Akademisk Radioklubb


Month: March 2017 (page 1 of 2)


This weekend we participated in the CQ WPX SSB contest. The conditions were rough, but we had a lot of fun in the multi-two category.

Øyvind LA3WUA (left) & Henrik LB5DH (right)

This was our first serious entry with the new Flex 6500 in parallel with the old IC-756 Pro III. The panadapter on the flex allowed us to find open frequencies swiftly and easily.  When operating multi-two previously, we have had a lot of trouble with self-interference. To amend the problem, we recently purchased bandpass filters. The 4O3A bandpass filters for 40 to 10 meters were used for this to great effect. This allowed us to transmit and receive simultaneously, as long as we remembered to stay away from the direct harmonics. 🙂

The conditions were difficult, with no real openings on 15m or 10m, and subsequently no QSOs on these bands. We had no serious runs towards North America, and were only able to obtain some  S&P contacts. We did obtain an Alaska contact on the 40m greyline, however, which was fun.

There were lots of different operators on our side this time, thanks to everyone who showed up and thanks for the QSOs!

Harsh conditions resulted in low rates.

Call: LA1K
Operating Time: ~46 hrs
Band QSOs Pts WPX Pt/Q
3,5 130 298 76 2.3
7 580 1 275 257 2.2
14 673 849 311 1.3
Total 1 383 2 422 644 1.8
Score 1 559 768

New hams

After the test last Wednesday there are 11 new amateur radio operators. We are proud to announce:

Martin Hergot Festøy: LB7AH
Ken Are Meisler: LB7CH
Ole Christian Tvedt: LB7DH
Håkon Eide: LB7EH
Haavard Knibe Fiskaa: LB7FH
Anders Liland: LB7GH
Anders Selfjord Eriksen: LB7HH
Dennis Skulbru Eriksen:  LB7IH
Einar Uvsløkk: LB7JH
Svein Ove Undal: LB7KH
Ragni Helene Halvorsen:  LB7RH

You will recieve a letter from NKOM with the final details.

Congratulations, we look forward to hearing you on the air.

Measuring coax length with burst generator and oscilloscope

I have a quite long Aircell 7 cable that I would like to know the length of, but didn’t want to uncoil. This is a good opportunity to showcase a technique for measuring the length and attenuation of a coaxial cable, using a function generator and an oscilloscope.

Fig 1: Time delay for RG58 patch cable

Measurement background

Using a function generator in burst mode we can measure the reflection from the open end of a coaxial cable. An oscilloscope is connected through a t-junction between the function generator and the test cable. Since the internal resistance of the oscilloscope is high, and current prefers the path of least resistance, and the burst signal will travel to the coaxial cable. A small amount of the signal will coupled to the oscilloscope. We denote this as the incident voltage, U.
Upon reaching the open end of the cable, the wave will reflect and travel back towards the function generator. As the wave passes the oscilloscope a small amount will be coupled. We denote this as the reflected voltage, Ur. The reflected wave finally dissipates when it reaches the function generator.

This is very similar to tying a rope to a pole, swinging it and having the rope reflect back.

Fig 2: Measurement setup

The time difference between Uand Ur is the time it takes for the wave to propagate to the open end of the coax and back again. Using this we can calculate the length of the coaxial cable using the following formula:

Vf is the velocity factor of the coaxial cable and c is the speed of light. Since the time between incident and reflected is the round-trip time we divide the result by two.

By seeing how much the voltage has dropped on the reflected wave relative to the incident wave we can calculate how much loss the coax has at generator frequency. Since the reflected wave passes through the cable twice we should divide by two to find the one-way attenuation.

Some measurements

As mentioned, I have quite long Aircell 7 cable that I would like to know the length of, but didn’t want to uncoil. To keep everything neat I used a short RG58 cable to patch it together. This is the setup shown in figure 2.

The two cables are made using different dielectrics, and will have different velocity factors.  Aircell 7 has a Vf of 0.83, RG58 has a Vf of 0.66. To account for this we should first measure the delay and attenuation caused by the RG58, and then subtract that contribution from the Aircell 7 measurement.

To be able to measure on the short RG58 cable I am using a 1 cycle 100 MHz sine wave burst. The burst is set up to repeat every second, this means that any remaining oscillations should have fully died out. A generator frequency of 10 MHz is sufficient to get accurate results, but only if the cable you are measuring is longer than 20 m.

Fig 3: Unknown length Aircell 7

Fig 4: Voltage drop of RG58 patch












Figures 1 and 4 show the measurement results from the RG58 patch, we put the results into the formula and get the following results:

I also measured the RG58 coax with a measuring tape, and found the physical length to be 1.55 m.

Fig 5: Voltage drop Aircell 7

Fig 6: Time delay Aircell 7










Finally using the results from figures 5 and 6 we find the length and attenuation of the Aircell 7 cable

In conclusion this method is a quick and efficient way to measure the loss and length of a coaxial cable. If you have a broken cable the breakage point will also reflect, so this can be a very useful tool to pinpoint where you need to mend the cable.
It should also be said that the accuracy of this method depends largely on the accuracy of the velocity factor given in manufacturer specifications, meter order deviations can easily arise from a wrong spec. The influence of the oscilloscope could also matter, some people connect 10X or 100X probes to the t-junction for these measurements, I found it to be fine using just the internal impedance.

NRRL Vintertest 2017

This weekend LA1K participated in the Norwegian contest ‘NRRL Vintertest 2017’.

The first two stages gave rather disappointing  results due to poor conditions and interference on the audio equipment in the building, but the third and final round gave us a nice score overall.

This was the first contest we’ve had with our Flex 6500 and we are more than happy with the results so far.

Henrik, LB5DH og Kristin, LB2NH kjører 40m

Henrik LB5DH and Kristin LB2NH on the 40m band

Mats LB0HG

Mads LB0HG on the Flex 6500 during the contest

Thanks to LA1BFA, LB6RH, LB0HG, LB5QG, LB5DH, LB2NH, LA2QUA, LB7RG,  LB7JG and LA3WUA for participating.

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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