El 13-11-14 21:39, FBMboomer escribió:
On 11/12/2014 1:50 PM, gareth wrote:
It is a truism that short antennae are poor inefficient radiators,
and no
amount of infantile bluster by Americanoramuses will change that.

The truth does not need the violence of abuse to force its way down
people's throats.



A perfect example is a G5RV on 75 meters. They suck. When someone
joins our group rag chew on 75, and they have a poor signal, The first
thing I ask is "Are you using a G5RV". We all have a chuckle when they
answer yes and then ask how we knew. :-)

Trying to prove with math that short antennae work as well as say a
1/2 wave dipole may give someone great sport. However, in the real
world, short antennae suck big time. I have been an American for most
of my life. Please do not paint us all with the same brush.



I agree that in many practical circumstances electrically small
antennas do not perform well. I also know that having an antanna is
better than nothing.

The smaller the antenna, the more difficulties you will experience to
get radiation out of it (heat radiation doesn't count).

In free space you can make a rather efficient antenna with say maximum
size of 0.03lambda, as long as you are a good electron tamer. If not,
electrons escape from the structure showing a nice corona, or full
breakdown occurs. Examples are tuned loops and short dipoles with
capacitive end plates and series inductors to arrive at some nice
impedance.

Tuning in the shack with lots of cable and a bad ferrite balun between
tuner and antenne mostly results in good VSWR but low efficiency (as
many people know).

In real world even a very small very efficient antenna may not perform
as expected Close to the antenna (say within 0.1 lambda), reactive
fields are very strong and they increase rapidly when reducing the
distance. This is also valid for "magnetic loops" (that Jennings HV
vacuum capacitor or thick potato cutter/slicer is for a reason). When
you put such a nice small antenna close to lossy dielectric (building
materials, ground, etc), significant part of the RF power may be
dissipated in that lossy dielectric materials.

In case of short monopoles, lots of power is mostly dissipated in the
ground/counterpoise system (saline wetlands, sea and large metallic
surfaces excepted). Of course we can solve this with lots of burried
radials, or somewhat less elevated radials, but such solutions don't
qualify for an electrically small antenna anymore.


--
Wim
PA3DJS
Please remove abc first in case of PM