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
