How do antenna's work the same when different?

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jassing

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based on a few posts, I've got a question..
The scanner antenna that I got is 17" tall.
But then I see others that are 18" up to 21" and then 3' and 4'

the larger ones seem to be coil wound all the way. the 18-22" ones seem to use a coil somewhere..

How can they all work similarly? or is that "more the better" when you're receiving only?

More to an application of antennas & mounting...

Suppose you mount a 17" antenna on your roof.
and then mount a 4' antenna on your mirror -- so that both antenna's have approximately the same amount of antenna is unobstructed above the roof line.

would they both end up receiving about the same?
 

737mech

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Antennas

based on a few posts, I've got a question..
The scanner antenna that I got is 17" tall.
But then I see others that are 18" up to 21" and then 3' and 4'

the larger ones seem to be coil wound all the way. the 18-22" ones seem to use a coil somewhere..

How can they all work similarly? or is that "more the better" when you're receiving only?

More to an application of antennas & mounting...

Suppose you mount a 17" antenna on your roof.
and then mount a 4' antenna on your mirror -- so that both antenna's have approximately the same amount of antenna is unobstructed above the roof line.

would they both end up receiving about the same?
Not as easy as you are thinking. There is a book that goes above and beyond antennas called the arrl handbook for antennas. Not wanting to get into the science of it? Ok here is a suggestion. Imagine signals as waves with tops and bottoms. Now imagine that wave passes over your antenna. Now if the very top of the wave exactly matched the very top of your antenna and the swell bottom of the wave matched the very bottom of your antenna. You would in this case get a clear signal on your scanner. If true you would have a full wave antenna, now chop it in half where the antenna is only half as high. Yes you get signals 1/2 wave for that freq. The example you gave you need to understand the science with antennas. The one on the roof has a ground plane (The roof) the one on the mirror has no ground plane.
It's not a contest as to what antenna is higher, there's much more to it than that. Length of antenna vs. what freq you want to hear. There's alot of info in the radiorefernce forum.
 

mmckenna

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Also, if you are comparing the antennas by looking at the gain claims, you really need to take that with a grain of salt.

First of all, some companies will stretch the truth on the claims.

Gain, measured in dB, or Decibels has to be in relation to something. What the base is is where it can get confusing.
dBi, or Decibels - isotropic is the antenna gain given in relation to an isotropic radiator, a perfect antenna.
dBd, or Decibels - half wave dipole.
Any time you see an antenna gain given as just dB, you've got to realize that they are not giving you the reference, so it's kind of a pointless thing.

Some companies will use dBi, since it will make the antenna look better than it really is. Most antenna manufacturers will us dBd. The difference between dBi and dBd is about 2.1dB, so using dBi will make a particular antenna look 2.1dB better than the same antenna that is measured as dBd.

A quarter wave antenna on a perfect ground plane will have 0 dBd gain, often expressed as "unity gain". The same antenna on a perfect ground plane will have 2.1dBi. So by using dBi when everyone else is using dBd will make one antenna look better than the other.

Here are some references for common single band antenna designs:
Quarter Wave with ground plane - 0 dB
Half Wave with no ground plane - 0 dB
Half wave with ground plane - 2.36 dB
5/8 wave with ground plane - 3 dB
5/8 wave over 1/4 wave (co-linear) - 5 dB

A quarter wave antenna on the VHF 2 meter amateur radio band will be about 19 inches tall
A half wave antenna on the VHF 2 meter amateur radio band will be about 41 inches tall or so
A 5/8 wave antenna on the VHF 2 meter amateur radio band will be about 47 inches or so

When comparing dB, when you increase your antenna gain by 3 dB, you are effectively doubling your emitted power. A 50 watt transmitter with theoretical lossless coaxial cable connected to a quarter wave antenna would have an "effective radiated power" or ERP of 50 watts.
Swap that quarter wave out with a 5/8 wave antenna and your ERP would jump to 100 watts.
 

jassing

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There is a book that goes above and beyond antennas called the arrl handbook for antennas
Thank you for the suggested reading.. Everyone has to start somewhere... Marketing claims are always optimistic (if not outright lies) -- which is why I have a bit of skepticism.

ultimately, I'd like to achieve a 'best case' for the conditions I'm working with; which for me, is trial and error.. as long as the book doesn't go too far over my head,maybe it will reduce my error count...

There's alot of info in the radiorefernce forum.
Indeed... but filtering it out, or finding what's applicable is the hard part-- I'm specifically looking at receive-only performance...I mean,the comtelco antenna I have looks like a coat hanger... then I look at a different antenna and it has a coil; both claim the same frequency coverage...

http://...
Thank you for the links.. 1st read was good reading, with most of it over my head.. hopefully the 2nd read after a cup o coffee will help...


Also, if you are comparing the antennas by looking at the gain claims, you really need to take that with a grain of salt.
Marketing claims are.... well; yea. I do.

Gain, measured in dB, or Decibels has to be in relation to something. What the base is is where it can get confusing.
Indeed... There are things I know, there are things I think I know, there are things I do not know. Antennas are somewhere between the last latter two. Which is dangerous; but at least I know to ask questions and learn something...

dBi, or Decibels - isotropic is the antenna gain given in relation to an isotropic radiator, a perfect antenna.
dBd, or Decibels - half wave dipole.
Any time you see an antenna gain given as just dB, you've got to realize that they are not giving you the reference, so it's kind of a pointless thing.
Some companies will use dBi, since it will make the antenna look better than it really is. Most antenna manufacturers will us dBd. The difference between dBi and dBd is about 2.1dB, so using dBi will make a particular antenna look 2.1dB better than the same antenna that is measured as dBd.
I hadnt noticed that; but it's a good point to remember -- compare apples to apples...

your emitted power.
I had to double check to make sure I posted in the correct area...

I'm talking strictly scanner/receiver stuff -- no transmit...

A 50 watt transmitter with theoretical lossless coaxial cable connected to a quarter wave antenna would have an "effective radiated power" or ERP of 50 watts.
Swap that quarter wave out with a 5/8 wave antenna and your ERP would jump to 100 watts.
This I knew, albeit not from a science point of view... People with 1/4 wave cb antennas could not get out as far as people with 5/8 - but usually (not always) this equaled "bigger is better" -- but after doing a base station with limited roof space, I learned "not always"....


Thanks for the info -- food for thought; after coffee -- I am looking to get a best-case scenerio recieving signals in a less than optimal antenna mounting location...

I'm trying the scannermaster antenna -- but it just seems (gut feeling) to me that I can do better by using a taller/larger (ie: 4') antenna mounted in a less than optimal position (rear of truck cab) than I can using a smaller antenna mounted in a more optimal, but still not optimal, position.

Given various mounting positions: (The roof is not available.)

I can go bumper, hood, mirror, or back of cab, near the roof line.

Current plan, from reading & suggestions, is to use the hood with the scannermaster antenna ... but it just seems to me, I'm doing more "listening forward" than "all around" since signals from the rear is blocked by the cab.

Mirrors (to me) is the least ideal, with the rear bumper right in there with the mirror.
Going off the rear of the cab, close to the roof line, with a taller antenna, just seems to make the most sense -- however, talking to folks, I get a "use a proper antenna with a ground plane" which brings me back to the engine hood as the best spot with a smaller antenna...

From the links/info here; seems to be; that's the best spot (available) ... I just wish I understood why.

Thanks
-j
 

zz0468

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First, start with this premiss... Antennas are reciprocal, meaning the radiation pattern is the same whether it's transmitting or receiving.

With that in mind, picture a perfectly round balloon. That represents the pattern of a theoretical isotropic antenna, with zero gain. For a given frequency, different antennas will squeeze the balloon in various ways. It'll compress in one dimension, but stretch out in another.

That represents antenna gain. If you could visualize the "balloon" created from various antennas, you would easily see that they're not really all alike. Maybe similar, but they all squeeze that balloon in various ways. As for "more is better" that depends entirely on what your definition of "better" is. Better for one application might be worse in another.

Antenna theory is extremely complex, and it's not easy to relate to vague simplistic questions. But to answer your last question about the 17" antenna and the 4' antenna behaving the same because of how much peaks over the roof, no, the patterns would be completely different. How and where an antenna is installed will effect how it squeezes the balloon.
 

prcguy

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None of the single band antenna gain "references" listed below are correct, where did you get them?
prcguy


Also, if you are comparing the antennas by looking at the gain claims, you really need to take that with a grain of salt.

First of all, some companies will stretch the truth on the claims.

Gain, measured in dB, or Decibels has to be in relation to something. What the base is is where it can get confusing.
dBi, or Decibels - isotropic is the antenna gain given in relation to an isotropic radiator, a perfect antenna.
dBd, or Decibels - half wave dipole.
Any time you see an antenna gain given as just dB, you've got to realize that they are not giving you the reference, so it's kind of a pointless thing.

Some companies will use dBi, since it will make the antenna look better than it really is. Most antenna manufacturers will us dBd. The difference between dBi and dBd is about 2.1dB, so using dBi will make a particular antenna look 2.1dB better than the same antenna that is measured as dBd.

A quarter wave antenna on a perfect ground plane will have 0 dBd gain, often expressed as "unity gain". The same antenna on a perfect ground plane will have 2.1dBi. So by using dBi when everyone else is using dBd will make one antenna look better than the other.

Here are some references for common single band antenna designs:
Quarter Wave with ground plane - 0 dB
Half Wave with no ground plane - 0 dB
Half wave with ground plane - 2.36 dB
5/8 wave with ground plane - 3 dB
5/8 wave over 1/4 wave (co-linear) - 5 dB

A quarter wave antenna on the VHF 2 meter amateur radio band will be about 19 inches tall
A half wave antenna on the VHF 2 meter amateur radio band will be about 41 inches tall or so
A 5/8 wave antenna on the VHF 2 meter amateur radio band will be about 47 inches or so

When comparing dB, when you increase your antenna gain by 3 dB, you are effectively doubling your emitted power. A 50 watt transmitter with theoretical lossless coaxial cable connected to a quarter wave antenna would have an "effective radiated power" or ERP of 50 watts.
Swap that quarter wave out with a 5/8 wave antenna and your ERP would jump to 100 watts.
 

mmckenna

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If you've got an aluminum tool box behind your cab, that can be a solution. I did that on my first truck, 20 years ago with a VHF antenna. Worked well, good ground plane, etc.

Rear bumper would be the worst place, in my opinion. Too close to the body sides, directional, etc. I always cringe when I see people with pickups that have mounted the fiberglass antennas on the rear bumper.

"I had to double check to make sure I posted in the correct area...

I'm talking strictly scanner/receiver stuff -- no transmit..."

Yeah, I understood, just using it as an example. I did go a bit overboard on my explanation.

I understand not being able to use the roof, on a full size truck. I'm usually only using VHF, so I've always put quarter wave antennas on the roof, and dealt with the issues of hitting things.

Front fender can work, but on some vehicles you can get RF hash off the ignition, computer or other electronics. Not saying it won't work, just suggest trying a temporary installation first.

Do you do a lot of VHF Low listening? If not, you could loose the longer antenna. On VHF and UHF, you wouldn't need a 4 foot antenna, necessarily, to get good results, unless you are really out in the sticks.
Also, something a little more flexible would give you some options. The dual band ham radio antennas work well.
 

mmckenna

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None of the single band antenna gain "references" listed below are correct, where did you get them?
prcguy
20+ years of experience doing LMR work, as well as the manufacturer websites. Feel free to look up the numbers.

My collinear numbers might be off, you likely have me there! I often confuse that one.
 

prcguy

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I don't have references in front of me but if you were using dBd for your references a 1/2 wave center fed or end fed is 0dBd, a 1/4 wave ground plane is less, a 5/8 over a ground plane is about 1.25dBd gain and so on.

Not very many manufactures websites are correct either, profit seems to sway antenna specs.
prcguy



20+ years of experience doing LMR work, as well as the manufacturer websites. Feel free to look up the numbers.

My collinear numbers might be off, you likely have me there! I often confuse that one.
 

jassing

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Antenna theory is extremely complex
that is the most comforting thing I've heard.... I was definitely feeling out of my comfort zone trying to understand / apply what I've read.

Better for one application might be worse in another.
... and it's not easy to relate to vague simplistic questions. But to answer your last question about the 17" antenna and the 4' antenna behaving the same because of how much peaks over the roof, no, the patterns would be completely different. How and where an antenna is installed will effect how it squeezes the balloon.
So it sounds like trial and error is my best friend here. Try antenna A in 3 positions, try antenna B in the 3 places, try antenna C... go with the one that performs the best in the available spots. I don't mind drilling holes -- I just don't want to make swiss cheese of the body....

Front fender can work, but on some vehicles you can get RF hash off the ignition, computer or other electronics. Not saying it won't work, just suggest trying a temporary installation first.
I get a LOT of rf off the engine...

Do you do a lot of VHF Low listening? If not, you could loose the longer antenna. On VHF and UHF, you wouldn't need a 4 foot antenna, necessarily, to get good results, unless you are really out in the sticks.
I monitor the marine bands, 150mhz, and 500 mostly.... My thought with the longer antenna mounted on the rear the cabin close to the roof would be that a spring and sturdy antenna would allow for substantially more gentle of pivoting (at the spring) when coming in contact with low hanging structures.

Also, something a little more flexible would give you some options. The dual band ham radio antennas work well.
I'll put it on the list of things to try...

I was hoping not to spend a fortune but $20 here and $20 there on various antennas -- going to end up with a couple hundred in mounting & antennas to try when I'm all done.

Thanks
-josh
 

mmckenna

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Collinear just means it's two radiators stacked on top of each other and the coil in the middle (or so) phases the top antenna in with the lower antenna. It's a way of increasing the antenna gain. That one looks like it may be a 5/8th wave over a half wave. Since it says "no ground plane needed", it's likely the half wave on the bottom.
 

jassing

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THanks -- I was stuck on a beam type collinear.. it finally dawned on me that you could stack as well; thanks for clarifying that.
 
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