Sunday, October 27, 2013

A Multi-Band horizontal dipole. Post #237

How would you like to build a simple, inexpensive multiband antenna that will give you hours of enjoyment chasing DX or "ragchewing" with your local amateur radio friends?  Sometimes, when it comes to "homebrew" antennas, you can't beat a 40 through 10 meter dipole or doublet fed  by ladder line into a suitable antenna transmatch.  One antenna can perform well on a variety of amateur radio bands if it is designed and built carefully.

Ideally, we could all use a 50-foot (15.24 meter) tower with a 4-element monobander on 20 meters, plus separate antennas for 160, 80, 40, 15, 10, and whatever other bands you can squeeze in.  However, most of the amateur radio operators I know can manage only one or perhaps two HF antennas on their small properties.  Add to this mix the growing trend of antenna restrictions found in many housing areas these days and you've got the one antenna scenario.  And that antenna must be hidden in many cases.

Enter the horizontal flat top half wavelength dipole and its immediate cousins the inverted vee, the sloping dipole, and the vertical dipole.  In past posts, I've described the inverted vees and sloping dipoles used at my station.  All of these antennas have performed well as multiband antennas using 450-ohm ladder line, a 4:1 balun, and a sturdy antenna transmatch.  My two antenna "tuners" are the trusty Drake MN-4 and a very old MFJ 941-E Versa Tuner which I use for 30 meters.

So, let's build a simple dipole half wavelength antenna and get ourselves on the air.

According to the late William Orr (W6SAI), the common half wavelength dipole fed at the center with two conductor feed line is an "inexpensive antenna to build and to get working."  If you use 450-ohm ladder line or television 300-ohm twin lead in combination with a 4:1 balun and robust antenna "tuner", you'll have a working multiband antenna at minimum cost.

MATERIALS:

If you're going to use the dipole for several bands, design your antenna for the frequency of the lowest band.  In my case, I wanted to work 40 through 10 meters.  So, using the frequency of 7.088 MHz (the frequency of the daily Hawaii Afternoon Net) and the general formula 468/f (MHz)=L (ft), I computed a half wavelength of 66.02 feet(20.13 meters).  Each dipole element would be half that amount or 33.01 feet (10.06 meters).
Some antenna experts recommend that you cut your antenna elements a bit long to allow for trimming and SWR adjustments.  For my purposes, I left each element slightly longer than 33-feet (10.06 meters).  The antenna transmatch would compensate for the mismatch found on each band.

Two masts to support the dipole.  I had two spare MFJ 33-foot (10.06 meters) telescoping fiberglass masts in the garage of my new home site in the Puna District.

Two ceramic insulators to tie off the antenna elements to the supporting masts.

Two 5-foot (1.52 meters) wooden stakes to support each mast.

One "homebrew" center connector to support each conductor of the 450-ohm feed line.  I used some plastic cut from an old "Clorox" (household cleaner) bottle for the center connector.

Two pieces of #14 AWG household wire, measuring slightly more than 33 feet (10.06 meters) each.  These wires would be the flat top horizontal half wavelength dipole.

Fifty feet (15.24 meters) of 450-ohm ladder line to feed the antenna.

One W9INN 4:1 balun.

Twenty five feet (7.62 meters) of RG-8X coaxial cable with UHF connectors.  The ladder line would be connected to the 4:1 balun and then the coax would be connected to the balun.  The coax would run into the shack for eventual connection to my transceiver, an old but trusty Ten Tec Argosy II.  If I wanted to operate on 30 meters, I would swap out the Drake MN-4 for the MFJ 941-E Versa Tuner.

Short pieces of dacron rope to attach the ceramic insulators to each mast.

Several 3-foot (0.91 meters) RG-8X coax patch cords to connect the Argosy II to the Drake MN-4/Versa Tuner, the dummy load, and the low pass filter.

Basic tools, soldering gun, tape, nylon ties.

ASSEMBLY:

The dipole was built on the ground.  I measured out the antenna elements, attached the ceramic insulators to the end of each element, and secured each end insulator to its respective mast with dacron rope.  The rope was further secured to each mast with several nylon ties.

The 450-ohm ladder line was secured to a "homebrew" plastic center connector with nylon ties.  Each antenna segment was soldered to a conductor of the ladder line.  Those connections were wrapped with several layers of vinyl electrical tape.

Each mast was hoisted on its wooden support stake and adjusted so that there was a slight "dip" in the horizontal flat top dipole towards the center conductor.

The ladder line was led to a W9INN 4:1 balun attached to the garage wall.  The balun was approximately 10 feet (3.04 metes) above ground level.  A 25-foot (7.62 meters) length of RG-8X with UHF connectors was attached to the 4:1 balun and fed to the radio room through a "homebrew" patch panel in the room window.

The coax was attached to the Drake MN-4 transmatch.  Short pieces of coax cable connected the Argosy II, the dummy load, and the low pass filter to the transmatch.  I also added a 33-foot (10.06 meters) "counterpoise" wire to the ground lug of the Drake MN-4.  So far, there have been no rf problems in the shack.

INITIAL RESULTS:

For a general antenna covering several bands, performance has been quite acceptable.  Although I would have preferred to have the dipole at a higher elevation, I couldn't find suitable trees nearby that would accommodate a height of a half wavelength above ground for 40 meters--66 feet (20.12 meters).  Even at this lower elevation, the antenna works well.  Performance on 20, 15, and 10 meters is good.

With the help of the Drake MN-4 transmatch and the MFJ 941-E Versa Tuner, I can get a SWR of 1.1 to 1 across each band.  Most of my mainland U.S. contacts range from 559 to 599 (cw) and from 54 to 59+(ssb), depending on the band and time of day.  Local Hawaii contacts often exceed 599 (cw) and 59 (ssb).
I usually run the old Argosy II at 20 to 30 watts.  The power supply is a solar panel/deep cycle marine battery combination.

Most of my materials were bought locally or were found at garage sales (#14 AWG wire, ceramic insulators, dacron rope).

Of course, your results will vary.  Soil conditions, available space, and property restrictions will affect your design.  But, for a simple, inexpensive antenna that delivers both excellent local contacts and sometimes decent DX, you can't beat the old half wavelength dipole. Besides, unlike a vertical antenna, you don't need a ground radial system to make the dipole work.  And you get the satisfaction of making the antenna yourself.

REFERENCES:

Orr, William I. (W6SAI) and Cowan, Stuart D. (W2LX).  The Radio Amateur Handbook.  Radio Publications, Inc., Wilton, CT, 06897.  First Printing 1978.  pp. 130-137.

http://www.ehow.com/how_4530_build-dipole-antenna.html.

http://voics.yahoo.com/how-dipole-antenna-340817.html.

http://www.buzzle.com/articles/dipole-antennas-build-a-dipole.

http://www.hamuniverse.com/easydipole.html.

http://www.stepbystep.com/how-to-build-a-dipole-antenna-54624.

Or, you can consult the latest edition of the ARRL Antenna Book.

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Aloha es 73 de Russ (KH6JRM)

BK29jx15--along the beautiful Hamakua Coast of Hawaii Island.