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

InsertionVSWR of Grebenkemper’s Tandem Match

The findings at InsertionVSWR of Revex W560 on HF and the suggestion that the low frequency problem is characteristic of poorly designed Sontheimer couplers (Sontheimer, C & Frederick 1966).

These couplers were popularised by (Grebenkemper 1987)  in his Tandem Match – An Accurate Directional Wattmeter and have appeared in ARRL handbooks over the decades, and may have inspired the many commercial implementations of the coupler.

Grebenkemper claims his meter is ‘good’ down to 1.8MHz, but does not clearly claim any particular InsertionVSWR. There is limited value in an instrument that can measure down to 1.05 when it causes significantly higher VSWR itself.

Screenshot - 24_07_16 , 15_31_41

Lets drill down on Gebenkember’s article, specifically the coupler design.
Continue reading InsertionVSWR of Grebenkemper’s Tandem Match

Optimal Zo for TLT sections challenge – a solution

Optimal Zo for Guanella balun sections left the reader with a though exercise, a transmission line transformer used by PA0V in a 144MHz power amplifier output network.

Clip 206

The pair of tabs to the left are driven by FET drains, the upper pink centre conductor is grounded, the lower end connecting to C1 is the output to a nominal 50R load. The network shown near OUT is for fine load adjustment. There are two coax sections making this TLT, shields bonded all the way around and the centre conductors connected as shown. What is the optimal value of Zo for each the coax sections?

PAdetail

Above is a pic of the PA, and we are looking at the network to the right of the dual FET. Continue reading Optimal Zo for TLT sections challenge – a solution