QST publishes a design by W0SJ for a nominal 50:450Ω EFLW matching device from 160-10m using the following circuit. The article is ‘in-brief’ as technical stuff that will not interest most hams is published privately on a members-only page. This article is based on the information in the QST article alone (ie not on the … Continue reading W0SJ matching transformer for an EFLW (Laird 28B1540-000)
I wrote recently of a flawed test of balance performance of an antenna system and an ATU, and some readers have taken up the issue, basically asking the question “then, how do you measure balance of a two wire line with a scope?” The first step is that you must define what you mean by … Continue reading Measuring common mode current with a scope
This article describes a current probe for use with a power meter calibrated in dBm (eg RFPM1 and RFPM2). For use with RFPM1 and RFPM2, both of which read to 16dBm max, it is convenient that the scaling factor for the probe is 0dBA/dBm, ie that those meters read dBA directly, implying a current range … Continue reading RFPM2 – current probe
At PD7MAA’s BN43-202 matching transformer for an EFHW I gave an estimate of the core loss in PD7MAA’s transformer. An online expert questioned the analysis and later measurements, and proposed his own transformer design as evidence. Notably, his transformer uses #61 material and a larger binocular core, a Fair-rite 2861006802 with 2t for a nominal … Continue reading WW1WW’s matching transformer for an EFHW
From time to time one sees discussion online about consistency of ‘measured’ VSWR at different power levels (on the same instrument). A question often asked is: I tune up at 10W and achieve VSWR=1.5, and when I increase power to 100W, the VSWR increases. Which should I believe? The first thing to note is that … Continue reading VSWR meter trap for the unwary
At FT82-43 matching transformer for an EFHW I wrote about the likely losses at 3.6MHz of a common design using a FT82-43 ferrite core with a 3t primary. In that case, expected efficiency (meaning PowerOut/PowerIn) of the transformer was less than 65% at 3.6MHz. I have been offered input VSWR curves for such a configuration, … Continue reading Small efficient matching transformer for an EFHW
A published design for an EFHW matching device from 80-10m uses the following circuit. Like almost all such ‘designs’, they are published without supporting measurements or simulations. The transformer is intended to be used with a load such that the input impedance Zin is approximately 50+j0Ω, Gin=0.02S. Analysis of a simple model of the transformer … Continue reading FT82-43 matching transformer for an EFHW
A common theme among online experts is to measure, or ask for measurement of a common mode choke connected between the centre conductor of a VNA’s tx and rx ports. That raises the question of whether |s21| with both ends shorted is meaningful, whether it in any useful way characterises the choke as a component … Continue reading Is |s21| measurement of a common mode choke meaningful to antenna systems?
An online expert recently expounded on detailed design of a balun, this is an excerpt about wire sizing. The wire gauge used limits the current handling capacity of the wire, run too thin a wire and it will heat up. Run much too thin of a wire for the power in use and it will … Continue reading Baluns – wire size insanity
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: Insertion … Continue reading An RF choke for a 1.8-30MHz coax power injector – LF1260 core