Get Horizontal on the 70 cm Band
(and find out that it’s not dead!)
by Dave Clingerman – W6OAL
By popular request I have been asked to dash out an simple instruction on how to “Get Horizontal” hear and be heard on the 70 cm band in particular. Why horizontal anyway? I will suggest a very simple design. I will tell you what is the better material to use. I’ll explain why such a design and materials are used. I will give you some construction tips. And, I’ll suggest where you should/could place your creation for it to be the most effective.
One of the main reasons to use horizontal polarization, not just VHF and UHF but all bands is “noise”. Noise appears to be vertically polarized according to some scientific research and is generally manmade. This type is usually from unintentional radiators; power lines, traffic lights, photoelectric light switches, fish tank heaters, faulty door bell transformers, electric blankets, computers and peripherals, etc… Other vertical interference is from many of these off shore, unlicensed products that are used in a multitude of applications. Much of the stuff does not have FCC approval. They are dirty and have little or no filtering making them very capable of radiating their harmonic content and other extraneous garbage.
By convention VHF and UHF, CW and SSB activity has from very early on used Horizontal Polarization as it is less susceptible to fading and picket fencing as in the case of mobile operation and has some immunity to other near band communications systems that are vertically polarized. The FM mode used vertical polarization as most of the activity is mobile or portable and the easiest way to operate FM is with a quarter wavelength spike on the roof of an automobile, backpack, bicycle or whatever.
A large horizontal array is not necessary nor is a lot of power necessary in order to be able to enjoy somewhat distant contacts on the VHF, UHF and Microwave bands. I’ll help you establish an objective for using a beam antenna. I’ll help you build a short boom length, four element yagi for the 70cm band using 10 to 50 watts that will get you up and down the Front Range of Colorado with a usable signal and allow you to hear the other folks quite adequately. All materials are available at your local hardware emporium or Radio Shack. Aluminum square or round tubing makes great booms. Aluminum clothes line wire makes great elements. A Radio Shack TV balun (300 to 75 ohms) makes a great matching device.
OK, are we ready? We have established the reason for going horizontal; noise and convention. Now, the objective, we want to place our signal and be able to hear more in one direction than another. This is called directivity, or in dB we call it gain. What sort of beam width do you want or is required? If it’s too wide the power will be spread out over a wider direction which may not put enough power in the direction of your intended audience. The power density will be spread too thin over the width of the pattern and your signal will be weak in the direction you are attempting to work. If it’s too narrow stations you want to receive your signal will be cut out. Establishment of a reasonable pattern is essential. From experience I have found out that in general a pattern of 60 to 90 degrees (o) should be sufficient for local and out to a 100 mile radius, depending also on the power being used. From this we can establish how large the yagi should be. How do we do this? Let’s take the geometric mean of the two suggested extremes 60o and 90o. Multiply the extremes and take the square root (square root of 60o X 90o = 73.5°). Short beams have almost a symmetric E and H field pattern, in that, their horizontal pattern is approximately equal to the vertical pattern. To find then how many square degrees the beam width will be we square the degrees of the horizontal pattern. Thus, our composite beam width will be 73.52 or 5,402.25 square degrees or correctly stated ‘steroidal degrees’ round degrees if you will. A sphere contains 41,253 steroidal degrees on its surface, (4 π R2, R= a radian or 57.3°) therefore our beam width occupies the difference of 41,253 ÷ 5,402.25 = 7.64. This 7.64 is how many times more power is in the main beam of a yagi than in any other direction. We can express this in dBi by taking the log of this number times 10 or 8.83 dBi. The “I” indicates the gain is over an isotropic source (a point source). To find the gain over a dipole or dBd we subtract 2.14 dB (the gain of a dipole over an isotropic source) or ~6.7 dBd. Next we need to determine how many elements a beam antenna will need to have to achieve 8.83 dBi or 6.7 dBd. If we add a reflector the gain increases by 2.8 dB. Theoretically doubling the size of an array yields a 3 dB increase but due to losses, in reality, the increase is more on the order of 2.8 dB. With a dipole for a driven element and a reflector the array will have a gain of ~4.94 dBi. If we double the size of the array again by adding two directors in front of the driven element we pick up another 2.8 dB gain or 7.74 dBi. This is within 1 dB of our target figure of 8.83 dBi, close enough. We have determined that a four element yagi will provide us with the gain we need to have a pattern of ~74o at the -3 dB points. The -3 dB points are where our gain drops by half on either side of the pattern from the gain in the center of the pattern.
All that said and done, lets build a four element yagi for the 70 cm band. Starting with the driven element and using the “constant” 5540, a dipole or folded dipole length is acquired from the equation 5540 ÷ F ( frequency in MHz) = L (length in inches) or 5540 ÷ 432.1 = 12.8″. The reflector by convention should be ~5% longer than the driven element or ~13.5″. The two directors generally are ~3% shorter than the driven element, again by convention, or ~12.5″ and in our case both can be the same length. Progressively shortening the directors tends to broadband the yagi.
Spacing between elements can be anywhere from 0.25 to 0.12 wavelength. I have found good results by equally spacing on a short yagi with a small number of elements 0.2 wavelengths along a boom. This would make the boom length 0.6 wavelength long or ~16.5″.
Now let’s get to materials. A boom should not be terribly large in diameter and not too small as it has to support the weight of the elements which in our case is fairly insignificant. Let me suggest a 1/2″ square aluminum tube (or even channel) which can be acquired from Home Depot or Loew’s in 3’ or 4’ lengths. While you’re there pick up a roll of aluminum clothes line or three, 3 foot lengths of aluminum rod about 1/8″ diameter and a 3‘ length of 1/8″ brass brazing rod. Cut the clothes line (or rod) to the dimensions of the reflector and the two directors. Straighten them out using a hammer or a mallet. Mark the brass rod at 25.6″ from one end and cut off the rest. Rather than just a dipole we are going to make a ’folded dipole’ driven element. This is going to make matching to the transmission line a lot easier than other types of matching systems.
Prepare the square boom material as follows; Cut the 3 foot length of material in two (18″ and 18″). Come in ¾” from each end and mark the boom for drilling. Between these two points divide up the space into three equal parts (5.5″ between each) this gives you four marked places for drilling. Drill holes equal to the diameter of the rod or wire you are using for the elements. On the adjacent side of the boom and inline with the holes just drilled drill four holes 0.089″ (#43 drill). These holes will be used to accommodate 4, ½” long #4 self tapping sheet metal screws that will hold the elements in place by grazing them when inserted. Mark the center of the reflector and two director elements. Insert the reflector at the hole at either end of the boom so that it is centered in the boom. Next, insert the two directors #2 at the other end of the boom and #1 in from it. Run a #4 sheet metal screw into the holes on the adjacent side of the boom in which the elements are inserted. The sheet metal screw will, at some point in being inserted encounter the element and hold it fast in place.
Preparing the driven element for the brass rod is next. Come in 25.6″ from one end of the rod and cut off the remaining piece. Taking the 25.6″ length, measure 6.4″ in from each end and mark it. Insert the rod in the boom in the hole between the reflector and the director #1. Make sure the center of this rod is aligned with the hole in which the sheet metal screw is to be inserted and screw in the fourth sheet metal screw holding the driven element in place. This element will be much longer than the other three. At the marks you’ve made previously on this brass rod, using a ½” mandrill or rod at the marks – bend the brass rod toward the boom so that the top and bottom lengths of the rod are parallel to each other. The ends of the brass rod will now be very close to the bottom of the boom but not touching each other or the boom. The balun you acquired from radio Shack now attaches via soldering to the two ends of the brass rod. The balun can be held in place with electrical tape or a nylon cable tie. An adapter will be needed to go from the “F” connector on the balun to a BNC connector or connectors on your 50 ohm transmission line. This balun will handle up to about 50 watts input NOTE: There are big differences in TV baluns from vendor to vendor. If the one you have employed does not perform to your expectations. Try a conventional half wave balun of coax. It will be about a 10″ loop with a 10″ pigtail.
THAT’S ALL FOLKS! You have just created a four element yagi for the 70 cm ham band that will provide ~8 dBi gain, a pattern of around 70°, A 20 dB front to ratio. Now mount it on a mast and get it as high in the air as you can or in your crawl space, attic or what have you but get it in the air. I’ll be listening!