Introduction: In relation to the previous jubilee article, which talked about LA1K/P, this time, we’re returning to the topic of portable operations, though with ARK’s very own touch. During this year’s Norwegian Constitution Day parade, a radio transmission was carried out on the 17th of May in Trondheim to celebrate ARK’s anniversary. The application to be allowed to walk in the front of the parade was approved by the municipality, and thus, work began on our radio-contributions for the event.
Transmitting radio from the parade was something several people in the club wanted to achieve now that we had the LA100K callsign, but a plan had to be laid out for how this would happen. In the first instance, the idea was to have a portable radio set up in a wheeled cart, with the radio operator sitting together with the radio and battery, as well as a wire antenna hanging from the top of our banner. This would have taken up quite some space. After some discussion, it was decided to go for a simpler and lighter setup consisting of a light hand-held antenna as well as a smaller radio for running digital modes. As with all of our projects, the work and completion is always postponed until the last moment, and with this initiative it was no different. LA8ITA provided the final 20m magloop antenna, and a Xiegu X6100 radio, as well as a Surface Pro 2 tablet to run WSJT-X and the FT8 mode. Below is a description of the mag loop.
Photo: LB0VG
The loop consisted of 3m of plastic-clad copper pipe, bent to a suitable round shape, a broom handle, and a capacitor construction.
The capacitive load had two challenges:
1) A selection of articles and documents mostly agreed that the formulas for short loop antennas were wrong in several antenna books and calculators, and little else.
2) This was meant to be a low-threshold project, “you take what you have”, and whatever variable capacitors you have on the scrap shelf were probably not able to handle a voltage which, if everything else goes right, can be many hundreds of volts.
As we enjoy theoretical investigation and practical exploration, the first challenge was no worse to solve than putting in time for testing and we chose to solve the second with some simple circuit engineering.
Photo: LB0VG
In short, one capacitor had been added in series, and one in parallel, where the serial one sets a working range that the other two can “pull down“, and the parallel one sets a minimum value so that a sufficiently large part of the voltage is always applied the series capacitor.
A feed loop was quickly made by soldering the center conductor of a piece of RG58 to the screen further back, and a choke was inserted a little further down (eight turns on an FT240-43 ring ferrite).
The first estimate indicated an operating range of around 65 pF, which gave a resonance of around 10.5 MHz.
Armed with this data point as well as one more obtained by cutting a capacitor leg, and after several old club members had managed to recommend instead using a variable vacuum capacitor (this work took place on Waffle Friday before ARK’s anniversary dinner) we calculated a working range of around 35 pF .
Photo: LB0VG
On Monday, only a couple of days before the day itself, everything was put together again with new values. Fortunately, the antenna now turned out to be tunable for our purposes, close to one in SWR on the entire twenty meter band. With the promising results, we relocated to the roof and tested the antenna with a radio on FT8. You could hear quite a few other radio amateurs, and after some CQ we even got in touch with an English radio amateur on a generous 0.7W. Quite impressive, but not past our goal just yet as the Xiegu is a 5W radio. Running out of time for now, we packed up and crossed our fingers for a better result in the parade.
On the day itself, we brought along all the equipment and took a small test just to confirm that things worked as they should. The start was promising, with one QSO locked in quite quickly. However, after 1 ½ hours, it unfortunately ended with only this one in the log. We were heard by a couple or three others but struggled to get through, presumably due to poor conditions in the middle of the Trondheim city centre. When we came around again, we almost had another QSO with an LA station, but unfortunately, as the parade advanced forward, it was not completed. Even with the efforts of the previous days, there were quite a few contacts in total. But with 1 completed (and 2 half) QSOs, we still managed to get the station running. Satisfied with the mission, we strolled one more round in the parade, this time with the rest of the Student Society, hoping to collect a couple more.
Harald holds the radio, and Oscar observes from behind the antenna
Photo: LA9GY
In addition to the radio operations, a rollable sign was also arranged for the parade. The lightbulb sign “LA100K”, which was made for the anniversary evening, suddenly got a portable power supply consisting of two 12V LiFePo4 batteries in series, as well as a Raspberry Pi to play the light sequences over DMX. Although the sign uses the same strong bulbs that can otherwise be seen on the lightbulb sign under the festival “UKA” every odd-numbered year, they were probably at their weakest today, with occasional sun (and rain) competing for illumination. However, the sign still attracted a lot of attention in the parade, and the spectators soon stopped to look at the beautiful patterns as ARK rolled through the streets.
Photo: LA9GY
The radio session in the parade was, despite the result with a mere QSO, still a resounding success, and many of ARK’s members defied the poor weather forecast and joined in on rolling the sign and carrying the radio setup under the drizzling rain. ARK also got a completely new banner, which was sewn and completed the night before by eager ARKers. It was probably a little cold for some who turned up without jackets and umbrellas, but people were mostly satisfied when they were served sodd at the clubhouse, a good tradition that ARK still maintains. Thanks to everyone who attended!
Photo: LA9GY
Featured image details:
Caption: ARK in the Constitution Day Parade
Photo by: LA9GY
– Heidi LB9WI, Kristoffer LB0VG, Snorre LA8ITA and Sondre LB5PI
Article first published in “Amatør Radio”, June 2023
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