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.
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:
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.
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.
6-element yagi (pointing towards azimuth 15°)
The sent CW signal consists of “LA2VHF JP53EG” and a long tone.
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.
1/2 wl vertical
The sent CW signal consists of “LA2VHF JP53EG” and a long tone.
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.
If you hear any of the beacons we would love to hear about it at la2vhf or email@example.com.
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.
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.
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.
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.
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.
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.
where: is the power at the receiver end (dBm)
is the transmitter power (dBm)
is the transmitter antenna gain (dBi) in the direction of the receiver
is the receiver antenna gain (dBi) in the direction of the transmitter
is the wavelength (meters) is the distance between the transmitter and the receiver (meters)
We fill in the measured data from the test site:
The theory was verified by direct measurements at 144.463 MHz which yielded about -8 dBm at the position seen in the photo below:
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.
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.
Previously we have written about troubles with spurious emissions from our LA1K CW Beacon based beacons (LA2VHF/4, LA2SIX, LA2UHF and LA2SHF). We first became aware of the issue when we discovered some very interesting sidetones on the LA2SHF beacon. Later we were also alerted by LA0BY that the same 63 kHz tones were appearing in the skirts of our 4 m beacon LA2VHF/4. These issues are now fixed (actually this has been fixed since we deployed LA2SIX in the start of July), and this post details how we found and eradicated them.
For a while we were stumped as to the origin of these birdies (spurious emissions), but then LA9SSA came to realize that the CW message in the birdie appearing in LA2SHF was different from the primary CW message. The birdie was actually pumping out “LA2VHF/4 JP53EK”, the morse code sequence of our 4 m beacon.
With one beacon appearing in the spectrum of another it was obvious that some sort of feedback was happening, but the exact mechanisms were unclear.
One helpful hints that we had were that the 63 kHz spurious sidebands were not appearing when we had the beacons on the lab bench. Further, we were not able to find any such spurs in the spectrum from our 2 m beacon, LA2VHF – indicating that the issue was isolated to beacons based on the LA1K CW Beacon platform (LA2VHF is an older beacon based on a modified FM-transmitter).
After finally putting two and two together, we figured that the issue only appeared when multiple beacons were co-located, and after further debugging that the issues only occurred when LA2VHF/4 was turned on.
We had at this point brought LA2SHF back to Samfundet, since its main signal was causing interference to a local installation. This allowed us to have a beacon to work on to test our hypotheses. Since the reference port and antenna ports are the only two that are available from the chassis, these were logical points to start at.
We started by trying to simulate feedback by coupling a combined signal consisting of a 10 MHz square wave and a 70.063 MHz sine wave into the reference port of LA2SHF. The result is seen below:
In the pictures above, we can see that the 63 kHz sidebands increase by more than 10 dB when affected by the 70.063 MHz signal. While the interference to target signal ratio might not be perfectly reconstructed in this orchestrated example, it gives an idea of how we might go about to amend the issue. Filtering at the reference, for example, successfully removed the injected signal at 70.063 MHz.
To shed some light on the actual mechanics of the birdie, we need to consider a couple of factors. Firstly, we consider the point where the RF output originates, the synthesizer ADF4351. In our design we copied the loop filter components from the reference design. This sets the loop filter bandwidth at an inconvenient 63 kHz. The synthesizer is vulnerable to noise within its loop filter bandwidth. This means that noise within +/- 63 kHz of the reference frequency is bad news.
Further, we must consider that the operation of a phase locked loop (a core component of a synthesizer) is primarily digital in nature, i.e. it works mostly with square wave signals. At the synthesizer input, however, it will accept either a sinusoidal or a square reference signal.
According to its Fourier series, a square wave is simply a sum of odd harmonics of a sinusoidal signal. This means that our reference frequency of 10 MHz will have a contributing component at 70 MHz.
Arriving at the end of our argument: the interferer LA2VHF/4’s frequency of 70.063 MHz may be considered a part of the reference oscillator 10 MHz square wave. However, it is shifted 63 kHz above where it should be. Since these 63 kHz are within the loop filter bandwidth of the synthesizer, we are in for a bad time. This creates a birdie which then propagates throughout the synthesizer, ending up at the output, and being keyed on and off as the 70 MHz beacon sends its sequence.
To alleviate our issues, we installed bandpass filters to the respective 10 MHz reference inputs (this happened back when we deployed LA2SIX, but was not detailed in that post for brevity). The keen reader might remember that we bought some 10 MHz band pass filters from SV1AFN a while back, and these were used to great effect here. After this we have had no further issues with 63 kHz spurs with our LA1K CW Beacon transmitters, mission successful!