Akademisk Radioklubb

LA1K / LA1ARK / LA1UKA

Author: LA3WUA (page 1 of 3)

New licencees H17

The results are in from the license exam last Tuesday. We are proud to announce five new amateurs:

Vegard Josvanger: LB0LH
Eirik Veløy Nadim: LB0MH
Håvard Gautneb: LB0NH
Jordi Frances: LB0OH
Sivert Bakken:  LB0PH

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

Microham Double Ten performance outside the specified frequency range

ARK has been expanding our antenna park with DK7ZB yagis for 4 m (70 MHz) and 6 m (50 MHz). In order to make the setup more practical we would like to have the antennas selectable via our Microham Double Ten antenna switch. The Double Ten is only specified up to a frequency of 30 MHz, so we wanted to investigate if it would be usable beyond this.

6m and 4m beam

The DK7ZB 6 m (bottom) and 4 m (top) yagis we want to use with the Double Ten switch.

To perform these measurements we used a HP 8753E network analyzer calibrated with a 85052D calibration kit.

Since we are intending to use this device outside its specified frequency range we decided to carefully measure the S-parameters between each output and input port for a sweep from 0 MHz to 200 MHz, focusing on the amateur radio bands covered.

The S-parameters (or scattering-parameters) are a way to measure the response of a RF system by applying a test signal to one port, and assessing the levels that result on the other ports. The results are categorised as S_xy, where S_xy denotes the power that appears at port x from an input on port y. S_xx is also a valid measurement, and gives an indication of how well matched a device is to the target impedance, in this case 50 ohm.

Schematic for a similiar switch, 6 to 2 switch by OK2ZAW. Source: Remoteqth.com

To better understand the internals of the device we wanted to check the circuit schematic, unfortunately none was available from Microham. Luckily OK2ZAW has made an open source 6 to 2 switch, that turns out to be very similar on the inside. Due to substrate and component losses we expect that the antenna ports that are placed further from the radio ports should be have more loss than the ones that are placed closer.

The inside of the Double Ten switch. Wires are attached to a lab power supply to manually select which ports are active/inactive.

To avoid noise interfering with our measurements we took care to re-mount the metal chassis between each change of ports.

Fewer measurements are required if the response between a given port and radio A/B is the same, so this was the first thing we checked.

As seen in the figures above the response from port 10 to radio A and radio B are nearly identical. We assume that this is also the case for the other ports.

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Insertion loss is a measure of how much signal is lost when translating through a device. For the switch the insertion loss varies quite a bit from port to port. Lowest insertion loss is observed at ports 1 and 10, which are on the edge of the switch. One possible explanation for this is that the edge ports have more series inductance, which could help match the relay better at high frequencies. At port 10 the insertion loss at 70 MHz is only 0.35 dB. This is a promising result, and could indicate that it might be useable for our purposes.

To ensure that the device can operate at 50 MHz and 70 MHz we also need to check that the reflectivity of the input port is not excessively high. Low reflectivity means that the device will translate most of the applied power to the output, and high reflectivity means that a significant portion of the input power is reflected back at the transmitter.

Return loss of port B when output is coupled to port 6

The return loss varies between -8 and -13 dB at 70 MHz, depending on which output is selected. The outputs with lower insertion loss have better match. A return loss of -13 dB corresponds to a SWR of ~1.6. At the antenna port we are not so concerned about the reflectivity, as the power levels are not a cause of concern when receiving.

The final characteristic that is investigated is the isolation between Radio A and Radio B ports. If two transceivers are operating on the switch simultaneously, the isolation between the ports must be sufficiently high to ensure that either radio is not saturated when the other transmits.

Isolation between Radio A and Radio B ports.

The isolation between ports is not very high. When transmitting with 100 W at 70 MHz, 2 mW will be coupled to the other radio. This can cause the receiver to saturate if the filtering is not sufficient. Typically the antenna will receive signals that are on the order of pWs, so the receiver would need tremendous dynamic range in order to cope with 2 mW coupled.

Practically this is not a large problem, as we don’t intend to use two radios at 4 m simultaneously. Furthermore, 70 MHz is heavily attenuated by the preselectors filters that are common in HF transceivers, so the intermediate frequency stages are likely to be sufficiently shielded from saturation.

At low frequencies (>30 MHz) the measurement is seen to be noise floor limited.

In conclusion, given that port 1 or 10 is used, the Double Ten will work well at 6 m, and decently at 4 m. Care should be taken to not run excessive power at these frequencies, atleast not without extra filtering on the other radio ports.

Thank you to LA1BFA for providing a Double Ten switch for testing! It was very convenient to not have to disassemble the one we already have installed.

CQ WW RTTY

Samfundet is currently closed down in preparation for UKA. In between the sound checks, rehearsals and other activities that occur in this hectic period, we managed to sneak in some contacts as LM100UKA in the CQ WW RTTY contest. We decided to run as Multi-Two to get the most out of the few timeslots when we could operate.

Henrik LB5DH working 20 m on Saturday.

One of the challenges we face when operating in contests from Samfundet is Electro Magnetic Interference (EMI) to the other groups at Samfundet. Particularly when operating legal limit on the low bands, audio and lighting equipment tend to get unwanted interference. Normally we try to locate the weak spots and seal them with ferrites and filters, but during UKA there is a lot of new equipment so it is hard to get to all the problems in time. The solution is often to cooperate with the groups that are responsible for audio and visuals and coordinate so that we do not transmit high power levels on the low bands while they are working.

Conditions on 20 m was also good on Sunday.

The contest was also our first good test of the Flex 6500 for RTTY. Thanks to the panadapter, finding an open frequency to work was a breeze. It also allowed us to quickly diagnose that we had set the wrong tone spacing (oops). We had a bit of trouble setting up SmartSDR to talk to MMTTY through N1MM+, but in the end we got it working through the help from some good resources online. What was not so great was setting RIT/XIT using mouse and keyboard. Hopefully our Maestro will arrive soon to address this problem.

QSOs per hour. Orange: 15 m, Red: 20 m and Blue: 40 m.

The QSO-rate was largely influenced by when we had a chance to operate in between different sound checks. As the graph shows, this was mostly during the afternoon on Sunday. Conditions were ok, but only one band was significantly open at a given time, typically either 20 m or 40 m.

Score and multipliers per band

We got 657 QSOs and 107 mults over the weekend. A good result considering the operating conditions, both on the air and at Samfundet. Thanks to LB5QG, LB1MH, LB5DH, LB7JG and LA3WUA for their operating efforts.

Repairing broken receiver on a USRP N210 WBX40 daughterboard

Kimmo Kansanen at NTNU recently donated some USRP N210 units they no longer had a use for. We have started to use them for various communications experiments, for example estimating the antenna patterns of our VHF/UHF antennas against the LA2VHF and LA2UHF beacons.

While operating one of the N210s we were sudddenly unable to receive the LA2VHF beacon that had previously been easily decodeable. We also saw that the noise floor had increased by 20 dB. Jens, LB6RH, decided to investigate matters further.

Inside the N210 a WBX-40 daughtercard provides the RF-frontend. The device functions as normal, except for in receive mode. The receiver section of the WBX-40 should be a good place to start looking.

The WBX-40 daughterboard after removing coaxial patches that attach to the front panel of the N210.

To further investigate what may be wrong we started investigating the board for any obvious short circuited connections. We were unable to find any such sources. The next step was to probe around the board with a multimeter to check the voltages being generated from the different voltage regulators on the board. Since Ettus Research provides the schematic for the WBX-40 online this process was greatly simplified.

LB6RH found that the 3.3 V rail on the output of voltage regulator U308 was only at 1.2 V. Suspecting that something was wrong with U308, he removed the component and attached a laboratory power-supply to the 3.3 V output pin. After turning on the supply (with the current limit set low, to avoid frying the circuit) he noticed that there was still a short circuit. To identify which component caused the short we borrowed a FLIR thermal imaging camera to check what components got hot when we turned up the current limit on the power-supply.

Unfortunately we did not get a good picture of the thermal test, but the chip that got hottest was the Low Noise Amplifier (LNA) U313. Since our problems are related to poor reception we thought this might be a likely candidate. After removing U313 we turned the power-supply back on, and saw that there was no longer a short, hurray!

A closeup after removing components U313 and U308.

We ordered new components for U308 and U313 from Digikey, and soldered them back in place.

U308 – Analog Devices Inc. ADP3336ARMZ linear regulator

U313 – Broadcom Limited MGA-82563-TR1G MMIC broadband LNA 

Replacements for U308 and U313 have arrived.

The soldering battlestation. A microscope helps when soldering small parts.

 

 

 

 

 

 

 

 

 

 

 

 

 

After replacing the two parts we connected everything back and tried powering on the device again.

We are now able to receive LA2VHF again!

Field day 2017

Last weekend ARK participated in IARU Region 1 Field Day. In Norway, this contest is more known as National Field Day and is coordinated by the Norwegian Radio Relay League (NRRL). ARK usually treats this as a full three-day social event where we travel to a cabin far away from Trondheim, and treat our members to a nice balance of antenna assembly, food, social exposure and a many new contacts on the radio. This year, we traveled to Fjellvær Gjestegård on Fjellværøya, Hitra.

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