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Properties of a 1/4 lambda loop.
pa0nhc 20160315

 

On the lowest position is a heated box under a roof. It contains the tuning capacitor, drive motor wit gearing and matching trafo.
Ferrite clamps are over the cables against mantle- and noise currents (best is at separate cables, not to combined cables).
Later on two rolls of 20m x 1m garden fence grid is laid left and right on the roof, and grounded to the satellite dish foot. This lowered the noise level coming from the apartments below.

In a 1/4 lambda circumference loop, is the RF loop current near the tuning capacitor much smaller than in the center of the loop. In other words: the loop current value is NOT constant around.

   Simulation shows that as a result :
- In the radiation pattern is a minimum at the position of the tuning capacitor.
- In the radiation pattern is a maximum near the RF current maximum.

                Conclusion:
When positioning the tuning capacitor at the lowest point :

1. The highest loop part radiates. Good for maximal efficiency.

2. That radiation is towards the sky, good for NVIS.

3. The minimum radiation (and sensitivity) is towards the roof. Resulting in less noise reception and less RF in the building.

According to 66pacific.com , a 10m loop (3m diameter) is efficient on 7.1MHz . If tuned to 3.65MHz, it operates as a less efficient (-7dB) 1/8 lambda loop. In such a situation the use of a thick radiator (>=22mm) is recommended.