Having just written again on skin effect and copper clad steel (CCS) conductors on HF, and the potential for less than copper performance, it was interesting to note a thread on QRZ where the OP asked for advice on the issue with budget CCS RG-11.
Two late posts as I write this were:
There really is no real issue with skin effect on HF bands with copper clad materials.
At 1.8 MHz, the skin depth in copper is 0.654 micro-meters (.0000654 mm), so the copper cladding on the center conductor of most RG-11 type coaxial cables is more than sufficient for any of our current bands.
(Stewart 1999) published a set of measurements of the popular Wireman windowed ladder line products. His measurements were in the range 50-150MHz. They form the basis for most calculators on quantitative analyses at HF, despite the fact that it is a dangerous extrapolation for CCS construction.
Nevertheless, the directly stated measurements at 50MHz are a useful calibration point for reconciliation.
Above is Table 1 from Stewart, it sets out measurements of four Wireman m.products and a plain copper line.
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
A correspondent recently wrote regarding the theory expounded in (Findling et al 2012), and their measurements and performance predictions of the AlexLoop Walkham, Portable Magnetic Loop Antenna by PY1AHD.
The authors give a formula for lossless Q (to mean no loss other than by radiation) without explanation or justification.
The formula is wrong, possibly a result of slavish acceptance of Hart’s two factor incorrectly applied (see Duffy 2015, and Antennas and Q). This error feeds into an optimistic estimate of antenna efficiency.
Analysis of measurement data
(Findling et al 2012) presents a table of measured half power bandwidth for the Alexloop.
Taking the 40m case, lets calculate to Q for a lossless loop, Qrad in Findling’s terms.
The roller inductor in a Palstar AT2K will be taken as an example to illustrate the technique. This tuner is popular and has a very good reputation amongst hams, though (Duffy 2012 was less enthusiastic).
Dodd espoused the merit of WSPR for antenna comparison in his article (Dodd 2011).
He documented a series of WSPR spots of his transmitter on 20m in a table swapping between antennas during the test period, one side of the table for each antenna. (Don’t be misled, the dipole is not half wave dipole but some non-descript multi band loaded dipole.)
He calculates the average for each data set and states:
The average from the dipole and the loop -16.74 and -17.0dB respectively meaning that the performances were very similar.
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.
A 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.