> 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 about 6
> 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.
You can not subtract either 6dB and/or 2.2 dB to get dBd freespace.
1.) There is no guarantee ground reflection gain is 6 dB, or any amount of
"dB". It might even be loss!
2.) If you subtract 2.2 dBi from the yagi's gain, you get a number 2.2 dB
less than the gain at the angle where gain was measured. You have no idea
how that number relates to dBi unless you measured the antenna in
freespace, or over perfect ground at the middle of the lobe maximum in a
test range.
73 Tom
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|