Dear Paul, LF Group,
At 14:08 08/04/2002 -0400, you wrote:
A total of 200 wires woven
specifically to be minimize crosstalk between pairs and common mode
interference.
I thought that this may approximate a "Poor Mans Litz Wire".
At RF, the current tends to flow on the outside of a conductor or bundle of
conductors. Litz wire counteracts this by weaving strands so that all
strands move between the inside and outside of the bundle, forcing the
current to flow through the whole cross section of the bundle, thus
reducing the RF resistance. But I don't think telephone cable is
constructed in the same way, so the resistance will not be reduced much, if
at all.
Also the slope either side of resonance was not smooth at all, there
seemed to be peaks among the slope.
Perhaps ambient LF signals being received by the SVM? See if they are there
with no generator connected.
When looking at the current and voltage
waveforms at the feed of the antenna, using the test tool described in the
LF handbook, it could be seen that a the current was a perfect sinewave but
the voltage waveform always had a flat top or a double peak.
Changing
resonance or turns on the matching transformer would only changeflat spot
slightly. Loop Rac measures 1 Ohm based on matching transformer ratio.
Now the loop is on a tower that is 130 feet tall. The guy wires are not
broken up with insulators. So is it possible that there is a resonance in
the guy wires causing the double peak?
/If so why only on the voltage wave forms?
In the LF range, the guys would have to be extremely long (hundreds of
metres) to resonate. With a Q of 61, the current in the resonant loop will
be almost sinusoidal whatever voltage waveform is applied - as with class D
and E amplifiers with resonant tank circuits. So the funny voltage waveform
probably originates in whatever you are using as a generator - perhaps it
does not like delivering power into the resonant load. You could try
inserting a resistive pad between generator and antenna.
A) When visualizing the losses from the earth in a vertical antenna system,
they would appear to be in series with the Radiation Resistance, thereby
limiting the current that thae antenna can draw. Is this statement true?
Current flows from the radiator and returns through the earth(limiting the
current) which is lossy and thus making a poor radiator.
B) How does the earth effect the a loop? Does it look like a lossy
secondary of a transformer? Since the loop is "closed" is cannot be truly
in series with the antenna.
Perhaps the affect is a reflected impedance from the ground causing current
to flow in the antenna. In a loop the Rac is determined by the radiation
resistance, in series with the loss of the capacitor, in series with the
loss in the inductor and an unknown series loss for the earth. Is this
true?
The magnetic and electric fields of the loop induce currents in the ground,
and it requires energy to maintain these, representing a loss from the loop
circuit. While not actually being connected to the antenna in any way, the
effect of these ground currents on the antenna impedance is equivalent to
having a resistor connected in series with a loop which absorbs some of the
power applied to the loop.
The loss resistance is not a physical resistor but the resistive component
of the equivalent circuit of the loop antenna that represents the sum of
the losses you mention. it is conventional to represent this as a series
resistance, but it could be equally be represented as a resistor (with a
different value) in parallel with the loop inductance, or coupled by a
transformer - in any case it would absorb the proportion of the power
delivered to the antenna that was not radiated.
The antenna I have radiates but not well.
.. Unfortunately, true of just about any amateur LF antenna!
Cheers, Jim Moritz
73 de M0BMU
|
