Current Projects - SARCNET

School Amateur Radio Club Network
School Amateur Radio Club Network
School Amateur Radio Club Network
School Amateur Radio Club Network
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Current Projects
At SARCNET we always have lots of projects on the boil. These projects, all inspired by our school Amateur Radio clubs, have been started, but have not yet been completed and documented on their own separate web page. We sometimes need to stop and re-evaluate our approach. We are always interested in what our readers think, too. So, any suggestions or ideas for new projects are always greatefully accepted.

Heavy-duty 300 Ohm open-wire feeder - Mk1
We needed to make our own 300 Ohm, heavy-duty, open-wire feed line for our folded-dipole and full-wavelength resonant loop antennas, which have a feed impedance of around 288 Ohms. We had been using 300 Ohm TV ribbon, but it is very light-weight, breaks, doesn't handle much power and its SWR is adversely affected when rain collects on the webbing between the conductors. So we decided to design our own open-wire feeder. An on-line impedance caculator indicated we could make our own 300 feeder if we maintained a spacing of 9.8mm with a conductor diameter of 1.5mm. We used Electra Cables SR1015 1.5mm^2, house-hold electrical cable.

Conductor: Plain annealed copper
Insulation: PVC 90
Voltage: 0.6/1 kV
Format: 7 x 0.50mm
Insulation Thickness: 0.8
Outer diameter: 3.1
Standard: AS/NZS 1125, AS/NZS 5000.1, AS/NZS3808
Standard Packing Length: 100m, 200m, 250m, 500m
Operating Temperature: 90°C

Our first attempt to make our own feeder was to use cable ties installed in an inverse manner, so that the cable-tie blocks work as a natural spacer. It is easy to make and works well provided you select the right cable ties for the job.

300 Ohm Open Wire Feeder Mk1
Heavy-duty 300 Ohm open-wire feeder - Mk2
Our second attempt to make our own feed line was to use adhesive (dual-wall) heat-shrink tubing. Although harder to manufacture, using a hot-air gun and small, flat pliers, it looked more professional, would be easier to pull through holes and might have the advantage of surviving prolonged UV exposure - something black cable-ties are not noted for, in our experience.

300 Ohm Open Wire Feeder Mk1
Balun for 300 Ohm, open wire feeder
Next we needed a 6:1 balun to feed the 300 Ohm balanced feeder from our radios with 50 Ohm unbalanced output. We started with the usual 4:1 (Guanella) balun. Noting the expert balun advice from Owen Duffy, VK1OD, we chose to use two cores and made a dual-core, current-balun. We tested it with a NanoVNA and it was flat from 1-60MHz. It also passed all five VK1OD Balun Tests. Very pleased with that result: Just for fun we added two extra turns to the balanced side and retested it: It remained flat from 1-60MHz, but matched an output impedance of 300 Ohms - Perfect for our needs.

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