A correspondent asked about the difference between two small untune loops mentioned in two of my articles, this article explains.
Firstly lets set the context, a small loop means less than λ/10 perimeter, and untuned is to mean that the loop is loaded directly, in this case by a receiver which we will assume has an input impedance of 50+j0Ω.
Let’s look at the two cases. The key difference is in the connection at the gap:
- the first has a short circuit coaxial stub of half the perimeter between the inner conductor at the right side of the gap and the outer surface of the outer conductor at the right side of the gap; and
- The second directly connects the inner conductor at the right side of the gap and the outer surface of the outer conductor at the right side of the gap.
Small single turn un-tuned shielded loop
Above is a diagram of the loop.
Simple Shielded loop with integral balun
The second case appears in the article Small untuned loop for receiving – it’s not rocket science.
Above is the detail of a simple shielded loop using 50Ω coaxial cable to a 50Ω receiver at the loop terminals.
Analysis
The additional coaxial stub inserts an impedance is series with the inner conductor at the gap, the impedance is frequency dependent, and its effect in this scenario is mainly in the inductive reactance of the short circuit stub. The influence of that reactance on the recovered power at the load will depend also on the reactance of the loop itself… which is sensitive to the diameter of the conductor.
In other respects, like depth of nulls, symmetry etc, there should be no difference.
Lets compare the two loop connections for two cases, a 1m diameter loop of RG58A/U another of RG213.
RG58A/U
Above is a plot of power out for the two scenarios to 10MHz (where perimeter is just over λ/10. The second configuration with the series stub is the lower output.
RG213
Above is a plot of power out for the two scenarios to 10MHz (where perimeter is just over λ/10. The second configuration with the series stub is the lower output.
The difference in this case is slightly more than for the previous case as the main loop inductance is lower by virtue of the larger conductor diameter.