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Beacons: collection of technical details

Recent additions of LA2VHF/4m (2015) and LA2SHF (2018) to our beacon park at Vassfjellet has increased the number of beacons ARK maintains to a total of 4 – from 2 and then 3 to “many”. Unfortunately, information about these beacons is scattered around the blog at la1k.no, and finding information about the frequency or transmitted signal is a challenging search activity (though luckily mostly contained within the beacon tag). But challenging no more! We’ve constructed a new page at https://www.la1k.no/beacons which lists the information in an orderly manner along with the expected transmitted signal and some history, which we hope will make life easier both for ourselves and others.

Beacon containment cabin at Vassfjellet. Photo: LA3WUA.

All our beacons transmit a morse signal at a regular interval. The beacons have been useful for the study of propagation conditions at the covered bands, and for debugging and measurements of our antennas.  We plan for the future to extend to a 6m beacon if we can obtain a license for it, as well as possibly covering the entire 1-10 GHz range. We’re also making plans for extending the transmitted signal from a simple morse signal to other digital modes like PI4, to enable easier decoding under weak propagation conditions.

Beacon rack: LA2VHF, LA2UHF and LA2VHF/4m from top to the center of the rack. LA2SHF has been left outside in the cold/on the table. LA2SHF’s sleeve dipole antenna can be seen in the white tube to the left. Photo: LA3WUA.

Like already mentioned on the page: If you hear any of these beacons, let us know! We appreciate reports on DX clusters, or direct contact through email. DX cluster reports or emails from operators who have heard our beacons are invaluable in investigating propagation phenomena.

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Beacons

ARK develops and maintains radio beacons from JP53EG at the top of Vassfjellet, a local mountain. Each beacon autonomously transmits a morse signal on a specific frequency. We use these for debugging of our radio equipment and for investigating propagation phenomena. The beacons can be heard on the following frequencies:

Not in service / Historical beacons:

  • LA2SHF 1296.963 MHz (23 cm) (QRT: 2019)

If you hear any of these beacons, we’d love to hear about it! We appreciate reports on DX clusters, or you can contact us directly. Details on the equipment and transmitted signals follow below.

LA2VHF

Latest posts on LA2VHF

Multiple incarnations of LA2VHF have existed throughout the times. The current beacon in use was built by LA3JJA and LA8TKA in 1999, and has faithfully and mostly uninterrupted pushed out a long stream of timed morse sequences since then.

It has a directive antenna pointing towards the North, with the intention that back-scatter from Northern lights should reach Europe.

QRG Power Antenna Polarization
144.463 MHz 25W 6-element yagi (pointing towards azimuth 15°) Horizontal

The sent CW signal consists of “LA2VHF JP53EG” and a long tone.

LA2VHF/4m

Latest posts on LA2VHF/4m

LA2VHF/4m was developed by LA7VRA and LA3JPA, and installed at Vassfjellet in 2015. The beacon was based on a 35-4400 MHz CW exciter board designed by LA3JPA Jon Petter in 2012, which has been made open source on GitHub.

QRG Power Antenna Polarization
70.063 MHz ~25 W 1/2 wl vertical Linear (vertical)

The sent CW signal consists of “LA2VHF JP53EG” and a long tone.

LA2SIX

Latest posts on LA2SIX

LA2SIX was developed in 2018 by LA1BFA and LA3WUA. It was licensed in early 2019, and deployed for the first time in the summer of 2019. The beacon is built on top of the same beacon platform as LA2VHF/4.

QRG Power Antenna Polarization
50.488 MHz ~25 W Diplexed with LA2VHF/4 Linear (vertical)

LA2UHF

Latest posts on LA2UHF

LA2UHF was QRT for five years, until it was refurbished by LA3JPA and LA7VRA in 2015 using the same beacon platform as LA2VHF/4.

QRG Power Antenna Polarization
432.463 MHz 22W Covered 12-element yagi (pointing towards Trondheim) Horizontal

The sent CW signal consists of “LA2UHF JP53EG” and a long tone.

LA2SHF (Not in service)

Latest posts on LA2SHF

LA2SHF is out of service for the foreseeable future due to interference issues with a primary allocation user.

The LA2SHF license was obtained already in 1979. A working beacon was made in the 1980s, but had to be taken down due to interference with an air traffic control radar at Gråkallen. In 2017, the need for a 23 cm beacon resurged due to activity in the 1 to 10 GHz project, finally culminating in a working beacon in January 2018 thanks to work done by LA3WUA and LA1BFA. The beacon was installed at Vassfjellet in June 2018, but had to be shut off permanently in 2019 due to new interference issues with a primary allocation user. The beacon was built on top of the same beacon platform as LA2VHF/4.

QRG Power Antenna Polarization
1296.963 MHz ~30 W Sleeve dipole Linear (horizontal)

The CW signal consisted of “LA2SHF JP53EK” and a long tone.

Beacons

ARK develops and maintains some radio beacons from JP53EG at the top of  Vassfjellet, a local mountain. The beacons can be heard on the following frequencies.

LA2VHF/4m 70.063 MHz
LA2VHF 144.463 MHz
LA2UHF 432.463 MHz

If you hear any of the beacons we would love to hear about it at la2vhf or la2uhf@la1k.no.

 

Back in 2012 Jon Petter, LA3JPA, designed a 35-4400 MHz CW exciter board (seen in the left picture below) that is the main building block used in the LA2VHF/4m beacon. The CW beacon project is made open source and can be found on this github page.

The cabin that houses our beacons is placed near the foot of a 196 m tall telecommunications tower. When icicles fall from this height they have a tendency to pierce the roof on our cabin, therefore we reinforced the roof with steel plates (building progress seen to the right above) in the summer of 2016.
We also got a working 5.8 GHz data link between the cabin and our main shack at Samfundet. As soon as the snow melts and we have access to the mountain top again we will work on improving the stability of this link.

Upcoming changes

We also have some other exciting changes to the beacon setup. In the recent years we’ve become particularly interested in the 6m band, dubbed the magic band for the way it suddenly opens and closes. To get an indication of when conditions are good we are hoping to expand our lineup with a 6m addition to LA2VHF in the summer of 2017.
Actually we’re well underway, we’re just missing power amplifier (PA), band allocation and final integration at this point. On the PA side Jotron donated some power transistors and matching 28 V supplies, speeding up the process immensely. Another blog on the design of this PA will pop up in the months to come.

On 70 cm we’re changing the antenna from a 10 element yagi to a big wheel antenna. This is because the main mode of propagation is likely to be via tropospheric ducting, where the antenna gain at each side is not the limiting factor. The big wheel antenna is an in-phase stack of three horisontal loops, yielding an omnidirectional horisontal pattern, with improved gain compared to a single loop. This we believe will improve the chances of this beacon being heard out there as the improved tropospheric volume coverage by going from narrow beam to omnidirectional is considerable.

The big wheel is also a prime candidate for the 6m and 4m beacons, this is primarily because most operators on these bands use horisontally polarised antennas. For 2m the main mode of propagation is aurora scatter, where the antenna gain does matter. So we will stick with a yagi for this band.

Vassfjellet remote access: part 1 – the link

As mentioned several times on our blog, we have a cabin up on a mountain top called Vassfjellet, sometimes referred to as VFJ. This cabin houses all our beacons, which provide amateurs all over the world the facilities to check band conditions towards Europe and other countries in reach.

The antenna at our QTH. The radio right behind it was used only for testing.

In addition to the beacons, we have also put up a 5.8Ghz WiFi-link to provide the cabin with internet access. This point to point link starts all the way back at our main QTH, Studentersamfundet. This is a fairly long stretch, and shooting the link through an urban environment adds additional challenges. The link margin is quite low because of this. There are a number of devices in our cabin which require a internet connection to operate, including an ADS-B and AIS-reciever, and more recently a couple of cameras and a relay-box.

The fairly long link, 18km in length

The cabin was mostly without any internet access for the duration of 2018 and up until early 2019. The main culprit was the enclosure we had chosen to put our wireless radio in: it was too waterproof. The problem was that when rainy weather and melting snow came, water followed the wires through the enclosure and to the radio, slowly filling the enclosure with water. We speculate that the radio was left this way for the majority of its time, as the end result didnt seem to prove otherwise.

Definitive signs of water. Photo: LA1WUA
Melted RJ45-connector, arguably not the best smell

Needless to say, the radio was as dead as it looked. The soloution for this could have been rather simple; by drilling holes in the bottom of the box we could have let the excess water drip out instead of flooding the radio. Luckily we had another backup radio, and used that to re-establish our connection to Samfundet. But lesson learned – proper care and attention will be taken the next time we mount anything outside up at Vassfjellet.

The new setup, consisting of the Rocket M5 radio, a heavy-duty Ethernet-switch and some power supplies. Please excuse the poor wiring.

In our next try, we decided to mount the radio indoors, again. The reason we wanted the radio as close to the antenna as possible the last time, was to minimize any output loss through the cable. The ‘old’-setup had RG58-cable feeding the antenna, which at 5m in length gave us a pretty low amount of power radiating out of our antenna. This time we went for a much better coax, and moved the WiFi-antenna to the other mast that was set up for the new beacon antennas in 2017.

The empty mast and WiFi-antenna, ready to be joined together

We are unable to recall whether or not moving the antenna increased the signal at our QTH, but we can report having no problems with it so far. That is, except for a few configuration mishaps that required us to take another trip up to the cabin. In the summer and early autumn this isn’t too much of a problem, but during winter and early spring we keep our trips to the mountain at a minimum.

“Risk of falling ice”

The conditions during the winter usually lead to ice forming on the 200m high mast right beside our cabin, which makes it a real hazardous operation to perform any maintenance here during the season. Driving up here usually is no-go due to the huge amount of snow usually present during the winter. This only leaves us with half of the year to actually visit the facilities, and requires us to think forward for any problems that might occur during the winter.

One last look at the cabin before leaving for the next season

This time we tried placing any untested projects up inside the cabin so that we have the winter to check for any flaws or conditional issues before deploying them fully. In a coming post we’ll get to the equipment we have behind the WiFi-link at Vassfjellet, including two web-cameras and a relay-box planned to control the beacons.

Wideband immunity issues at an RF-crowded site

Recently, we were made aware of plans another radio club has to install a voice repeater at the Vassfjellet mountain summit where our beacons reside. They have performed a survey at the site, and it seems they will try to set up a six meter repeater there.

As mentioned in previous posts, we operate four beacons at the site. The beacons operate in the 6 m, 4 m, 2 m and 70 cm bands.

We don’t know too may details from their survey, as they haven’t made contact with us. We became aware of it by getting hints of a blog post they wrote after doing the survey. The survey was primarily performed with an FT-857D and a four element phased vertical dipole. They claim that the beacon splatters all over the 2 m band – resulting in 2 m being unusable both for repeaters and other users at the summit.

We take complaints and criticism seriously, and we want to work with all interested parties to find good solutions. We immediately went up to measure the beacons properly, and to address any issues with unwanted or excessive radiation.

Our survey indicates that LA2VHF, or any of our beacons, are not the cause of the interference, and that the issues observed by the other club are spurious components generated by some receivers when exposed to high signal levels.

Survey equipment

Our own survey was performed with an R&S FSH3 portable spectrum analyzer, a reference 2 m dipole antenna, and a set of different 2 m ham radio transceivers as control (FTM-350, TH-D72E, VX-8DR, VX-5R).

LA3WUA operating the FSH3 spectrum analyzer with attached 2 m dipole antenna. Credit: LA3WOA

The 2 m beacon (LA2VHF)

The current incarnation of LA2VHF was installed in 1999/2000, so it has been operating for about 20 years. The exciter is semi-commercial, based on a modified AIS-transmitter board. We know it has some phase noise close to the carrier, and that it is not capable of fully suppressing the FM modulation at the start of every “dit” and “dah” in the CW sequence. The added FM modulation leads to a slightly wobbly sound.

To check the purity of the beacon, we attached it directly to the spectrum analyzer via 60 dB of attenuation. Samples were taken using the max hold function of the spectrum analyzer, and sampled until all spurious signals reached maximum level.

After correcting for the attenuation at the spectrum analyzer input, the phase noise is found to be better than -140 dBm once 100 kHz off from the carrier.

With 100kHz span (right picture), we clearly see the FM modulation component in the upper sideband from the carrier. We also see that the FM portion is not unreasonably wide, with a deviation of approximately +5 kHz. Finally, the phase noise 10 kHz off from the carrier is roughly -95 dBm.

Since the beacon has a phase noise below -140 dBm at 100 kHz it is safe to assume that it is not the cause of the issues observed by the other radio club. Only the best transceivers available on the market can boast a sensitivity rating of -140 dBm in the VHF section.

Handheld transceiver behavior at Vassfjellet

While doing our measurements at Vassfjellet, we also took the opportunity to get some clarity on the issue observed with handheld transceivers. From 40 years of operating beacons at the site, it is well known to us that some 2 m radios are susceptible to strong interference at the beacon site, while others are not.

To make controlled tests, we turned off the beacons, and powered them one-by-one to see what might be causing the issue. Interestingly, the transceivers that have noise issues in the 2 m band still experience the same noise when the 2 m beacon LA2VHF is turned off. In one of the transceivers, careful inspection revealed that the CW sequence actually belongs to LA2SIX.

With strong signal levels, there are many potential sources of saturation or intermodulation in a receive chain. One source is that quite a few modern rigs have a first intermediate frequency at frequencies such as 48.6 MHz and 68.4 MHz. As we operate beacons at 50.488 MHz and 70.063MHz, these may inject directly into the mixer, and cause wideband problems.

Experiment

To prove that a strong signal out of band is capable of causing such issues we set up an experiment in the lab.

A signal generator was used to generate a leveled 50 MHz tone with an output power of -15 dBm into a Yaesu VX-5R handheld transceiver. The audio output of the transceiver was monitored using an oscilloscope. The VX-5R was operated in narrow FM mode, tuned to a frequency of 145.0 MHz and set to squelch level 4. As seen in the photo below, turning the generator on and off brings the squelch wide open. We also repeated the experiment with less output power, and got similar results, but less repeatability once the 50 MHz tone power was low enough.

50 MHz immunity experiment.

This experiment shows the rudimentary functions of the fault. In order to fully diagnose the interference issue, we would need direct access to the various stages of the receive chain.

Some theoretical notes

In order to see that the signal levels from our lab experiment is in line with what might be expected at the site, we must perform some calculations.

The Friis’ equation allows us to calculate how strong the beacon signals are at some point away from the transmitter. In this case, the original survey was done at a distance of 127 meters from the beacon antenna, with a 4-element stacked dipole array. The beacon antenna is a 10 element yagi, pointing at 15 degrees. The angular distance between the beacon main lobe direction and the survey site is 47 degrees.

We estimate 6dBi gain in the vertical plane from the stacked dipole, and a loss of somewhere around 20dB due to polarization. The beacon antenna gain is, conservatively, 10dBi in the front direction, with about 8dB less at 47 degrees.

Friis’ equation:
P_r = P_t + D_t + D_r + 20  log_{10}{(\frac{\lambda}{4 \pi  d})}
where: P_r is the power at the receiver end (dBm)

  • P_t is the transmitter power (dBm)
  • D_t is the transmitter antenna gain (dBi) in the direction of the receiver
  • D_r is the receiver antenna gain (dBi) in the direction of the transmitter
  • \lambda is the wavelength (meters) is the distance between the transmitter and the receiver (meters)

We fill in the measured data from the test site:

P_r = 44 + 4 + 2.15 + 20 log_{10}{(\frac{2}{4 \pi 120})}
P_r = 50.15 - 57.54 = -7.39dBm

The theory was verified by direct measurements at 144.463 MHz which yielded about -8 dBm at the position seen in the photo below:

LA3WUA measuring signal strength in the direction of the stacked dipoles, which are also seen in the background. Credit: LA3WOA

Similar calculations for 6 m and 4 m yield received results on the order of -10 dBm to 0 dBm (~S9+90 dB), assuming – 10 dB gain in the stacked dipole antenna. This means that our 4 m, 6 m and 2 m beacons are all capable of delivering powers that are beyond what we used to reproduce the issue in our lab experiment.

Conclusion

In conclusion, the beacon LA2VHF is not the source of the problems observed by the other radio club. The problems relate to wideband immunity issues within the receive chain of some radios when exposed to very high signal levels. In general, these issues are best solved by sufficient filtering, or attenuation at the receiver input.

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