A model for current distribution in a conductor is that for a homogenous conducting half space with surface current parallel to the interface. Current density at depth d is given by the expression J=Js*e^(-(1+j)*d/δ) where δ is the skin depth (δ=(ω*µ*σ)^0.5, σ is the conductivity).
Copper round conductor – 1.024mm (#18) single core
I saw a question posed online about the merits of a proposed antenna system which used a hybrid feed arrangment as 15′ (sic) of the feed line needed to be buried.
Above is the poster’s diagram, and his posting lacked some important details so let’s make some assumptions. Lets assume the antenna is at 150′ in height above average ground, and since the dipole is long enough to be usable on 160m, let’s study it at 1.85MHz.
Input impedance of the dipole under that scenario is around 45-j400Ω.
Let’s consider two options:
a tuned feeder option (ie open wire line all the way to the ATU); and
We often learn more from failures than successes, this exercise is one of those opportunities.
An online poster tried to validate his newly purchased MFJ-918 by measuring Insertion VSWR.
That is done preferably by measuring a good termination (dummy load) to validate that it has a very low VSWR, then inserting the Device Under Test (DUT) and measuring the VSWR as a result of insertion of the DUT.
The poster did not mention measurement of the dummy load alone, and it is a type that warrants validation.
A ham in the need of help recently asked for advice on eHam about the use of Wireman Ladder Line and the like.
After a fairly well considered, detailed and lengthy answer from on online expert, another online expert stepped in to confuse the matter with conflicting advice:
Wire resistance (loss due to current ) is not a factor with higher voltages typically seen in high impedance antenna feed applications. Attenuation loss is a factor depending on dielectric properties in VHF and UHF frequencies. Their is little skin effect below 50 MHz in wire antennas and feeds.
This comes down to line strength. I would go with the solid Copperweld for HF antenna work.
I recently purchased a Surecom SW-102 VSWR meter. It looked a little like a supercharged RedDot copy.
Above the Surecom SW-102 VSWR meter with backlight and photographed under normal interior lighting. The display lacks contrast, and overall is difficult to read due to size of text, fonts used, and lack of contrast. (The pic is taken with a screen protector installed, but the evaluation is based on the bare meter with original protective film removed as it degraded readability.) Continue reading Surecom SW-102 VSWR meter review
The term “common mode current” applied to coaxial transmission lines is bandied about with abandon these days in online fora, awareness of its existence has increased if not understanding.
A simplistic analysis is that in TEM mode, ONLY differential current is supported inside a coaxial line, ie that at any point the current on the outer surface of the inner conductor is exactly equal to a current in the opposite direction on the inner surface of the outer conductor.
But, lets look at the wider context of the meaning of common mode current when a uniform coaxial line is connected to an antenna system. Whilst an antenna might have an obvious two terminal connection to the feed line, in the presence of ground, the current in those two terminals are not necessarily equal and opposite. Continue reading Does common mode current flow inside coax?
Find coax cable velocity factor using a very basic analyser
A common task is to measure the velocity factor of a sample of coaxial transmission line using an instrument that lacks facility to backout cable sections or measure OSL calibration (as discussed in other articles in this series). The older models and newer budget models often fall into this category.
The manuals for such instruments often explain how to measure coaxial cable velocity factor, and the method assumes there is zero offset at the measurement terminals (whether they be the built-in terminals or some fixture / adapters). In fact even the connectors are a source of error, especially UHF series connectors.
It is the failure to read exactly Z=0+j0Ω with a S/C applied to the measurement terminals that adversely impacts efforts to measure resonant frequency of a test line section.