The half waves of coax rule for measuring VSWR accurately

Lots of hams recite a rule that accurate measurement of VSWR can only be made at the feed point or an integral number of electrical half waves from the feed point.

It is one of those ‘rules’ that the proponents cannot usually explain… they would regard themselves as experts, but blindly follow folk-lore that they do not understand. Continue reading The half waves of coax rule for measuring VSWR accurately

Exploring VK2XSO’s transmission line example

VK2XSO posted a sweep of “Return Loss (SWR) (the lower plot) from 500 to 2500MHz of a 50Ω load through ~5m of RG59” apparently to demonstrate his knowledge of transmission line basics. As he says “here are also many other things we can deduce from looking at these two lines.”

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For students of transmission lines, some deductions… Continue reading Exploring VK2XSO’s transmission line example

Feed line length affect on VSWR

This article explores the way in which VSWR varies along a feed line.

Actual VSWR

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The graph above shows R and X, and VSWR along a feed line with a 100+j0Ω load at 3.6MHz. The feed line is Belden 8262 50Ω coax, manufacturing tolerances are taken to be zero, and the displacement is relative to the feed point, ie -ve distance is distance before the feed point, the feed point then is at the right hand side of the graph.
Continue reading Feed line length affect on VSWR

End fed wires – another matching case study

A reader of my article End fed wires – new hams love ‘em asked about the performance of the matching transformer described at (Osborne 2014).

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(Osborne 2014) gives the transformer design above using a FT82-43 ferrite toroid with three turns on the primary. The concept is to adjust the antenna length and C1 for a ‘perfect match’. Continue reading End fed wires – another matching case study

End fed wires – new hams love ’em

Hams impoverished for space to erect a ‘full size’ antenna for HF pursue a range of options and end fed wires figure highly.

New hams who heard the maxim that ‘any antenna is better than no antenna at all’ will try to use bed frames, window frames, balcony railings, anything metallic to get on air. This cry for mediocrity is to prevail over a quest for understanding, ham radio as it has evolved.

The ‘net abounds with conflicting advice:

  • (Yates 2010) recommends a T50-2 powdered iron core;
  •  (VK2AVR 2014) states do not use a powdered iron toroid.. they won’t work.

They cannot both be correct. Continue reading End fed wires – new hams love ’em

Spoiling balun action with ‘shielded twin’

In a QST column in 2008, a correspondent asked the question

… I have the ladder line terminated to double coaxes that run about 12′ (4m) inside the house to an antenna tuner. Should this pair of coaxes be grounded at one end or both ends?


The Doctor gave a detailed diagram (above) and his advice was… Continue reading Spoiling balun action with ‘shielded twin’

Analysers – help or hindrance

A ham posted online:

I spent several happy hours this weekend building the DE of the 6M Quad described in the June 2014 QST, p 30. When I got it completed, I put the antenna analyzer on it, expecting to find a nice resonance in the 50-51Mhz region and an impedance of 120 ohms or thereabouts. To my surprise, the radiation resistance in the couple of dozen ohm range, and resonance, if that is what I can call it, depends on how am I holding the loop.

After a bit of QST bashing in the thread, he later reveals:

The trial with the analyzer was about 2′ of RG-8X with PL-259s on each end, to BNC jacks on both antenna and analyzer with adaptors.


Much as the chap expressed his lack of confidence in modelling tools, NEC reveals what is happening. Continue reading Analysers – help or hindrance