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radiation patterns of different types of antennas

A

azepp

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Jul 28, 2009
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Okay, correct me if I am wrong. Am signals radiate out in sine wave form. On a whip without a coil the radiation patteren would leave straight out and in a perfect set up the sine wave would peak at the tip of the antenna thus reducing the resonating signal returning down the antenna. With a coil on said antenna the signal no longer travels straight up and therefore radiates differently as it comes around the coil. However the actual length of the antenna is shorter but the length that the signal is radiated from remains the same.

If that is true, then is a 1/2 or 1/4 wave better than a 5/8 wave because of where the sine wave is when it gets to the tip of the antenna?

Also if you need a coil antenna is it better to be coiled (or loaded)- I assume these are the same thing- at the bottom, like the Wilson mag mount, or middle like the Wilson 5000 trucker on a 10 inch shaft before the coil, or top loaded like many of the fiberglass whips?

Also does the length of the coax effect the antenna output- it must. Does coiling the coax effect anything?

Just some things I have been trying to figure out. Thanks

Alex
 

There are a few misconceptions in there about how an antenna works.

Let's start with this one first and then go from there:

With a coil on said antenna the signal no longer travels straight up and therefore radiates differently as it comes around the coil.
Click to expand...

RF does not travel around the coil in a circular fashion like water through a hose. Rather, it passes straight through the coil. The coil does cause some electrical changes with the antenna, though. Usually coils are added to allow the antenna to be shortened from what it would normally be, but that comes at a price. Generally, a full size 1/4 wave mobile will work better than a shortened one.

is a 1/2 or 1/4 wave better than a 5/8 wave because of where the sine wave is when it gets to the tip of the antenna?
Click to expand...

First, don't confuse some manufacturer's claims (like Firestik) that their mobile antenna is a 5/8 wave antenna. It is not...it's a 1/4 wave. The one that is better (1/4 wave, 1/2 wave, or 5/8 wave) depends on your purpose and where you are trying to talk. For mobile applications, you only ever be able to use a 1/4 wave antenna for CB.

Also if you need a coil antenna is it better to be coiled (or loaded)- I assume these are the same thing- at the bottom, like the Wilson mag mount, or middle like the Wilson 5000 trucker on a 10 inch shaft before the coil, or top loaded like many of the fiberglass whips?
Click to expand...

Generally it's better to be middle or top loaded than bottom loaded. However, the fiberglass whips are really another animal completely. Yes they are top loaded, but they are also what is called a helical antenna becasue the wire is wound around the fiberglass stick from the bottom all the way to the top. In other words, the whole thing is one big coil. That is the worst case scenario for an antenna as far as radiation performance is concerned. However, using a CB antenna in a mobile environment is often about choosing which compromises you're willing to make so the fiberglass antenna often is the choice of many. I use one on my Jeep for various reasons.

Also does the length of the coax effect the antenna output- it must.
Click to expand...
Generally in a mobile environment it will not. However, if the length of coax is overly long, you will have signal loss before it ever gets to the antenna. But in a mobile environment you never have a run long enough to worry about that. The length can also some effect on the overall system if the antenna is not adjusted properly...this is where the misconception comes from that you need to adjust the coax length to address SWR issues. In reality, you should fix the antenna and then you don't have to worrry about the coax.

Does coiling the coax effect anything?
Click to expand...

No, not if you're just coiling up the excess. It won't hurt a thing.
 
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moleculo i was of the impression just coiling up excess coax can creat a choke and cause problems . that that coil should be tied in the middle so it looks like a bowtie . was i missled or maybe missunderstood what i read ?

from what i understand if youre down in a hole , low spot , or valley a 1/4 wgp with its higher angle of radiation can get more of your signal up out of the low area and i think there would also be some reciprocal benefit to the recieve also .
but i could be wrong .

height is might sp5it . ;)
 
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moleculo i was of the impression just coiling up excess coax can creat a choke and cause problems
Click to expand...
You can create a choke by coiling up the coax, but it has to be done correctly. I'm sure you've seen pictures chokes at the feedpoint of a base antenna? The point of the choke is to help "choke off" unwanted feedline radiation that is on the shield of the coax, which is where you don't want it. It's possible to create a choke anywhere along the line if you had some reason for wanting to do it that way. Coiling up the exceess might create some type of choke (which wouldn't hurt anything), but in reality it won't have much effect because the coils are probably going to be done too messy to accomplish much.

Rob from what i understand if youre down in a hole , low spot , or valley a 1/4 wgp with its higher angle of radiation can get more of your signal up out of the low area and i think there would also be some reciprocal benefit to the recieve also .
Click to expand...

Yes, that is the case, especially if the station you're trying to talk to is on the high spot above you :). It works on RX, too...it's called the law of reciprocity when you're talking about antennas.
 
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Whoa, ok, thanks for helping me to get a better understanding of whatis happening, I read and reread that article and totally smoked my brain out...

Thanks again

Alex
 
BOOTY MONSTER said:
moleculo i was of the impression just coiling up excess coax can creat a choke and cause problems . that that coil should be tied in the middle so it looks like a bowtie . was i missled or maybe missunderstood what i read ?
Click to expand...



I asked this a while ago and was told it won't affect the signal along the center conductor, but will prevent common-mode currents from traversing along the shield.
 
Another 'way' of thinking about coax is that what happens inside (the shield layer) isn't the same as what happens outside (of that shield layer). And since that shield has two sides to it, inside/outside, and since RF tends to flow on the surface of things rather than 'all the way through it', you can have two signals flowing on the same coax, one inside, and one outside. If those two RF signals don't get together someplace, no big deal. The 'trick' is keeping them away from each other. Typically, since the one inside the coax isn't the one causing a problem, you want to deal with the one on the outside. An easy way of doing that is by disrupting the 'flow' on the outside of the coax. Put something there that an RF signal can't easily get past, a 'choke' point on that 'RF' road.
One of those 'choke' thingys is a simple coil, another is ferrite 'donuts', or a length of conductor that that particular 'size' of RF just really cant resist such as an electrical 1/4 wave tube. They all work as a stop sign on that RF highway. The coil thingy works cause until you bend that coax past it's ability to bend, the inside of that coax never 'sees' a coil, that coil you bent the coax into only affects the outside, not the inside. Until you bend that coax to the point where you break the continuity of the shield, there's no way for that outside RF to 'mix' with the inside RF. (Why the diameter of those choke coils make any difference at all.)
Does all that sound sort of 'childish'? Yes, it is, but it dang sure is an easy way for me to remember/understand it.
- 'Doc

(There are other analogies you can 'do' with this. They generally change the 'rating' from 'PG' to something worse, so I won't go there. Sort of like the 'phrase' to remember the color code?)
 
Another handy way of thinking about the shape of radiation patterns is that they are sort of 'fuzzy'. Never really 'sharp edged'. Another one is that those radiation patterns can't just 'touch' each other, they have to overlap the 'other' antenna for that station to hear it. Does that one sound like a 'no brainer'? It is, sort of, but I can remember when it 'dawned' on me, one'a them 'epiphany' thingys that make you wonder why you ever thought differently, you know?
- 'Doc
 
It takes a long time to get antenna concepts. You have to ignore DC theory.

Antenna gain occurs due to multiple current nodes which must be separated by distance.

A 5/8 electrical wavelength antenna fashioned with a loading coil that is physically 1/4 wave long will have the same exact gain as a 1/4 wavelength antenna minus the 5/8th's coil losses. It will also have less SWR bandwidth.

Yes, they can claim 5/8th wave but it's electrical length and does nothing for performance.
 
"Am signals radiate out in sine wave form"

initial radiation is in the form of an electric field.

http://www.worldwidedx.com/cb-antennas/19996-e-plane-h-plane.html#post30424

even though the thread topic is about stacking the last several paragraphs applies here in regard to the generation of an electromagnetic field.

E-Field = plane of polarization

if we follow one cycle/hertz of transmitted energy for the full 360 degrees, here is what is happening.

at 360/0 degrees the transmitter and the antenna are energized and the electric field charge begins to develop.

at 90 degrees the electric field charge in the E-PLANE reaches maximum potential.

at 180 degrees the electric field charge collapses, generating a developing magnetic field charge in the plane perpendicular to the collapsing electric field charge. this plane is referred to as the H-PLANE.

at 270 degrees the magnetic field charge reaches maximum potential.

at 360/0 degrees the magnetic field charge collapses, generating another developing electric field charge and the entire process begins all over again.

if the operating frequency is 27 mhz then this entire process occurs 27 million times a second. this is how the antenna radiates a signal. this is the process responsible for generating an ELECTRO-MAGNETIC wavefront.

(pertaining to helically wound antennas)
"That is the worst case scenario for an antenna as far as radiation performance is concerned...."

wrong.

"Also does the length of the coax effect the antenna output- it must."
it does if the feedpoint swr is anything but 1:1, especially in the case of a transmitter source with a fixed impedance.

"In mobile antennas size does matter. Longer is better"

wrong.

"Does coiling the coax effect anything?"
except in the case of a properly constructed choke all excess feedline should be formed into a skein, not a loosely coupled coil.

"from what i understand if youre down in a hole , low spot , or valley a 1/4 wgp with its higher angle of radiation can get more of your signal up out of the low area and i think there would also be some reciprocal benefit to the recieve also but i could be wrong ."

that's correct.

"Coiling up the exceess might create some type of choke (which wouldn't hurt anything), but in reality it won't have much effect because the coils are probably going to be done too messy to accomplish much."

loose coiling of excess feedline defeats the counterpoise effect called for in most mobile installations where body metal is at a premium.

"RF does not travel around the coil in a circular fashion like water through a hose. Rather, it passes straight through the coil."

wrong.

"Antenna gain occurs due to multiple current nodes which must be separated by distance."

does not apply to single monopole designs.

"A 5/8 electrical wavelength antenna fashioned with a loading coil that is physically 1/4 wave long will have the same exact gain as a 1/4 wavelength antenna minus the 5/8th's coil losses."

wrong.

some have a few "misconceptions" of their own.
 

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