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
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
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:
- IC-7410 transceiver with CIV;
- 40dB power attenuator;
- Return Loss Bridge (RLB);
- RFPM1 with USB data logger (A prototype data logger for RFPM1); and
- a PC orchestrating the test.
- measurement of a large number of data points;
- improved accuracy by reducing the risk of recording errors; and
- reducing the tedium of a measurement task.
Continue reading Return Loss sweep using IC7410, RL bridge, and RFPM1
I recently created a map from APRS archives of a recent trip by some friends over about eight weeks through central and north west Australia and back by the southern coast.
Above is a graphic of the created map, but the ‘real’ map is not simply an image, but it is a kml file for Google Earth which you can view / zoom / scroll, for example in Google Maps by clicking on the map above.
Continue reading Mapping trips from APRS archives
This is a 2015 update of an article written originally in October 2005, earlier editions published on VK1OD.net which is now offline.
Over recent years to 2002, the number of issued amateur licences was declining, the trend was about 2.8% pa decline over the five years to 2002.
This has concerned some people, who took the view that the decline was a harbinger of the impending demise of Amateur Radio. Continue reading Australian amateur population trends 1998 – 2015
Adjusting KISS TNC AFSK tx level using an isochronous test packet explained a technique to drive a KISS TNC with a specially constructed packet that contains an ISOCHRONOUS test packet, a packet that will produce equal high and low tone alternation in the transmitted AFSK signal. The improved packet should be repeated by most digipeaters, allowing observation of their modulation performance.
Above is the waveform recovered from a receiver without de-emphasis (a Motorola R2009D communications analyser in this case).
Continue reading Adjusting KISS TNC AFSK tx level using an improved isochronous test packet
A correspondent having seen recent discussion and models on eHam regarding steel dipoles, asked about the accuracy of my articles:
Galvanised steel wire CF dipole; and
Galvanised steel wire OCF dipole.
The eHam article gives the gain of a low half wave steel dipole on 160m as 0.5-1dB less than copper depending on steel composition. (The thread was entitled “galvanised steel wire”, but the model was clearly labelled steel. For discussion of the effect of galvanising, see Galvanised steel wire OCF dipole.)
The model used is not fully exposed, but the results are unlikely unless perhaps the permeability of the steel was ignored (NEC-2 does not natively model µr>1).
Above are the gain plots from NEC-4.2 for three different material types, copper, steel, and steel resistivity with µr=1 (-wrong). Continue reading Steel wire CF dipole on 160m