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Nitro
| | Posted on Thursday, July 11, 2002 - 9:46 pm: | 
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Can anyone explain to me what happens to the TX and RX signals when they reach the beam antenna, and what part does the reflector play in the radiation and receiving of the TX and RX signals?
It's shocking to know so many of my friends with beams don't really know how they operate and how the beam handles the signal, they just assemble the beam and start talking.
Nitro. |
ChillyDog
| | Posted on Friday, July 12, 2002 - 11:06 am: |
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Directional antennas work by constructive and destructive interference between the phases of the radio waves radiated from the individual elements of the antenna.
Now, that's not a very helpful description, is it? I'll try to explain in clearer language.
A familiar way of describing radio waves is to compare them to the waves formed by dropping a rock into a pond. When the rock hits the water, a series of peaks and troughs are formed, radiating out in a circle around the point the rock entered the water. In a general sense, radio signals radiating from a single element antenna look the same.
As an example: consider a vertical antenna. If you were to be floating above it, and looking down, you would just see the tip of the antenna as a point. Radio waves would be radiating with peaks and troughs around that point the same as the water ripples around the rock in the pond.
Now imagine what would happen if you dropped two rocks, separated by a few inches, into the pond at the same time. Each would hit the water and start generating peaks and troughs. These would travel outward just the same as the single rock, until the ripples from each rock came together.
Where the peaks from each ripple came together, the peaks would add, making a bigger peak. That's called constructive interference. When a peak and a trough came together they would cancel each other, leaving the water flat. That's destructive interference. And where the troughs came together they would also add (like two negative numbers) to make a bigger trough. And that, again, is constructive interference.
To relate this to radio antennas, imagine the common dual "trucker" antenna array used in CB. Two antennas, separated by a half-wave length, fed by a "harness" the splits the radio energy in half and drives the antennas in phase. These are equivilant to the two rocks dropped into the pond.
The radio waves emanate from each antenna, and through constructive and destructive interference add and subtract the peaks and troughs so that the field pattern of the radiated signal becomes directional.
With the typical dual antenna, ideally installed, the directionality concentrates the field strength to the front and rear (in more technical terms, perpendicular to the plane of the antennas) and reduces the signal to the sides (along the plane of the antennas.)
This same process applies to other types of directional antennas. With a "beam" antenna, usually only one element is driven, rather than the two driven elements in the dual antenna example, but the process of constructive and destructive interference still applies.
What happens in the beam antenna is that the radiated signal from the driven elements induces current in the reflector and director elements which then re-radiates the signal. The effect of the phases of the signals from all the elements results in a concentration of the field strength into a particular direction. It's not perfect, in that some signal strays "off-bore" and causes secondary lobes in the radiation pattern, but a beam antenna can be very effective.
One way to explore wave interference is by using a wave tank. I don't know if schools still teach this in physics class, but back when dinosaurs roamed the earth and I was in school, a common classroom demonstration was to put a wave tank (a flat container of water) on an overhead projector and tap the surface to make ripples visible on a screen. There were even devices to provide a rythemetic tapping that simulated radio waves. It was a very good way of understanding a hard to describe phenomena.
You can do the same with any still surface of water and a couple of pencils. The larger the surface the better the effect, but even just a casserole pan is effective.
If you experiment this way and are careful with your timing, you can easily see the directional effects. You can vary the frequency of tapping and the distance between the points of contact, and try tapping out of phase. If you're a physics geek like me, it can be entertaining!
Best Regards,
Bob |
DeadlyEyes
| | Posted on Friday, July 12, 2002 - 1:26 pm: |
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The easiest way to describe it is as follows.
For transmissions think like a flashlight and how it aims a beam of light.
For receive just think of what happens when you cup your hands in back of your ears. The hands act to focus the incomming sounds making the weak noises sound louder. |
Tech833
| | Posted on Friday, July 12, 2002 - 1:36 pm: |
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This is a very complicated subject, so I will try to condense a small amount of the info for you here. Much more in-depth information is available everywhere.
Let's look at a 3 element yagi 'beam' antenna. The driven element is self explanatory. The reflector is inductive compared to the driven element at operating frequency and is slightly lower in resonance also since it is longer than the driven element (which should be a resonant length at operating frequency). The director is capacitive in relation to the driven element and resonates at a slightly higher frequency than the driven element.
When transmitting, this effect creates a radiation pattern which draws the major lobe (and the majority of the transmitter energy) toward the capacitive element from the radiating (driven) element. Additional energy is reflected by the reflector element my virtue of its mutually coupled inductance. This also creates more energy to leave the antenna in the direction opposite of the reflector, toward the director.
When receiving, the current induced upon the elements creates the same effect, which is like focusing the antenna much the way a camera lens focuses light.
Adding more directors at the proper phase will 'focus' the yagi more and more. Adding more
reflectors to a yagi makes an undetectable difference. |
Bigbob
| | Posted on Friday, July 12, 2002 - 10:32 pm: |
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All we(us dummies) know is they work,really what difference would it make in the universe or to our hobbies if we knew how they work,the more directors you have the narrower the beam width.Adjusting the spacing can improve gain or give you a tighter back door,heck a friend of mine had stacked 4's,he added extra reflectors 90* to the main and he said it tightened the corner rejection to a null just what he wanted. |
Nitro
| | Posted on Sunday, July 14, 2002 - 10:17 pm: |
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Really Bigbob??? If you don't know how something works how do you know that it's working properly?
Anyway... This is what I know about the RX signal when it hits the antenna.
As a signal comes in it strikes all three elements hence generates a current on each element. Current on a wire causes it to radiate. Even though the current is very low, this current induced on the antenna actually re-radiates off the antenna again! The signals are re-radiated by the director and reflector and arrive at the driven element in-phase with one another (the two re-radiated signals and the original signal). This basically means, the signals reinforce each other...and make the incoming signal much stronger.
When the signal comes from the sides, the same thing happens, except the signals arrive at the driven element out-of-phase with one another which simply means they cancel each other out, significantly reducing signal strength.
This very useful effect (signals arriving in-phase/out-of-phase) is caused by the special spacing and length of the director and reflector element in relation to the driven element.
Thanks ChillyDog and Tech833 you've been a great help. I wanted to verify what I read on the RX signal and get some idea on how the TX signal behaves since the site I got this info from doesn't refer to the TX signal, thanks again.
Hey Bigbob have you learnt anything?
Nitro. |
Highlander
| | Posted on Sunday, July 14, 2002 - 11:09 pm: |
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I sure have a better understanding than I did before and I appreciate you guys posting. |
Tech833
| | Posted on Sunday, July 14, 2002 - 11:48 pm: |
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Glad to help! Anyone who would like to learn more about how antennas work is welcome to drop by my shop when I am designing an array or something. When you see an antenna or array being created in the design process, many students have said that it made the theory much clearer.
Resonance is only one part of the equation. Resonance only represents reactance zero when inductance and capacitance are balanced so as to cancel each other out of the equations. Resistance, phase, vectors, mutual inductance, mutual capacitance, inductive coupling, capacitive coupling, and velocity factors are just as important when creating an antenna that will do what you want it to. The more elements you add, the more you calculate the same formulas with numbers.
Antennas are not accidental. Antennas are merely science. (Chet- You may quote me if you wish) |
Insider
| | Posted on Sunday, July 14, 2002 - 11:58 pm: |
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Well Bigbob, with all this good information, and curiosity it doesn't sound like there are any dummies here. I think knowing something about how this stuff works makes the hobby more enjoyable.
The more you know, the more you can experiment and get hands on with your gear.
I can also see your point, in that there are times when you want to just talk on your radio without thinking about your radio.
But this forum, including the open forum section, is primarily for questions related to radios, antennas, and how they work. |
Bigbob
| | Posted on Monday, July 15, 2002 - 6:52 pm: |
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As I said before opinions are like elbows everybody has at least 2 of them. |
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