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Best (or highest gain) 1900MHz directional antenna?

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EVDOforums.com Forum Index -> EVDO Signal Enhancers with Antennas & Amplifiers

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Scott
EVDO Junkie

Joined: 18 Jul 2005
Posts: 530
Location: Central Coast of California

Posted: Thu Mar 23, 2006 1:55 pm Post subject:

Pick wrote:

DBi indicates gain in DB over an isotropic radiator, which is a theoretical antenna that radiates equally in all directions, like a globe.

There is also a DB over a dipole, which is a more real world indicator. A dipole is 2.14 DB above an isotropic radiator, so just subtract 2.14 under the 14DBi figure to get a "real world" figure of 11.86 DB gain over a dipole. That is how it is SUPPOSED to work. Your mileage may vary.

A dipole antenna is usually the driven element in a yagi or dish antenna.

YEEEES! The best short laymanesque explanation of dBi (isotropic) vs. dipole I have read yet. Thanks Pick! (may I quote you?) Sometimes users in our world of EVDO and Wi-Fi may come across dBm and dBw as well. Pick, do you have another superb layman's explanation for dBi vs. dBm (or dBW even)?

Outside of the difference between dBi and dB, those comparing dual-band antennas should keep in mind that some "dB" specifications are "system gain" (combined 850/1900) and not individual frequency band gain. I find it helpful when a reseller indicates those details.

Last edited by Scott on Sat Mar 25, 2006 5:12 pm; edited 1 time in total

kaefer
EVDO User

Joined: 05 Dec 2005
Posts: 66

Posted: Thu Mar 23, 2006 2:56 pm Post subject:

Scott wrote:

Outside of the difference between dBi and dB, those comparing dual-band antennas should keep in mind that some "dB" specifications are "system gain" (combined 850/1900) and not individual frequency band gain. I find it helpful when a reseller indicates those details.

I wondered about this as well, especially comparing the single-band Digital Antenna 1900MHz Yagi vs. the maximumsignal.com dual-band Yagi.

Scott
EVDO Junkie

Joined: 18 Jul 2005
Posts: 530
Location: Central Coast of California

Posted: Thu Mar 23, 2006 3:27 pm Post subject:

Ok. Well, my point wasn't specific to antenna vendor, just that antenna shoppers reading numbers and looking for an "Easy Button" comparison shouldn't just rely on numbers (though honestly, what else are we to go on besides user experience here on the forums, etc.). Not that I expect all of you to compare antennas like I do... but the idea is that a "dB?" number may or may not be a useful reference. How well an antenna functions for our intended use in relation to its cost and our expectations is a better "yard stick" and ultimately what we're after. As well, regarding single-band/dual-band compatibility with amplifiers, I would heed any vendor's recommendations, since they usually "know what they sell". So match things accordingly and maybe ask questions before buying from different vendors and configuring your own mixed/matched setup.

jstjohnz
EVDO Fledgling

Joined: 04 Apr 2006
Posts: 25

Posted: Wed Apr 05, 2006 8:16 am Post subject:

Scott wrote:

Pick wrote:

My 2 cents worth...

The term 'db' by itself, is really meaningless, a db value is just another way of indicating a percentage difference. To say an antenna has a gain of 300% is obviously meaningless unless we know, % gain compared to what?

The db is simply a logarithmic percentage scale. Adding 3db DOUBLES the gain. Adding 6db QUADRUPLES the gain, etc.

0db = 100%, in other words the same, no gain
3db = 200%, double the signal strength
6db = 400%, 4 times the signal strength, etc..

It's the letter *after* the db that tells what we are comparing to.

As Pick stated, for antennas:
dbi is gain compared to an isotropic radiator, or 'point source' antenna.
dbd is gain compared to a dipole antenna.

The term db, by itself, though seen frequently, is absolutely meaningless since it does not give a refernce point for the comparison.

The other terms you mentioned, dbm, and dbW, are measurements of POWER, not antenna gain. dbm is power referenced to 1 milliwatt (m), and dbW is power referenced to 1 Watt (W). dbm=30+dbW (not exactly but very close) since a watt is 30db (each doubling is 3 db, and 10 doublings is very close to x1000) stronger than a milliwatt.

Finally, antenna gain comes from one way and one way only: focusing the beam to a small area. A perfect analogy is a flashlight bulb. A bare flashlight bulb in a dark room shines light in all directions, the whole room will be lit, but very dimly. Put that same bulb in front of a reflector and you focus a very bright beam of light in one direction while leaving the rest of the room dark.

The bare bulb is the isotropic radiator. The flashlight is the high gain antenna. Although it's easiest to visualize this analogy for transmitting antennas, the principle is the same for receiving antennas.

If you want antenna gain, you must sacrifice beamwidth. If you wide wide angle coverage, you must sacrifice gain.

-jim-

Last edited by jstjohnz on Wed Apr 05, 2006 11:35 pm; edited 1 time in total

jstjohnz
EVDO Fledgling

Joined: 04 Apr 2006
Posts: 25

Posted: Wed Apr 05, 2006 11:04 pm Post subject: Re: Best (or highest gain) 1900MHz directional antenna?

MaximumSignal wrote:

$715 actually. Still very very expensive.

Bear with me here....here comes a hypothetical situation:
The Arc antenna is about 18.66DB but is directional while your marine antenna is 17DB but is omni-directional. If both antennas were 17DB would they both operate equally as well in a given enviroment (assuming the directional antenna is aimed correctly)?

Or, is a 17DB directional antenna more efficient than a 17DB omni when aimed properly, since the energy is focused at the tower versus radiating in all directions?

I would say in theory the 17 db directional should be more efficient[/quote]

Antenna gain comes from reducing the radiation angle or beamwidth, either in the vertical or horizontal planes, or both. An omni antenna gets gain by reducing the vertical radiation angle only. Directional antennas (yagi, panel, corner, parabolic) produce gain by reducing the radiation angles in both planes.

*If* the numbers were correct with regard to the gain of the antennas in the example above, the performance of the two antennas would be the same when aiming at a single tower.

There's a big BUT, however: There is *no way* that an omnidirectional antenna can have that much gain! The vertical beamwidth would have to be so small as to be unuseable, and the antenna would have to be significantly taller.

FWIW, the provided specs page for that antenna makes *no* mention of gain claims.

Finally, you would *never* want to put such a high-gain antenna on a boat! Boats roll, so marine antennas must have a fairly wide vertical beamwidth, so high-gain for marine applications just isn't practical.

-jim-

digitrip
EVDO User

Joined: 04 May 2006
Posts: 35

Posted: Wed May 10, 2006 12:57 pm Post subject:

Good stuff. Sounds like we may have some more amateur radio operators in here....

jstjohnz
EVDO Fledgling

Joined: 04 Apr 2006
Posts: 25

Posted: Wed May 10, 2006 1:34 pm Post subject:

kaefer wrote:

MaximumSignal wrote:

All our measurements are in DB not DBi so the number is the number. Our unit is 24 inches long Includes 20 feet of low loss cable and mounting hardware . It is self contained. The Digital unit if you read down the listing is just the antenna everything else is extra .

Wow. Now that I'm educating myself on terminology (db, dbi, dbd...) you sell very nice antennas at very nice prices!

So your 17DB omni marine antenna should blow the Digital Antenna omni antenna (6.86db actual) out of the water. No pun intended. Well, maybe...

The term db by itself is meaningless when describing an antenna. Specifying gain without indicating what it is relative to has no meaning.

There are only 2 accepted references when specifying antenna gain, dbi which indicates gain relative to a zero-gain theoretical antenna, and dbd which indicates gain relative to a dipole antenna. As has been previously stated dbi is, by definition, just over 2db greater than dbd.

A 17dbd (or dbi) gain omni antenna 24" long at 1900 mhz is a physical impossibility.

-jim-

Scott
EVDO Junkie

Joined: 18 Jul 2005
Posts: 530
Location: Central Coast of California

Posted: Wed May 10, 2006 6:17 pm Post subject:

jstjohnz wrote:

...A perfect analogy is a flashlight bulb. A bare flashlight bulb in a dark room shines light in all directions, the whole room will be lit, but very dimly. Put that same bulb in front of a reflector and you focus a very bright beam of light in one direction while leaving the rest of the room dark.

The bare bulb is the isotropic radiator. The flashlight is the high gain antenna...

Which is very similar to how I try to explain omni vs directional antennas to customers. Directional antennas don't increase gain so much as they focus it in one direction while reducing it in other directions (though the net result on your antenna cable is more signal from a specific direction).

I appreciate your breaking things down further, jstjohnz as well as Pick's simple description noted above, as many posts are either technically dense or so general that they're unhelpful.

These forums can always use information that sheds light on the technology and theory behind the products. Most users just want something that works... others want to know why.

Now that we have covered some of the "behind the lines" details... let's get back to the reason for this thread in the first place: what IS the BEST (or highest gain) 1900MHz directional antenna?

Perhaps we ought to add 'under $50' 'under $100' and 'under $200' to the 'Best 1900 MHz' question to keep with the spirit of 'doubling'. ;] I should note though that many folks I speak to would choke on antenna costs over $100, generally.

Also we may want to introduce the very real FCC limits in place for cellular/PCS antennas sold in the USA. I'd love to know what various wireless company RF engineers/techs think of all this external antenna business, but they RARELY if ever chime in here... unfortunately.

jstjohnz
EVDO Fledgling

Joined: 04 Apr 2006
Posts: 25

Posted: Wed May 10, 2006 10:42 pm Post subject:

Scott wrote:

jstjohnz wrote:

One of the difficulties in comparing antennas is that manufacturers gain figures are often suspect, to put it mildly. Frequently gain figures are quoted which are a physical impossibility for the specified antenna size. An example is the 17db gain 24" omni antenna mentioned earlier in this thread. Not physically possible. As another posted stated earlier, some manufacturers apparently are quoting 'system gain' whatever that means (maybe sum of gains in the rcv and xmit directions?) and that may be the case with this antenna. Personally I think this is why some manufacturers don't identify their gain spec as being dbi or dbd, since the term db by itself can mean just about anything the manufacturer wants it to mean. It is also suspect when a dual-band antenna has only a single stated gain figure, as the gain will always be different for the two bands.

So, the bottom line is that manufacturer's gain figures may not be a reliable indicator of true antenna performance. With yagi antennas, one can make a pretty accurate gain estimate just by looking at the number of elements, as number of elements and gain go hand in hand. Likewise, with omni antennas, length and gain go hand in hand.

With regard to FCC limits, well the FCC power limits are always expresed in terms of effective radiated power (ERP) which includes the device (phone, wireless card, or amplifier) and the antenna. That's why the booster amplifiers all contain, on the FCC ID sticker, a warning specifying the maximum antenna gain that is legal for use with that amplifier. All that I am aware of specify maximum antenna gains in the range of of 3-7dbi. So, if you want to get technical, no booster amplifier is FCC approved when used with a high-gain (ie yagi) antenna. Now practically speaking the FCC probably has better things to do than track down folks breaking this rule.

-jim-

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