Category: Small Transmitting Loops

A prototype 50:75 RF transformer

The design is of a 50:75Ω autotransformer using a single Fair-rite 5943003801 (FT240-43) core. Note that this data is NOT for an Amidon #43 core.

The winding is a total of 6t tapped at 5t, wound as two close wound layers. The prototype used a single conductor stripped from LAN cable. The winding configuration is an attempt to minimise leakage inductance for improved broadband performance.

This is unconventional, but then the conventional designs on Youtube etc do not usually have credible evidence of low InsertionVSWR or other performance. Continue reading A prototype 50:75 RF transformer

Last update: 12th January, 2025, 7:16 PM

What passes as an optimised small transmitting loop?

Whilst researching another article, I came across a Youtube presentation on  the subject of “Optimum Magnetic Loop Antenna.” It described the authors design of the “10-40m “epicenter” 42″ square 1.5″ dia loop” as an example optimised loop.

This article attempts to analyse the presented data to estimate the performance of the loop, specifically radiation efficiency at the lowest operating frequency.

Loop parameters:

  • shape: square
  • conductor diameter : 38mm
  • perimeter: 4.47m

The presentation gives a measured VSWR curve at maximum capacitance. Minimum VSWR is just below the 40m band, so we might expect that performance on 40m is very similar.

I did not see description of the measurement environment (height above ground, soil type, proximity to structures etc) in the presentation, they are all relevant if the experiment was to be replicated.

Above, the VSWR curve at maximum capacitance. Unfortunately it is not matched for low VSWR50 so we must make assumptions. Lets assume the impedance at resonance is lower than 50Ω, we can estimate the half power bandwidth from the VSWR=10 bandwidth of 480kHz, see below. Continue reading What passes as an optimised small transmitting loop?

Last update: 22nd July, 2023, 12:24 PM

Review of “The Truth About Magnetic Loop Antennas – MYTH BUSTING!”

A reader referred me to a video of a presentation to a radio club, the subject being “The Truth About Magnetic Loop Antennas – MYTH BUSTING!”

The presentation includes prediction and measurement of a small transmitting loop. This article tries to reconcile the claimed radiation efficiency between prediction and measurement.

Radiation Efficiency

The presentation liberally uses the term “efficiency”, let us take that to mean Radiation Efficiency:

the ratio of the total power radiated by an antenna to the net power accepted by the antenna from the connected transmitter.

Note that Radiation Sphere requires that radiated power must be measured / determined / summed in the far field.

Radiation fields decay inversely proportional to distance, other fields immediately around an antenna decay more quickly and are insignificant for the purpose of radio communications at great distances. Hence, Radiation is the usual objective of radio communications antennas.

The calculation tool used in the presentation defines:

Efficiency (%): The percentage of input energy that is actually radiated and not lost as heat.

The prototype loop antenna

The prototype loop antenna is described:

A table is given which appears to be predicted performance based on https://miguelvaca.github.io/vk3cpu/magloop.html . Continue reading Review of “The Truth About Magnetic Loop Antennas – MYTH BUSTING!”

Last update: 19th July, 2023, 8:48 AM

KB0YH’s STLcalc v2.05

In the light of Small transmitting loop calculators – a comparison a reader asked my thoughts on yet another small transmitting loop calculator, KB0YH’s STLcalc v2.05.

There are lots of small loop calculators published, and yes, I have added to the number. Most are some form of elaboration of formulas published by (Hart 1986), and given ‘imprimatur’ by ARRL (Straw 2007). These formulas are deeply flawed, see Reconciling W5QJR’s loop formulas.

For that reason, my first step in reviewing any small loop calculator is to look for hints of Hart. Continue reading KB0YH’s STLcalc v2.05

Last update: 29th December, 2022, 5:28 AM

4NEC2 plots of STL VSWR II (v5.9.3)

At 4NEC2 plots of STL VSWR  and 4NEC2 plots of STL VSWR II I explained a method of working around a limitation of 4NEC2 values for Zo that can be applied using the Settings menu.

I can advise that exactly the same change works in 4NEC2 v5.9.3

It appears that 4NEC2 enforces a requirement that Zo>=0.1, so having discovered that by trial and error, one wondered if it was possible to change that threshold by hacking the exe file. Continue reading 4NEC2 plots of STL VSWR II (v5.9.3)

Last update: 7th March, 2022, 7:33 AM

Performance of a small transmitting loop with varying height – NEC-5.0

Around 2015 I constructed a series of models exploring the effect of ground proximity on a small transmitting loop (STL).

At frequency 7.2MHz, the loop was octagonal with area of 1m^2 equivalent radius a=0.443m, ka=0.067rad, 3.15mm radius copper conductor, lossless tuning capacitor, and centre height above ground (σ=0.007  εr=17 ) was varied from 1.5 to 10m (0.036-0.240λ).

The model series was run in NEC-2, NEC-4.1, NEC-4.2 and NEC-5.0, and the results varied. NEC-4.1 showed serious problems, eg negative input resistance at some heights. The problem was discussed with Burke, and he explained that there was a known problem in NEC-4.1 for small loops near ground, and sent me an upgrade to NEC-4.2 to try with the GN 3 ground model, but that the better solution was in NEC-5 if it was ever released.

NEC-4.2 solved the negative resistance problem, but some issues remained.

With the recent release of NEC-5.0, opportunity arises to compare all four approaches.

(Burke 2019) p45 discusses loop antennas over ground and NEC-5.0.

The plot above of radiation efficiency gives an overall comparison of the different model techniques. (Burke 2019) states Since the mixed-potential solution ensures that the approximated integral of scalar potential around the loop is zero, whether the potential is accurate or not, it might be expected to do better than NEC-4. Continue reading Performance of a small transmitting loop with varying height – NEC-5.0

Last update: 9th September, 2021, 12:03 PM

Review of W8TEE, AC8GY STL (Radcom Feb 2020)

(Purdum 2020) describes a small transmitting loop (STL) which is a little novel in that it uses an arrangement of four circular conductor loops, two in parallel in series with the other two in parallel.

The article goes on to claim some pretty extraordinary efficiency calculated from radiation resistance for a loop structure that is shown at a height of perhaps 2m above natural ground. Continue reading Review of W8TEE, AC8GY STL (Radcom Feb 2020)

Last update: 30th November, 2020, 6:03 PM

MFJ-1786 loop antenna – a study of the matching scheme

The article MFJ-1786 loop antenna – measurements and NEC-4.2 model at 10.1MHz observed of the plot of loop impedance:

It looks quite different to the expected behavior of the underlying loop, but it does contain an arc albeit rotated and offset. In fact it can be transformed in two simple steps.

Continue reading MFJ-1786 loop antenna – a study of the matching scheme

Last update: 27th December, 2019, 3:01 PM

MFJ-1786 loop antenna – measurements and NEC-4.2 model at 10.1MHz – analysis tools

Further to MFJ-1786 loop antenna – measurements and NEC-4.2 model at 10.1MHz, the question arises as to what commonly used tools readily permit the transformations and analysis.

Some relveant theory: for a load where R is approximately constant and X varies, the half power points occur where R=|X|, and following on from that s11=0.2±j0.4, s11=0.4472∠63.43°, |s11|=-6.99dB, ReturnLoss=6.99dB (yes, the +ve sign is correct), VSWR=2.618 etc.

Finding the points where ReturnLoss is approximately 6.99dB with the cursor on the above diagram is quite easy. Continue reading MFJ-1786 loop antenna – measurements and NEC-4.2 model at 10.1MHz – analysis tools

Last update: 2nd February, 2020, 7:12 AM