Windowed ladder line loss – G3TXQ #2

I tried to glean some useful information from G3TXQ’s measurements of windowed ladder line loss at Windowed ladder line loss – G3TXQ.

In reviewing his article today (05/02/14), there is new information on a further series of measurements of the same line.

450_mll

The shape and position of the two lines does not reconcile with the formulas stated, so I digitised the data points and analysed the data set to try to find the most appropriate model for the reported measurements. Note that although the chart above is in imperial units, my work is usually in ISO metric units, and usually basic units.

The digitised data points were converted to loss in dB/m, and fitted to the model MLL=k0+k1*f^0.5+k2*f using regression techniques. Note that the digitisation process introduces some noise, but it is estimated to be small compared to the noise in the underlying measurement data.

The coefficients k0, k1, k2 were reviewed to test that there was sufficient data to support the hypothesis that they were not zero, and all three passed that test, the standard error of the coefficient was significantly less than the coefficient. Note that k0 is not derived from a DC measurement of resistance as done by some modellers, but from the measurement data over the range of 3.6 to 48MHz in this case, and extrapolation beyond that frequency range increases uncertainty.

Clip 141

The above chart shows G3TXQ’s measurements as digitised from his published graph, and it shows the components of loss indicated from the model I built (the k0 component is allocated as conductor loss).

The “G3TXQ model” line is equivalent to his MLL=0.063+0.063*f^0.5 dB/100′ converted to dB/m, and as you can see it is not a good fit to the measurement data points, nor does MLL=0.063+0,063*f^0.5 dB/100′ reconcile with the blue line on G3TXQ’s chart earlier in this article.

G3TXQ’s measurement points (as digitised) are quite a good fit to the model MLL=0.001456+1.499e-6*f+5.631e-11*f dB/m where f is in Hz, and provide a good predictor of MLL over 3.6 to 48MHz.

 

Foundation manual on power measurement

My article Foundation watts explained triggered some discussion on the thorny issue of compliance with power limits of the LCD.

One correspondent was confident that the Foundation candidates are properly trained, which leads to examining the training materials.

SWRMeter

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Do SWR meters make you stupid

Eric Nichols, KL7AJ, published a manifesto entitled “SWR Meters Make You Stupid” announced on eHam in March 2010. Eric is clearly a disciple of Walt Maxwell (W2DU SK), his first announcement states By popular demand, my complete treatise on transmission lines. Approved by Walt Maxwell, W2DU as if Walt’s blessing was an imprimatur from the ham radio church.

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Cobwebb antenna impedance matching scheme

From time to time, correspondents have asked how the Cobwebb antenna works, and particularly how the impedance matching scheme works.

Firstly, what is the Cobwebb?

It is an innovative antenna for small spaces, quite compact and as I recall originally intended to cover five amateur bands from 20-10m.

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Common mode current and coaxial feed lines

An antenna feed line is intended to convey energy from the transmitter to the antenna, and usually without giving rise to radiation itself.

The term “common mode” comes from consideration of the currents on an open two wire line, and it refers to the net  or unbalance current, ie the current that would give rise to external fields, to radiation.

This article looks at the equivalent common mode current in a coaxial transmission line.

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Speaker cable as RF transmission line

Speaker cable and similar Figure-8 twin in various sizes is popular as RF transmission line, particularly with QRP operators, especially those operating portable in the field (eg SOTA activators). Indeed, SOTA seems to have triggered a revival in the use of these cables. With the large number of online recommendations, one could be forgiven for thinking that a dipole and feed line formed entirely from #24 speaker wire is the antenna of choice.

With that in mind, this article looks at the performance of Jaycar WB1702 speaker cable, 14×0.14mm conductors (#24) with PVC insulation, cost $0.50/m.

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Measuring matched line loss

Matched Line Loss (MLL) is the loss or attenuation of a transmission line terminated in its actual characteristic impedance, and usually give for some length, eg dB/100′, dB/100m, dB/m.

One method that occurs in ham radio articles is to measure the input resistance of a resonant s/c or o/c stub, and to calculate MLL as 8.686*Rin/Zo/length dB/unitlength. (This is not the only method, or even a good one, but it is commonly used.)

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