10647Re: Shielded Resonant HF Loop
- Jun 24, 2014Hi Gerry;
I agree with your concerns 100%. Maybe it's because of a cruddy image, but care was taken to ensure that shielding gaps were maintained in this kluge. In no place does the shielding foil make a complete turn around the loop windings; there is a 1/2" gap which hopefully is visible there. There is also a gap at the top of the loop so that the shield does not form a shunting turn in parallel with the windings.
I expected to take a major hit on the Q but I was pleasantly surprised to find that the Q was diminished by only about 20% (it dropped from around 250 to around 200 at 6900 kHz). While this does knock down the effective height proportionally, the loop still has plenty of output. Likewise, this Q reduction will degrade the minimum discernible signal somewhat; that point is probably moot since it was intended to be used during times of high ambient noise.
Also interesting is the fact that the tuning range of the shielded loop was translated upwards a bit as compared to the free space loop. I surmise that this is because the loop windings are now running in proximity to a ground plane, and their self-inductance has been knocked down a bit, but I am always open to other theories!
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