K4PP described his Small Transmitting Loop (STL), including details of its construction and measured VSWR response.
The loop is a 1m diameter circle of 12.7mm dia copper tube with a high Q vacuum cap for tuning.
Using a quality capacitor and copper tube, this loop should be as efficient as they come for its size and location. Continue reading K4PP’s 1m Small Transmitting Loop
Always interesting to look at glitches on APRS maps and investigate the cause. In this case, a local NMEA log provides evidence that the APRS track is faulty.
Above is an image of a track, red is from APRS and blue from a local NMEA capture. Two ‘backtracks’ are evident on the red trace and not on the blue trace, they are artifacts of the design of APRS. Continue reading Backtracking in APRS
(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:
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:
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
In Calculation of equivalent self capacitance of Small Transmitting Loop I mentioned that (Straw 2007), The ARRL Antenna Book 21, gave an expression for equivalent self capacitance of a Small Transmitting Loop of one turn:
C=HD where C is in pF, D in cm, and H comes from a given table of length/diameter ratios from 0.1 to 1.0. ARRL cites (Medhurst 1947) for this expression. Medhurst’s work was for solenoids.
Values of the Constant H for Distributed Capacitance Length to Diameter Ratio H ===== ===== 0.10 0.96 0.15 0.79 0.20 0.78 0.25 0.64 0.30 0.60 0.35 0.57 0.40 0.54 0.50 0.50 1.00 0.46
A 1m diameter loop of 10mm diameter conductor has l/d=0.01, so it is not covered by the table, and you might form the view from the table that H tends to 1.0 or thereabouts as l/d approaches 0, but that is an extrapolation and dangerous. Continue reading Calculation of equivalent self capacitance of Small Transmitting Loop – ARRL
Small Transmitting Loops behave fairly much like an ideal inductance in series with some small resistance. They do however exhibit a self resonance at a frequency where the perimeter is approximately a half wavelength. This can be expected to slightly alter the Xl vs frequency characteristic below the self resonant frequency (SRF), more so as the SRF is approached.
This departure can be compensated for to some extent by addition of a small equivalent shunt capacitance. Continue reading Calculation of equivalent self capacitance of Small Transmitting 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 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
This article demonstrates an automated Return Loss scan of an antenna using:
Automation facilitates:
Continue reading Return Loss sweep using IC7410, RL bridge, and RFPM1