Tuesday, 19 March 2013

Further work on the 472kHz antenna

After last night's pleasing results on 472kHz I though I would make some further improvements to the antenna. It was a 7m ( approx) vertical, using the lowest four sections of my MFJ 40/80m vertical. As it is 472kHz, radials are not used as they would hardly be effective, being limited to 10m long.
Instead it had just one ground rod. The soil here is sandy ( the area is known as the Sandlings!) so it is very poor for an antenna ground. On the recommendation of Alan, G3NYK, I have increased the number of ground rods from one to 4 and used thick green/yellow earth wire to connect them to a common earth plate and hence to the antenna mounting ground.

Using my HP8753C network analyser I was able to measure the vertical self capacitance as approximately 90pF. This was a little higher than expected from a 7m vertical, but the capacitance was confirmed by resonating the vertical with a series inductor and measuring its inductance value. The ground loss resistance was far too high at around 200R! It was not possible to determine the real value due to 'noise' in the measurement. With the cursor sitting out on the edge of the Smith chart even small changes, e.g. due to antenna movement or interference, caused the value to change dramatically.

The photo shows the cursor out on the right side of the VNA display.

With an extra section of vertical taking the total height to 8.1m and the extra ground the antenna capacitance has gone up to 114pF and the ground loss resistance now measures approximately 110R. Coming down but more to do......!

For matching I am just using a substantial ferrite rod matching/base loading inductor, with 4 turn overwind to couple the coax into the inductor, in order to tune out the vertical self capacitance. This gives me 1.4:1 (38R) SWR at mid band. I will need to add another 1 turn to the overwind To further improve the match before firing up the transmitter.
The SWR is currently (just) within 2:1 across the band.

Just how critical the adjustment of the ferrite inductor core is came as a big surprise.
Who said LF adjustments were less critical than at microwave!

I will leave the receiver on overnight and see what WSPR stations can be received. Last night's was quite good.


- Posted using BlogPress from my iPad


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