Akademisk Radioklubb


Month: June 2017

New beacon antennas for LA2VHF (4m) and LA2UHF (70cm)

For a while now, the beacons on 4m and 70cm have had sub-optimal antenna systems.  We decided to install new Big-Wheel antennas for both bands, as an experiment.    The “Big Wheel” is a horizontally polarized omni-directional antenna.

Getting ready

Getting ready for departure. A 6 meter long steel mast, 3 Big Wheel antennas, and 4 hams

The installation was performed in typical Norwegian summer weather – rain and wind.

Antenna assembly

Øyvind (LA3WUA) and Arne (LB7JG) assembling a Big Wheel

In addition to the 70 cm and 4 m Big Wheel antennas, we put up a 6 m version as well.  We don’t currently have a beacon license for 50 MHz, but since there has been allocated a new beacon segment above 50.400 MHz, there might be room for us there.  Stay tuned – perhaps LA2VHF will show up on 50.463 some day.

Final adjustment on the 6m Big Wheel. Arne (LB7JG) and Øystein (LB7IG) in intense competition – manual or electric?

6 m and 70 cm big wheels were bought from Wimo. The 70 cm version is a very neat design that comes pre-assembled. Some assembly is required for the 6 m version.  We had a lot of trouble getting the aluminum tubes to fit inside the square aluminum pieces. In the end we had to file the tips so that they would fit. If you are planning to build this antenna, make sure to have some lubricant and proper tools at hand.

For 4 m there are no commercial designs. Our solution is a custom design based on the build notes OE5MPL and OE5VRL provide on 70MHz.org.

70 cm and 4 m Big Wheel is mounted. Now, the 6 m remains. Øystein (LB7IG) and Sveinung (LB1SH) receives the antenna from Arne (LB7JG)

If you hear any of the beacons we would love to hear about it at LA2VHF or LA2UHF @ la1k.no.

Installing an AIS receiver at Vassfjellet

This weekend we put up a Automatic Identification System (AIS) receiver while making some improvements on our internet link to Vassfjellet.

Improving the link

Last year we installed a 5.8 GHz radio link between Samfundet and Vassfjellet, where we have our radio beacons. The radio link will allow us to remotely check status of the beacons, and allow for several exciting monitoring applications.

We found out that we had done a mistake in choosing the feedline between the Ubiquity rocket M5 and the antenna. The mistake has nagged us over the entire winter, so when the snow on the road finally melted, we bolted up the mountain.

LB0VG terminating RJ45 plugs for the link. LA1BFA inspecting important assets in the background.

The feedline was around 6-7 meters of a RG-58 type. At 5.8 GHz this turns out to have a massive attenuation, approximately 13 dB. By simply replacing with a shorter and better cable, we could get a huge improvement in link quality.

To get as low cable loss as possible we decided to mount the Rocket right behind the antenna. We bought some 15 cm RP-SMA to SMA pigtails that use RG174 cable, which should give a cable loss of only 0.56 dB. The resulting improvement is seen below.

When we took the link down at 12.00 UTC the link margin was 10 dB. We had it up again an hour later, and the link margin is now 22 dB. This is very much in line with the cable loss improvement mentioned above.
This improvement in link margin will be very nice when we start adding more services up there.

Marinetraffic AIS receiver

Boats over a certain size are required to report their position using AIS. This makes for very interesting listening, as you can effectively stalk the movements of large boats.

Marinetraffic is a website where reports from a network of AIS receiving stations are gathered.  Marinetraffic are also interested in unique sites that will allow them to expand their coverage, see their application form here. We got in touch with them, and they were interested enough to send us:

We tuned the antenna to 162 MHz using our AA-170 antenna analyzer, and got it to resonate with about 1.2 VSWR. The antenna was plugged to the SLR350Ni, which surprisingly is based on a Raspberry Pi 3 with a radio daughterboard. After a small power struggle with the software and trying to set it to a static IP, we started receiving ships.

We were a little worried about the receiver getting a lot of interference from LA2VHF, as they are in the same band, and very close. But it looks like everything is working smoothly.

LB0VG handing LA3WUA the Rocket modem. Behind LB0VGs head is the newly installed AIS antenna.

Below is a display of the ships that we have received. On average we get 200 AIS messages a minute from about 100 ships, with a maixmum reception distance of 463 km. I’m confident that by adding some filtering, an LNA and maybe a small yagi antenna, we can get more than double of this.
You can also find live information on our Marinetraffic station page.

AIS messages from boats near the Trondheim coastline. Vassfjellet receiving station in lower right.

It’s very nice to finally get some traffic over the link. Over summer we’re hoping to expand with more monitoring services, but that’s a story for another blogpost.

LA3WUA demonstrating a patented LA1K antenna hoisting method.