Akademisk Radioklubb

LA1K / LA1ARK / LA1UKA

Category: Projects (page 1 of 4)

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.

Soldering course

From time to time we hold internal courses on technical topics in order to increase the general knowledge of our members. Recently,  Henrik – LB5DH, took great initiative in organizing an introductory course in soldering for our members. Soldering is an activity that we do a lot of in ARK, for example when building beacons, antennas or doing general maintenance. It is a very useful skill to have when doing work on projects for the club.

Henrik got in touch with experienced solderers Øystein – LB7IG and Snorre – LA8ITA, who were willing take responsibility for the academic content of the course.  The theory portion consisted of topics such as:

  • Getting optimal heat transfer between soldering iron and PCB/components.
  • How to solder components.
  • What does a good solder-joint look like.
  • Flux – what is it and why do you need it.
  • Solder alloys and their effect on performance and usability.
  • Care and maintenance of a soldering iron.
  • Health and safety hazards and procedures to avoid them.

Øystein – LB7IG demonstrates how to apply solder to a through-hole component.

For the practical portion of the course, the attendees each got a Pixie CW 40 m transceiver kit. The Pixie is a very simple fixed frequency CW radio that will output around 1 watt at 7.023 MHz. As an introduction to soldering for an amateur radio club, we could not have asked for a better kit!

Partially assembled Pixie kit.

Originally, the plan was to solder a through-hole component kit (the Pixie) first, and then move on to surface mounted device (SMD) kit afterwards. This part of the course was postponed due to delivery problems with the SMD-kit.

We are very grateful for being able to loan soldering equipment and a place to use it from Omega Verksted. This came in handy, as fitting all eight attendees into our workshop at the same time would have been very crowded. In addition, we got to loan portable fume extractors from the Elprolab at NTNU through Ingulf, LB1VF. They were indispensable for the continued health of the participants. Thanks to Omega Verksted and LB1VF at the Elprolab for making the course possible!

LA6XTA removing some excess solder.

LB5QG showing off a custom case he made for the Pixie transciever.

LA2QUA brought a display and microscope unit.

Jens – LB6RH placing a throughhole component on his Pixie.

LB6RH doing a test-powerup.

Back to consulting the schematic after the test-powerup failed.

Einar – LB7JH checking out the bill of materials and placement instructions.

Testing the Pixie on a spectrum analyzer.

Local oscillator leakage is quite bad!

TX mode gives approximately 24 dBm out, note that the carrier is a little offset from the receive leakage to produce a 600-800 Hz tone. It also turns out that the output power is roughly 0.25 W (24 dBm) for this specific Pixie (-6 dBm measured on the spectrum after a 30 dB attenuator).

LB6RH and LA6XTA tuning the Pixie to see if they are able to receive each other.

LB7JH inspecting his solder joints.

 

Overall, the course was a great success, and got a lot of ARKs members more interested in soldering. A big thanks to LB5DH for organizing the course, LB7IG and LA8ITA for sharing what they know, and LB1SH, LA3WUA, LB1MH and LB7RG for acting as teaching assistants during the practical portion. Hopefully, we will repeat this course several times during the upcoming semesters.

Parsing a DX-Cluster using python and Club Log

LA1K is always interested in working new DXCCs. To assist with this we wanted to see if we could make a piece of software that could alert us when these opportunities occur.

A DX-cluster is a telnet server where clients can report different callsigns that they have heard or worked. To listen to the DX-cluster we will use Python’s telnetlib library. We parse the cluster output with regular expressions using Python’s re library.

Sample output from DX-cluster at LA3WAA.DDNS.NET:8000

The code snippet below opens a connection to the DX-cluster running on LA3WAA.ddns.net, using telnetlib in Python.

import telnetlib
import re

# Open connection to telnet
tn = telnetlib.Telnet("la3waa.ddns.net",8000)
tn.read_until("login: ")

Once the telnet server replies with login:, a callsign can be sent as a reply:

tn.write("LA1K \n")

The output from the telnet server follows a regular structure, as seen in the image at the start of the post. For our DX-cluster, the structure is “DX de [CALLSIGN]:   [Frequency]  [SPOTTING_CALLSIGN]  [Comment] [Time]”. To parse this, we use some straightforward regular expressions. First is the callsign, which is simplified to any combination of the letters A-Z, numbers 0-9 and the “/ ” operator.  Similarly the frequency contains the numbers 0-9 and the “.” operator:

# Define regular expressions
callsign_pattern = "([a-z|0-9|/]+)"
frequency_pattern = "([0-9|.]+)"
pattern = re.compile("^DX de "+callsign_pattern+":\s+"+frequency_pattern+"\s+"+callsign_pattern+"\s+(.*)\s+(\d{4}Z)", re.IGNORECASE)

Once a new line is found in the DX cluster output, it is checked against the previously compiled pattern. Matches are grouped so that the 0th match contains the whole string, the 1st match contains the first regular expression match (if it exists), 2nd match contains the second regular expression match and so on. We sort these into named variables.

# Parse telnet
while (1):
    # Check new telnet info against regular expression
    telnet_output = tn.read_until("\n")
    match = pattern.match(telnet_output)

    # If there is a match, sort matches into variables
    if match:
        spotter = match.group(1)
        frequency = float(match.group(2))
        spotted = match.group(3)
        comment = match.group(4).strip()
        spot_time = match.group(5)
        band = frequency_to_band(frequency)

Processing DX-cluster information

We are going to further process this information using some features from ClubLog’s Application Programming Interface (API). ClubLog is an online tool to manage your amateur radio logs. To access this API, you need a couple of things:

After you have uploaded your logs, ClubLog provides a very neat DXCC-matrix that shows which entities you have worked across the various bands. By comparing the infomation from a DX-cluster with this matrix, we are able to alert the user of potential DXCC opportunities.

We start by retrieving the DXCC matrix from Clublog’s API. The DXCC matrix is fetched from a URL as a JSON structure. Our solution is to use a cron-job that fetches the JSON file at regular intervals, as the matrix might be updated once we upload more logs.

#!/bin/bash

API_key="your_api_key_as_string"
callsign="LA1K"
email="your@email.com"
password="password"
curl -s "https://secure.clublog.org/json_dxccchart.php?call=$callsign&api=$API_key&email=$email&password=$password&mode=0" > ./dxcc_matrix.json

Save the file and make it executable using

chmod +x

Finally register the file with cron by typing

crontab -e

This will open the crontab file in your favorite text editor. Next we will add a line that runs the script once every hour:

0 * * * * [YOUR_PATH]/get_clublog_dxcc_matrix.sh

Save the file and run the script once manually so that you have some data to work with for the next step.

The DXCC matrix sorts countries by DXCC number.   As an example, the entry for DXCC number 1 (Canada) may contain

{"1":{"10":2,"12":3,"15":1,""20":1,"40":1,"60":2,"80":1,"}

This means that you have worked Canada on 10 m, 12 m, 15 m, 20 m, 40 m, 60 m and 80 m. The missing bands are 6 m, 17 m, 30 m and 160 m.  Conveniently, ClubLog also provides an API to get DXCC info for a given call. We retrieve this data for spotted callsigns and check it against the matrix. First we define functions to query a callsign for DXCC info, and to check whether the DXCC is contained in the logs for the specific frequency band.

def query_dxcc_info(callsign, api_key):
    return json.load(urllib2.urlopen("https://secure.clublog.org/dxcc?call=%s&api=%s&full=1" % (callsign,api_key)))

def dxcc_in_matrix(dxcc, band, matrix_filename):
    try:
        with open(matrix_filename) as dxcc_json_data:
            dxcc_data = json.load(dxcc_json_data)
            dxcc_data[dxcc][band]
            return True
    except KeyError:
        return False

We can check whether the spot is a new DXCC and print relevant information to the terminal using the following snippet:

       
# Get DXCC information from clublog API
spotter_data = query_dxcc_info(spotter, api_key)
spotted_data = query_dxcc_info(spotted, api_key)
spotted_dxcc_route = str(spotted_data["DXCC"])

# Compare DXCC number to DXCC matrix, if there is an error the band has not been worked before
if band and spotted_dxcc_route and not dxcc_in_matrix(spotted_dxcc_route, band, dxcc_matrix_filename):
        print "New DXCC! %s (%s, %s) at %s by %s (%s, %s) %s" % (spotted,spotted_dxcc_route,spotted_data["Name"],frequency,spotter,spotter_data["Name"], comment, spot_time)

The DXCC is new if there’s no data for the given band. Maybe you can work it!

After running the script for some time, we noticed that it would give a lot of unnecessary data. An example is Asian calls spotting other Asian calls, which is useless in Norway, as it is unlikely that we would be able to hear them. To make the software more functional, some filtering with respect to distances and locations should be employed. This is possible using information obtainable from the ClubLog API. We will come back to this in a later post.

The script currently prints all information to the command line. The information can be pushed further to notify remote users in real-time through e.g. IRC or email, and can serve as a building block for several nice applications.

For now you may find the code and usage instructions available on github.

A quick sleeve metal dipole for 23 cm

We have been looking for a horizontally polarized omnidirectional antenna that could be used on a future 23 cm beacon. While a perfectly omnidirectional antenna would be best we are able to cope with some nulls in the antenna-pattern.  A quick and easy antenna that does this is a half-wave dipole, which is largely omnidirectional, but may have some nulls at certain elevations.

The half-wave dipole has a feed impedance of around 72 Ohms. To get even better performance the dipole can be matched to 50 Ohms with a balun. We chose to use a quarter wave impedance transformer, which is often called a sleeve or bazooka match. The sleeve also prevents unintentional radiation from the coaxial  feed cable.

The rest of this post is a tutorial on how to build the 23 cm sleeve dipole. For outdoors mounting it is made extra waterproof, this adds some additional design considerations.

Required materials:

  • 10 cm or longer coaxial cable (we use RG-223 for this guide)
  • 20 cm of 6 mm inner diameter 7 mm outer diameter copper tubing. If you use a thicker cable than RG-223 you can use a wider copper tube. This will increase the bandwidth of the antenna but lower the efficiency somewhat.
  • 20 cm adhesive lined shrink tubing 5-6 mm shrunk diameter
  • 5cm self-amalgamating tape (heat activated)

Required tools:

  • Hacksaw
  • Cable cutter
  • Knife
  • Soldering iron
  • Heat gun
  • Measuring tape or caliper
  • SWR-meter or network analyzer covering 23 cm (up to 1300 MHz, preferably higher)

Start by cutting three 55 mm long sections of copper tubing. I used 6 mm inner diameter and 7 mm outer diameter for my copper tubing in order to fit around the cable I decided to use. If you decide to make the antenna for a different frequency the cut length can be calculated roughly as (where f is the frequency in Hz):

The copper tubing is used for each leg of the dipole, as well as the quarter wavelength sleeve.

The next step is to prepare the coaxial cable. Three cuts should be made, one to remove 5 mm from the end of the jacket, where the dipole legs should be soldered. The two remaining cuts should be made at 55 mm and 60 mm, where the sleeve will be soldered to the coaxial shield.

The next step is to prepare the end of the coaxial cable for soldering to the copper tubing. If you are using a coaxial cable that is already terminated you must make sure to thread the sleeve tube and a piece of heat shrink that is 7-10 mm long over the coax before soldering the dipole legs.

Take extra care to avoid solder bridges between the left and right leg of the dipole, they should be electrically separate. When soldering the sleeve in the next step, make sure to avoid contact between either leg and the open end of the sleeve.

Next up is tuning the resonance. This is done by cutting the dipole legs until the antenna resonates at the desired frequency, for me this is 1296 MHz. One very interesting thing to note when adding heat shrink tubing on the outside of a radiator is that the resonant frequency will shift down. This is due to the increased dielectric constant of the heat shrink plastic compared to air. To mimic this behavior during measurements I place the heat shrink tubing over the copper tubing, but I don’t shrink it until I am satisfied with the resonance. Keeping in mind the dielectric effect, I tune the antenna a little bit too high to compensate for the dielectric constant, which gets a little larger when the plastic is fully shrunken.

Now all that remains is to add heat shrink to the dipole legs. In order to make it waterproof at the middle and at the ends I use a type of self-amalgamating rubber that melts into a solid mass when heat is applied.

The end result is a cheap and simple antenna that performs well. In total the antenna took about one hour to make.

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.

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