Ham House
 

"Sum Ting Wong" wrote in message
...
On Fri, 22 Sep 2006 07:24:46 -0500, "H. Adam Stevens, NQ5H"
wrote:

Logs are Passe'; Use a SteppIR.

Yep... a SteppIR AND one of these...

http://www.teleskopmaste.com/optibeam/ob804020e.htm

I see the gain figures on that beauty and I puzzle over why the dBd and dBi
gain figures are so far apart -- about 7 db. I expect a difference
somewhere in the 2 - 3 dB range. IIRC it's 2.2 dB. What am I missing?

Before I sent this, I did a Google search using some of these terms and
discovered "ground reflection gain," a term I have never seen before, except
in connection with EME with the moon near the horizon. Is this a realizable
gain or damnable inflation of gain numbers?

TKS.

Ham House
 

On Fri, 22 Sep 2006 20:07:39 -0700, "Sal M. Onella"
wrote:


I see the gain figures on that beauty and I puzzle over why the dBd and dBi
gain figures are so far apart -- about 7 db. I expect a difference
somewhere in the 2 - 3 dB range. IIRC it's 2.2 dB. What am I missing?

Before I sent this, I did a Google search using some of these terms and
discovered "ground reflection gain," a term I have never seen before, except
in connection with EME with the moon near the horizon. Is this a realizable
gain or damnable inflation of gain numbers?

TKS.

Yep. Put a 1/2-wave dipole 1/2-wavelength over sea water and compare
its gain to the gain to a isotropic in free space.

Danny, K6MHE

Ham House
 

Danny Richardson wrote:

Yep. Put a 1/2-wave dipole 1/2-wavelength over sea water and compare
its gain to the gain to a isotropic in free space.

Actually, the gain of a horizontal antenna at reasonable height and
reasonably low angles is nearly the same over plain ground as it is sea
water.

But do what Danny says, and you'll find that a dipole over ground has a
gain of around 6 - 8 dBi. While dBi is rigorously defined, "dBd" isn't.
Using a common conversion of 0 dBd = 2.15 dBi (the gain of a dipole in
free space), you find that a dipole over ground has a gain of about 4 -
6 dBd. At least one major antenna manufacturer quoted the gains of their
antennas over ground in "dBd". Naive people assume that it means
"compared to a dipole at the same height", while it actually means
"compared to a dipole in free space". Gives their antennas an instant 4
- 6 or so dB boost over the competition.

Roy Lewallen, W7EL

Ham House
 

Sal M. Onella wrote:

SNIPPED

I see the gain figures on that beauty and I puzzle over why the dBd and dBi
gain figures are so far apart -- about 7 db. I expect a difference
somewhere in the 2 - 3 dB range. IIRC it's 2.2 dB. What am I missing?

Before I sent this, I did a Google search using some of these terms and
discovered "ground reflection gain," a term I have never seen before, except
in connection with EME with the moon near the horizon. Is this a realizable
gain or damnable inflation of gain numbers?

TKS.



BOTH!!

Ground reflections CAN add to the 'freespace' gain at certain antenna heights
and angles of incidence. The ground reflected wave is is phase with the incident
wave.

'damnable inflation' ... well if one manufacturer uses the term in it's
marketing literature then the competitors will follow. Then 'gain' comparisons
go to H..L!

The best comparison is free space gain [calculated] as dBi or dBd.

I used to mamange a microwave antenna test range. Out antennas were installed
between 20 to 30 wavelengths above ground to reduce 'ground effect confusion'.