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LF and MF Experiments etc - 6
July 2007
Further to my evaluations, research and small back garden I have constructed a smaller self supporting capacitive top of about 2.0 square metres and lightweight form so there is no drooping of the load and no reduction of the effective antenna height (previously mentioned), I have also introduced a lightweight elevated inductance to enhance the small capacitive top load as my research showed that an elevated inductance will have little effect on an antenna which already has sufficient top capacity. Look at ON7YD’s antenna pages for this and other useful information, access from my links page.
May 2007
As previously mentioned I already have the G0MRF 300 watt 136 kHz transmitter, so I liaised with David G0MRF and found that I was able to carry out some simple changes to allow its use on 500 kHz. Not only that, but in carrying out the changes I modified the transmitter hardware to allow easier band change from 136 to 500 kHz.

I found that my external DDS VFO gave insufficient drive to allow normal use to 500 kHz due to IC2 ampilifier failing beyond 950 kHz (for times 2 frequency drive) but I saw a possible resolution to my problem by placing copy of the IC2   pre-amp circuit at the input from my DDS VFO (before IC2) and it did the trick, the 300 watt Tx now will now operate correctly with a VFO input of higher than 1200 kHz giving me the required x2f input for use on 500 kHz.

A second modification I carried out was to remove the 136 kHz LPF components from the PCB and fit solder pins on the board at the LPF input and output points and wire a short length of coax to link the two points. I built the removed 136 kHz components into an aluminium box and the 500 kHz 7 element LPF (David Bowman - G0MRF) also into an aluminium box so I can now connect the 7 element 500 KHz LPF or the 5 element 136 kHz LPF externally for operation on either band when required.
The antenna should be suitable for operation on 500 kHz and 136 kHz by selection of the correct inductance in the bottom loading coil unit .
VFO post-amp
Onboard 136kHz
LPF removed
External 136kHz
And 500kHz LPF’s
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LF & MF experiments 7 LF & MF experiments 5