Co-Phased Antennas

Mosley Pro 67-B

Mosley + Moxon Co-Phased for 20meter.

Co-Phasing 

Co-phasing or “stacking” has long been a way to get high gain from antennas. Co-phasing involves placing two (or more!) identical or even not identical (in my situation)  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 (or out – phase).The result is 3 db more than just a single antenna. I have do a lot of experiments with stations in North America on air and most of the time the result is between 3-6db more with the Co-Phased Antennas (Mosley + Moxon).

I think Iam the only one (maybe in the world?) who is using this HF antenne system ,because also that the antennas are not identical !

Co-Phased is meaning that the antennas are standing next to eachother and not above eachother , most of the big contest stations using 4 over 4 or 5 over 5 stacked yagi’s above eachother .

Getting Down to Business

Stack - 2 Yagi System

Stack – 2 Yagi System

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 . In my situation Iam using 1.5 wavelenght between the antennas for the 20meter band.The rules go like this, for higher gain antennas larger stacking distances are needed to realize the full 3 db gain increase. This means , to get the whole 3 db increase you should get them as far apart as practical. There is no sense in putting up huge antennas and keeping them so close.  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.

 

Figure 3 – On the left is the radiation pattern of co-phased beams that are too close to each other . The pattern on the right show the pattern of single beam. When stacking distances are too close, there is not really any effect from stacking, the pattern remains the same as having a single antenna.

 

overlap

 

 

 

correct

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4 – The result of stacking beams with a wide enough spacing. You can see that the near field patterns of the single antennas just touch (this requires wide spacing ), the resulting pattern is re-shaped and has the full 3db increase.

 

coming soon more… under construction

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