Roy Lewallen wrote:
A plain dipole 76 feet above perfect ground has a gain of about 8.6 dBi

Is that true? I've always heard it was slightly over two dB. I re-read
your post and I don't think I'm taking what you said out of context. Did I?

73,
kz1o

http://www.radio-electronics.com/inf...ivity_gain.php
http://www.tmeg.com/tutorials/antennas/antennas.htm
http://www.starantenna.com/omni_dire...le_antenna.htm
http://www.softwright.com/faq/engine...Y%20UNITS.html
http://www.ventenna.com/faq.html
http://tinyurl.com/5zste

On Wed, 15 Dec 2004 16:27:40 -0500, Dave Bushong wrote:

A plain dipole 76 feet above perfect ground has a gain of about 8.6 dBi

Is that true? I've always heard it was slightly over two dB. I re-read
your post and I don't think I'm taking what you said out of context. Did I?

Hi Dave,

Wrong context. Not many can hoist a dipole that high, and finding
perfect ground is even more difficult to find - but once they do, that
2+ dB is undoubtedly measured in dBd (which is to say, compared to a
dipole in free space) which at least one of your links uses also.

73's
Richard Clark, KB7QHC

No, you read it correctly.

The gain of a dipole *in free space* is about 2.15 dBi. The gain of a
dipole over ground (compared to the standard reference of an isotropic
antenna in free space, which is what dBi means) is considerably greater.
You get an automatic 3 dB in field intensity when you put a dipole over
ground just because the radiated power is concentrated in a hemisphere
rather than a whole sphere. Then there can be additional gain due to
pattern shaping by interference between the direct and reflected signals
-- the effect is exactly the same as you get from a two element array
with the elements fed out of phase. This additional directive gain from
pattern shaping can be as high as about 5 dB. (See the two element array
patterns in Chapter 8 of the ARRL Antenna Book.) So the gain of a
horizontal dipole over ground is typically between about 5 and 10 dBi.
This is, of course, at the angle at which the pattern is maximum, which
is never at the horizon for a horizontally polarized antenna, and in
some cases is at a fairly high and perhaps useless angle.

Except for very high radiation angles, ordinary ground is nearly as
reflective as perfect ground for horizontally polarized fields. So the
above conclusions apply also to real ground as well as perfect ground.

I've argued many times against the use of "dBd" as a measure of antenna
gain, unless the meaning of dBd is carefully spelled out. A good deal of
misunderstanding results if one person assumes it means the gain
relative to a dipole in free space (2.15 dBi) and another believes it
means relative to a dipole at the same height as the antenna being
discussed (in which case it means about 5 - 10 dBi). I've seen at least
one antenna manufacturer cleverly make use of this misunderstanding to
make their antennas sound like they have a lot higher gain than they do.
A casual reader might have gotten this same impression from reading
Art's data, although I know he's not intentionally trying to mislead anyone.

If any of the sources you referenced contradict what I've said, I
recommend dropping them a line and suggesting that they correct what
they've published.

Roy Lewallen, W7EL

Dave Bushong wrote:
Roy Lewallen wrote:

A plain dipole 76 feet above perfect ground has a gain of about 8.6 dBi


Is that true? I've always heard it was slightly over two dB. I re-read
your post and I don't think I'm taking what you said out of context.
Did I?

73,
kz1o

http://www.radio-electronics.com/inf...ivity_gain.php
http://www.tmeg.com/tutorials/antennas/antennas.htm
http://www.starantenna.com/omni_dire...le_antenna.htm
http://www.softwright.com/faq/engine...Y%20UNITS.html
http://www.ventenna.com/faq.html
http://tinyurl.com/5zste