Antennas – Page 26 – owenduffy.net

Cadweld Plus Control Unit

The CADWELDPLUSCU is the igniter for the Cadweld Plus line of electrically initiated thermite weld capsules. The plug seen on the end of the cable slides over the flat connection strip to the capsule.

The environment around the weld crucible is subject to a sudden ejection of hot gases and smoke, much of which is emitted out of the slot in a Oneshot crucible intended for the connection strip, and this is only a 45g capsule. There is probably less gas emitted towards the connector on reusable moulds as the gap for the connection strip is very narrow.

Despite dressing the plug away from the stream of hot gasses emitted, it can be seen above that the plug has suffered heat damage. Of particular concern is deformation of the parts at the cable clamp rendering it less effective. The plugs come on a new lead set that is relatively expensive.

Since one wants to be able to yank the cord away from the crucible as soon as possible to minimise heat damage, the cord grip needs to be effective.

This article describes two measures taken to protect the plug and ensure the cord grip works. Continue reading Cadweld Plus Control Unit

Last update: 3rd October, 2016, 5:52 PM

MFJ-993B internal balun review

The MFJ-993B auto antenna tuner includes an internal balun, this article is a review of that balun.

The schematic shows the balun as a Guanella 4:1 balun with the usual external link to one of the coaxial antenna sockets. (The label “Z balanced” is misleading, clearly one of the terminals is grounded and this is the unbalanced connection to the coax connector via a link. The antenna connects to the left hand terminals.)

Unlike almost all ATUs with an internal balun, this is a current balun (to their credit), but a 4:1 balun.

There are two aspects of balun behaviour that are of particular interest:

choking or common mode impedance; and
impedance transformation.

Continue reading MFJ-993B internal balun review

Last update: 11th August, 2018, 9:10 AM

Shack entry / ATU configuration for my G5RV with tuned feeder

At MFJ-993B on my G5RV with tuned feeder I discussed first impressions of the replacement ATU.

This article documents the physical layout.

The antenna is a G5RV with tuned feeders (Varney 1958). The tuned feeder is home made two wire line using 2mm diameter copper spaced 50mm.

Above at the right, the open wire line terminates on a home made balun on the feed line entrance panel, see
Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #4 for details of the balun. This is all under the building eaves, but it is waterproof… the area is regularly jetted with high pressure water to clean insects away. Continue reading Shack entry / ATU configuration for my G5RV with tuned feeder

Last update: 30th September, 2016, 7:28 AM

Is the premises main earthing conductor ok as a lightning down conductor

Upon reading Rationale for sizing of lightning down conductor a correspondent asks whether his premises 4mm^2 main earthing conductor ok as a lightning down conductor.

Intended purpose of electrical installation ground electrode

The usual source of current on the premises main earth conductor would be a fault energised by the incoming supply. To understand the implications, lets review the supply system. Continue reading Is the premises main earthing conductor ok as a lightning down conductor

Last update: 22nd September, 2016, 1:14 PM

Rationale for sizing of lightning down conductor

The lightning ground conductor shown at Mast ground rework might at first seem excessive, this article sets out the rationale.

The connection to a 2.4m copper clad steel driven electrode (under the green cover) is 35mm^2 copper.

The nature of lightning protection sizing

Lightning protection sizing is a risk management regime driven by the mechanisms of lightning and variation in distribution.

It is not surprising then that regulatory standards in different distributions broadly use similar design methods but set different practices for implementation in the jurisdiction.

So, let’s go standards shopping… what we are looking for is guidance on the energy (or work) that is directed to heating the down conductor, and choosing a conductor size that will sustain not just a single stroke, or an average stroke, but most events that may include many strokes in a short period of time. Continue reading Rationale for sizing of lightning down conductor

Last update: 23rd September, 2016, 12:33 PM

Demonstration video of Cadweld Oneshot Plus ground rod connection

I showed in Mast ground rework the use of a Cadweld Oneshot Plus thermite weld of the ground conductor to the ground rod.

Responding to reader interest, I have made a little video demonstrating the process.

Above is a pic of the demonstration piece with crucible and slag broken away. Continue reading Demonstration video of Cadweld Oneshot Plus ground rod connection

Last update: 21st September, 2016, 6:40 PM

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.

Above 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

Last update: 13th September, 2016, 7:58 AM

Mast ground rework

When I moved here about eight years ago, I quickly installed a small mast and associated ground system for the station. The grounding of the mast itself for lightning protection was a temporary solution, and less temporary than planned. This article documents the rework.

Above is the temporary solution. A 2.4m copper clad ground rod was driven into the clay, and a couple of short 25mm^2 tails connected to the mast tube. The long term solution was to be tidier and allow the mower / brushcutter to be used to trim grass without fouling the earth rod or cables.

The plan is to cut to bent top of the earth rod, drive it below ground level, and make a new tail of 35mm^2 (#2) cable and Cadweld it to the ground rod. Continue reading Mast ground rework

Last update: 23rd January, 2017, 9:08 PM

MFJ-993B on my G5RV with tuned feeder

This article is an analysis of why my recently acquired MFJ-993B will not match my multiband antenna system on most bands above 20m. The MFJ-993B replaces an Ameritron ATR-30 which was capable of matching the antenna system on all HF amateur bands.

A detailed analysis is performed 18.15MHz on the first problem band.

Configuration

The antenna system uses a tune feeder configuration.

The alternative tuned feeder arrangement described at (Varney 1958).

In this case, the open wire line is 9m of home made 450Ω line (2mm copper wires spaced 50mm air insulated), a 1:1 current balun and 0.5m of RG400 tail to the ATU.

A 470K 1W metal film resistor and a 2095-100 gas discharge tube in parallel are connected in parallel with the ATU antenna terminals to reduced static build up and modest transient protection.

Impedance was measured looking with a Rigexpert AA-600 into the cable end that plugs onto the ATU, at 18.15MHz is is 4.7-j69.5Ω.
Continue reading MFJ-993B on my G5RV with tuned feeder

Last update: 20th January, 2020, 7:33 AM

Exploiting your antenna analyser #25

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

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

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

Last update: 5th September, 2016, 9:06 AM