LAWRENCE MAYHEAD wrote:
Hi All,
When I first set up my 136 station I put up a Marconi "T" vertical
ant. 15M high 30M long.The best ant. current I could get from my 400
watt amplifier was 1.8 amps.The measured ground /coil/copper loss was
125 ohm! Over the past 6 months I have tried to improve this, I have
increased the top to 3 wires,increased the grounding from 2x4 foot
rods plus house wiring/water etc to 10x 4 foot rods, 20 radials each
about 50 feet long plus one radial 150 feet long ending up at an earth
rod in sea water! I also re-made the loading coil with thicker
wire.The total result of all this effort was 2 Amps !!! Not very
encouraging.This weekend I decided to start again.I replaced the "T"
with an inv.L using the same support (tree) at one end but 60M long to
another tree.SO I now have less wire in the air 60M instead of 3x 30M
but stretched out.The loading inductance has not changed so the
capacitance is the same. BUT the ant current is now 3 Amps!
Does this mean that the antenna wire needs to have a longer "footprint
" on the ground?,presumably the current density is less than under the
3 wire top,but the new ant stretches beyond my radial system,and it is
still much better!.Does this also explain the reported good
performance of so called "long wire" antennas on this band?
Food for thought.
I am hoping to
improve things further but I dont have room to extend the top wire in
a straight line,perhaps I will try a crooked L Hi.
Checks with Eznec indicate a R.Res of about .06 ohms for the "T "
and .0675 for the "L" so with the increased current I hope to have
improved my signal by at least 3db.
73s Laurie.
Some time back I did some impedance measurements on my top loaded
vertical by testing the upwire, and also combinations with each top
loading on or off. Of course the capacitance increased as wires were
added, but it was changes in the resistive component that I was most
interested in. As each top loading wire was added, the result was a
generally lowering resistance. Lower resistance means more current for
a given applied power, and obviously loss resistance is unwelcome in
terms of improving LF transmission efficiency.
At the good suggestion of Andrew ZL2BBJ, I constructed a spreadsheet
that converted all the R+jX data to admittance (parallel conductance and
suseptance, G+jB), as this is physically what happens with connection of
top loading to the upwire. Having the basic information in parallel
format strongly suggested that each "sector" added with coverage by a
top loading wire was like adding lossy capacitors in parallel. A fresh
sector covered by a top loading wire was like adding more capacitance
with a similar "power factor" as could be scaled from the length of top
loading wire. There would have been some proximity effect around the
top of the upwire, reducing total capacitance when all wires were
connected compared to each wire alone, but that can not be avoided.
Note that I had several copper radials but it was far from being "copper
plated".
The net result was that the top loading model appeared to be very like
connecting lossy capacitors in parallel. The input resistance (in R+jX
thinking) at the base of the vertical should decrease somewhat in
proportion to the extent of top loading width and fresh sectors
covered. This suggests the wider the top loading the better, and that
an X top should be better than for a T as it obviously covers "sideways"
over "fresh ground". This also has a lowering current density per
square (cubic?) metre of ground.
In the case described by Laurie, he has increased top loading coverage
from 3x30 metres T to 1x60 metres inverted L. The 3x30 metres of
parallel wires has higher capacitance than one wire, but the ground
covered is approximately a little more than 30 metres. The L top
loading covers 60 metres of ground. This gives a distance ratio of a
little under 2. As the current changed from 1.8 to 3 amps (a ratio of
1.67), then the end result is very similar to what I observed.
Modelling using NEC-2 software does not support the practical data
observed by Laurie or myself. However, NEC-2 software is rather
hopeless at modelling real ground.
Thus the parallel lossy capacitance model seems to have validity. I
have not seen this described in an antenna theory book, but it would
seem to be a plausible explanation for input resistance of amateur
vertical LF antennas over ground.
Bob ZL2CA
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