Gain dBd and dBi - Be Careful

Hello amateurs from Derek in Locust, NC.  Many hams out there (especially
those just joining us in the hobby) are not fully aware that antenna
companies rate antenna gain using two different measurements - those
being dBd and dBi.  dBd means reference to a 1/2 wave dipole.  Most
commercial antenna companies use dBd to rate their antennas.  dBi, which
is used on some ham antennas (mainly foreign made) is refering to
dB gain over a 1/4 wave.  A 1/2 wave dipole is said to have 3dB gain 
over a 1/4 wave.  At the hamfests you may see a new antenna just made by some company you
never heard from and on the sign beside it you see,

         9 DB GAIN!! WOW!!   ... and then beside it you see

it says dBi..  So subtract 3 dB from that and you get 6 dbd (referenced
to a dipole)..  In other words 9 dBi and 6 dBd are the same thing.
So next time you see a cheap antenna that has a claimed very high gain,
be careful -- it may be less than you think.

73 DE KC4FWC @ KD4LV.NC.USA.NA.


Technical Note:   Technically speaking the above referenced 3dB gain difference
between dBd and dBi is actually 2.1512dB.   However, from a practical 
standpoint there is very little noticeable difference, which is why 
its simply referred to as a 3dB difference.