13 May 2013

Saamrus Magaliesberg and EFHW revelations

We went to one of our favorite places for the weekend of May 11th and 12th, Saamrus in the Magaliesberg. This is an area where we can go on some great walks and let our dog run free. What a great time we had although a little cool.

Om Monk ZS4SF had mentioned to me that he thought that I had a problem with my EFHW set-up since I had measured the coupled wire as being approx 62 feet long. Meaning it was resonating at around 7.5Mhz. When I arrived at Saamrus I measured and adjusted my new thin wire EFHW to exactly 66ft 8inches. Since the weather was cold I was forced to sit inside in the small kitchen. I set up my K2 on the kitchen table and had about 5 feet of the EFHW inside. The far end was launched up at a height of about 25 feet into a tree. My new launcher worked beautifully. The slippery thin cord had no problem sliding over the tree branch and the weights dropped the far end to the ground without incident.

On connecting my T-200-2 coupler to the EFHW I could not get a decent match at 7020Khz. The SWR was basically off the scale. The resonant point for the system was at 6.8MHz where the SWR was down at 1.2:1. I then trimmed a few inches carefully off the RG58 coax that I was using as a capacitor and was able to bring the system to resonance at 7020Khz. I then noticed that the system was still unstable. When I touched my hand to the rig the SWR varied wildly. However the resonant point remained much the same. I then connected about a 20ft piece of wire to the ground connection on my K2 and ran it along the concrete floor. This made a huge difference and the system became very stable. I then increased the Link coupler by another turn to about 2.8 turns. This brought the SWR right down to 1:1 and a stable system.

I noticed also that the Bandwidth of the system was much better and that I could tune right up to 7.2MHz and still be in under 2:1 SWR. Why was this?

The next morning, Saturday, I erected my portable 7m mast and deployed an inverted Vee 40meter antenna. Fed at the center with a coax cable. The SWR was a bit high at 2:1 on 7020Khz and I did not have a Balun.

On a QSO and exhaustive tests with ZS4SF I received slightly better reports from the Inverted Vee. Propagation conditions were very bad with high QRN and much QSB. The conclusion was that the report differences were marginal at best. I did notice significantly more noise on the EFHW as would be expected. The Inverted Vee was nice and quiet.

The coupler new turns ratio was 6:1. This means that the impedance seen at the end of the wire was more like 50*36=1800ohms. I could verify this by connecting 1800ohms across the coupler and measuring the SWR. According to the books with a counterpoise of 20feet the end resistance should be more like 5000ohms. Why the difference?

When I called in to the AWA net at 2pm local I was disappointed to note that I was receiving 579 reports. After the net I discovered that I did not have the counterpoise connected. Ok so you can still communicate but the difference was at least 2 S points down (12db).

On Sunday morning I had a great ragchew with Barrie ZS6AJY and ZS5SF who both gave me 599 plus reports. FB.

Next steps: I will adjust my small T-50-2 field coupler to resonate at 7020Khz. The link coupling needs adjustment to reduce the turns ratio. This is a huge lesson learnt for me. I will experiment with different length counterpoise wires in order to establish a stable deployment scenario. Next weekend we will go to Manyane again. This will be an opportunity to test the new field system.

View of Inverted Vee 40m dipole and the Saamrus cottage 'Piet my Vrou'

ZS6RSH. Rugged explorer at the summit

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