On Sat, 10 May 2014 09:44:44 +1000, Clifford Heath
wrote:
On 10/05/14 04:21, Jeff Liebermann wrote:
Good explanation and you're correct about the directivity. Having
gain in the wrong direction isn't very useful. In the case of the
shortened monopole, the peak gain is in roughly the correct direction
needed to be useful, so it's not an issue. However, other antenna
configurations can cause problems. For example, here's an animated
GIF of a common discone antenna.
Where?
Sorry. Here's the missing link:
http://802.11junk.com/jeffl/antennas/Discone/slides/discone-animated.html
http://802.11junk.com/jeffl/antennas/Discone/index.html
Up to about 400 MHz, the major lobe
(i.e. maximum gain) is roughly horizontal, making the antenna quite
useable. However, between 400 and 1000 MHz, most of the RF goes
straight up. There's little RF left at the horizon, where it's
needed. Such an antenna might be good for listening to airplanes, but
not terrestrial stations.
wavelengths gain Efficiency Radiation
(db) Efficiency
0.250 5.19 100% 99.93%
0.125 4.85 100% 99.66%
0.050 4.75 100% 99.09%
Again, the problem is matching the impedance of the shortened antenna.
The losses are not in the antenna. They're in the matching circuit.
wavelengths gain(db) VSWR
(50ohms)
0.250 5.19 1.74:1
0.125 4.85 158:1
0.050 4.75 5954:1
I'm curious how you calculate these values?
4NEC2 does the number crunching for me. I would hate to do the
calculations by hand or with a calculator. I'm not even sure that I
could do them.
The VSWR and Efficiencies are show in the "Main" page. More on
radiation and power efficiency:
http://www.antennex.com/w4rnl/col0504/amod75.html
0.250 wavelength:
http://802.11junk.com/jeffl/antennas/Monopole/monopole_0_250/index.html
http://802.11junk.com/jeffl/antennas...ole_0_250.html
0.125 wavelength
http://802.11junk.com/jeffl/antennas/Monopole/monopole_0_125/index.html
http://802.11junk.com/jeffl/antennas...ole_0_125.html
0.050 wavelength
http://802.11junk.com/jeffl/antennas/Monopole/monopole_0_050/index.html
http://802.11junk.com/jeffl/antennas/Monopole/monopole_0_050/slides/monopole_0_050.html
Mo
http://802.11junk.com/jeffl/antennas/Monopole/index.html
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558