Dipole mode of Small Transmitting Loop per King

(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.

Screenshot - 11_11_2015 , 01_31_53

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

Calculation of equivalent self capacitance of Small Transmitting Loop – ARRL

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
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

Calculation of equivalent self capacitance of Small Transmitting Loop

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

An NEC-4.2 model of VK5BR’s 1m square loop for 20m

(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

Underhill on Small Transmitting Loop efficiency

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

Return Loss sweep using IC7410, RL bridge, and RFPM1

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.

Automation facilitates:

  • 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

Mapping trips from APRS archives

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.

Google Earth googleearth 01/11/2015 , 08:43:23

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

The BLHeli sojourn is over

I was an early embracer of BLHeli ESC firmware on a Silabs based ESC, the Hobbywing Skywalker 40A UBEC.


The ESC looked to have quality hardware, and BLHeli was choice in aftermarket firmware.

The Skywalker did perform better with BLHeli than the stock firmware.

When BLHeli was released for the AVR based ESCs, I gave it a trial both on the bench in instrumented tests and in the air on a couple of quadcopters with different ESC / motor / prop / FC combinations.

I cannot say that BLHeli for AVR was any better than SimonK, and generally poorer though not much poorer.

More recently, users have reported some serious issues through BLHeli 14.0 and 14.1, and I removed the Skywalkers from service out of concern there were problems to be fixed. Continue reading The BLHeli sojourn is over

NBN is coming… it seems

The NBN was the Australian government’s response to a dominant carrier that would not engage in government’s attempts to guide industry to develop an equitable access network beyond that delivered off the telephony copper that existed in the street (the 2007 RFP).

That ‘initiative’ sees us return the good old bad old days when government owned most of the street infrastructure used to deliver telecommunications, and of course it has been in their interest as a provider to weaken consumer controls that otherwise gave some measure of protection of service levels.

The above graph shows the performance of my ADSL Internet access which reliably achieved  greater than 7Mb/s when I moved here in 2009 but fell to around 0.5Mb/s. A recent upgrade has seen around 2Mb/s fairly reliably, though not enough for reliable IPTV, a stunning improvement on the previous 3+ years. Continue reading NBN is coming… it seems

Zig zags in APRS tracks

There are many causes of zig zag errors in APRS tracks, and they fall generally into two main categories:

  1. incorrect positions (ie the tracker was never there); and
  2. correct positions with incorrect timestamps.

The first is common and has a number of causes, but principally defects in software used in iGates, most of which is not maintained either by the iGate operator or original developer. APRS is pretty static, but most old software has significant defects.

The second category is again common, and mostly the result of the design of the APRS radio network and its vulnerability to network delays (some of which can be caused by defective equipment).

This article looks at a case in the second category where a vehicle appears to have done a U-turn on the highway, travelled back some distance then another U-turn and caught up their original track speed at the next posit. It is clearly an out of order packet. This article shows how to diagnose the cause from the raw packet log for the tracker.


Screenshot - 30_10_2015 , 08_36_44

Above is a map of the glitch that is not uncommon in APRS for one of several reasons. No, VK2HJ has not done U-turns on the highway, the zig zag track is incorrect. Continue reading Zig zags in APRS tracks