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LF and MF Experiments etc - 7
November 2007
Upgrade To Vertical. As the previously mentioned antenna seems to be a valid improvement I have carried out a more permanent upgrade of the current LF/MF vertical antenna by increasing the size of the inductance to 500 uH, the capacity hat to 5.75 square metres and more rigid supporting arms made from recycled polypropylene (normally for use as garden canes) , I have also separated the vertical element from the mast by a greater distance. The vertical section is supported by a fibreglass standoff arm which is attached just below the elevated coil. The new lightweight elevated coil shown below is wound on an air-spaced former made from a ribbed arrangement using plastic rods. The spreaders for the capacity hat top load are fixed to the top of the coil assembly.
April 2008
Although the previous antenna worked quite well I was determined to make any possible improvements. The antenna evaluation software MMANA-GAL was used to create various options using the existing components. The conclusion was that I could obtain about a 1db increase in gain by placing the elevated coil at a third of the way down the vertical section. I carried out the required modifications as shown in the drawing on the left and my impression is that the changes were beneficial. I was able to reduce the bottom inductance slightly for resonance and needed lower transmit power for the same ERP as with the earlier setup.  In the photograph on the right the LF/MF vertical is at the left of the mast, a 3 band VHF quad in the centre, the PA0RDT Mini-Whip and the 4 metre band vertical at the right of the mast. The fibreglass support for the vertical allows the VHF quad to rotate between the outer antennas.
Whilst all of this was going on I felt that I should also improve the matching transformer to replace the existing inefficient one wound on a monitor scan coil core. I obtained two 58 mm 3C90 toroids and made my new transformer as before based on the design by Steve GW4ALG. It does work very well without getting warm even at the high antenna current required for 136 kHz.
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