Zo of two wire line

I saw a recent discussion where the blind were leading the blind on the dimensions of a twisted two wire line for Zo=50Ω for use in a balun.

The poster had used an online calculator which used the well known log function for estimating Zo of an air spaced two wire line… the calculator, like most quotations of the formula do not state clearly that it is only an approximation of limited validity, and the calculator returned results for ridiculous inputs (like negative spacing).

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The graph above (Duffy 2008) shows the log approximation, and the underlying acosh based estimate. I say estimate because the acosh function does not account for proximity effect which becomes significant at the very closest spacings, and internal inductance which becomes significant at lower frequencies. Proximity effect depends on more than just the spacing/diameter ratio and so cannot be shown on the above graph.

So how did our poster find dimensions for wires for Zo=50Ω when the log graph above shows that as the wire centre to centre spacing approaches the wire diameter, it the wires approach touching, Zo approaches 83Ω? Continue reading Zo of two wire line

The sign of Return Loss

I was browsing a ham forum recently when I came across a Return Loss plot apparently from a ham grade miniVNA Tiny.

Lets just remind ourselves of the meaning of the term Return Loss. (IEEE 1988) defines Return Loss as:

(1) (data transmission) (A) At a discontinuity in a transmission system the difference between the power incident upon the discontinuity. (B) The ratio in decibels of the power incident upon the discontinuity to the power reflected from the discontinuity. Note: This ratio is also the square of the reciprocal to the magnitude of the reflection coefficient. (C) More broadly, the return loss is a measure of the dissimilarity between two impedances, being equal to the number of decibels that corresponds to the scalar value of the reciprocal of the reflection coefficient, and hence being expressed by the following formula:

20*log10|(Z1+Z2)/(Z1-Z2)| decibel

where Z1 and Z2 = the two impedances.

(2) (or gain) (waveguide). The ratio of incident to reflected power at a reference plane of a network.

Return Loss expressed in dB will ALWAYS be a positive number in passive networks.

Return Loss according to the miniVNA Tiny

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Above, the miniVNA Tiny presents Return Loss as a negative value. Continue reading The sign of Return Loss

Effective RF resistance of a braided solenoid – Gilbert’s coil measurements

(Gilbert 1996) gave a set of measurements of impedance of several inductors wound as a single layer close spaced solenoid of RG-213 coaxial cable.

Of particular interest is the measurements of the 6t solenoid as there are several measurements well below the self resonant frequency of the inductor.

Key geometry details used in this analysis are:

  • cable OD 10.287mm;
  • conductor OD 8mm;
  • mean solenoid diameter 117.4mm (ASTM D-2729 pipe + RG-213);
  • cable length 2.213m; and
  • solenoid length 6*10.287mm.

clip-218Above is a plot of Gibert’s measurements from 1 to 5MHz, and curve fits.
Continue reading Effective RF resistance of a braided solenoid – Gilbert’s coil measurements

Exploiting your antenna analyser #25

Find coax cable velocity factor using an antenna analyser without using OSL calibration

A common task is to measure the velocity factor of a sample of coaxial transmission line using an instrument without using OSL calibration.

Whilst this seems a trivial task with a modern antenna analyser, it seems to challenge many hams.

We will use a little test fixture that I made for measuring small components, and for which I have made test loads for OSL calibration. We will find the frequency where reactance passes through zero at the first parallel resonance of an O/C stub section, this is at a length of approximately λ/2 (a good approximation for low loss coaxial cables above about 10MHz).

We will use a little test fixture that I made for measuring small components, and for which I have made test loads for OSL calibration.

The text fixture used for this demonstration is constructed on a SMA(M) PCB connector using some machined pin connector strip and N(M)-SMA(F) adapters to connect to the instrument.

VfMeasurement01

Above is a pic of the test fixture with adapters (in this case on a AA-600). Continue reading Exploiting your antenna analyser #25

Exploiting your antenna analyser #24

Find coax cable velocity factor using an antenna analyser with OSL calibration

A common task is to measure the velocity factor of a sample of coaxial transmission line using an instrument that supports OSL calibration, an AIMuhf in this example.

Whilst this seems a trivial task with a modern antenna analyser, it seems to challenge many hams.

There are a thousand recipes, I am going to demonstrate just one that suits the instrument and application.

We will use a little test fixture that I made for measuring small components, and for which I have made test loads for OSL calibration. We will find the frequency where reactance passes through zero at the first parallel resonance of an O/C stub section, this is at a length of approximately λ/2 (a good approximation for low loss coaxial cables above about 10MHz).

The text fixture used for this demonstration is constructed on a SMA(M) PCB connector using some machined pin connector strip and N(M)-SMA(F) adapters to connect to the instrument.

VfMeasurement01

Above is a pic of the test fixture with adapters (in this case on a AA-600). Continue reading Exploiting your antenna analyser #24

Post mortem review of a 144MHz combiner / splitter

This article is a post mortem review of a 144MHz splitter combiner that was made using RG6 coax. It is post mortem (ie post death) because the combiner was stored outdoors without checking that the connectors were protected from weather.

Fig01

The combiner was used successfully for over 10 years on a 144MHz four over four antenna system (above) without any maintenance problems.

Fig06

Above is a close up of the Tee point of the network. The coax cables are protected by HDPE sleeving to reduce the chance of damage at the hands of Sulphur Crested Cockatoos, in the event there was no damage.
Continue reading Post mortem review of a 144MHz combiner / splitter

Tuning combiner lines

A common method of combining two 50Ω antennas to a single 50Ω feed is using a quarter wave transformer using 75Ω line from the common feed point to each antenna.

A recent posting to one of the ham fora raises the posters problems with making this really simple feed system work.

Screenshot - 25_08_16 , 07_25_16

Above is his measured input characteristic with good 50Ω loads on each leg. Reading a hundred posts, it seems that he attributes this to legs of 0.167m length of RG11. The problem is that RG11 as most of us know it has a solid PE dielectric giving it a vf=0.66 and that 0.167m is 63° at 207MHz… so why the response above. Continue reading Tuning combiner lines

Exploiting your antenna analyser #23

Seeing recent discussion by online experts insisting that power relays are not suitable to RF prompts an interesting and relevant application of a good antenna analyser.

Screenshot - 03_08_16 , 14_12_56

Above is a sweep of an A/B changeover relay intended for HF application at up to 100W and lowish VSWR. The sweep is actually from 1 to 61MHz to be confident that there is not poor behaviour just outside of the HF range that might present on another implementation of the same design. Continue reading Exploiting your antenna analyser #23