MismatchLoss of severely mismatched EFHW transformer – system response

It is easy to become focused on the behavior of a component, but don’t lose sight of the fact that it is but a component of a system where components interact, and the system response is the bigger / more complete picture.

In the article MismatchLoss of severely mismatched EFHW transformer , a caveat stated and restated was:

Transmitters are not necessarily well represented as a Thevenin source, so measurements using such sources (VNA, SA with TG) and application of linear circuit theory are not necessarily applicable.

So, can we estimate a likely system response to a 30+j0Ω load, good 1:49 transformer and modern HF 100W (20dBW) SSB transceiver designed for a nominal 50Ω load?

The following analysis gives a likely solution and it deals with a common implementation where the source is anything but a Thevenin source.

PA VSWR protection

Most transceivers of this type incorporate several PA protection measures, and one of them is commonly to reduce IF gain so that reflected power measured in a directional coupler near the antenna jack is not more than say 4W. This accommodates VSWR up to 1.5 without power reduction due to VSWR protection.

So, with an extreme mismatch, Pref=4W due to the PA protection system.

30+j0Ω load via an ideal 1:49 transformer

The scenario of 30+j0Ω load if that of the quoted measurement in the previous article, used without comment on its merit.

Lets use the calculation from the previous article, the equivalent case of a 50Ω Thevenin source with load of (30+j0)/49=0.6122+j0Ω.

The quantity 1/(1-|s|^2) is the MismatchLoss, \(MismatchLoss=\frac{P_{fwd}}{P_{fwd}-P_{ref}}=\frac1{1-\frac{P_{ref}}{P_{fwd}}}\) and it is 20.92, so we can calculate that if Pref=4w (by virtue of PA protection), that \(P_{fwd}=\frac{P_{ref}}{0.952}=\frac4{0.952}=4.202 \text{ W}=6.235 \text{ dBW} \). Continue reading MismatchLoss of severely mismatched EFHW transformer – system response

Last update: 14th April, 2024, 1:34 PM

PVC speaker twin – loss model applied

One of the many gems of ham lore that I was fed as a beginner almost sixty years ago was that 23/0.076 (0.67mm^2) PVC insulated twin flex was suitable as an RF transmission line at HF, and that it had a Characteristic Impedance Zo close to 75Ω.

It seems that these claims have been extended to apply to lighter gauge cables often called speaker cable or bell wire.

This article explores two cases of the application of a light grade of speaker twin to a G5RV antenna. The scenarios is a G5RV Inverted V with 7m of speaker twin from dipole to the coax section, and loss is calculated for the speaker twin section at 14.1 and 3.6MHz.

PVC speaker twin copper / PVC 0.2mm^2 characteristics

The following articles report measurement of a sample of speaker twin, and derivation of a simple loss model:

From those articles, the loss model is copied for reader convenience.

Above is a plot of the calculated MLL (red dots) based on the s11 measurements, and a curve fit to the model \(MLL = k_1\sqrt f+k_2f \text{ dB/m}\). Continue reading PVC speaker twin – loss model applied

Last update: 29th August, 2023, 4:28 PM

MismatchLoss of severely mismatched EFHW transformer

In an social media discussion about loss of EFHW transformers under mismatch conditions, one of the gathered experts said:

It doesn’t even have to be highly complex Z. Just presenting an impedance other than 2450 sends the loss through the roof. The back to back transformer test is misleading unless the antenna presents something very close to 2450 on each band for which it is used.

giving this graphic to quote someone else’s work in support.

Interpreting this graphic is fraught with risks, the author obviously does not understand and accept / follow the conventional meaning of term loss. Continue reading MismatchLoss of severely mismatched EFHW transformer

Last update: 14th April, 2024, 1:34 PM

Repair of the valve of a Dramm “touch ‘n flow” water wand

I have a Dramm “touch ‘n flow” water wand, a moderately expensive thing which worked very well, except that like so many similar things, the lever valve failed after a few years of age, but little use I might add.

Above, the wand. Note that the handle is molded rubber over some structure that contains the valve… it is designed to be unserviceable. Continue reading Repair of the valve of a Dramm “touch ‘n flow” water wand

Last update: 19th August, 2023, 8:12 PM

Toro (Loncin) 24.5HP V-twin spark plugs and crossfire

I recently replaced the spark plugs in a Toro (Loncin) 24.5HP V twin with new non-resistor plugs, the original were Champion RN9YC resistor plugs. The replacement was solely scheduled maintenance as per the book.

The engine was then difficult to start, it took a long time cranking and often backfired whereas previously it had started quickly and certainly. Low speed idle was rough and slow.

So, could the non-resistor plugs be the cause of this sudden change?

A pair of new NGK BPR6ES (resistor plugs) was installed and the engine starting was restored and idle speed was smooth and book speed.

So, the non-resistor plugs did look like the problem, and being of an enquiring mind, I captured the HT waveform with one resistor plug and one non-resistor plug (#2).

It was again difficult to start and idle was slow and rough.

Above is the capture, red is #2 and blue is #1. The lack of symmetry can be seen, there is a distinct blue spike at the beginning of the red burn phase. It seems like the red spike (non-resistor plug) that caused the spark burn has somehow triggered an impulse on the blue circuit. Continue reading Toro (Loncin) 24.5HP V-twin spark plugs and crossfire

Last update: 17th August, 2023, 9:07 AM

Where is the best place to measure feed point VSWR – error in Zo

At Where is the best place to measure feed point VSWR I discussed location of the VSWR meter and projection of its reading to another point on a known transmission line.

One of the conclusions drawn in that article is:

Feed point VSWR can be estimated from measurements made at another place if the transmission line parameters are known. It, like all measurements, is subject to error but it may be a manageable error and indeed possibly better overall than direct measurement.

This article discusses some issues that may arise in referring measurements from one place to another (eg near transmitter to antenna feed point).

Characteristics of transmission line categories

Let’s consider two categories of transmission lines in terms of characteristic impedance Zo and propagation constant γ:

  • Lossless line; and
  • practical line.

A lot of theoretical analysis uses lossless line for simple explanations, and whilst for a lot of purposes, approximation of practical line as lossless line serves well, at other times the error may be significant.

Lossless Line

A Lossless Line has imaginary part of Zo equal to zero and the real part of γ equal to zero.

Practical line

A practical line has non-zero imaginary part of Zo and non-zero real part of γ, and these are frequency dependent.

Under standing waves, attenuation along a practical line is not uniform, in most practical applications conductor loss/m is higher than dielectric loss so loss is higher near current maxima than near current minima.

For the purpose of this article, it is the frequency dependence of Zo, particularly the non-zero imaginary part that is significant.

A model

A model of a load similar to a 7MHz half wave dipole fed with 10m of RG58A/U was created in Simsmith to provide a basis for discussion. Whilst the model is subject to some errors computation, it is much less than comparing two field measurements at both ends of a transmission line.

VSWR at each end of the transmission line

Let’s look at the ACTUAL VSWR. Actual means that if you were to observe the standing waves on the line (eg with a voltage probe), this is the VSWR you would expect to observe.

Firstly, observe that the source end VSWR (orange) is a little lower than the load end VSWR. This is by virtue of the attenuation on the line. The difference between the two can be calculated, but it is moderately complicated. Continue reading Where is the best place to measure feed point VSWR – error in Zo

Last update: 13th August, 2023, 6:52 PM

Implementation of G5RV inverted V using high strength aluminium MIG wire – 7 year review

This article continues on from Implementation of G5RV inverted V using high strength aluminium MIG wire documenting review after 7 years operation under a wide range of temperature, humidity and wind conditions. It adds some contemporaneous pics of some parts of the system.

Support structure

Above is a view of the steel mast (2021) with the Inverted V G5RV rigged from the top of the 11m mast using a halyard through a purchase on a small gibbet to offset the antenna and feed line from the mast. There are lateral guys at 7m height, and the left hand one is non-conductive synthetic fibre rope. Atop the mast is a 2m/70cm vertical.

Above is a view of the feed point (2023) of the G5RV at 1mm AGL. The galvanised telescopic TV mast is 55 years old and starting to show signs of rust, likewise for the gibbet which was painted with cold gal back then. Continue reading Implementation of G5RV inverted V using high strength aluminium MIG wire – 7 year review

Last update: 5th August, 2023, 9:33 AM

Thoughts on the ARRL EFHW antenna kit transformer

Several readers have asked my thoughts on the ARRL EFHW kit.

I have not built and measured the thing, but have done the first step in a feasibility study.

The transformer design is not novel, it is widely copied and this may be one of the copies. The design is usually published without any meaningful performance data or measurements.

The article Select a ferrite core material and sufficient primary turns for a low InsertionVSWR 50Ω broadband RF transformer – comparison of measured and predicted laid out a method for approximating the core loss of a EFHW where the load is adjusted to that input VSWR50=1, ie input Z=50+j0Ω.

That method will be applied here for a good initial estimate of core loss.

I will present calcs for 80m and 40m since there are lots of articles and videos encouraging people to extend the antenna to 80m (with and without a loading coil).

It is quite practical to build an EFHW transformer with less than 0.5dB (11%) core loss.

Amidon FT240-43 toroid with 2t primary

The first point to note is that Amidon’s 43 product of recent years is sourced from National Magnetics Group, and is their H material. It is not a good equivalent to Fair-rite’s 43 mix.

Let’s make a first estimate of core loss at 3.5MHz.

We can estimate the complex permeability which is needed for the next calculation. Continue reading Thoughts on the ARRL EFHW antenna kit transformer

Last update: 14th April, 2024, 1:34 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