Amplifiers and high gain antennas

[Skip Talbot]

Howdy folks,

I've been looking at various cellular amplifiers and antennas, and noticed that there are legal limits, as definied by the FCC, as to how high the gain can be on an antenna when connected to a particular amplifier. The 3 watt amplifier I saw was listed as having a legal limit of 7 dbi for the antenna. Why is there a limit on antenna gain at particular wattages, and what is the adverse affect if you exceed it? Does it cause harmful interference, damage equipment, or is it a saftey hazard to people near the antenna?

Also, the amplifier said it works best with a 3 dbi antenna. I was under the impression that the higher the gain the better, especially for our applications where we are a long distance from the cell tower. What about the amplifier would hinder performance with a higher gain antenna?

 

[Jerry Prsha]

Also, the amplifier said it works best with a 3 dbi antenna. I was under the impression that the higher the gain the better, especially for our applications where we are a long distance from the cell tower. What about the amplifier would hinder performance with a higher gain antenna?

Skip, I'm more of WiFi Guy but gain doesn't always work that way. The reason for the lower db rating is that depending on the angle of the antenna to the source/target, the antenna beam can get too narrow vertically and miss the target; say going up or down a hill. The lower the db, the shorter the distance but more tolerance for tilt. The higher the db the more focused the beam and less tolerant of the movement of the source/target.

You might end up getting less coverage and spending more money... Especially on a moving vehicle. Personally, I wouldn't go any higher than a 5db antenna on moving vehicles. Even though the plains states are relatively flat, you'd be amazed at the angle your car will tilt while driving and how it relates to the surrounding landscape.

Feel free to drop me a line if you need more info.

 

[Jesse Risley]

The FCC also has to regular maximum permissible exposure (MPE) limits emitted from transmitters. The type of antenna, frequency and location of the antenna are taken into consideration when evaluating MPE limits, including those for cellular products.

 

[Rob Wadsworth]

Db gain on an antenna has to do with the design efficiency of that radiator/resonator. Cell phones are basically encrypted UHF Ham radios with repeaters/towers. They need towers and height to attain line-of-sight for your phone to work. As one gets too far away with a weak transmitting phone; the harder for that tower to hear it. Having more gain from an 'omni-directional' antenna is certainly the better way to achieve a communication; but having the signal strength boosted with an amp helps even more.

We are dealing with 900-1200mhz frequency usage with only milliwatts in output from the phone; these freq's tend to be only as good as you are close to/from the tower. Spurious frequency amplification and intermodulation distortion are good reasons for not using more power than one needs, as the ill effects of these undesireable noise may impair the tower/repeater system. I think that the 3db antenna and 12 watt booster I got does the job quite well. Often, the antenna does well enough on its own.

You can get a Yagi-type 'uni-directional' antenna for your home with mega db gain. If you really wanted to and felt inventive - you could even mount it on your vehicle. The only down side to that is that Yagis are beam antennas - meaning they need to be pointed in the right direction for full magnification of signals to occur. Anything to either side of the antenna has a great deal of signal rejection. So, an antenna rotor would need to be used. Not exactly impossible to do - but a little impractical to accomplish...

 

[Gordon Spencer]

I know the antenna limits listed on our amplifiers are a FCC thing . Basically they do not want the setup to work to well . The 5 DB magnetic mount that comes with our CA819 kit or Rockies ( CHA819) is usually all you need when used in conjunction with the amplifiers. I can only speak for our amplifier and the software driven amplification . I cannot speak for hardware driven amplifier . But they do tend to cause more havoc on the cellular networks so a more powerful antenna used with one of them may indeed have an even greater negative effect on the network .

Howdy folks,

I've been looking at various cellular amplifiers and antennas, and noticed that there are legal limits, as definied by the FCC, as to how high the gain can be on an antenna when connected to a particular amplifier. The 3 watt amplifier I saw was listed as having a legal limit of 7 dbi for the antenna. Why is there a limit on antenna gain at particular wattages, and what is the adverse affect if you exceed it? Does it cause harmful interference, damage equipment, or is it a saftey hazard to people near the antenna?

Also, the amplifier said it works best with a 3 dbi antenna. I was under the impression that the higher the gain the better, especially for our applications where we are a long distance from the cell tower. What about the amplifier would hinder performance with a higher gain antenna?

 

[Jarrod Cook]

The reason they have antenna gain limits for a given amplifier gain is to stay under the maximum legal EIRP (effective isotropic radiated power). EIRP is the actual power out from the antenna. To get EIRP, you have to do a link budget and take into account your transmitter power, amplifier gain, coax losses, and antenna gain. I did a quick search on the FCC page and couldn't find the max EIRP limits off the bat.

The main reason is to prevent interference with other wireless users. Of course, safety should be taken into account too with higher EIRPs at these frequencies...