Akademisk Radioklubb

LA1K / LA1ARK / LA1UKA

Category: Equipment (page 1 of 2)

3m parabole dish ready

We have finally finished one of the major goals of the 1 to 10 GHz project. The mast is installed, the rotor is mounted and the 3m parabole is built.

Below is a narrated selection of pictures from the build. You can find the full album here.

Espen Molven, LA2USA, advised that the mast would be easily liftable by two persons.

We barely made it with 4 people.

The mast was easily fitted to the lamp post.

First hoisting of the mast.

The mast hoisting mechanism is really strong. Hydraylic lifting allows us to easily lift a grown man.

The rotor (SPID BIG-RAS/HR) mounted on the mast.

To get the rotor running some outdoors soldering was needed.

The center hub of the parababole fully assembeled.

Two of twelve spokes mounted. The spokes came ready assembeled.

Progress on spoke mounting.

A view of the attachment point of spokes to the center hub.

All spokes in place.

Mounting circular bracing rings around the dish. In the background, you can see our 5.8 GHz wifi link and the four bay array for 144 MHz.

The twelve pieces of mesh that the dish is made of.

One of twelve mesh pieces mounted.

Clamps were very helpful when mounting the mesh.

Half way there. In the background are our homebrewed DK7ZB antennas for 4m and 6m.

We had time to do a re-enactment of the photo from the original 5 m parabole.

Twelve hours after starting mounting the mesh the parabole is complete!

 

A big thanks to everyone who helped make this possible. Stay tuned as we attempt to make our first contacts with the dish, and progress on the RF hardware.

Preparations for ADS-B reception at Vassfjellet

We’ve been planning to set up a receiver for ADS-B  at Vassfjellet for some time, and after after the upgrade of our 5 GHz link, the time has come to finally do something about it.

Automatic Dependent Surveillance Broadcast (ADS-B) is a service that is required onboard planes above a certain size. We’re already running a receiver at Samfundet (JP53ek) powered by an RTL-SDR and dump1090. This setup is giving us around 180 nautical miles in maximum reception distance (according to fr24 receiver statistics). We’re hoping to improve this by installing another receiver at Vassfjellet, which is at over 700 m above sea level. The setup for Vassfjellet will consist of the following parts:

  • Raspberry Pi
  • RTL-SDR
  • 1090 MHz collinear antenna purchased from eBay.
  • 1090 MHz bandpass filter purchased from eBay.
  • LNA4ALL

We had the opportunity to go a little overboard with the measurements of the setup: both horizontal and vertical antenna pattern, S-parameters, noise figure and gain were measured. Many thanks to Jens (LB6RH) at the Department of Electronic Systems for helping us measure the antenna in the anechoic chamber at NTNU.

Antenna mounted vertically in chamber.

Radiation pattern of antenna mounted vertically.

 

 

 

 

 

 

 

 

 

 

The figures above show the mounting position and the measured antenna pattern for the vertical plane. We can see that the antenna performs excellently with an almost omnidirectional pattern.

Antenna mounted horizontally in chamber.

Radiation pattern of antenna mounted horizontally. The diagram is oriented so that 0 degree corresponds to the colinear antenna pointing straight towards the reference antenna.

 

 

 

 

 

To assess the take off angle of the colinear antenna we mounted it horizontally and changed the reference antenna to a horizontal configuration. The results and mounting position can be seen above. The large lobes at 80 degree and 270 degree correspond to elevation angles 10 and 0, respectively. This means that the horizon is well illuminated and that we will have good reception for multiple planes. Further we see that the sidelobes are more than 10 dB below the main lobes, ensuring that the antenna has good gain.

Filter insertion loss.

Colinear antenna return loss

The S-parameters for filter and antenna were measured using a miniVNA Tiny and are displayed in the figures above. The performance of the filter and antenna are similar to what their manufacturers claim.

Measurement setup for Noise Figure and Gain measurements.

The next measurement shows the measured noise figure and gain of the LNA4ALL. This was done using the FSV-K30 option on a R&S FSQ signal analyser with a HP 346B noise source, as seen in the figure above.  We will get back to this measurement in a separate blog post.

Measured noise figure (blue) and gain (black) for an LNA4ALL

We are very satisfied with these measurement results, and look forward to seeing how the equipment will perform once installed at Vassfjellet. We hope to be able to receive many planes.

New beacon antennas for LA2VHF (4m) and LA2UHF (70cm)

For a while now, the beacons on 4m and 70cm have had sub-optimal antenna systems.  We decided to install new Big-Wheel antennas for both bands, as an experiment.    The “Big Wheel” is a horizontally polarized omni-directional antenna.

Getting ready

Getting ready for departure. A 6 meter long steel mast, 3 Big Wheel antennas, and 4 hams

The installation was performed in typical Norwegian summer weather – rain and wind.

Antenna assembly

Øyvind (LA3WUA) and Arne (LB7JG) assembling a Big Wheel

In addition to the 70 cm and 4 m Big Wheel antennas, we put up a 6 m version as well.  We don’t currently have a beacon license for 50 MHz, but since there has been allocated a new beacon segment above 50.400 MHz, there might be room for us there.  Stay tuned – perhaps LA2VHF will show up on 50.463 some day.

Final adjustment on the 6m Big Wheel. Arne (LB7JG) and Øystein (LB7IG) in intense competition – manual or electric?

6 m and 70 cm big wheels were bought from Wimo. The 70 cm version is a very neat design that comes pre-assembled. Some assembly is required for the 6 m version.  We had a lot of trouble getting the aluminum tubes to fit inside the square aluminum pieces. In the end we had to file the tips so that they would fit. If you are planning to build this antenna, make sure to have some lubricant and proper tools at hand.

For 4 m there are no commercial designs. Our solution is a custom design based on the build notes OE5MPL and OE5VRL provide on 70MHz.org.

70 cm and 4 m Big Wheel is mounted. Now, the 6 m remains. Øystein (LB7IG) and Sveinung (LB1SH) receives the antenna from Arne (LB7JG)

If you hear any of the beacons we would love to hear about it at LA2VHF or LA2UHF @ la1k.no.

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.

Flex 6500

After a long wait, we finally received our sample of the Flex 6500 this Wednesday. This was purchased as a replacement of our 756 Pro III as it has reached 10 years of use, and therefore will be put to use as our HF-radio number 2.

The package included a quick start guide, a CD for the SmartSDR software, a power and Ethernet cable and a FHM-2 microphone.
Flex 6500 radio

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