Akademisk Radioklubb

LA1K / LA1ARK / LA1UKA

Month: December 2017

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.