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Antenna Gain


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#1 taco6513

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Posted 05 May 2019 - 06:46 PM

I have a question about antenna gain. Is gain really gain? A base antenna rated at 6db gain vs. A mobile antenna with the same 6db gain?

#2 BoxCar

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Posted 05 May 2019 - 07:08 PM

Simply put, gain is gain. Gain is measured as an increase over an isotropic dipole of the RF field strength. From my understanding the difference between the two, a mobile and base antenna has to do with the height of the two antennas and the amount of radiating surface of each. You really can't compare one of these against the other but you could compare the mobile to another or a base against another base antenna.


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#3 marcspaz

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Posted 05 May 2019 - 09:22 PM

BoxCar is correct.  DB scales (I think there are 7 of them) are all based on a random agreed upon standard. RF is typically measured in DBi.  The " i " in DBi stands for isotropic, referencing the standard performance of an isotropic dipole antenna (omni-directional antenna).  The power gain of an isotropic antenna is 1.0.

 

Here is where the problem comes into play... true DBi gain is comparing a isotropic dipole antenna to a directional antenna.  So, the makers of omni-directional antennas that claim a specific amount of gain, are actually comparing the general performance improvement of their antenna, to that omni-directional dipole baseline. 

 

They use size, shape, materials, coils, etc, to get a net improvement in the transmit and/or receive performance and use that measured improvement to advertise its relative DBi gain.  That improvement is, for every 6 dBi in gain, you double the range of the antenna when compared to an isotropic dipole antenna tuned for the same frequency.

 

That said, you should be getting a legit improvement that matches that advertised DBi improvement.  Mobile vs. base is not relevant.  To borrow BoxCar's phrase... gain is gain, and the standard you are testing against doesn't change.

 

Also, keep in mind that as atmospheric conditions and terrain/obstacles change overall performance.  If you plan on doing any field testing, you are going to have to test both antennas at the same time, with both stations being in the same location, on the same mount/height/cables/radio/etc.



#4 RickW

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Posted 05 May 2019 - 10:04 PM

Some manufacturers use the theoretical isotropic antenna as a baseline and then compare their antenna to the isotropic. If they are honest they will specify their antenna gain in dBi. A dipole antenna has a gain of 2.15 dB above the isotropic and some manufacturers use the dipole as the baseline and specify their antenna gain in dBd. You can subtract 2.15 dB from dBi gain to match up with the dBd gain.

 

Another consideration with gain is that it depends upon the angle of radiation. Some antennas may radiate with rather high angles, such as using a quarter wave vertical on the third harmonic which can give you a fairly close match. A good example being a 2 meter ham antenna being also used as a 440 MHz antenna. Even though the match might be reasonable, this type of antenna will have a higher angle of radiation on the odd harmonic and may not be as useful for VHF and higher frequencies, especially on flat land. The manufacturer could claim this level of gain without being specific as to the angle and makes the antenna appear to have higher gain than you might expect. There are cases where a higher angle might work better, such as if you were in a deep valley and wanted to communicate with a station at a higher elevation. But, most of the time we want the antenna to radiate toward the horizon for maximum communications distance.


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#5 taco6513

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Posted 06 May 2019 - 06:54 AM

  That improvement is, for every 6 dBi in gain, you double the range of the antenna when compared to an isotropic dipole antenna tuned for the same frequency.

 

 

 

Each 3db of gain in a double effective radiated wattage and receive gain. However to a certain point to get the next 3db gain is not feasible or cost effective. 



#6 marcspaz

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Posted 06 May 2019 - 09:54 AM

Each 3db of gain in a double effective radiated wattage and receive gain. However to a certain point to get the next 3db gain is not feasible or cost effective. 

 

For an omnidirectional, I would tend to agree, generally speaking.  There are only so many "tricks" you can do to make up for a lack of redirecting/focusing energy.

 

Keep in mind that dBi is originally focused around directional antennas.  Seeing 100+ dBi gain in a directional antenna is not uncommon.  Especially in low power services such as cellular and Ham radio 1 watt global comms.


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