On Fri, 05 Nov 2004 07:45:16 GMT, Richard Clark
wrote:
First I will start with a conventionally sized quarterwave and by
iteration approach the short antenna and observe effects. I am using
the model VERT1.EZ that is in the EZNEC distribution and modifying it
by turns. For instance, I immediately turn on the wire loss.

For this progression, I have amended the design through the addition
of 1 wire, 20M long, 21 segments, Vertically polarized, center loaded
with a 73 Ohm Resistor, 4000M remote from the test antenna, and
elevated 2127M to sample the radiation lobe at an angle of 27° which
represents the Best gain angle from previous results (or nearly so).

I further perform readings of the 73 Ohm load under two conditions of
the test antenna. Those conditions are when it is excited by 1A (the
constant current mode) and when it is excited by 36.65W (the constant
power mode). I also include the power into the antenna for the
constant current mode.

40mm thick radiator 10.3 meters tall:
Impedance = 36.68 + J 2.999 ohms
Best gain is
-0.03dBi
Power = 4.214E-05 watts for 1 A excitation
Power = 4.214E-05 watts for 36.65W

next iteration:
cut that sucker in half:
Impedance = 6.867 - J 301 ohms
best gain
0.16dBi
Power = 7.979E-06 watts for 1A excitation
Total applied power = 6.856 watts for 1A excitation
Power = 4.266E-05 watts for 36.65W excitation

next iteration:
load that sucker for grins and giggles:
load = 605 Ohms Xl up 55%
Impedance = 13.43 + J 0.1587 ohms
best gain
0.13dBi
Power = 1.559E-05 watts for 1A excitation
Total applied power = 13.41 watts for 1A excitation
Power = 4.262E-05 watts for 36.65W excitation

next iteration:
cut that sucker down half again (and remove the load):
Impedance = 1.59 - J 624.6 ohms
best gain:
0.25dBi
Power = 1.849E-06 watts for 1A excitation
Total applied power = 1.585 watts for 1A excitation
Power = 4.274E-05 watts for 36.65W excitation

next iteration:
load = 1220 Ohms Xl up 55%
Impedance = 3.791 + J 1.232 ohms
best gain:
0.23dBi
Power = 4.407E-06 watts for 1A excitation
Total applied power = 3.78 watts for 1A excitation
Power = 4.272E-05 watts for 36.65W excitation

Now, all of this is for a source that is a constant current generator;
we've monkeyed with the current distribution and put more resistance
(Rr?) into the equation with loading; and each time loading craps in
the punch bowl.

So much for theories of Rr being modified by loading. I would
appreciate other effort in kind to correct any oversights I've made
(not just the usual palaver of tedious "explanations" - especially
those sophmoric studies of current-in/current-out).

Well, now we can review this data in light of my previous
editorializations.

We begin with the premise that Rr is evidenced by the power expressed
by a known current through an unknown (Rr) resistance. We needn't
concern ourselves with the constant power mode as it closely mimics
the former data. In essence, it serves as a validation of the two
models (the previous post and this post).

However, the constant current mode does show a variation in power
received at the sniffer antenna. For a shorter antenna, there is a
corresponding fall in the power. Counter to my editorial observations
above there is an increase in this power received at the sniffer
antenna when a load is applied. The contrast in my former editorial
observation and this data reveals that Yes the Rr is impacted by
loading and that the drivepoint Z is the Rr.

This comes as no surprise to many.

Now, let us return to a point of analytical bias that lead me to
believe no apparent change in Rr was observable. In fact there was no
way to make it observable except through the artifice of my sniffer
antenna. For the model of the constant current generator, it is a
truism that gain (that is true gain for a system and not simply
antenna directivity) must increase for the same excitation. After
all, we are changing the Rr either through the actuality of modified
length, or the artifice of a moving, variable load along the short
radiator. Such gain is only observable through a circuit (broadcaster
lingo for a transmit/receive pair).

In the back of my mind I was troubled about comparing situations in
dBi. Yesterday I expressed this as a possible source of confusion for
the effects sought in evidence against the obvious gain differential.
dBi is a dimensionless relation such that true gain is washed out of
the result. When I attempted to confirm my suspicions through field
expressions of mv/M for 1KW, I was struck that that too forced the
results to a constant power (not constant current) and thus hid the
gain demonstration in the same way. I then fell back on my practice
of employing a sniffer antenna to test reality and the data is found
above confirming the gain that would be expected. In other words, the
far field's power followed the diminution of Rr with a positive
correlation. It also followed the subsequent increase of Rr (with a
load applied to that shortened radiator) with a positive correlation.

73's
Richard Clark, KB7QHC