Readers of my articles occasionally ask for explanation of the distinction between meanings of:
These terms apply to linear circuits, ie circuits that comply with linear circuit theory, things like that impedances are independent of voltage and current, sources are well represented by Thevenin and Norton equivalent circuits. Continue reading Definitions of important loss terms
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.
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.
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
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.
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
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
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
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
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).
Let’s consider two categories of transmission lines in terms of characteristic impedance Zo and propagation constant γ:
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.
A Lossless Line has imaginary part of Zo equal to zero and the real part of γ equal to zero.
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 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.
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
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.
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