Hello Jim and the group,
Thanks for responding! I will read your email again beacause it addresses
exactly what I was looking for as far as the ground questions are concerned.
As far as the performance of my antenna it seem in this as in many other
things I am always looking for a complex answer and continue to overlook the
obvious. This loop is located about 6 feet away from the 130 tower with its
18 guy wires.
Bill Ashlock (WA) along with others pointed out the tower and its massive
array of guy wires would have very detrimental effect on the pattern, "Q",
feedpoint impedance, etc.
So I took down the antenna which again weighs 60 lbs and put up a #12 AWG
wire in its place. Much more managable.
Same location, even higherfeedpoint impedance as I expected and a lot less
capacitance to resonate, also expected.
Next step was move the antenna away from the tower by 25 feet. This made a
big difference. To achieve resonance I had to add more capacitance and I
had to add 2 turns to the primary of the matching transformer indicating
lower feedpoint impedance.
So tonight I will move it even farther away and get the feed poit above the
ground, it is only 6" high right now.
Lesson learned..... Do not overlook the obvious!!!
Thank again Jim.
----- Original Message -----
From: "James Moritz" <[email protected]>
To: <[email protected]>
Sent: Tuesday, April 09, 2002 9:39 AM
Subject: Re: LF: Ground loss diffferences between loops and verticals
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
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