Antennas are always a hot topic on 6 meters because there is more antenna experimenting that goes on on this band than any other. The antenna size is manageable and many things that can be done on 6 meters are impossible (or highly difficult) to do on HF.
Today’s topic is stacked dipole arrays. We’ll use EZNEC to model some dipoles.
Remember that antenna gain is nothing more than taking radiation from the “standard” isotropic radiator and concentrating it in desirable directions. You can load up a straightened out paperclip on 160 meters and it will radiate every bit of power put into it. But it won’t have any gain. The sole purpose of your antenna to direct radiation where you need it the most and this is called “gain”.
There are three types of gain in antennas:
- dBi – gain over isotropic
- dBd – gain over a 1/2 wave dipole in free space
- dBadv – huge random numbers thought up at the bar by commercial manufacturers of yagi antennas – most often expressed as simply “dB” which we might assume is gain over a rock (or something)
This is the radiation pattern for a single 1/2 wave dipole in free space – 2.14 dB gain over the venerable isotropic radiator that only exists in theory. This shows the “figure 8” pattern that most folks associate with a dipole antenna. Unfortunately, this is not how your dipole works in the real world, and I don’t know anybody that operates a ham radio antenna in free space.
Put that same dipole at 1/2 wave above ground – gain is 8.4 dBi and radically different than the figure 8 pattern that everybody uses as the “example” for a dipole. This is due to ground gain:
You move that same dipole to 3/4 wave above ground, this is what you get – a little bit lower radiation takeoff angle but less gain on the side lobes than you had at 1/2 wavelength because now you’re losing a bunch of radiation straight up where the max gain is. The earth as another “element” is too far away. You can put a dipole too high and actually lose performance unless you’re trying to talk to a spaceship in orbit. You need something on top to direct that wasted radiation downwards.
Move this same antenna closer to the ground than 1/2 wave and you lose the sidelobes altogether at less than 1/4 wave and the radiation pattern looks like a big balloon – it becomes an NVIS antenna. But the above is modeled at 3/4 wave above ground.
Stack two dipoles with the bottom element 3/4 wave above ground, with 5/8 wave stacking height between the dipoles and it changes to this – 3.26 dB gain over a single dipole at 3/4 wavelength above ground with lower radiation takeoff angle and a couple minor higher elevation lobes. It gets rid of that big balloon on the top and compresses the radiation towards the horizon.
Although not modeled here, a 3 element yagi at one wavelength above ground develops 9.13 dBi gain in the forward lobe at a takeoff angle of 17 degrees. And height with a yagi is better to prevent what actually amounts to ground losses and higher takeoff angle. So two stacked dipoles actually develop more gain, omni-directional, than a 3 element yagi develops in the forward lobe.
Of course, all this assumes no losses in the antennas and earth in the models, and things are slightly different in the real world due to obstructions, RF conductivity of the soil, etc.. But still it gives you an idea of what’s going on with your antennas.
So how do we apply this to building a stacked dipole array for 6 meters? Well, one the simplest antennas that most new Technician operators learn to build is a 2 meter vertically polarized dipole. Nothing more than a piece of PVC pipe with two aluminum or copper rods or wires cut to 19.5″ long on the end of it. Hook 50 ohm coax up to it with the center conductor going to one element and the shield to the other, and you have an SWR of 1.5:1 and it “just works”.
The length of a 1/2 wave dipole is given by the formula 468/frequency in MHz = length in feet of the antenna.
OK, so you just build two of these for 50 MHz . And then you have to co-phase them.
Now, I keep talking about “wavelengths” in the above modeling of antennas. What is this? A wavelength is the length of a complete sine wave. Since the speed of light is 299,792.458 km/sec and radio waves travel at the same speed, the wavelength in meters is easy. For 50 MHz, which means the frequency of your sine wave is 50 million cycles per second, you only have to know the propagation speed to figure out how long it is. 299,792.458 km/sec divided by 50,000 kilo-cycles per second = 5.996 meters. That’s your wavelength. Guess why the band is called the “6 meter band”?
5.996 meters x 3.28 feet/meter = 19.67 feet
Notice that if you do the math the wavelength of your radio waves traveling thru space are different than what you get when you do the calculations to build an antenna. This is due to something called “velocity factor” in the wire, and due to the fact that the energy travels slower in copper or aluminum than it does in space. The standard formulas for cutting a dipole have a velocity factor of 0.9 built-in. But we’ll get into velocity factor when we build our phasing harness for our new dipole array.
The optimum spacing for two stacked dipoles is 5/8 wave. This is 19.67 feet x .625 = 12.29 feet. Optimum spacing for three stacked horizontally polarized dipoles is 3/4 wave. This is 19.67 feet x .75 = 14.75 feet. Obviously three stacked horizontal dipoles are going to require a minimum 44 foot tower if you have the bottom element 3/4 wave from the ground.
All antennas in the array have to be fed exactly in phase. This is called co-phasing. If you simply run your feedline to one antenna and then a jumper to the other one, the second one is going to be out of phase by the wavelength of the jumper line. So you have to use equal length feedlines to each antenna in the array that all come to a common feedpoint where they are connected to the single feeder from the shack. And you want all antennas in the array to be fed at a current node.
So let’s say we have two stacked dipoles. We run a 400 ohm parallel feeder to the bottom one. To be in phase, the top one has to be fed with a full-wave jumper. A full wave is 19.67 feet @ 50 MHz. But that is not the length of your jumper or phasing line. The length has to be shortened by the Velocity Factor of the wire. 14ga THHN wire has a VF of .88 (measured with my antenna analyzer). Good quality RG-8/U coaxial cable is .81. So if we phase our array with parallel feeder made with 14 AWG THHN wire the length of the phasing line is 19.67 ft x .88 = 17.31 feet. Since the stacking height between the antennas is 12.29 feet the 17.31 foot long phasing line will reach between the two dipoles.
If you have three or four stacked dipoles you must build a phasing harness – read the sentence above that is in bold again. Phasing harnesses are built with 1/4 wavelength multiples of feedline. Odd 1/4 wave multiples will act as transformers to transform low impedance at the antenna feedpoint to high impedance at the feedline (like matching a dipole to parallel feedline). Even 1/4 wavelength multiples repeat the impedance at the antenna. So this all depends on whether you chose to use coaxial cable to feed your array, or twinlead (parallel feeder).
So there you are – stacked dipoles are simple to build, a little more complicated to phase them, they take a bit of tower space. But they are VERY effective antenna arrays on 6 meters, usually matching yagi’s pretty easily with the added advantage of having gain in all directions.
Stacked dipoles are also very common high-gain antenna arrays for VHF repeaters, except in vertically polarized configurations. In a vertically polarized setup this is called a “collinear” antenna array. A topic for another day but the principal is the same.
Chris Olson – AC9KH