LF,
I shall be transmitting on VLF again over the weekend.
The main purpose will be to test (and adjust) my new
loading coil therefor the transmitted frequency may vary.
I shall keep the reflector informed as to frequency and
times that transmissions occur should anyone wish to take a
look for my signals.
PRESENT STATION SET UP
TX
I have the option to use either an internal OCXO or an
external off-air standard frequency, both of 10.000 . . MHz
which is then passed through a programmable divider to give even
divisions e.g.
10.000/1110 = 9.090 909 kHz 10.000/1112 = 8.992 806 kHz
etc. . .
The PA is capable of giving several hundreds of watts but
it has a low output impedance and the aerial system has a high
impedance (several kilohms).
To achieve reasonable matching I use a large laminated-core
mains transformer with a step up ratio of 1:24
LOADING COIL(S)
To tune out the aerial's capacitive reactance I use a
series loading coil (inductor).
The original coil was single-wound, about 600 mm diameter
and has an inductance of ~ 1.25 H
With the above TX, loading coil and my 40m inverted 'L' I
can generate an aerial current of 0.15 - 0.2A
The original coil's losses seemed high so to improve
matters I have wound another unit, this time using heavier
gauge wire and of double-layer construction.
Initial results have been disappointing as I generate less
aerial current than with the old coil.
The inductance necessary to achieve resonance has also
fallen: from ~ 1.25H to ~ 0.75 H implying that the new coil
has considerably more self-capacitance.
There are a few hundred turns on it which are not used and
may be causing the problem so I intend slowly to remove these,
checking the aerial current at intervals.
AERIALS
I generally use my 40m inverted 'L' aerial, height 10m but
have the facility (wind conditions being suitable) to launch
balloon or kite-assisted wires.
My property borders a public playing field so I need a wind
direction SE - S or SW for kite operations and near still air
conditions for balloons.
ADVICE
I should be much obliged for information which will enable
me to improve the station's ERP and detectability.
Can someone point me in the right direction to improve the
matching system, maybe using ferrite cores which are readily
available ?
Link coupling to the loading coil may be an option but my
early attempts at this merely shorted out the loading coil and
damaged the PA, so I have avoided it.
73
Chris G3XIZ
