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Beam antennas?

roadrage

roadrage

Active Member
Aug 3, 2011
497
109
38
In the middle of nowhere. Central/northern Maine
I may regret asking this question. I was thinking of trying to build one in the near future. I first must understand how it works. I may be over my head.

Can someone explain the workings of a beam antenna? I don't know anything about them except they seem to have a lot of gain. I would love to have one for my base instead of using an amp. Looking at one makes me think that it is a series of multiple horizontal dipoles connected together as one antenna. Is this so, or am I way off?

Also, I have a .64 wave ground plane antenna. I figure the horizontal polarization may take away or limit my ability for close transmissions. Can you have a ground plane mounted above a beam? I gave my wife the house in our divorce, but I am going to buy another in the spring. I have 1 50' tower I am putting up. I would like a beam for DX and to use the GP when talking local. I would like the GP as high as I can get it. If it isn't a good idea to mount it over the beam, I will have to get a good tripod and mast for the roof of the house.
 

Having 5 element Yagi for example gives you about 13dB of forward gain. That means 2s units over a dipole. It is a lot. Being 2s stronger than others, to hear stations when others hear only background noise.
Great thing is F/B ratio. On 5 ele it is about 25dB. It means, that you can cancel unwanted signals coming off your direction.
Go for that.
Mike
 
Hi Roadrage,

The beam (or more correctly, the YAGI) antenna is a multi-element array. You start with a dipole antenna. Then you can add a reflector element (sort of like a mirror that reflects the dipole signal a bit) about .25 or so wavelengths away. The reflector is 5% longer than the dipole, or 1.05 x the dipole length. This is one form (and the most common type) of a two element yagi. Add a director element another .25 wavelength away from the dipole on the other side (the first director is 5% less than the dipole, or 0.95 x the dipole length) and you now have a three element yagi. Adding more directors will increase the gain. This is a very simplistic description of a yagi. I would suggest you get a copy of the ARRL Antenna Handbook that will give you more detailed (a lot more detailed) information on all antennas. You will find an entire chapter dedicated to the yagi antenna, and some examples of 10 Meter yagi's (these can be lengthened to work the 11 Meter band). It will look daunting at first, but read, (or rather inspect areas of interest in the book) over-and-over again and it will begin to stick.

As you read the book you will discover how gain is formed, and find out other factors like Front-to-Back (FB) Ratio, Beamwidth, optimum element spacing, feed point matching, and practical construction tips. You can also look this information up on the Internet too, but the Antenna Handbook is a one-stop source for information not only on antennas, but also provides information on propagation, transmission lines, towers, etc.

You can mount your Ground Plane Antenna above the Yagi, it's done all the time. You need at least 10 or 11 feet of seperation between them. The easiest way to do this is to install the yagi horizontal (for horizontal polarization) with the Ground Plane mounted above. You need to take the Square Foot Area of the two antennas and the mast into account to make sure the tower can take the load.

Have fun learning, take your time, and it will come to you.

73,
Mike
 
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I think 'binrat's links are not bad for a very simple explanation of how a yagi works. Just keep in mind that they are very simple, and use analogies which you can't take literally. To honestly understand exactly why 'fields' work as they do requires some 'steep' learning. Have at it till you get absolutely, totally lost. Then back up a step or two and try it again. :) At some point in there you are going to decide that you know enough to do what you want, then do it.
- 'Doc
 
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No; I would not mount any GP above a Yagi beam. It can be done, but it gets a little complex and a bit more expensive. I would run a Imax 2000 above it. That would be cheap and easy and work mounted directly above the Yagi beam.
 
Robb said:
No; I would not mount any GP above a Yagi beam. It can be done, but it gets a little complex and a bit more expensive. I would run a Imax 2000 above it. That would be cheap and easy and work mounted directly above the Yagi beam.
Click to expand...

ROBB???????

An IMAX 2000 is a ground plane. IT just uses the mast or the shield of the coax as a counterpoise.


I have mounted a 5\8 wl vertical ground plane above a four element yagi.

No ground radial needed as the yagi provided the ground radials.

Any type of vertical that DOES NOT require a radial will work above a yagi.

IF you have a GP with radials mount it at leas 1/4 wl above the yagi and it will not have to much effect on the yagi performance,
 
Mounting any two antennas close to each other, above or on a side, will make each antenna affect the other, there's just no getting around that. The affect may not be 'large' but it will always be there. In most cases it can be 'got around', compensated for. If those two antennas are for the same frequency/band then they will affect each other more than if they were on unrelated frequencies/bands. It's a matter of how much affect is too much, or what's acceptable to you.
The easiest 'cure' for that affecting thingy is distance. Doesn't matter in what direction, up or side ways, just more of it. That can get very unpractical real quick. You'll have to decide the 'best' alternative for your situation. (One of those 'relative' thingys, and if you run out of relatives I got some you can have!)
- 'Doc
 
Reviewing the books and references some have provided here is a great way to understand the inner workings of the beam antenna. Which could be a Yagi, Quad or even a Quagi.

On the other hand if your goal is simply to build an effective beam quickly, just go to the Maco website and pick how may elements you want. Download the assembly manual and copy it.

There is an art to designing these types of antennas. The simple formulas for element length and spacing are guidelines which often compromise between forward gain and front to back rejection.

It's not too hard to mess one measurement up that could impact performance negatively. If this is your first build, you may have an easier time with better results working from a proven design.
 
wavrider said:
ROBB???????

An IMAX 2000 is a ground plane. IT just uses the mast or the shield of the coax as a counterpoise.


I have mounted a 5\8 wl vertical ground plane above a four element yagi.

No ground radial needed as the yagi provided the ground radials.

Any type of vertical that DOES NOT require a radial will work above a yagi.

IF you have a GP with radials mount it at leas 1/4 wl above the yagi and it will not have to much effect on the yagi performance,
Click to expand...
Yes, the Imax 2000 is a ground plane that doesn't come WITH the ground plane kit unless you purchase it separately. But if it is mounted above a Yagi beam, the beam itself will act like a ground plane for it - no ground plane kit needed to be purchased/used. Just killing two birds with one stone this way - is all. There are a lot of people that have done this, our Aussie cousins down under too . . .
 
Robb said:
Yes, the Imax 2000 is a ground plane that doesn't come WITH the ground plane kit unless you purchase it separately. But if it is mounted above a Yagi beam, the beam itself will act like a ground plane for it - no ground plane kit needed to be purchased/used. Just killing two birds with one stone this way - is all. There are a lot of people that have done this, our Aussie cousins down under too . . .
Click to expand...

:D

I have one above my Mosley PRO67B and Both work great. I do not notice any interaction with either one.

no change in resonance verified by the analyzer.
 
The math makes my head hurt. There are literally thousands of variations of measurements that go into designing a yagi . After checking out the basics ,down load one of the yagi design programs .Let the computer do the work as well learn along the way. I use Yagi cad simple and free.
 
Simply copy a design as someone above suggested, unless you are really interested in understanding the how and the why, I wouldn't bother worth the ARRL antenna manual ;)
 
I really appreciate all the replies. It has given me much to consider. I may end up just buying one for simplicity. I also will give a hand at trying to build one as well for the experience and knowledge. I always need to know the how and why. I am not the kind of person who is happy just knowing the answer. There is no wisdom in the answers alone. Only having the ability to come up with the answer is worthy of appreciation.

I am sure I will run into problems and have to research and ask advice. But helps things stick in my brain a bit better than if all goes well. Maybe I'll get a start in the spring.
 

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