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Tom Miller

wrote in message
...
On Fri, 09 May 2014 11:21:13 -0700, Jeff Liebermann
wrote:

On Fri, 09 May 2014 20:41:15 +0300, wrote:

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.

It is not that far away, assuming perfect ground, which explains the
lower lobe to reflected upwards, which would explain 3 dB of gain. The
remaining 2.19 dB sounds much like the dipole gain over an isotropic
radiator. So actually, we should talk about 5.19 dBi gain or actually
_directivity_.

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. 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.

For electrically small antennas, the efficiency can be
much less than 100 %, thus the _gain_ would be less.

The radiation efficiency and internal losses are included in the gain
calculation. For example, if I made the antenna from lossy material,
it would show up as a loss in gain. However, for a fairly close to
ideal antenna, the radiation efficiency barely changes.

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

Most of this VSWR is due to the fact that the small antenna has a high
(capacitive) reactance. Tuning it out with a loading coil, you only
have a smallish resistance mismatch and hence much lower VSWR.

The 0.050 wavelength monopole shows a feed impedance of 1.52-j707
which is going to be verrrrrry difficult to match efficiently to 50
ohms. Got a 500:1 transformer handy?

You tune out the capacitive -j707 ohms with a loading coil +j707 ohms
and you only have to worry about the resistive 1.52 ohms. To match
this to a 50 ohm input, the impedance ratio would be 1.52:50 or 1:33
or less than 1:6 turns ratio.

A few cm long "rubber duck" (normal mode helix) is a reasonably
efficient antenna for 433 MHz.

But only for certain values of "reasonably"

.