Having spent a good deal of time building, setting up and adjusting LF antennas and transmitters for what I thought were best results, I realised that watching the SWR, PA and antenna current just wasn’t the ultimate answer to an efficient system. I trawled many amateur LF enthusiasts web sites and decided that finer adjustments and settings were required to make improvements to the antenna system in the form of coil Q, top load capacity and ground loss. So my first goal was to make reductions in ground loss as this appeared to be one of the most beneficial improvements to a LF antenna system.

The next step of course is to construct an LF Ground Loss Bridge to assist me in this area.

I studied 3 sites with designs, notes and ideas published on the internet by Alan Melia (G3NYK), Johan Bodin (SM6LKN) & Lloyd Butler (VK5BR) then set about the task of building it.

I first made up the necessary toriod couplers and a simple rf detection meter and fed a signal from my audio signal generator. Using known values of resistance and capacitance on the antenna port I was able to determine that I was on the right track by seeing the required null. One of my requirements was to make the bridge self contained or as near as possible. The Marathon 5 watt LF transmitter VFO and case therefore became a building block for this criteria. The bridge was constructed using RF drive from the Marathon to the bridge circuit but the waveform was far from sinusoidal. Using my oscilloscope enabled me to detect the point in the driver were the sine wave was ok and I coupled to a BC549 output stage at this point and achieved a reasonable output. At this time there was only a 470R carbon potentiometer available so I conducted my tests using that. Being happy with the bench test I transferred to the antenna and tested from there. The null was easy to see on the meter but only if the resistance was quite high, when reading lower R using the 470R potentiometer it was near impossible to null with the track being near the end. Next then I obtained a 220R and a 50R linear carbon pot but I found that I was unable to get a proper dip at low resistance. The output transistor was getting hot and at this point failed, a TIP41A higher power specification replacement was fitted with heat sinking to the die cast case and this has proved OK. When I checked the output on my ‘scope I found that when connected to a load the wave form was distorted possibly, I thought, due to the greater output acheived with the TIP41A, I tried different toriod cores and windings and found a core/winding combination which produced a good sine wave on load but I was still unable to get a satisfactory dip when connected to the antenna. Checking again on the bench with the ‘scope it was OK so I decided to use the ‘scope as a detector. So out to the antenna base again this time with the ‘scope, I connected up and was very surprised to see a substantial sine wave even before switching the power on. There was an interfering signal being received by the LF antenna and this was preventing the bridge from detecting the null at 137KHz.

The next stage was to use my JRC NRD-345 receiver as tuned detector (which I was hoping to avoid) and on carrying out the tests again I found that the null was easily seen on the desired frequency, it was very sharp and very deep, in fact I had great difficulty in finding an absolute stable dip until I realised that a slight breeze was moving the top load wires and slightly changing the dip minimum position.

Next day I carried out further checks to find the source of the interfering signal suspecting that it might be the Royal Naval transmitter on 81Khz which is only about 12 miles away and is always an end stop signal. The sine wave came in at 1 volt peak to peak and the timing 5 micro-seconds, I was very surprised to find that it to calculated to the frequency of the BBC Radio 4 transmitter on 198Khz at Droitwich..........Continued next page.