A correspondent wrote suggesting that he had seen online NEC patterns showing a 30″ square small untuned loop to have a gain of around 10dBi, more than 30dB better than given by Calculate small loop Antenna Factor.

Firstly, lets describe a loop for study, a square diamond with sides of 760mm (30″) of 2mm diameter copper fed in one corner at 7.1MHz.

## Calculate small loop Antenna Factor

Calculate small loop Antenna Factor models a small loop in free space (therefore does not include ground losses).

Above is the calculator result, the key figures are Antenna Factor 31.75dB and Gain -44.5dBi.

## NEC-4.2 model

An NEC-4.2 model was constructed with external excitation (1V/m) incident on the loop which has a 50+j0Ω load inserted at the feed point to represent the receiver load.

Here is the model source.

CM Small square untuned loop
CM NEC-4.2
CM
CM 1. Plane wave excitation
CM
CM Owen Duffy
CM Note: rotations might not work properly in various NEC-2 versions, beware of segment size issues in NEC-2.
CE
GW 1 5 -0.38 0 -0.38 0.38 0 -0.38 0.001
GM 1 3 0 90 0 0 0 0 1
GM 0 0 0 90 0 0 0 2 1
GE 0
LD 5 0 0 0 58000000
LD 4 1 1 1 50 0
GN -1
EK
EX 1 1 1 0 45 0 0 0 0 0
FR 0 0 0 0 7.1 0
EN

The key result to be extracted from the model run is the current in the 50Ω resistor in segment 1 of wire 1. The magnitude of the current is 5.1204E-04, so the voltage developed in the resistor V=5.1074-04*50=0.02554V. Antenna Factor is the ratio of the E field excitation to the terminal voltage of the receiver, so in dB it is 20*log(1/0.02554) =31.83 dB/m.

The NEC model’s 31.83 dB/m is close to the calculator prediction of 31.75dB/m, but includes the benefit of the lossy ground reflection . Continue reading Small untuned loop for receiving – NEC model

Last update: 19th May, 2017, 7:56 PM