(Dodd 2010) describe a small transmitting loop (STL) and gave some meaningful performance measurements. It is rare to see such measurements and he is to be congratulated.

The loop is an octagon of perimeter 4.7m which at 14.2MHz is 0.224λ so although many will consider it meets the requirements of an STL, the common formula for radiation resistance Rr of a STL fail for perimeter above about 0.1λ (see Accuracy of estimation of radiation resistance of small transmitting loops).

Dodd gives calculations of one of the many simple loop calculators which gives Rr as 0.422Ω, it is probably closer to 160% of that value. This is an important quantity as it has direct bearing on calculated efficiency.

Dodd’s NEC model should have used a better figure for Rr, but it seems unlikely that the structural losses were fully included and its bandwidth prediction will be impaired.

Above is Dodd’s measurement of antenna VSWR at 20m. This is most useful as it allows estimation of the half power bandwidth of the antenna. In this case, the antenna is not perfectly matched at its centre frequency, the residual VSWR is 1.07. The graph allows scaling off the VSWR=2 bandwidth as approximately 42kHz.

Continue reading Review of G3LDO STL (Radcom Sep 2010)