UHF antenna rating dBi

[Utemad]

Hi folks

What is with UHF antenna ratings going from dB to dBi?

It looks like a 4.5dB antenna is now rated at 6.6dBi.

I googled and found no explanation. Just everyone has changed over to the different rating system.

Thanks

[waynep]

Recalling my 30 year ago radio theory but here goes.

I'm pretty sure "dBi" refers to dB gain over an isotropic antenna, which is a theoretical antenna with zero gain. You can't make an isotropic antenna.

The other way to reference gain is to a standard half wave dipole antenna which has about 2.2dB gain over an isotropic (again only theoretical ). This "dBd" measurement is more practical because a dipole antenna can actually be built and other antennas tested against it as a reference. A quarter wave ground plane antenna looks electrically like a half wave dipole.

If you Google isotropic antenna you will get more on the theory.

[slug_burner]

your 30 year recollection has stood the test of time.

db is a ratio (power ratio) dbi is a ratio with respect to an isotropic antenna.

an isotropic antenna is theoretical and it sends power out in all directions evenly spread. A bit like the energy that is radiated away from the sun, just as much out the sides as out the top, you get the picture

[Utemad]

So why would they change the rating for consumer antennas across the board?
I think it is weird not even GME mention the change on their website.

[miky]

Read what is said above!

If dBi is 2.2dB "more" than using dB over a dipole they get a bigger number.
Bigger the number means it must be better, yes? Actually no it doesn't of course, they are just using a different reference point.


.

[Hoges]

Read what is said above!

If dBi is 2.2dB "more" than using dB over a dipole they get a bigger number.
Bigger the number means it must be better, yes? Actually no it doesn't of course, they are just using a different reference point.

.

quite correct...to get a bigger number 'cause size counts:wasntme:

[Utemad]

This came up for pretty much that reason. A friend of mine who lives in outback Qld was buying a UHF and antenna. He wanted a 6db as there is nothing resembling a hill for hundreds of kms from where he lives.

The ARB guy said that this is the one he needs as he shows him a 2ft or so 6.6dBi antenna.