OwenDuffy.net 


Small square un-tuned loop for field strength measurement - NEC models 

This article documents a theoretical model of the Antenna Factor of a small square loop in free space, connected by a balun and specified length of coaxial transmission line to a 50Ω receiver.

Antenna Factor is the ratio of electric Field Strength to voltage on the receiver input terminals.

Model

The models are created over a range of frequencies using:

The number of model segments in each side of the loop varies with frequency, the number of segments is the greater of 5 and int(<side>/(299.792458/<freq>/20)*2)+1.

Table 1: Sample NEC deck
CM Small square untuned loop
CM Extended thin wire kernel used
CM 1. Free Space
CM 2. Plane wave excitation
CM
CM Parms 12.109 1 5
CM
CM Owen Duffy (VK1OD)
CE
GW 1 5 -0.500 0 1.000 -0.500 0 2.000 0.000800
GW 2 5 -0.500 0 2.000 0.500 0 2.000 0.000800
GW 3 5 0.500 0 2.000 0.500 0 1.000 0.000800
GW 4 5 0.500 0 1.000 -0.500 0 1.000 0.000800
GE
EK
FR 0,1,0,0,12.109
EX 1 1 1 0 90 0 0 0 0 0
LD 5 0 0 0 1.35E6
LD 4 1 1 1 25 0 0
LD 4 4 5 5 25 0 0
GN -1
XQ
EN

 

The Antenna Factor is calculated as AF=-20*log(I*R).

The loop described in this article has 1m sides and consists of 1.6mm diameter bare 316 stainless steel conductor.

The analysis assumes / includes:

 Loop load impedance may be stabilised by a minimum level of Z-matched attenuation ahead of the receiver.

This loop will be self resonant at about 75MHz. Use near resonance is to be avoided because of the steeper slope of the AF curve and the resultant increased sensitivity to small physical implementation details.

Fig 1: Modelled Antenna Factor.

The self inductance of the loop tends to compensate the loop to within +/- 1dB over about a decade from 3 to 30MHz, or within +/- 0.5dB over the range from 4 to 20MHz.

For more exact measurement results, the AF can be looked up and entered into the measurement system automatically, or the loop calibration curve loaded for automated correction. FSM has been enhanced in V1.0.12 to read an antenna calibration table and automatically perform a cubic spline interpolation at the frequency of measurement.

Links

Changes

Version Date Description
1.01 05/10/2006 Initial.
1.02    
1.03    
1.04    
1.05    

© Copyright: Owen Duffy 1995, 2017. All rights reserved. Disclaimer.