Skin depth in copper at 1.8MHz according to QRZ

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.

and…

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.

The specific advice above looks interesting, convincing even… but thankfully, the skin depth in copper is nowhere near either of the figures he gave. Continue reading Skin depth in copper at 1.8MHz according to QRZ

Reconciliation of Duffy CCS model with N7WS ladder line measurements

In developing and implementing A model of current distribution in copper clad steel conductors at RF reconciliation against some other published data was important.

(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.

The table below compares Stewart’s measurements with the CCS model and with TLDetails results (where available). Continue reading Reconciliation of Duffy CCS model with N7WS ladder line measurements

A model of current distribution in copper clad steel conductors at RF

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

Fig 1:

Fig 1 is a plot of the current distribution in a 1mm dia (#18) round copper conductor at 1.8MHz as implied by the model. Note that while the magnitude of current decays exponentially with depth, there is an imaginary component that hints a curl of the E and H fields within the conductor. Continue reading A model of current distribution in copper clad steel conductors at RF

QRP quarterly on small transmitting loop efficiency

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.

Note that Q for the lossless loop is about half that of Findling. Continue reading QRP quarterly on small transmitting loop efficiency

Workup of G5RV inverted V using high strength aluminium MIG wire

This article is a workup of replacement of my 2mm HDC G5RV and feedline with high strength 1.6 aluminium MIG wire to evaluate practical issues with use of an aluminium conductor.

The G5RV configuration is an inverted V, and although half a G5RV is 15m, the supports result in a 20m length of wire to the support. The configuration has a central support and simple spans for each leg of the G5RV to their respective supports. Continue reading Workup of G5RV inverted V using high strength aluminium MIG wire

Effect of shorting turns on a tapped air cored solenoid at RF

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).

Above a pic of the internals of the AT2K. The unused roller induction section is shorted. Continue reading Effect of shorting turns on a tapped air cored solenoid at RF

Review of Dodd’s WSPR based antenna comparison

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.

You might reasonably interpret this to mean that there was no significant difference between the antennas, one was as good as the other. Continue reading Review of Dodd’s WSPR based antenna comparison

Loss of ladder line: copper vs CCS (DXE-LL300-1C)

DXE sell a nominal 300Ω ladder line, DX Engineering 300-ohm Ladder Line DXE-LL300-1C, and to their credit they give measured matched line loss (MLL) figures.

They make the common ham mistake of writing loss figures as -ve dB where in fact by definition they are +ve (MLL=10*log(Pin/Pout)).

The line is described as 19 strand #18 (1mm) CCS and the line has velocity factor (vf) 0.88 and Zo of 272Ω.

Let us calculate using TWLLC the loss at 2MHz of a similar line but using pure solid copper conductor with same conductor diameter, vf and Zo. We will assume dielectric loss is negligible at 2MHz Continue reading Loss of ladder line: copper vs CCS (DXE-LL300-1C)

Loss in open wire + coax hybrid feed arrangements

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
  • the hybrid feed arrangement shown.

Let’s get serious and use home made open wire line made from 2mm diameter copper spaced 150mm. The very popular 19strand windowed ladder lines using CCS do not have sufficient copper to give copper like performance, the single core CCS is marginal. Continue reading Loss in open wire + coax hybrid feed arrangements

Exploiting your antenna analyser #27

An Insertion VSWR test gone wrong.

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.

Above is the poster’s test setup, his Rigexpert AA-170 is connected to the balun’s input jack using a M-M adapter. The output wires on the balun form a rough circle of about 550mm perimeter by eye. Continue reading Exploiting your antenna analyser #27