Akademisk Radioklubb

LA1K / LA1ARK / LA1UKA

Category: Projects (page 1 of 3)

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.

Updates on the SSTV project

After a long break, Henrik LB5DH and I decided to start working on the SSTV project again.

This time we wanted to look on how we wanted to mount and secure the antenna up on the roof.

Handmade foot

We found an old foot on the loft, which seems to work well for our installation.
We later found that we could rest the antenna on the roof without it, but in the finished installation we will be sure to make use of it.

The antenna after taking it down

The mounting solution

For mounting the guy wires we drilled two holes through the pipe on the opposite sides of each other.
This way we got four points for securing the mast. Luckily we already had structures on the roof to secure it to.

Temporary length of wire added

The issues came when trying to tune the antenna. Even though the antenna tuned fine inside, it gave us different results with the best SWR around 100Mhz when we took it outside.

This drop was the closest to 1:1 ratio we could find.
We tried adding more spools, extending the wire etc with not too good results. We did have a drop at about 3-4 in SWR around 10-11Mhz, so we seem to be close.
It does seem like the antenna isn’t reading it’s full length, so we’ll have to do some more calculations before proceeding with mounting the antenna.

We’d also like to mention that the SDR dongle has arrived, and initial testing has showed us that the direct sampling technique will work adequately for us.
More info about this and the antenna is coming soon!

-LB0VG

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.

Introduction to the libpredict API

libpredict is an ANSI C library for predicting satellite orbits based on TLEs, developed by ARK. This was primarily developed for use in flyby, but can also be useful on its own. If you just want to track a satellite, flyby is usually a better choice, but if you want to go down to a deeper level and be able to apply satellite prediction to more advanced and complex usecases in a more flexible way, libpredict might be suitable.

The goal of libpredict was mainly to separate the satellite calculations from predict for use in its fork, flyby, and enable reuse of the API in other satellite applications. C implementation became a requirement due to the well-defined binary compatibility for C libraries and the use of C in both predict and flyby. While the core routines are in C, we will also at some point be providing high-level bindings for other languages like python. See also: Development of flyby and libpredict.

This post outlines in detail how libpredict can be used to track satellites in a programming language, and is long and technical and probably mostly for those with special interest in the topic. If life gets too frustrating and boring, you can scroll down to the plots and rest your eyes on colorful satellite tracks:-). An earlier post, Satellite tracking using flyby, gives a top-down motivation for why we are doing this at all and a more user-friendly approach to satellite tracking.

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