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  #151  
Old 12-06-2010
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There is nothing finer than technical brawl between radio freqs......er, I meant geeks.
  #152  
Old 12-06-2010
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Antenna spatial diversity goes back a long time, to World War II I believe. Automatic tuning and steering started in the 70s at HF, and at microwave in the late 70s and then 80s with the US Navy AEGIS radar program. It's cool stuff.
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  #153  
Old 12-06-2010
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I think part of the confusion over the gain of an omnidirectional antenna is that we are really dealing with signal-to-noise ratios, not just signal strength.

The high-gain omni-directional antenna does not have any extra signal bounced back to it by a reflector. But it does have a tight beamwidth where it's expecting signal (perpendicular from the antenna, all 360 degrees around it). All other areas, like above and below the beamwidth, have reduced sensitivity to signals. Since most of the access points and all of the boats are at sea level, (as oppoosed to 1000 feet up), the tight beamwidth works well. The world is a very noisy place, so the narrow beamwidth gives you a much better signal-to-noise ratio.

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Last edited by Bene505; 12-06-2010 at 10:52 PM.
  #154  
Old 12-07-2010
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I think the big issue is the statement by mlahrkamp that there are antenna designs with different transmit and receive gain.

Jim and I and others maintain there is no such animal.
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  #155  
Old 12-07-2010
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Quote:
Originally Posted by Bene505 View Post
I think part of the confusion over the gain of an omnidirectional antenna is that we are really dealing with signal-to-noise ratios, not just signal strength.
No, it's gain, pure and simple. And since antennas, being passive devices, have no way of discriminating between in-band noise and the desired signal, noise is "amplified," too. The only time a gain antenna improves S/N ratio is when a significant portion of the noise is off-axis to the antenna's gain pattern. This might be true for a unidirectional antenna, but less likely to be so for an omni-directional.

Jim
  #156  
Old 12-07-2010
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Quote:
Originally Posted by richardhula View Post
Sorry Mike but someone with your experience should know better. Antenna gain (dBi) is its improvement relative to that of an isotropic radiator - it doesn't matter how its constructed or whether its being used for transmit, receive or both. Check out this article.
I just had a look at the article you referenced: It only talks about output power and output signal radiation. We're exploring gain during receive.

Do you have any other website you'd recommend that refers to the gain that an omni exhibits during receive, not gain for output or radiating signal?

Mike
  #157  
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Excerpt from the article referenced a few posts back (here):
"Most antenna manufacturers specify gain as dBi, which is the gain relative to an isotropic source. In other words, dBi is how much the antenna increases the transmitter's power compared to using a fictitious, isotropic antenna. dBi represents the true gain that the antenna provides to the transmitter output."

You'll notice they define gain in terms of increasing transmit power only.

So, anyone have any articles or data that shows that rated gain is bi-directional in an omni? I'm eager for fact.

Mike
  #158  
Old 12-07-2010
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When I queried the real range of the Bullet2 and Bullet 2 high power it was because I wanted to know in the real world with a typical masthead omnidirectional aerial what I could expect to pick up and from what distance.Assuming I would be in a harbour with wifi available within a mile or so from my position.
I am not in the world of just popping up my mast to change the aerial or directing it towards a particular transmitter. Neither do I expect to receive at most 2 mb broadband reception.
Problem with discussion now as is is that its about aerials in the purist world.
Its a bit the same with Ham radio.At home no problem playing with my home brewed roller coaster ATU and trying out different aerials but on my boat at sea all I want is something that delivers the best signal and sends out the best signal with minimum standing wave etc.
All I want is to arrive in a harbour switch on my laptop and be able to get useful stuff like local weather reports or over here the Met office isobaric charts etc. and perhaps watch a bit of online TV in a part of the world where medium and long range wireless communicatios are difficult.
  #159  
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Quote:
Originally Posted by mlahrkamp View Post
So, anyone have any articles or data that shows that rated gain is bi-directional in an omni? I'm eager for fact.
*sigh* So the fact that I used to teach this stuff apparently isn't enough for you?

This is basic antenna and transmission line theory. I don't mean to offend, but anybody that truly understands how antennas work knows this. But, tho it's "basic," it's also very, very complicated. It's not the kind of thing that can reasonably be taught on this site or a simple web page.

Best I can do in a short space is this: Examine the radiation pattern of an isotropic antenna element in free space. It'll be a balloon. Examine the radiation pattern of a half-wave dipole in free space. Notice how it's a big, fat donut-shaped thing--with the hole along the axis of the antenna. (This dipole has 3dB gain over the isotropic source, btw, at right angles to the element.) Now examine the radiation pattern of an omni-directional "gain antenna" in free space. You'll notice it's a flattened along the antenna's axis and elongated at right angles to the antenna, as compared to the simple dipole. Now, in any of the two latter instances: Imagine an RF source that's outside the previous antenna's radiation patterns. E.g.: A source that's outside the isotropic antenna's pattern, but w/in the dipole's. A receiver connected to the lower-gain antenna would not be able to "hear" that RF source.

The power, or "value," of a signal is in its S/N ratio. A signal theoretically never truly disappears, it just becomes so weak (dissipated--in every sense of the word) its S/N ratio at the receiver has it buried in the noise. A gain antenna improves the energy received (more technically: The energy density), in a given direction, or, in the case of an omni-directional antenna, along a given plane (kind of--it's not really a "plane," per se). (In this respect, Bene505 was correct: It's all about S/N ratio.)

That's the best I can do. (And the most I'm willing to do.) I've forgotten more than I remember about this stuff. Like I said: I used to teach it. I could post my CV, but, if you don't believe me now, that probably wouldn't do any good, either.

Jim

Last edited by SEMIJim; 12-07-2010 at 03:31 PM.
  #160  
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Quote:
Originally Posted by ffiill View Post
When I queried the real range of the Bullet2 and Bullet 2 high power it was because I wanted to know in the real world with a typical masthead omnidirectional aerial what I could expect to pick up and from what distance.
You're not going to get a definitive answer because there is no such thing. There are too many variables.

Quote:
Originally Posted by ffiill View Post
Problem with discussion now as is is that its about aerials in the purist world.
That's because the only way to intelligently discuss such things is to remove extraneous variables. Thus we discuss the theoretical behaviour of radiators in free space, which is an impossibility to achieve in reality, even if we were in free space, because the theoretical models don't even account for the presence of a feed line .

It has to be that way. It's the only way it can be.

Jim
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