[TowerTalk] 2.2dBi vs dBd

[Ward Silver]

> Just got my copy yesterday, and it's a real winner at only $15.

Thanks!

> Especially interesting and informative is the protocol used. The guys
> did a good job to eliminate just about as many variables as possible.
> 
> Will wait for a few others before discussing it, but would like to 
> ask a question for my own benefit. Something I've accepted for years,
> and it was used in the study. The report wisely used a reference dipole,
> but at one point, remarked that 2.2 dbi of free space gain, and about6
> db of ground reflection gain should be subtracted from total dbi to
> obtain dbd. That's pretty much how I've always done it ( I definitely do
> not like comparing antennas in space to those in the back yard ), but it
> got me to wondering.
> 
> Is it necessary to subtract the 2.2 dbi gain of free space? Obviously,
> a dipole in free space, and one here, would already have the
> compensation
> by default. To further subtract the 2.2 dbi from the gain of a yagi
> would seem to be crediting the real dipole with an additional 2.2.
> 
> Again, the book is a real winner, and as the authors say, an excellent
> starting point for further work.
> 
> 73
> Ed

The 2.2dB correction between dBi and dBd should *NOT* be made to the
figures in our report.  I discuss that correction only to the extent that
manufacturers may include it in their published figures.  If a
manufacturer such as Force-12 or Mosley specifies forward gain in dBd,
there should be no dBi-to-dBd correction.

Furthermore, if the manufacturer follows the dBd definition exactly, the
figure should be referenced to a dipole at the same height, as well.  dBd
is shorthand for how much louder the antenna under test would be if
replaced with a full-sized dipole, resonant at the same frequency, and
mounted at the same height and azimuth.

73, Ward N0AX


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