Akademisk Radioklubb


Author: LA3WUA (page 1 of 2)

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.

An update on the 1 to 10 GHz project

ARK is making an open source ground station that is compatible with GENSO and SatNOGS. We want to focus on documentation, making it easy for others to adopt.

We are attempting to approach this problem from a radio system perspective. We want to give a guideline to set up a modular distributed network node. The node should support common ham activities in the bands covered, such as EME and satellite work.

The ground station will cover 1 to 10 GHz and be RX and TX capable.

Block schematic of the project

The reference ground station will support up- and downlink covering 1 to 10 GHz using a cross-polarised dish feed. A dual channel software defined radio is used as the central transceiver, allowing for a wide range of applications to be programmed on the fly.
By using two radio channels and two polarisations we want to control polarisation schemes dynamically in software. This will allow for optimization of a large range of satellites. Using this setup we aim for access to linear vertical and horizontal, right and left handed circular as well as some elliptical polarisations.
This ground station (and network) could be interesting for amateur radio groups, CubeSat initiatives and research projects.

The Antenna(s)

We are going to use a parabolic dish antenna with around 30 to 45 dBi gain, increasing with frequency. The parabolic dish will be a 3 m kit by rfhamdesign. In the long run we might attempt a custom parabole design that aims to lower costs even further, utilizing machined or 3D-printed parts and metallic mesh.

At the core of the project at this stage is the choice of feed antenna for the dish. We want a feed that has the following attributes:

Continuous coverage over 1 to 10 GHz, S11 < -10 dB: Covering the ham bands from 23 cm to 3 cm. There are also a lot of interesting satellites in these frequencies.

Stable phase center over the frequency range: The phase center is the apparent center of radiation, for a feed antenna this is particularly important as it decides where the antenna should be placed in relation to the dish. If the phase center shifts with frequency the main lobe of the antenna will also change with frequency, making calibration and gain optimization hard.

Constant antenna pattern over the frequency range: Similarly to the previous requirement it is desireable to have a stable main lobe. Distance to the dish, as well as a backwards ground plane will have to be considered.

100 W power handling: In Norway this is the legal limit output power for the bands 23 cm to 3 cm.

Dual linear or dual circular polarisation:  By having either of these combinations it is possible to combine the remaining polarisations as well as a set of elliptical polarisations.

We are looking into several antenna topologies. Luckily there’s a lot of research available for ultra wideband (UWB) dish feeds, as they are very common in radio astronomy. Our design differs a bit from these however, since we also intend to use the antenna for transmit applications. The main difference is the power handling, as radio astronomy applications usually are RX only.

So far we have found two promising designs, and are running simulations to get the exact dimensions.

Dual linear spiral antenna

The first of the two is the dual linear spiral antenna. This is a planar antenna that is realizable on printed circuit boards. For additional power handling machining the elements may be a better choice. For high frequencies the antenna has to be scaled in a way that leads to lots of tight gaps between metallic elements, for high power levels this will cause spark-gaps, and the antenna will not function as intended.

Dual linear spiral antenna

Dual ridge vivaldi antenna, alternately called quad ridge horn antenna:

The more promising of the two designs is the horn antenna, as several commercial designs that fit our specification already exist. One such design by Schwarzbeck Mess-Elekronik is shown in a picture below. This antenna covers 0.4 to 10.5 GHz with an efficiency of 90% or better. It also handles 200 W.

We are looking forward to learn more about the design as we progress with our own.

A dual polarised horn by Schwarzbeck Mess-Elektronik

Getting funding

The other thing that we have been working with at this stage is getting funding for development. Transistor costs alone are so high that we couldn’t do this on our own.

We would like to give a big thanks to NTNU IES, KSAT, Marlink, Sit, Radionor, Jotron and Kongsberg for making this happen!

What’s next?

We have prepared a  lamp post mast that will serve as an attachment point for a tiltable mast, which LA2USA is designing. The lamp post previously housed ARKs 5.5m spoil in the early 90s, seen in the photo below.

The story of the old spoil ended with a winter storm, which is why we’re making some changes to the mast. Hopefully by tilting it down during windy periods it will catch less of the harsh winds. This also comes with the benefit of easier maintenance since the feed will be more accessible.

We have also ordered a rotor and 3 m mesh parabole kit from rfhamdesign. Building will commence when parabole kit and the LA2USA mast arrive here mid June.

As the design on LNA, PA, antennas and software continues there will be more updates.

New hams

After the test last Wednesday there are 11 new amateur radio operators. We are proud to announce:

Martin Hergot Festøy: LB7AH
Ken Are Meisler: LB7CH
Ole Christian Tvedt: LB7DH
Håkon Eide: LB7EH
Haavard Knibe Fiskaa: LB7FH
Anders Liland: LB7GH
Anders Selfjord Eriksen: LB7HH
Dennis Skulbru Eriksen:  LB7IH
Einar Uvsløkk: LB7JH
Svein Ove Undal: LB7KH
Ragni Helene Halvorsen:  LB7RH

You will recieve a letter from NKOM with the final details.

Congratulations, we look forward to hearing you on the air.

Measuring coax length with burst generator and oscilloscope

I have a quite long Aircell 7 cable that I would like to know the length of, but didn’t want to uncoil. This is a good opportunity to showcase a technique for measuring the length and attenuation of a coaxial cable, using a function generator and an oscilloscope.

Fig 1: Time delay for RG58 patch cable

Measurement background

Using a function generator in burst mode we can measure the reflection from the open end of a coaxial cable. An oscilloscope is connected through a t-junction between the function generator and the test cable. Since the internal resistance of the oscilloscope is high, and current prefers the path of least resistance, and the burst signal will travel to the coaxial cable. A small amount of the signal will coupled to the oscilloscope. We denote this as the incident voltage, U.
Upon reaching the open end of the cable, the wave will reflect and travel back towards the function generator. As the wave passes the oscilloscope a small amount will be coupled. We denote this as the reflected voltage, Ur. The reflected wave finally dissipates when it reaches the function generator.

This is very similar to tying a rope to a pole, swinging it and having the rope reflect back.

Fig 2: Measurement setup

The time difference between Uand Ur is the time it takes for the wave to propagate to the open end of the coax and back again. Using this we can calculate the length of the coaxial cable using the following formula:

Vf is the velocity factor of the coaxial cable and c is the speed of light. Since the time between incident and reflected is the round-trip time we divide the result by two.

By seeing how much the voltage has dropped on the reflected wave relative to the incident wave we can calculate how much loss the coax has at generator frequency. Since the reflected wave passes through the cable twice we should divide by two to find the one-way attenuation.

Some measurements

As mentioned, I have quite long Aircell 7 cable that I would like to know the length of, but didn’t want to uncoil. To keep everything neat I used a short RG58 cable to patch it together. This is the setup shown in figure 2.

The two cables are made using different dielectrics, and will have different velocity factors.  Aircell 7 has a Vf of 0.83, RG58 has a Vf of 0.66. To account for this we should first measure the delay and attenuation caused by the RG58, and then subtract that contribution from the Aircell 7 measurement.

To be able to measure on the short RG58 cable I am using a 1 cycle 100 MHz sine wave burst. The burst is set up to repeat every second, this means that any remaining oscillations should have fully died out. A generator frequency of 10 MHz is sufficient to get accurate results, but only if the cable you are measuring is longer than 20 m.

Fig 3: Unknown length Aircell 7

Fig 4: Voltage drop of RG58 patch












Figures 1 and 4 show the measurement results from the RG58 patch, we put the results into the formula and get the following results:

I also measured the RG58 coax with a measuring tape, and found the physical length to be 1.55 m.

Fig 5: Voltage drop Aircell 7

Fig 6: Time delay Aircell 7










Finally using the results from figures 5 and 6 we find the length and attenuation of the Aircell 7 cable

In conclusion this method is a quick and efficient way to measure the loss and length of a coaxial cable. If you have a broken cable the breakage point will also reflect, so this can be a very useful tool to pinpoint where you need to mend the cable.
It should also be said that the accuracy of this method depends largely on the accuracy of the velocity factor given in manufacturer specifications, meter order deviations can easily arise from a wrong spec. The influence of the oscilloscope could also matter, some people connect 10X or 100X probes to the t-junction for these measurements, I found it to be fine using just the internal impedance.

New website

We’re proud to present  our new website based on the wordpress platform!

New features include:

  • What you see is what you get editor
  • Comment section
  • Tags and categories
  • Sensible archive
  • And more…

While prototyping we’ve been writing a series of blog posts to test the new functionality,  we hope that you take the time to read these. We’re very satisfied with the new editor, and will be blogging regularly about our activities.

If you need any of the content from the old site it can be found at old.la1k.no.

We would love to hear any feedback about the new site in the comments section.

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