Co-phasing or “stacking” has long been a way to get high gain from antennas. Co-phasing involves placing two (or more!) identical antennas either side-by-side or one over top another (“stacking”) at a certain distance apart (usually a 1/2 Wavelength or more) and feeding the antennas in-phase. The result is 3 db more than just a single antenna. In my opinion, this is the absolute way to go with beam antennas instead of going with say, 8 elements beams, it would be much better to go with co-phasing two 4 element beams. If you look at the gain figures for a 8 element beam, you see that you will end up with more gain if you co-phasing 4 elements instead. If you are considering co-phasing antennas for your mobile, better check out the “Mobile” section first.
Before I get into the details of co-phasing, lets consider what antennas we should considered co-phasing. The reason I say this is because co-phasing would not make sense on certain antennas, when you could just use another type of antenna (with more gain) that would be simpler. For instance, you could take two A99’s (1/2 Wave verticals) and co-phase them, but, why? It would be more simple to get a 2 element yagi, and mount it vertically. It would have more gain than two co-phased A99s (and I hope you are not taking their advertised gain figure of 9.9db and adding 3db onto it, if you think co-phasing two A99s gives you 12.9 db (more than a 4 element beam!) you better go back and read how much gain your A99 really has in the “Verticals” section).
Lets look at the pattern of a single 1/2 wave vertical and the effect that co-phasing has (by showing the pattern of two co-phased 1/2 wave verticals). As you can see in figure 1 the pattern is now focused mainly into two directions. If you want to have a pattern that is focused into two directions only and do not want the single direction only that the 2 element yagi gives you, you could make a yagi that does not have a reflector element, but two director elements. This would have more gain than two co-phased A99s and take up a lot less space. Figure 2 show a possible arrangement for phasing two 1/2 Wave vertical antennas.
The only antennas I could really recommend co-phasing are beams with 3 or more elements. The work involved is serious, and with other antennas, there are simpler solutions to stacking.
Why co-phase antennas then? Well, co-phasing beams with 3 or more elements results in seriously high-gain. If you are serious about phasing your antennas (any antennas, do not let my ideas and opinions stop you from co-phasing your antennas) then lets get started.
Getting Down to Business
First off, stacking takes a lot of planning, time and money. More planning than anything. Starting off with the distance you should use, let me discuss what good stacking distances are. Most text books say that the spacing between co-phased antennas should be at least 1 wavelength (36 feet!). But in practice at 27MHz, we see that stacking at 36 feet is tough. The rules go like this, for higher gain antennas larger stacking distances are needed to realize the full 3 db gain increase. This means for your 4 element beam, to get the whole 3 db increase you should get them as far apart as practical. I have instructions from Antennas Specialists (the maker of the Moonraker 4) on how to stack 2 Moonraker 4’s. The distance they recommend is at least 24 feet. If it were me, folks, If I were going to attempt this, I would get it out to at least 27 feet. There is no sense in putting up huge antennas and keeping them so close. For lower gain antennas (the A99 (1/2 vertical) for example) it would be perfectly acceptable to stack at 1/2 wavelength apart (18 feet). I would not recommend co-phasing any CB antennas under 1/2 wavelength though (except for mobile antennas where you can only go so wide). What happens is the radiation patterns overlap so much, see figure 3, that there is no effect from co-phasing. The pattern combines and ends up the same shape as a pattern from the single antenna. It is necessary to separate the two patterns far enough that overlap is not great, and then you will get the effect of the pattern reforming and creating a much narrow front lobe as shown in figure 4. Its as simple as that.
Lets look at some possible physical arrangements for co-phasing. Figure 5 shows two cubical quads stacked side-by-side. This is the most common arrangement. A stacking boom must be made to support the two antennas. HERE is the plan for a stacking boom designed by Antenna Specialists. I do not know of anybody that commercially makes a stacking boom. Another possible combination of stacking two antennas is shown in figure 6. This is okay if you only need really high gain in one direction, because if you require that this arrangement be able to be rotated, the WHOLE tower will have to turn! It has been done, but the construction problems are formidable. You can even stack more than 2 antennas at a time. Any even combination of antennas can be co-phased. Figure 7 shows an example.
I will leave the construction planning of supporting such huge antenna setups to you! I would suggest using aluminum for the stacking boom and use guy cables (3 of them) on the stacking boom made of phillystran. Phillystran is a insulating material that will not stretch and is invisible to RF. Hey, if you are going for it, do it right! Minimize the effect your supporting structure has on your antennas patterns. Steel guy cables cause undesirable pattern interference. HERE is another document from Antenna Specialists on how to orient your Moonraker 4 antennas on the stacking boom. HERE is one last document of a crazy guy climbing a skinny tower to put rather large stacked Moonraker 4’s up.
The All-Important Step
Now that we have down the distance we should use and the arrangements we can use to stack antennas, most importantly we must get the feedline right! You must feed your antennas properly. It is not called co-phasing for nothing. If you use random amounts of coax, your beam will be fed out-of-phase with one another..let’s just say this is not what you want!
Figure 8 shows the all important arrangement of coax you must use to feed your phased antennas right. A odd multiple of 1/4 Wavelength 75 Ohm coax must be used. Check out the “Coax Basics” section for information on cutting coax to certain lengths.
I will go over some common coax types here to help you out. Please, please, do not “assume” that you can just cut your coax close enough and you can just adjust the antennas. This step will make or break your setup. You must verify your coax type, get the right velocity factor and cut it right. Again pick a frequency in the middle of where you talk…or pick the exactly frequency you use most.
Here are some examples (for common 75 Ohm coax):
|1/4 Wavelength for channel 40 (27.405 MHz)|
6′ 8 3/4″
7′ 2 3/16″
7′ 1 1/8″
More example. Say we have built our stacking boom for our Moonraker 4’s 27 feet long (our Moonrakers will be 27 feet apart boom to boom) and we got RG-59 Foam Coax to use for the co-phase harness. We must make each leg to the antenna (from the Tee connector) 21′ 3 1/4″. As you can see, if we stretch that out straight across we are going to have a ton of excess coax, we only need 13 1/2 foot for each leg, but we have to use 21′ 3 1/4″ on each side to match them up right and feed them in phase. Take the excess and spiral it down the stacking boom. Do not do it in a tight coil, but make a long wide-spaced spiral around the stacking boom towards each antenna starting after the Tee connector.
Tuning the antennas
Before you place your antennas up on the stacking boom with the co-phase harness you should adjust each one individually. I would place it up on the stacking boom where it will be, then hook the coax straight from the radio to it (preferably use 1/2 multiples of 50 Ohm coax from the radio to the tee connector, or antenna in this case) then tune the antenna for whatever ever frequency you are centering your design on. Remember, your antennas have to be adjusted for the exact same frequency and the co-phase harness must be cut for that frequency also. So after you get both antennas adjusted to have the exact same SWR curves as one another, you are ready to connect up your co-phase harness. Be sure that you have oriented both antennas in the same manner. For instance if the gamma match is pointing right on the right antenna, be sure that it is ALSO pointing RIGHT on the left antenna. If you have followed these directions, your array should be close to where you tuned the individual antennas for. It will not come out exactly where you tuned the individual antennas for because the odd 1/4 wavelength multiple of coax is going to induce confusion to your SWR meter again (only 1/2 wave multiples give accurate readings)… but remember the actually SWR at the antennas feedpoints in theory will be were you set them for. If SWR is way out of whack, you may have to go back and readjust your antennas again. Or, try rewinding your co-phase harnesses again in a different way. Instead of making wide space wraps around the stacking boom, run the coax straight down to the antenna from the tee, and wrap a tight coil around the stacking boom right where it connects onto the antenna. The wraps of coax should be touching each other. This acts as a RF choke (prevents RF from flowing on the outside of the coax) and prevents feed line from interacting with the antenna pattern. This may be the cause of your problems. If you antenna uses a gamma match or balun you should not have this problem though. But, may the force be with you!
Article by Scott scott2RP789 originally available at http://signalengineering.com/ultimate/co_phasing.html