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SIMPLE ANTENNAS THAT YOU CAN BUILD

By Billy Williams, N4UF

INTRODUCTION

THE HERTZ DIPOLE

THE VERTICAL

SWITCHING FOR MULTIPLE ANTENNAS

SIMPLE BEAM ANTENNAS

INTRODUCTION

Some Amateur Radio operators accumulate contacts with hundreds of countries & territories using huge multi-element antenna arrays. They qualify for difficult awards and win radio contests.  Others work moon bounce (EME), troposcatter and long-distance VHF.

But large antennas and towers are beyond reach of the majority of Amateur Radio operators because of small lots and pervasive restrictions.

Simple, low-profile antennas can be made inexpensively.  These antennas and principles work for frequencies up to 1000 MHz or so.  

Keep in mind that making contacts with simple antennas requires patience and persistence.  Those who prize instant gratification will not be satisfied.   You will fare better operating on radio nets.  You will not bust through pileups of dozens of operators calling a distant station.   But when propagation conditions are favorable, contacts will be plentiful.

 
AVOIDING A MATCHING NETWORK

In order to radiate effectively, the impedance (Z) rating of the transmitter output circuit, the transmission line and the antenna feed point must be similar.  Slight variations of a few ohms are not critical.  

Most transmitter/transceiver output circuits tune 50 to 72 ohms.  Common coaxial cable such as RG-8, RG-11 and RG-58 has a 50 or 52 ohm impedance rating.  RG-59 cable TV coaxial cable has a 72 ohm impedance rating which is close enough for most applications.

To avoid the need for a matching network at the feed point, build an antenna with an impedance in the 50 to 72 ohm range.  Antennas with other impedances can perform well but require odd-type feedline or a matching network.  

These networks can be tricky to adjust and add more maintenance considerations. Slight variations in dimensions cause problems.  So keep things simple and design your antenna to have a 50 to 72 ohm impedance and avoid the matching network.  

In theory, the  ½ wave Hertz Dipole has a 72 ohm impedance in “free space” which trends downward toward 50 ohms in practical installations.   Thus it is an excellent choice to avoid the added complexity of a matching network.

-------------------------------------x  x-------------------------------------

THE HERTZ DIPOLE

The Hertz Dipole starts as  a single piece of wire which is cut into equal halves.  It is mounted horizontally (parallel to the ground).  If cut to a length of ½ wavelength, the “natural” impedance is in the 50-72 ohm range.

For VHF, UHF or even upper HF bands, lightweight aluminum tubing is practical instead of wire. Antennas made from tubing can be designed to be rotatable, either by hand from ground level or by using a small rotor.  Read below for more about metal tubing and antenna rotation.

If you want a 10 meter dipole, use a length of about 5 meters.  A meter is slightly longer than a yard (39+ inches).  For 40 meters, use a wire which is 20 meters long.  Actually, the wire should be slightly shorter due to several factors.  To figure the wire length in feet, use the formula 468 divided by the frequency in MHz. For example, an antenna designed for 7.2 MHz should be  468 / 7.2 or 65.0 feet.

TOTAL LENGTHS FOR HALF WAVE DIPOLES

2 Meters (146 MHz)----------3.2 ft.

6 Meters (52 MHz)----------9.0 ft. 

10 Meters (28.6 MHz) ------16.5 ft.

15 Meters (21.3 MHz)------22.0 ft.

20 Meters (14.2 MHz)-------33.0 ft.

40 Meters (7.2 MHz)--------65.0 ft.

75 Meters (3.9 MHz)-------120.0 ft.

80 Meters (3.6 MHz)---------130.0 ft.

160 Meters  (1.85 MHz)------253.0 ft.

The exact length depends on the characteristics of the area surrounding your antenna, including height above ground.  For wires, you will need extra inches to accommodate one or more insulators; so add 5 or 10% to the length.   You can trim excess wire much easier than you can splice additional wire to the ends of an antenna that is too short.

Take your single piece of wire (65 feet for 7.2 MHz for example) and cut it in half (32 ft. 6 inches per half).  Then connect the outer shield of your coaxial cable transmission line to one of the halves and the coax center conductor to the other.  This puts the feed point in the center.  Use a ceramic insulator to accommodate these connections or take a three-inch or so piece of thick-walled PVC pipe and drill two holes in it to hold the two halves.

In theory, one should use a balun at the feed point since coax cable is an unbalanced feedline and the dipole is a balanced antenna.  But over the years, I have found the advantage of a balun to be very marginal but not worth the extra complexity and maintenance.

SIZE AND TYPE OF WIRE

If you are subject to antenna restrictions and live close to other households, it is advisable to run low power. The odds are very good that high power will cause interference to at least one neighbor. Unless you intend to run power in excess of 200 watts or so, any size wire can be used.  

The thinner the wire, the higher the American Wire Gauge (AWG) number.  AWG #20 is only slightly bigger than thread and is difficult to see if you live with tight restrictions.  I have used dipoles made from AWG #20 for temporary set-ups.  A better choice is AWG #14 or #12 stranded “copperweld” wire which is available from many sources.

From an electrical standpoint, any conductor is acceptable to use but it is best to avoid wire which is stiff such as ROMEX electrical cable, stretches easily or contains aluminum.   Copper-based wire is easy to solder. It does not matter if the wire is bare or has insulation such as lacquer or rubber.

Wire with thick insulation usually lasts longer but is more expensive.

INSTALLATION

Take the half of the wire that is hooked to the center conductor of the coaxial cable and elevate it as much as possible.  Hang it from a tree limb or other convenient point.  A height of 20 feet or so will work.  The other end of the antenna can be secured to a similar support to make a horizontal dipole or the wire can be sloped down near the ground to form a sloping dipole or sloper.   Keep the low end a few feet off the ground and use rope to secure it. Mark low points (under 7 feet or so) prominently with surveyors ribbon or by other means to avoid injury. 

To get the antenna over a tree limb, use thin nylon cord or fishing line. Attach a weight such as an old padlock or sinker to the cord or line and toss over the limb.  As the weight descends, take it loose and tie a more permanent support such as poly rope or clothesline and pull it back over the limb.  Then attach that rope to the end of the antenna (using another stub of PVC as an insulator if you wish), pull it over the limb (in the original direction) and tie it off.  

Leave enough rope so that the antenna can be lowered.  If a tree limb is not available, a 20-foot support can be made using a telescoping mast or two 10 foot sections of chain link fence toprail stacked on top of each other.

Another method is to attach the antenna center feed point to the support (at the highest point) and slope the two ends downward.  This is known as an Inverted Vee and produces an even less directional pattern. If using a metal support mast, get the feed point at least a couple of feet away from the metal to prevent detuning.

ALUMINUM TUBING

Dipoles, especially those for 14 MHz and above, can be designed using lightweight aluminum tubing.  When mounted to a mast, this dipole can be rotated using a small TV antenna rotor or the “Armstrong” method--turning the mast by hand from the ground.  Attach the 20-foot mast (or two toprail sections) to the eave of your house and only slightly tighten the eave-mount screws.  Eave mounts can be found near the external TV antennas at most hardware stores such as Ace, Lowe’s and Home Depot.

If your restrictions are tight, it is possible to mount a dipole in an attic.  Try to keep the wire away from metal as much as possible.  Another option is a portable or temporary antenna which can be taken down when you are not on the air.  In some cases, restrictions do not apply to portable items.  If you have access to a balcony, a portable antenna might be your best choice.

RADIATION & DIRECTION PATTERNS

Horizontal dipoles favor two directions and sloping dipoles favor one direction. 

A level horizontal dipole favors the two directions that are broadside to the wire.  The sloper favors the direction of the lowest end.  For 40 and 80 meters, directional characteristics are minimal.  If a stretched-out 80 meter dipole/sloper won’t fit on your lot, you can dog-leg (bend) the dipole without major degradation.

If possible, put up two dipoles at ninety degree angles.  Have one face east-west and the other north-south, for example.  You can also put up dipoles for different bands.  It is best to use a separate coaxial cable feedline for each one.  Otherwise one antenna may tend to detune the other.  However, with extra effort and patience, one coaxial cable can feed more than one dipole.  In many cases, a half-wave dipole will not only work well without matching at its design frequency but also will work at triple the design frequency.  A 40-meter (7 MHz) dipole usually will work reasonably well on 15 meters (21 MHz.).

TUNING THE DIPOLE

After installing your antenna, check the tuning.  Use a Standing Wave Ratio (SWR) bridge meter.  The lower the reading, the better your antenna will radiate and the less heat that will generated.  Excessive heat caused by “reflected power” can damage the final stage of your transceiver/transmitter.

The SWR should be 1.5 to 1 (1.5:1) or less which represents 96% being radiated and only 4% of your power being reflected and converted into heat.  A perfect match with 0% heat generation is indicated by a 1 to 1 (1:1) SWR.  An “antenna analyzer” can be used to measure SWR but these are considerably more expensive than an SWR meter.  Do not use an SWR meter at frequencies above 30 MHz unless the device is rated for VHF use. Readings may be incorrect.

If your SWR is higher than 1.5 to 1 at the design frequency, determine at what frequency the SWR reading is the lowest.  If the minimum SWR (the dip) is on a lower frequency, you need to prune each end of the antenna equally to raise the resonant frequency.  If the SWR reading is minimum (dips) at a frequency higher than the design frequency, you need to add an equal amount of wire to each end.  For example, an antenna with the lowest SWR at 7.3 MHz would need to be lengthened to get down to 7.2 MHz.  An antenna designed for 14.15 MHz. would need to be reduced in length if it dips at 14.0 MHz.

If the SWR reading is very high (2 to 1 or higher) and does not dip to an acceptable level anywhere in the band, you probably have a bad connection or your wire dimensions are incorrect.  First, check the coaxial cable by disconnecting the antenna at the feedpoint and connecting a “dummy load” to the far end of the coax.  

If you are using an antenna analyzer, you can put a 50 ohm carbon resistor across the line instead.  The SWR should be 1 to 1 and certainly no higher than 1.2 to 1 or so.  If your SWR is high, the coax is suspect.  Pay special attention to the connectors which can easily be installed incorrectly and be intermittent.  Shake the connector and flex the coax near the end to see if the SWR reading becomes erratic.  Unless the coax is very old, the odds are high that the problem is near the connectors.  Look for cuts in the coax or water infiltration.  Substitute another run of coax and hook it to the antenna.

If the coax is good, make sure that your measurements are correct and that no part of the antenna is within two feet or so of any metal.

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THE VERTICAL

The horizontal dipole works best when extended in a straight line between two supports or sloped from one support down to ground level.  If your lot size is not favorable for an antenna occupying this amount of space, a vertical antenna may be a better choice.  A dipole can be transformed into a vertical by standing it up on its end.  

A half wavelength of wire (as determined by the formula 468 divided by the design frequency in Megahertz) can be hung perpendicular to the ground (vertically) from a tree limb.  While this may be feasible for antennas designed for frequencies above 14 MHz. (20 meters), the wire length exceeds sixty feet for 40 and 80 meters.

Half of this length can be eliminated if a good ground is available as a substitute.  This is the basis for a vertical or Marconi antenna--a quarter wavelength antenna that relies on a ground.  The center conductor of the feed line is connected to the bottom of the quarter wave wire.  

The shield is hooked through a length of heavy wire to a ground rod hammered six feet or so into the dirt.  Impedance is usually in the range of 35 to 50 ohms which may be close enough to match to 50 ohm coax such as RG-58 without producing excessive reflected power.  Matching networks can be used to provide a closer match if SWR exceeds 1.5:1

If your ground is not good, complications will occur in the form of high or erratic SWR readings.  Sandy, dry soil produces bad grounding while muddy, wet soil works best for an effective ground.

RADIALS

For an area with a bad ground, either a half wave dipole mounted vertically (metal rods or wire) can be used or the ground connection can be replaced by additional elements called radials.  

Radials may take the form of  elevated aluminum rods for shorter antennas or wires buried in the ground for longer ones.  A  quarter wave vertical antenna with radials is called a ground plane.  A ground plane needs at least three or four radials extending horizontally from the feed point and each radial should be at least a quarter wavelength (234 divided by the frequency in Megahertz).

RADIATION PATTERN

Vertical antennas do not favor any direction.  Verticals work equally well (or equally poorly) in all directions. This may be an advantage for general monitoring but  may also be a shortcoming when operating on a busy frequency where several stations are being received.  A dipole can filter out or weaken signals from unfavorable directions.  

Since most noise is vertically polarized, vertical antennas tend to receive more noise generated by electrical power lines and appliances. Since the vertical stands straight up, it has a minimal “footprint” which is ideal for small lots.

1/4 WAVE VERTICAL DIMENSIONS

2 Meters (146 MHz)----------1.6 ft.

6 Meters (52 MHz)----------4.5 ft. 

10 Meters (28.6 MHz) ------8.25 ft.

15 Meters (21.3 MHz)------11.0 ft.

20 Meters (14.2 MHz)-------16.5 ft.

40 Meters (7.2 MHz)--------32.5 ft.

75 Meters (3.9 MHz)-------60.0 ft.

80 Meters (3.6 MHz)---------65.0 ft.

160 Meters  (1.85 MHz)------126.5 ft.

REDUCING LENGTH OF VERTICAL ANTENNAS

The length of a vertical antenna can be reduced by winding up some of the wire into a “loading coil.”  Instead of  32.5 feet of wire for 40 meters, a ten-foot vertical may match if a proper loading coil is used.  Mobile antennas on vehicles use this technique.  To determine the number of turns of wire and the diameter for a loading coil, reference charts or software is needed.

Calculations can be difficult and considerable trial and error may be required.

For best results, a loading coil should be mounted near the top of the antenna.  However this may be unwieldy from a mechanical standpoint. To simplify design, loading coils are generally mounted at the bottom near the feed point or sometimes in the center of the vertical element.  The shorter the length of the antenna, the more performance suffers and the narrower the bandwidth will be with loading coils, especially on 40 and 80 meters.  So try to maximize the conductor length.

A vertical antenna feed point can as low as a few feet above the earth.  If adjacent to a body of water,  a vertical can transmit and receive very well. Verticals with low elevations seem to work better than very low dipoles when near bodies of water.

SWITCHING FOR MULTIPLE ANTENNAS

Generally, horizontally-polarized antennas perform better than verticals on frequencies below 30 MHz.  But at times, a vertical may allow you to receive a better signal.  When propagation is unstable and when a band is closing or opening, a vertical may outperform a dipole, even if only for brief periods of time.  Also, the non-directional pattern of a vertical may be better for signals in directions that your dipole does not favor.

To take advantage of the benefits of both, get a two-position antenna switch--also called an A-B switch. Run feedlines from both a vertical and a dipole to your operating position.  Connect each feedline to a jack or port on the antenna switch.  Use a short length of coax (a coax jumper) to connect the switch to your transceiver.

To improve your station, install two dipoles at 90 degree angles with separate feedlines in addition to your vertical.  By using three feedlines and a three-position antenna switch, you can select the best antenna for receiving a desired signal and rejecting unwanted signals.

For weak signal reception, re-check for the best antenna often when conditions are changing.

With a few exceptions, antennas follow a rule of reciprocity.  Most antennas have the same pattern and favor the same directions when transmitting signals as when receiving. If you receive a better signal on an antenna, you will probably transmit more effectively using that antenna.

An exception is a long wire antenna.  Long wires work much better for receiving signals than for transmitting. The longer the wire, the better, even if there are several bends in the antenna. Long wires need not be high.  A few feet off the ground is sufficient.   A long wire is a good addition if a three or four position antenna switch is available.

------------------------------------------------------             Reflector

-----------------------x x-----------------------    Driven Element

-----------------------------------------              Director

SIMPLE BEAM ANTENNAS

Gain an advantage by focusing your transmissions in one direction by using a beam antenna.  You also will receive signals better from that direction.  Unwanted signals from other directions will be attenuated (reduced in strength) on receive and less RF will be transmitted in those other directions.

The Yagi beam uses a half-wave dipole as the radiating (driven) element. Using the formula 468/frequency in Mhz, determine the length.  For beams above 14 Mhz, lightweight tubing is a good choice.  Wire beams are feasible for lower frequencies but are not easily rotated.

I will discuss construction of beams made from tubing.  First, design and tune a dipole for the frequency band desired.  As part of a beam, this section becomes the driven element and the transmission line is attached to the center.

To gain effectiveness in one favored direction, you add a second element called a director. The director will be about 10% shorter than the dipole.  There is no need to cut the director into halves.  It is mounted parallel to the driven element with about .1 to .2 wavelengths spacing between the two elements.  This would be about 25% of the length of the driven element.   To hold both elements parallel to each other, a boom is used.

The boom can be larger lightweight tubing or even rigid thick-walled PVC for VHF/UHF antennas.  Clamp the two elements to the boom using U-bolts or other hardware.  For VHF/UHF beams, you can melt or drill holes in the PVC boom and push the elements through and then hold in place with epoxy or putty.  You also will need to make provisions to attach the boom to the vertical mast pole, again using a U-bolt or similar hardware.

A two-element beam will gain you about 3 to 4 decibel (db) in the favored direction and will attenuate signals coming from other directions.  The attenuation factor may be the more important characteristic on a crowded band.  Your transmit output also will be beamed or focused in a similar manner.

To make a better beam, add a third element called a reflector.  It is 10% or so longer than the driven element and is mounted in a similar fashion as the director.  The three-element beam provides a 6 to 8 db gain over a dipole.

To gain more effectiveness, extra directors can be added.  The antenna will favor the direction broadside to the director(s).  So point the short end of the beam toward the area where you wish to contact.  

If you make the extra directors the same length, you have a Yagi beam.  A variation is the Log-Periodic-V (LPV) beam in which the directors are made progressively shorter.  Many outdoor TV antennas are LPVs.  These have lower gain but cover a wider range of frequencies.

The favored direction is changed by rotating a beam.  Rotation can be accomplished with a rotor or by turning the beam by hand from ground level by leaving the mounting screws slightly loosened and/or attaching a rope to the antenna.

Mount your beam as high as possible.  Telescoping masts mounted to the eave of a building are effective for smaller beams.  Overlap the extendable sections for added stability and use guy ropes for heights above 25 feet.  For heights over 40 feet or so, use a tower.

Tune your beam by adjusting the driven element length for minimum SWR.  Do this first with the beam jjust a few feet off the ground.  The SWR should be less than 2:1 across the band for which the beam is designed.  Then put the beam up to its full height and verify that the reading is still acceptable.

Most beams will work equally well on transmit and receive.  To test your beam, point the director broadside to a distant station and note the receive signal strength as reading #1.  Then rotate the beam 90 degrees and note that signal strength as reading #2.  Then rotate another 90 degrees and note the signal strength as reading #3.  

Obviously reading #1 should be the highest.  Reading #2 should be much weaker and reading #3 an intermediate value.  One S-unit equals about 5 Db although this can vary widely.  The comparison between reading #1 and reading #3 is your front-to-back ratio (f/b).  The more elements, the larger the f/b ratio.  

The comparison between reading #1 and #2 is the front to side ratio and should be at least 20 Db (4 S units).  You also can ask a distant station operator to give you these three signal reports as you transmit and turn your beam.

More director elements=more directional response=more boom length, larger & heavier antenna.

For beam dimensions, use the length for a half-wave dipole listed above to determine driven element length.

Add 5 to 10% to determine reflector element length. (x 1.05 to 1.1)

Deduct 5 to 10% to determine director length(s). (x .9 to .95)

Separate the elements using a boom consisting of lightweight aluminum tubing.  Spacing between elements is 20 to 40% of the driven element length (.1 to .2 wavelength)

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