most resonant wire dipoles display a measured feed point impedance **

**of about 65 ohms – a good match for 50 ohm coax. **

**The Basic Vertical Antenna **

**The ¼ Wave Marconi **

**Image Antenna **

**Surface of the Earth **

**Ground Radials **

• **Half a dipole sticking up in the air **

• **Length (feet) = 234/f (MHz) **

• **Ground reflection forms 2d half of **

**dipole– the “image antenna” **

• **Ground return through earth increases **

**loss resistance – ground radials decrease loss resistance **

• **How many radials **

– **Ground mounted: As many as possible - **

**120 is about optimum – One will work **

**(classic 30 over 30 or 60 over 60) **

– **Mounted above ground (Ground Plane): 3 or **

**four but one or two will work **

• **Make of wire, coil it up, hang one end in **

**a tree for portable or emergency operation **

**Dipole Variation: the Folded **

**Dipole **

short short

• **More broadband than a single wire; can be **

**operated on its fundamental and odd harmonics **

• **Length = 438/frequency (Mhz); length between **

**shorts = 492 x k/frequency where k = velocity factor (typically 0.82 to 0.85) **

• **Feedpoint impedance = 300 ohms: Make out of and **

**feed with cheap**

**300 ohm TV twin lead. **

**The Basic Multi-band Wire: the **

**Center–Fed Zepp or McCoy Dipole **

• **Length = as long as possible – cut it to fit between your **

**supports or to be resonant on the lowest frequency band on which you plan to operate **

• **Feed it with open wire feeders or ladder line **

• **Feed point impedance will vary with frequency **

– **The feedline will operate at a high SWR **

– **Will require a matching network/transmatch for off-resonance **

**operation **

• **Off resonance. **

– **A short antenna looks like a capacitor and a resistor in series. **

– **A long antenna looks like an inductor and a resistor in series. **

**If you have only one support **

• **Inverted Vee **

• **Resonant length = 490/frequency in MHZ (per 1964 ARRL **

**Handbook) **

• **Feedpoint impedance about 50 ohms at resonance if legs droop 45 **

**degrees **

• **Feed with 50 ohm coax for resonant operation, ladder line for multi **

**band operation **

• **Warning: High Voltage at ends – keep ends well out of reach. **

**If you have only one support **

• **The sloped dipole **

• **Feedpoint impedance about 50 ohms **

**at resonance **

• **Slightly directive from the pole to **

**the antenna if the support is metal **

• **Caution: High Voltage at the ends! **

**If you have limited space between supports: **

B

A

• **The Bent Dipole **

• **Resonant length about 490/frequency in MHZ **

• **Feedpoint impedance should be close to 50 ohms at resonance **

• **Feed with 50 ohm coax for resonant operation, ladder line for multi **

**band operation **

• **Warning: High Voltage at ends – keep ends well out of reach. **

• **Note: few people are eight feet tall. Try to keep ends at least 8 **

**feet above the ground **

**Option: End fed or Zepp **

• **Designed to trail out of WWI Zeppelins **

• **Traditionally, a half wave fed at one end with a quarter wave of **

**open wire or ladder line **

• **High impedance at antenna feedpoint; low impedance at end of 1/4 **

**wave feedline **

• **Feed with open wire or ladder line **

• **Multiband operation possible with appropriate transmatch: per 1964 **

**ARRL Handbook, following lengths support matching **

– **Antenna: 135 feet, feedline 45 feet (80 thru10 meters) **

– **Antenna: 67 feet, feedline 45 feet (40 thru 10 meters) **

**The Multi-wire or “Fan” Dipole **

•

**Separate resonant elements for each band **

•

**Feed with coax **

•

**Each set of parallel elements affects the resonant length of all of the others – cut to formula and prune, beginning with the lowest frequency band **

•

**Novice special (80, 40, 15): Resonant at 3.7 and 7.12 MHz. Operates 80m and 40 m on the fundamental, 15m on the 3d harmonic of 40m. **

–

**91 feet of window line or 300 ohm twin lead. **

–

**Cut one conductor 31 feet from one end, cut second conductor 31 feet from other end **

–

**Separate ladder line between cuts **

–

**Connect conductors at “square” end; hook to center insulator **

–

**Attach insulators to free end of long elements, haul up, prune as needed. **

–

**Get on the air and enjoy! **

•

**Novice special plus 20 m: Weave resonant 20 m radiator (16’ 6” or 8’ 3” either side of center) through the windows line between the 80 m and 40/15 m elements. **

•

**Novice special plus 30 m: Weave resonant 30 m radiator (43’ 4” or 22’ 10” either side of center) through the windows in the ladder line between the 80 m and 40/15 m elements. **

**NOTE: Use the same idea for a multi band wire vertical from ladder line, TV lead in, rotator cable, zip cord or whatever. Have fun with it! **

**The Three Element Ground Plane for 10, 15, and 20 meters **

**16’ 5” **

**11” **

**8’ 3” **

• **Use 3 wire rotator **

**cable for the vertical elements. **

• **Connect all three **

**elements at the bottom **

• **Four 17’ radials (ground **

**plane) or more (ground mounted) **

• **Feed with 50 ohm coax **

• **For portable use, tie a **

**rope to the longest element and suspend from a tree! **

**The Windom: Single wire offcenter fed ½ wave **

**L=438/f (Mhz) **

**.36L **

**Single-wire feed against ground **

• **“The antenna will operate satisfactorily on second **

**harmonic frequencies” (1964 ARRL Handbook) **

• **Single wire feeder shows an impedence of about **

**600 ohms to ground at resonance **

• **Invented by Loren Windom who also invented the **

**product detector **

**The off-center fed dipole **

**(1964 ARRL Handbook) **

**136’ **

**44’ 4” **

**Feed with 300 ohm twin lead **

• **Claimed to offer a good match on 80, 40, 20, and **

**10 meters **

• **“Widely used and does work satisfactorily” **

• **Some feed line lengths “awkward” **

• **See QST index for in depth treatment **

• **Theory behind Cushcraft R4, R5, R7, R8 and R9 **

**vertical antennas **

**Advantages of Wire Antennas **

• **Simple and effective – they just **

**WORK. **

• **Can be made to fit almost anywhere. **

• **Cheap – a hank of wire and three or **

**four insulators is all you need. **

• **Lightweight for portable operation. **

• **Just plain fun! Throw up a wire, get **

**on the air, and work the WORLD! **

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