2dBi or 2dBd, that is the question
Wednesday 30 August, 2006, 08:06 - Amateur Radio
Something that I found confusing when first learning about the 'black art' of radio is that there seemed to be some argument about the gain figures quoted for antennas. Take a look at this example: The gain figure quoted for this Comet GP-3 dual-band 144/432 MHz antenna is shown as 4.5 and 7.2 dB respectively for the two bands. But any 11th grade physics student will tell you that dB or 'deciBel' is just a ratio between two numbers. So a figure of 4.5 dB is meaningless unless it is 4.5 dB with respect to some reference or other - and this is where the confusion arises...To circumvent this confusion, an additional letter or letters is usually added after the dB to indicate what the reference point is. So 'dBuV' is 'dB relative to 1 microVolt (uV)', and 'dBm' is 'dB relative to 1 milliWatt (m)' and 'dBW' is 'dB relative to 1 Watt'. So what is the reference point for our antenna gain, and why is there potential for a mix-up?
Antenna gain can be measured in 2 different ways. In it's purest form, antenna gain calculations are made with reference to an 'isotropic source' - the measure being dBi. This theoretical antenna radiates signals equally in all directions, up and down, left and right, backwards and forwards. However an isotropic source is just that - theoretical - as if one could be constructed, the maths of antenna design tells us that it would have to be infinately small. So there is another reference point, a standard half-wave dipole (as this is the most basic form of antenna) - indicated by dBd. Now dBd is, in my mind, a more sensible measure as a simple dipole has a gain of... 0 dBd. However the same antenna could be said to also have a gain of 2.15 dBi as a dipole is somewhat larger than our theoretical isotropic source and thus radiates better.
All very straightforward you might think, so what's the problem? As long as I know that I need to subtract 2.15 dB to convert from dBi to dBd then everything's hunky dory isn't it?
Well, no. Click again on the link to the antenna specification. It shows the gain as 4.5 and 7.2 dB... it doesn't say whether this is dBi or dBd so in fact, the antenna could have a gain of only 2.35 and 5.05 dBd, which doesn't sound quite so good. Indeed if we check the manufacturer's web-site we discover that the quoted gain figures are, indeed, in dBi.The upshot of all this is that when comparing manufacturers' quoted gain figures to decide whether one antenna claims to be better than another, make sure you're not comparing apples and pears by making sure that you convert all the quoted gain figures to be relative to the same reference point. It doesn't matter whether it's dBi (which manufacturers prefer as it gives a bigger number) or dBd (which 'feels' more sensible) but always be careful if the gain figure is just quoted in dB.
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