On Fri, 9 May 2014 12:31:01 -0400, "Tom Miller"
wrote:
"Jeff Liebermann" wrote in message
.. .
On Fri, 09 May 2014 12:02:50 +0100, Mike Perkins
wrote:
http://802.11junk.com/jeffl/antennas/Monopole/index.html
Do you believe the numbers it is giving you? 5.19 dB for a 1/4 wave ground
plane antenna? Also, gain is in dB, not dBm. Just a little nit.
Sorry about the dB and dBm mixup. My sloppiness. Thanks.
Yes, I believe the gain figures. However, it doesn't matter because
my point was that the CHANGE in gain with reduced antenna length is
minimal. I've seen that in bench and field tests, so I know that
works. The catch is that the antenna has to be impedance matched,
which becomes increasingly more difficult and lossy as the antenna
becomes shorter.
In general, unless I hit one of the limitations of NEC2, the patterns
and gains of the models are fairly accurate. For this monopole, the
added gain comes from reflections off the ground plane. The situation
is similar to a mobile HF antenna, where the length of the antenna is
considerably shorter than 1/4 wavelength. As long as the antenna is
reasonably well matched and the matching losses are included in the
calculations, then a short antenna can work almost as well as a real
1/4 antenna.
However, reality is never as good as the models predict. In this
case, I'm using an infinite perfect ground. I have yet to find one of
those outside of an antenna range. The matching losses for the
shortened antennas will add more losses. At best, the 5.19dB gain
will be a best case maximum, with reality being somewhat less.
You can sorta see the effects of height above ground in:
http://802.11junk.com/jeffl/antennas/vertical-dipole/index.html
http://802.11junk.com/jeffl/antennas/vertical-dipole/slides/animated-v-dipole.html
The animation was originally intended to show the effect of antenna
height on the pattern, but it also shows the changes in gain. Closest
to the ground, it shows 5.24 dB gain. Highest shows 7.08 dB. Both
are much larger than the traditional 2.18 dB gain of a free space
dipole.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558