Akademisk Radioklubb

LA1K / LA1ARK / LA1UKA

Author: LA3WUA

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 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.

New website

We’re proud to present  our new website based on the wordpress platform!

New features include:

  • What you see is what you get editor
  • Comment section
  • Tags and categories
  • Sensible archive
  • And more…

While prototyping we’ve been writing a series of blog posts to test the new functionality,  we hope that you take the time to read these. We’re very satisfied with the new editor, and will be blogging regularly about our activities.

If you need any of the content from the old site it can be found at old.la1k.no.

We would love to hear any feedback about the new site in the comments section.

Shipment from Rohde & Schwarz

We were recently in touch with Bjørn Sveum at Rohde & Schwarz Norway asking if they would be interested in helping us out with some measurement equipment. I was astounded by the generosity Bjørn showed, and today the package arrived!

tung_pallewrapping_tape

The hardware enthusiast in me got really excited seeing the packing tape labeled Rohde & Schwarz, an even more impressive sight when we realized that they had sent an entire pallet.

The centerpiece is a FSH 3 handheld spectrum analyzer with tracking generator and preamp. A tracking generator allows the spectrum analyzer to perform scalar network analyzer measurements, allowing for filter and simple antenna measurements. This will be very helpful when working with projects both on the lab bench and when debugging in the field, such as working on the beacons at Vassfjellet.

We also got four NGPE 40/40 power supplies. Using two of these in series will allow us to power most FETs up to 50 V (LDMOS and GaN are usually max 50 V drain voltage), this will be very useful in developing amplifiers up to the legal limit for amateur radio – 1000 W (given that they have reasonable efficiency).

The two other supplies will be part of the 1-10 GHz ground station project. More on this in a separate blogpost.

Finally here’s a glamour shot of all the equipment and some of the goodies Bjørn included.

all_equipment

Beacons

ARK develops and maintains some radio beacons from JP53EG at the top of  Vassfjellet, a local mountain. The beacons can be heard on the following frequencies.

LA2VHF/4m 70.063 MHz
LA2VHF 144.463 MHz
LA2UHF 432.463 MHz

If you hear any of the beacons we would love to hear about it at la2vhf or la2uhf@la1k.no.

la2uhf

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.

imag174214355763_10153799842871440_6641142721484082045_n

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.

Upcoming changes

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.