Author: Owen

(Austin et al 2014) made measurements of feed point impedance of a small transmitting loop using a calibrated transformer, and discussed the loss mechanisms. They also extrapolated their measured data to a larger conductor.

This article is an update of my article of 05/02/2015 which contained some errors as a result of my incorrect interpretation of the legend in Austin’s Fig3. This article is a rework to correct that error and those that flowed from it, my apologies to the original authors and readers… Owen.

Let me focus on their measurements, the loop was:

• a 1m diameter circular loop of 6.3mm copper;
• tuned with a low loss tuning capacitor (ATC chip capacitor);
• at heights of 3m and 6m above ground;
• from 3.3 – 12.8MHz.

They did not report the capacitor loss, but gave some likely range from manufacturer’s data.

They used ground parameters G-0.007, εr=17 at 7MHz for their NEC models..

Above is a reconstruction of the measured data from their Fig 3. They measured Rin to their matching transformer and used calculated inductance (relying on the calibrated matching transformer) to calculate Q at each measurement frequency. Continue reading Review of Austin et al paper “Loss mechanisms in the electrically small loop antenna”

I mentioned in An NEC-4.2 model of VK5BR’s 1m square loop for 20m that Butler’s ~1m square loop was too large to be considered strictly a Small Transmitting Loop (STL):

Note that the loop is sufficiently large that the current is not uniform around the loop

(Butler 1991) gives a design for a Small Transmitting Loop (STL) for 14MHz and some other bands.

He gives key design data:

Tube Diameter d   0.75 inch
 Circumference S  12.7 feet
 Area A =   10 square feet
 Frequency f =  14.2MHz
 Power P   100 watts
 Radiation Resistance Rr =   0.137 ohm
 Loss Resistance RL =   0.064 ohm
 Efficiency n =  68%
 Inductance L =   3.27 micro-henry
 Q factor =   723
 Inductive reactance XL =   291 ohms
 Bandwidth B =   19.6kHz
 Distributed capacity Cd =   10.4pF
 Capacitor potential Vc =   4587V
 Tuning capacitor Ct =   28pF

The data above appear to ignore some important factors, and estimate some others based on an assumption of uniform current. Continue reading Dipole mode component of VK5BR’s 1m square loop for 20m

(Revised 12/11/2015)

Small Transmitting Loops (STL) are loops with approximately uniform current around the loop.

(King 1969) gives us expressions for an equivalent circuit of the ‘loop mode’ and ‘dipole mode’, it consists of parallel branches for each mode of series R and X:

• R0: radiation resistance of loop mode;
• X0: reactance of loop mode;
• R1: radiation resistance of dipole mode; and
• X1: reactance of loop mode.

Plotting these and the combined total Rt and Zt for a 1m diameter (perimeter p=3.14m) lossless circular loop of 20mm diameter conductor from 2-30MHz in free space gives an insight into their relative magnitudes at different frequencies. Continue reading Dipole mode of Small Transmitting Loop per King

(Butler 1991) gives a design for a Small Transmitting Loop (STL) for 14MHz and some other bands.

He gives key design data:

Tube Diameter d   0.75 inch
 Circumference S  12.7 feet
 Area A =   10 square feet
 Frequency f =  14.2MHz
 Power P   100 watts
 Radiation Resistance Rr =   0.137 ohm
 Loss Resistance RL =   0.064 ohm
 Efficiency n =  68%
 Inductance L =   3.27 micro-henry
 Q factor =   723
 Inductive reactance XL =   291 ohms
 Bandwidth B =   19.6kHz
 Distributed capacity Cd =   10.4pF
 Capacitor potential Vc =   4587V
 Tuning capacitor Ct =   28pF

The data above appear to ignore some important factors, and estimate some others based on an assumption of uniform current. Continue reading An NEC-4.2 model of VK5BR’s 1m square loop for 20m

The meaning of the terms efficiency and radiation resistance are often critical to understanding written work on antennas, yet different authors use them differently, often without declaring their intended meaning.

Mike Underhill (G3LHZ) is an enthusiastic proponent of Small Transmitting Loops and in his slide presentation (Underhill 2006) challenges the proposition that their efficiency is low.

The line taken broadly is to introduce his own interpretation of efficiency and to challenge by experimental evidence other views on expected efficiency. Continue reading Underhill on Small Transmitting Loop efficiency