A recent online discussion on common mode feed line current was directed to Thompson’s article with the recommendation that is ALL basically needed to discuss the common mode current.
Above is Thompson’s diagram of currents in a feed coax, and it contains two significant errors that could / would lead to formation of the wrong concepts in a learner’s mind. Continue reading Thompson’s coax common mode explanation
This article describes a prototype RF choke (RFC) for use in a power injector for 50Ω coax over range 1.8-30MHz. Power injector / extractors are often used to connect power and / or signalling on a shared common RF coax feed line to accessories such as remote antenna switches and ATUs.
Design criteria are:
The core chose is a LF1260 ferrite suppression bead from Jaycar. It is a medium / high µ core readily available in Australia at $7.50 / 6.
Above is the prototype RFC wound with data cable wire for the purpose of measurement. In application it could be wound with 1mm enamelled copper or PTFE insulated wire (Curie point is lowish at 120°+, but it still benefits from higher temperature insulation). Continue reading An RF choke for a 1.8-30MHz coax power injector – LF1260 core
A prototype broadband transformer for a End Fed Half Wave operated at fundamental and first, second, and third harmonic is presented.
The transformer comprises a 32t of 0.65mm enamelled copper winding on a FT240-43 ferrite core, tapped at 4t to be used as an autotransformer to step down a load impedance of around 3300Ω to around 50Ω. The winding layout is unconventional, most articles describing a similar transformer seem to have their root in a single design.
Continue reading End Fed Half Wave matching transformer – 80-20m
In a discussion about using a 40m centre fed half wave dipole on 80m, the matter of feed line loss came up and online expert KM1H offered:
Use this to help make up your mind. Add it to the normal coax loss. http://www.csgnetwork.com/vswrlosscalc.html
This is to suggest that the feed line loss under standing waves can be calculated with that calculator.
He then berates and demeans a participant for commenting on his recommendation, bluster is par for the course in these venues.
The calculator in question states this calculator is designed to give the efficiency loss of a given antenna, based on the input of VSWR (voltage standing wave ratio) and other subsequent factors.
This is a bit wishy washy, efficiency loss
is not very clear. The usual meaning of efficiency is PowerOut/PowerIn, and the usual meaning of loss is PowerIn/PowerOut, both can be expresssed in dB: LossdB=10*log(Loss) and EfficiencydB=10*log(Efficiency). Continue reading Line loss under standing waves – recommendation of dodgy tool on eHam
The so-called End Fed Half Wave antenna system has become more popular, particularly in the form of a broadband matching transformer in combination with a wire operated harmonically over perhaps three octaves (eg 7, 14, 21, 28MHz).
The broadband transformer commonly uses a medium µ ferrite toroid core, and a turns ratio of around 8:1. Flux leakage results in less than the ideal n^2 impedance transformation, and a capacitor is often connected in parallel with the 50Ω winding to compensate the transformer response on the higher bands.
David, VK3IL posted EFHW matching unit in which he describes a ferrite cored transformer matching unit that is of a common / popular style.
Above is David’s pic of his implementation. It is a FT140-43 toroid with 3 and 24t windings and note the 150pF capacitor in shunt with the coax connector.
The article End fed matching – analysis of VK3IL’s measurements gives the following graph showing the effects of compensation for various resistive loads. Continue reading End fed half wave matching – voltage rating of compensation capacitors
The sign of reactance – a challenge posed a problem, a set of R,|X| data taken with an analyser of a quite simple network and asked readers to solve the sign of X over the range, ie to transform R,|X| to R,X.
The sign of reactance – challenge solution gave a solution to the challenge, and The sign of reactance – challenge discussion provided some discussion about the problem and solution.
Some correspondents have asserted that the challenge (see above Smith chart) contains a response that is contrived for the purpose and not representative of real world antenna systems. Continue reading The sign of reactance – challenge reality check
The sign of reactance – a challenge posed a problem, a set of R,|X| data taken with an analyser of a quite simple network and asked readers to solve the sign of X over the range, ie to transform R,|X| to R,X.
It is widely held that this is a trivial matter, and lots of software / firmware implement algorithms that fail on some scenarios. Though the scenario posed was designed to be a small set that provides a challenging problem, it is not purely theoretical, the characteristics of the data occur commonly in real world problems and the challenge data is derived from measurement of a real network.
Above is a Smith chart plot of the measured data that was transformed to the R,|X| for the challenge. Continue reading The sign of reactance – challenge discussion
The sign of reactance – a challenge posed a problem, a set of R,|X| data taken with an analyser of a quite simple network and asked readers to solve the sign of X over the range, ie to transform R,|X| to R,X.
It is widely held that this is a trivial matter, and lots of software / firmware implement algorithms that fail on some scenarios. Though the scenario posed was designed to be a small set that provides a challenging problem, it is not purely theoretical, the characteristics of the data occur commonly in real world problems and the challenge data is derived from measurement of a real network.
Imported and rendered graphically in ZPlots we have:
The network measured is comprised from analyser, a 2.8m length of RG58/CU, a tee piece feeding a 50 resistor on one branch and on the other branch, another 2.8m length of RG58/CU with a 4.7Ω resistor termination.
The challenge is: what is the sign of X across the frequency range? Continue reading The sign of reactance – challenge solution
Over time, readers of The sign of reactance have suggested that determining the sign of reactance with an antenna analyser that does not directly measure the sign is not all that difficult, even for beginners. The article shoots down some of the most common algorithms as failures on simple cases.
This article gives measurements made from a simple network of two identical lengths of 50Ω coax, a 50Ω resistor and a 4.7Ω resistor. It is a network designed to offer a challenge to the simple algorithms, and it IS solvable analytically… but not with most algorithms and software,
Here is the data from measurement made with an AA-600 and then all – signs removed, so in fact the Xs column is |Xs|.
"Zplots file generated by AntScope" "Freq(MHz)","Rs","Xs" 9.000000,78.13,53.66 9.250000,82.12,51.10 9.500000,86.10,47.83 9.750000,89.46,44.00 10.000000,92.30,39.90 10.250000,94.53,35.39 10.500000,96.21,30.71 10.750000,97.17,26.14 11.000000,97.49,21.54 11.250000,97.30,17.12 11.500000,96.54,13.04 11.750000,95.47,9.14 12.000000,93.92,5.68 12.250000,92.16,2.70 12.500000,90.25,0.17 12.750000,88.13,2.50 13.000000,85.94,4.50 13.250000,83.67,6.15 13.500000,81.45,7.36 13.750000,79.29,8.38 14.000000,77.22,9.21 14.250000,75.21,9.78 14.500000,73.23,10.16 14.750000,71.44,10.37 15.000000,69.70,10.25 15.250000,67.99,10.23 15.500000,66.50,9.99 15.750000,65.10,9.68 16.000000,63.81,9.27 16.250000,62.65,8.72 16.500000,61.59,8.15 16.750000,60.55,7.54 17.000000,59.69,6.86 17.250000,58.97,6.20 17.500000,58.20,5.43 17.750000,57.66,4.68 18.000000,57.14,3.81 18.250000,56.77,2.98 18.500000,56.47,2.16 18.750000,56.22,1.22 19.000000,56.04,0.38 19.250000,56.07,0.50 19.500000,56.02,1.38 19.750000,56.12,2.29 20.000000,56.41,3.15 20.250000,56.68,4.03 20.500000,57.11,4.86 20.750000,57.51,5.72 21.000000,58.06,6.61 21.250000,58.77,7.45 21.500000,59.54,8.22 21.750000,60.47,8.95 22.000000,61.44,9.75 22.250000,62.52,10.34 22.500000,63.77,10.97 22.750000,65.11,11.55 23.000000,66.56,12.02 23.250000,68.11,12.38 23.500000,69.82,12.64 23.750000,71.75,12.82 24.000000,73.67,12.84 24.250000,75.96,12.67 24.500000,78.12,12.27 24.750000,80.40,11.72 25.000000,83.05,10.69 25.250000,85.56,9.68 25.500000,88.29,8.09 25.750000,90.92,6.21 26.000000,93.63,3.91 26.250000,96.17,1.13 26.500000,98.61,2.16 26.750000,100.68,5.92 27.000000,102.51,10.11 27.250000,103.87,14.90 27.500000,104.65,19.98 27.750000,104.71,25.32 28.000000,103.98,30.95 28.250000,102.58,36.48 28.500000,100.14,41.97 28.750000,97.08,47.32 29.000000,93.07,51.86
Imported and rendered graphically in ZPlots we have:
The challenge is what is the sign of X across the frequency range? Continue reading The sign of reactance – a challenge
Following on from KL7AJ on the Conjugate Match Theorem, KL7AJ on the Conjugate Match Theorem – analytical solution asked the question Is a ham transmitter conjugate matched to its load?
The answer speaks to the relevance of Walt Maxwell’s Conjugate Mirror proposition to ham stations. Continue reading Is a ham transmitter conjugate matched to its load?