Akademisk Radioklubb

LA1K / LA1ARK / LA1UKA

Category: Equipment (page 1 of 3)

Securing the antenna park for Ylva

Norway is often/sometimes plagued by fierce autumn/winter/spring/summer storms. Here in Trondheim, we’re usually well sheltered from the worst storms due to the city’s location within a fjord. This results in a rather tempered climate (as per Norway’s standards), with a mixture of mild rain, mild winds, mild snow and occasional sunlight being the norm throughout a normal hour in Trondheim. This is usually more of a concern for the frustrated inhabitants rather than a concern for our antennas on the roof of Studentersamfundet. Once a fierce storm actually hits Trondheim, however, the worries intensify.

Storms in Trondheim often coincide with the late fall. Shorter days and the leaves falling off the trees function as a pavlovian trigger for all former station masters of ARK to start waking up in a cold sweat every slightly windy night. According to previous station master LA5GKA, it takes approximately 4-5 years for a station master to stop worrying about the antennas at ARK when the wind speed increases. (This also has a weak correlation with the average time it takes for the average station master to get married and start producing children and get other concerns in his life. :-))

ARK has some experience with bad storms. In 1992, for example, we had “Nyttårsorkanen”, which bereaved ARK of our previous attempt at a satellite dish. During the recent years,  we’ve had the storms named “Dagmar” in 2011 and “Ivar” in 2013. Dagmar blew some of the elements of our VHF/UHF array off the roof, requiring us to do a major rebuilding of the entire array during 2012. This was not finished until early fall 2012, and the array was not fully operative until much later. (The array was later replaced by a brand new UHF/VHF-array in connection with a pre-project for LA3WUA’s master thesis at NTNU).

The previous UHF/VHF array during a rehaul in 2011. Maybe extra bitter that parts of it got blown down half a year later. Photo: LA2USA Espen Molven, from http://bilder.la1k.no/antennejobbing11.

With the long period during which the UHF/VHF array was QRT in mind, this naturally made us a bit nervous the next time a major storm hit Trondheim. We probably went a bit overboard with the antenna securing. For “Ivar” in 2013, for example, we secured all of our arrays with extra ropes. This was probably not necessary, ropes wouldn’t have saved the arrays from becoming destroyed, but they would at least have ensured that no arrays could fall off the roof and damage something/someone else. Our antennas survived, however. LA2T, the other local ham radio organization here in Trondheim, weren’t so lucky.

We have later taken a less nervous approach to antenna securing, but we always do measures if the wind reaches above 20 meters per second. The next major storm was going to be “Ylva” around the 23rd of November. The forecast said that the temperature would increase from -10 degrees Celcius to +10 degrees Celcius, and that the wind speeds would reach above 30 m/s. Our station master, LB5DH Henrik, therefore made the necessary preparations on Wednesday the same week.

Antennas aligned with the expected direction of the wind.

The HF array mast was winched down to its lowest position, and all array antennas were aligned with the expected wind direction, to lessen the impact of the wind and reduce any vibrations. The satellite dish was rotated in such a way that the wind load would be as little as possible, i.e. with the dish pointing straight upwards towards the sky.

There was also one extra problem with the satellite dish: The rotor pole had already gotten slightly bent.

Photo: LB5DH Henrik Dobbe Flemmen

With the experience with the previous satellite dish in 1992 in mind, we therefore were extra nervous, and were determined not to let the satellite dish be destroyed by the first and best storm, like what happened to the previous dish in 1992. We therefore made some special preparations for the satellite dish by securing it to its pole using straps.

Photo: LB5DH Henrik Dobbe Flemmen

Photo: LB5DH Henrik Dobbe Flemmen

In the end, it turned out that the northern parts of Norway got the worst parts of the storm, and that the winds in Trondheim were only slightly mild at best. We’re still on the guard, however, as always. Remote control over the antennas makes it more convenient to adjust for changing wind directions, and we’re also thinking to invest in a rotating webcam to better monitor the antennas.

Aligning the antennas with the wind speed to avoid vibrations and resonance effects, and doing regular maintenance to avoid loose bolts is probably our best bet for preparing for storms. While not exactly excited to see whether our precautions are enough, we still have mild optimistic feelings for the future and a vague hope that our antennas should survive to see another year.

Anechoic measurements of RF Hamdesign’s 5 band ring dish feed

To quickly get operational on the amateur bands between 1 and 10 GHz with our new 3 m dish, we have purchased a 5 band ring dish feed from RF Hamdesign. The ring feed antenna is a loop over a ground plane, and a multiband version of this antenna can be made by stacking several loops inside each other. A good technical article on the construction and theory of these antennas is presented by Galuscak and Hazdra, and build details are found in the long version.

We purchased the 5 band ring feed as it seems to be a decent compromise between multi-band coverage and ease of use. It even allows for legal limit operation (in Norway) on the bands that require it. However, beyond frequency coverage, return loss and power handling there was not much in the way of specifications available. Therefore we have conducted some measurements in order to allow other amateurs to make a more educated guess at what they can expect from this multiband dish feed.

Thanks to LB6RH we were able to measure the antennas in an anechoic chamber at the local university, NTNU. We have attempted to find the 3 dB and 10 dB radiation angles in order to see how well it should perform when used to illuminate a dish.

We set up the test using a ETS Lindgren Model 3117 horn as the test antenna. The pattern is measured by turning the Device Under Test (DUT)- platform, where the 5 band ring dish feed is placed on in the image above, by use of a mechanical rotor. At each azimuth point a network analyzer measures the isolation between the test antenna and the DUT-antenna. Once the DUT has turned 360 degrees a relative pattern is derived.

Horizontal polarisation:

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Vertical polarisation:

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Finally we compare these results to the 3 dB and 10 dB values that come attached when you buy the antenna.

Comparison of measured and specified horizontal radiation angles:

Frequency [MHz] Specified H 3 dB angle Measured H 3 dB angle Specified H 10 dB angle Measured H 10 dB angle
1296 67 65.3 111 N/A
2320 59 65.3 104 124.0
3400 46 53.7 107 97.6
5760 44 40.7 109 108.6
10300 44 N/A 109 N/A

Comparison of measured and specified horizontal radiation angles:

Frequency [MHz] Specified V 3 dB angle Measured V 3 dB angle Specified V 10 dB angle Measured V 10 dB angle
1296 72 80.2 144 155.2
2320 68 95.0 132 163.6
3400 43 67.9 94 119.0
5760 44 37.9 91 90.4
10300 44 N/A 91 N/A

Although it is possible to read a radiation angle out from the plots, we chose to let a script perform the measurement for us. The script will fail if it is unable to determine a single peak, which indicates lobe skew or split. Some caution should be shown when considering the measurements that yielded N/A results.

The measurements fall reasonably close to the specified values, although there is a significant difference in the cleanness of the horizontal and vertical patterns. Based on these results we will be using the ring feed in vertical mode. In conclusion the RF Hamdesign ring dish feed seems to be a good and reasonably priced way to get  operational on many GHz range bands quickly.

Kuhne amplifiers and 1 to 10 GHz stage three

ARK is currently working on a project that will allow us to work Earth-Moon-Earth, satellites and various scattering modes on the amateur bands between 1 and 10 GHz. Our solution uses a 3 m parabolic dish together with a set of discrete amplifiers, the entire system is excited by a USRP SDR.

We have split the 1 to 10 GHz project into four sections. Up until now we have completed stage one and two, while stage three and four are still remaining. The stages are roughly:

Stage 1: Literature study, ordering of components – Further details in “An update on the 1 to 10 GHz project”

Stage 2: Construction of the parabolic dish and mast – Further details in “3m parabole dish ready”

Stage 3: PCB development and integration onto parabolic dish – Further details in this blogpost 😀

Stage 4: Long term projects with the dish, software, amplifiers, new antenna feed – Further details in the future.

This is a good time to elaborate more on our plans for stage 3 as the amplifiers that will be used for the project just arrived! We have purchased:

144/432 MHz IF to 10.5 GHz mixer – MKU 10 G4, 3 cm Transverter

To keep the work more organised we have split stage three into four sections. The sections are three PCB-design projects and one final assembly of all the components at the back of the parabolic dish.

1: Wideband driver amplifier
The Kuhne amplifiers and transverter seen in the previous section, will bring the output power to the level that is required to achieve Earth-Moon-Earth communications in the amateur radio bands. In order to be able to excite the Kuhne amplifiers and transverter from a USRP SDR with 10 mW max output power, an intermediate stage is required. The next step is to design and construct this amplified.

Very simplified schematic of 1 W driver.

We have conducted a study of available parts, and concluded that it is indeed possible to create an amplifier that will deliver 1 W across the frequency band from 0.1 GHz to 6 GHz.

In the figure above an example schematic using Guerilla RFs GRF4001 together with Analog Devices HMC637LP5 is shown. The device will deliver 1 W across 0.1 GHz to 6 GHz. This will allow exitation of all amplifiers, as well as the transverter that enables 10 GHz coverage. The gain should be on the order of 30 dB in order to avoid operating the USRP at its saturation power, where it is known to be quite noisy.

2: Wideband low noise amplifier
Another component that we want to develop ourselves is the low noise amplifier (LNA). There are not many good and cheap LNAs available for the amateur radio market, despite there being integrated circuits that boast very good performance for this application. If we are able to make an LNA and provide the design notes as open source, it will likely be beneficial for many people.

The LNA is one of the more challenging circuits. It needs to work using relatively cheap equipment while being largely immune to electromagnetic noise. A lot of work will likely be spent on making the supply-lines that power the HMC753 LNA circuit noise free, as well as ensuring that the metallic shielding is sufficiently tight. Another consideration is that the amplifier must be able to sustain relatively high input powers that will leak through the coaxial relays during transmit.

Outline of LNA.

The figure above shows a draft for a test assembly for performance testing of HMC753, which is a device that could be used in our LNA.

3: Controller board
Interfacing with the amplifiers will be handled through a controller board that communicates with either a computer or the USRP directly over the serial protocol RS232. The interface board is responsible for managing power supply states for all amplifiers as well as startup sequencing.

A set of RF relays are used to select which of the discrete amplifiers should be connected to the different points along the circuit. These are available as surplus devices on auction sites such as eBay.

Essentially, the interface board is responsible for ensuring that all connections and devices in the figure below are connected and powered correctly for a given configuration. It should also be able to alter the configuration in a rapid way.

Relays, amplifiers, transverters and SDR connection diagram.

 

4: Mechanical integration

After the three sections above are complete, the mechanical integration of the RF system onto the dish can start. This is an extensive effort as there are many concerns to deal with. Thermal management and waterproofing are two likely issues. So far we have an idea revolving around a gutter heater solution to keep the system from freezing during the winter. To keep the system cool enough we are experimenting with different heatsinks and weatherproof fans (IP68).

We hope to have the first three stages finished by the end of november, and the mechanical work started and delivered some time early next year. The time it takes to develop the PCBs gives us a good chance to secure the final funds that are required for the mechanical work (cabinets, fans, heatsinks). Overall we are really excited to see the project taking shape.

Before we finish all the sub-projects in stage three we might try to work some contacts using the 23cm module on our IC-9100, a MKU 131 AH 23 cm LNA, coaxial relays and the 200 W 23 cm PA we just bought. More on that in a later post.

Repairing broken receiver on a USRP N210 WBX40 daughterboard

Kimmo Kansanen at NTNU recently donated some USRP N210 units they no longer had a use for. We have started to use them for various communications experiments, for example estimating the antenna patterns of our VHF/UHF antennas against the LA2VHF and LA2UHF beacons.

While operating one of the N210s we were sudddenly unable to receive the LA2VHF beacon that had previously been easily decodeable. We also saw that the noise floor had increased by 20 dB. Jens, LB6RH, decided to investigate matters further.

Inside the N210 a WBX-40 daughtercard provides the RF-frontend. The device functions as normal, except for in receive mode. The receiver section of the WBX-40 should be a good place to start looking.

The WBX-40 daughterboard after removing coaxial patches that attach to the front panel of the N210.

To further investigate what may be wrong we started investigating the board for any obvious short circuited connections. We were unable to find any such sources. The next step was to probe around the board with a multimeter to check the voltages being generated from the different voltage regulators on the board. Since Ettus Research provides the schematic for the WBX-40 online this process was greatly simplified.

LB6RH found that the 3.3 V rail on the output of voltage regulator U308 was only at 1.2 V. Suspecting that something was wrong with U308, he removed the component and attached a laboratory power-supply to the 3.3 V output pin. After turning on the supply (with the current limit set low, to avoid frying the circuit) he noticed that there was still a short circuit. To identify which component caused the short we borrowed a FLIR thermal imaging camera to check what components got hot when we turned up the current limit on the power-supply.

Unfortunately we did not get a good picture of the thermal test, but the chip that got hottest was the Low Noise Amplifier (LNA) U313. Since our problems are related to poor reception we thought this might be a likely candidate. After removing U313 we turned the power-supply back on, and saw that there was no longer a short, hurray!

A closeup after removing components U313 and U308.

We ordered new components for U308 and U313 from Digikey, and soldered them back in place.

U308 – Analog Devices Inc. ADP3336ARMZ linear regulator

U313 – Broadcom Limited MGA-82563-TR1G MMIC broadband LNA 

Replacements for U308 and U313 have arrived.

The soldering battlestation. A microscope helps when soldering small parts.

 

 

 

 

 

 

 

 

 

 

 

 

 

After replacing the two parts we connected everything back and tried powering on the device again.

We are now able to receive LA2VHF again!

Unified software rotor control over the local network

ARK has recently collected all rotor controllers on a single Raspberry Pi-device and made these available for software control from all Linux machines on the local network. We’ve also enabled rotor control from N1MM on Windows. This post outlines how we did it.

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