[Elecraft] dBd vs dBi

[Bob Nielsen]

Antenna Measurements 101 (short version):

The dBi reference is compared to an isotropic radiator in free space,
one which has an equal signal intensity in all directions (the pattern
is spherical).  This is the reference typically used when calculating
antenna patterns and gain.  Compared to an isotropic reference, a
dipole in free space has a directivity of 1.64, or 2.15 dB.  The gain
is equal to the directivity times the efficiency of the antenna (which
is usually ignored).

Since true isotropic radiators do not physically exist, measurements
are often made with a dipole reference.  Knowing the directivity of an
antenna compared to a dipole, you add 2.15 dB to get the directivity
relative to an isotropic reference.  Therefore 3 dBd would be
equivalent to 5.15 dBi.

However, when measuring gain under other than free-space conditions (or
in a suitable anechoic chamber, not very practical at HF), ground and
other reflections affect the measurement.  Since many companies handle
this in different ways, it is often difficult to make an exact
comparison.  Usually a dipole at the same height is used as a
reference, but if the antenna in question has a narrow elevation
pattern, the effect of the ground reflection may vary between the
antenna being tested and the reference dipole.  Screens are typically
used on antenna ranges to minimize the effect of reflections, but one
can get slightly different results when using different test ranges. 
It is probably safe to say that no two manufacturers use exactly the
same measurement criteria and a few cannot be trusted very much at all,
which is why you seldom see gain figures in QST.

About 40 years ago, an engineer I worked with said in regard to
measurement accuracy, "tenths of a dB don't count." Unfortunately, this
still often applies.

73, 

Bob, N7XY
Bainbridge Island, WA

On Sun, Oct 20, 2002 at 07:42:26AM -0400, kc4kgu@ENTERZONE.NET wrote:
> 
> Can someone tell me what how these two figures would compare to each
> other?
> 
> 3-dBd over a dipole
> 
> 3dBi freespace
> 
> I'm trying to compare antenna specs from two different companies but, as
> usual, they can't use the same _exact_ measurement criteria.
> 
> 73 de John - KC4KGU
>