RX antenna Gain

[productionguy85]

I apologize for my ignorance, but I want to make sure of something before purchasing an antenna. The gain rating of an antenna is for RX and TX, NOT just for TX? So in theory, a 10db antenna will receive better then a 0 or unity antenna? Is this accurate, (of course with exceptions)?

 

[prcguy]

Antennas are reciprocal, so whatever gain they have on transmit will be the same on receive except for active receive only antennas of course. A lot of companies really hype their antenna gain figures so look for the gain referenced to something when comparing.

Gain in dBd would reference to a half wave dipole and dBi would be referenced to an isotropic antenna, which is and industry standard model that only exists on paper but has 2.4dB less gain than a half wave dipole.
prcguy

I apologize for my ignorance, but I want to make sure of something before purchasing an antenna. The gain rating of an antenna is for RX and TX, NOT just for TX? So in theory, a 10db antenna will receive better then a 0 or unity antenna? Is this accurate, (of course with exceptions)?

 

[LtDoc]

And if a gain figure is quoted with out that 'd' or 'i' on the end of it that references some antenna for comparison, the figure is meaningless. For all they're telling you it could be in comparison with a beer can in the back yard.
That's called advertising!
- 'Doc

 

[productionguy85]

Thanks. So an antenna with 10dbd will RX "better" then 2.5dbd? And always look for dbd and NOT dbi?

 

[jim202]

One thing to bear in mind here on antenna gain is where they are going to be used. An important factor to keep in your mind with an omni antenna is the higher the gain, the more compressed it's pattern will be in the vertical plane out to the horizon. This is like viewing the coverage pattern like a donut. The more gain, the less the donut is in height, but the greater the diameter of the donut.

The real point to look at here is if the antenna is mounted on a high mountain and has no electrical down tilt, you could have a so called "cone of silence" under the antenna near the mountain. If your out on fairly flat ground, then it's an I don't care. What the "cone of silence" really does is cause the signal from the antenna to go over the users that are trying to operate lower than what the antenna pattern allows coverage for. In other words, the signal coming out of the antenna goes way over the head of the users below the coverage pattern. This goes for both TX and RX .

Lower gain antennas have a more broad coverage pattern in the vertical plane (thicker donut) and cover better close in. The down side is that you loose the ability to cover far away from the antenna to some degree.

 

[n4yek]

Thanks. So an antenna with 10dbd will RX "better" then 2.5dbd?

Yes
Here is a link explaining the difference between them:
http://wireless.fcc.gov/outreach/2004broadbandforum/comments/YDI_understandingdb.pdf

Antenna makers like giving you gain in 'dbi' because it makes their antenna's sound better than they really are in the real world.
Like stated earlier, subtract 2.15 from gain given in dbi to get a 'real world' gain factor over a dipole, dbd.
Example: 7 dbi gain - 2.15 = 4.85 gain in dbd

 

[prcguy]

Yes and no. Higher gain numbers based on the same reference would generally receive better if the gain is where you need it like at the horizon for a vertical antenna. If one company has an honest gain figure listed in dBd and another is in dBi you can add 2.14dBd when going from dBd to dBi or subtract going the other way.

You can fit a 10dBd gain omni UHF antenna in about 20-21ft, or about 6dBd gain in VHF in the same size. If you see some fantastic claims like 10dB gain for a omni VHF antenna that's 7ft tall then don't believe it.
prcguy

Thanks. So an antenna with 10dbd will RX "better" then 2.5dbd? And always look for dbd and NOT dbi?

 

[productionguy85]

Thanks everybody for your input. My question is geared toward an antenna or RX only, specifically scanning, and and planning to have separate VHF, UHF and 800mhz antennas going to my scanner. But the setup is for another forum. I have heard of the "cone of silence" and am not (real) worried about it, but thanks for the info.

 

[n5ims]

Gain is generally referenced based on two types, dBi and dBd. dBd is gain measured against an actual antenna (a dipole) while dBi is calculated against an antenna that only exists in theory, but can't actually be created. The formula for gain is dBd = dBi - 2.15 (or the converse of dBd + 2.15 = dBi). dBd is what I like to call "real gain" since it's something that you can actually get. dBi gain is only there to make the theory and formulas work out correctly.

dBi is often the gain that is quoted by folks trying to sell their antenna. Why you might ask? The answer to that is simple, it makes the gain numbers LOOK better while not actually being better. You know, like the EPA estimated MPG of that new car you're looking at, looks good, but you'll never get that much in normal driving.

To sum up, a 10 dBd gain antenna would be better than a 12 dBi antenna (it would have .15 extra gain). If they don't list what they measure the gain in, assume it's in dBi since you must assume that they're simply trying to make the numbers look better. If you're trying to compare antennas where some are listed in dBd and others are in dBi and still others do not list what the gain is reported in do the smart thing. Add 2.15 to those listed in dBd and compare away. They should now be on an even setting (yes, you may underestimate some that don't list what they report gain in, but it's really their own fault for not listing it).

 

[prcguy]

Antenna gain listed in dBi is not a bad thing and pretty much all satellite antennas, commercial point to point microwave, radar antennas, etc are listed in dBi. In fact you will probably not find any reference to a dipole for most types of antennas except for amateur and land mobile since dBi is a more common reference.
prcguy

Gain is generally referenced based on two types, dBi and dBd. dBd is gain measured against an actual antenna (a dipole) while dBi is calculated against an antenna that only exists in theory, but can't actually be created. The formula for gain is dBd = dBi - 2.15 (or the converse of dBd + 2.15 = dBi). dBd is what I like to call "real gain" since it's something that you can actually get. dBi gain is only there to make the theory and formulas work out correctly.

dBi is often the gain that is quoted by folks trying to sell their antenna. Why you might ask? The answer to that is simple, it makes the gain numbers LOOK better while not actually being better. You know, like the EPA estimated MPG of that new car you're looking at, looks good, but you'll never get that much in normal driving.

To sum up, a 10 dBd gain antenna would be better than a 12 dBi antenna (it would have .15 extra gain). If they don't list what they measure the gain in, assume it's in dBi since you must assume that they're simply trying to make the numbers look better. If you're trying to compare antennas where some are listed in dBd and others are in dBi and still others do not list what the gain is reported in do the smart thing. Add 2.15 to those listed in dBd and compare away. They should now be on an even setting (yes, you may underestimate some that don't list what they report gain in, but it's really their own fault for not listing it).

 

[n5ims]

Antenna gain listed in dBi is not a bad thing and pretty much all satellite antennas, commercial point to point microwave, radar antennas, etc are listed in dBi. In fact you will probably not find any reference to a dipole for most types of antennas except for amateur and land mobile since dBi is a more common reference.
prcguy

True, but since this is a scanner/land mobile/ham radio site, my comments were pretty much right on. Had this been another site aimed at the satellite/microwave/radar/broadcast antenna crowd, I would've given my comments a much different slant.