Johnson's Techworld

 Yesterday I was trying to check in to AIRNET India at 7.30 pm and I could hear the net control very strong, at 5,9+20 dB. Pressed the PTT to announce my callsign when the call for any more check-ins came through. Immediately 'HI SWR' was displayed in the meter in red colour and there was no output from the radio. Tried tuning with built-in auto tuner of FT-710 and the tuner refused to tune. Switched to 21 MHz and tried auto tuning and it did tune to one frequency. I was confused and went outside to check the antenna. As it was dark, I first checked the portion of the antenna wire and mast from the front side and it seemed OK. At second thought, I went on to the first floor terrace and checked and there it was, the other limb of the 40m dipole was down! The nylon thread which had been used to tie the antenna had given way. So the lesson is, whenever you see a 'HI SWR' display on your screen with a previously functioning antenna, go out and check the antenna in full.  Wa

  I am quite happy to receive this picture from Amateur Radio on International Space Station using my homebrew Delta Loop VHF antenna just now. One of the best pictures I have decoded ever with my setup! In case you wish to have a look at the antenna, it is here: https://youtu.be/ySYL96zvvOs . It was a 75 degree pass.

  Demo of the method to receive SSTV from Space Station on 145.800 MHz during a pass of the space station. Timing and direction of the pass can be found from https://www.n2yo.com/passes/?s=25544# . Doppler tuning needed can be found at: https://mail.amsat.org.ar/pass.htm . In good elevation passes, simpler antennas and reception without Doppler tuning might work. Decoding is by Robot 36 app in mobile phone kept near the speaker of the radio. Alternatively, recorded audio can be decoded later using the Robot 36 app. Directional Yagi antennas will decode better pictures if moved along the direction and elevation of the pass.

 An SSTV (Slow Scan Television) event from International Space Station is scheduled to start at 11:50 UTC on 11th November 2024 and will continue up to 18th November 2024. So there is plenty of opportunity to tune in to 145.800 MHz during the space station passes in our region and try to record the audio as well as decode SSTV images using Robot 36 app in our mobile phone in PD 120 format. As the transmissions are at relatively high power, most VHF reception setups will be able to get at least part of the SSTV images. Last time I could get partial copy even with a magnetic mount VHF antenna kept on my linear power supply. Ideal method of course, is to use a directional antenna pointed in the direction and elevation of the pass of space station continuously. But many of us may not have that. Doppler correction is also useful, but not mandatory to receive some images in high elevation passes. I had used my recently homebrewed VHF Delta Loop antenna for local repeater contacts and a sing

 Some time back we have a Google Meet on Fldigi in our LEO Satellite Group, conducted by VU2TUM. Thought of brushing it up once again when VU2NXG told me yesterday that Fldigi can be used for RTTY mode, though I am yet to try it out. Fldigi known in full as Fast Light Digital, is a cross-platform modem application which supports most of the peer-to-peer digital modes used on amateur radio bands with live keyboard. Fldigi allows the sound card of a computer to be used as a simple two-way data modem. It is a free and open source program which can be downloaded from sourceforge.net . Fldigi supports CW or Morse Code operation using a computer keyboard and Radio Teletype known in short as RTTY and many other modes which I am not familiar with. The program was written by W1HKJ who has also authored several other amateur radio programs. There is a nice Beginners' Guide to Fldigi written in simple language. But to make the best use of the program, one has to read the Online Documentati

 I had posted about LMR SDR a couple of days back. LMR SDR covers from 160m to 10m band and gives an output of up to 50 mW which has to be amplified further using a linear amplifier, if needed. It is based on Arduino Nano microprocessor and Si5351 programmable clock generator. A computer and sound card are needed for the operation of the basic version of LMR SDR and uses PowerSDR software. Yesterday VU3GEX told me about standalone versions of LMR SDR which do not need a computer. He mentioned that ESP32 and STM32 can be used in Standalone LMR SDR. STM32 is a family of 32-bit microcontroller integrated circuits by STMicroelectronics. ESP32 is a series of low-cost, low-power microcontrollers. To my great surprise, a search for ESP32 Standalone LMR SDR brought me to RDvibes , a website by VU3ZOF, whom I have been hearing and talking to on 40m for the past few days. According to the website, the standalone variant was developed by YD1GSE and uses ESP32-A1S processor. The exciter front end

 

 When there is no signal being received, a sensitive receiver can produce significant noise because automatic gain control disappears in the absence of any carrier. Receiver becomes more sensitive and amplifies any electrical noise present in the input. This especially more in FM receivers which produces a hissing noise when the squelch is kept open. The circuit used to quieten the noise while no signal is being received is known as squelch. Usually a volume control like mechanism is used to adjust the squelch level. While receiving weak signals of amateur radio satellites, squelch is intentionally kept open as the weak signals from satellites may not be enough to open the closed squelch. Powerful signals from satellites like the amateur radio on international space station may be able open squelch in high elevation passes when the satellite comes near overhead.

 Some important features to be looked at in a radio receiver are its sensitivity, selectivity, fidelity, adjacent channel and image interference, automatic volume control (AVC), squelch and signal to noise ratio. Sensitivity is the ability of the radio receiver to pick up weak signals. Lower the signal needed in the antenna circuit to have good audio output, higher the sensitivity of the radio. Sensitivity may range from microvolts to millivolts of signal in the antenna circuit and depends on the number of amplification stages in the radio and their quality. Selectivity is the ability of a radio receiver to separate the signal of the intended station from those operating on other frequencies. We need the reception bandwidth to be narrow. Yet it should not cut off the upper and lower audio frequencies modulating the received signal. Selectivity can be improved by having more tuned circuits of higher quality in the receiver. Fidelity is the quality of reproduction of the received signal

 We have seen that usual superheterodyne radio receivers have single conversion of received frequency to the intermediate frequency using a single local oscillator and mixer. In many of the modern communications receivers, multiple successive stages of frequency conversions with multiple intermediate frequencies are used. A receiver with two frequency conversions and two intermediate frequencies is known as a double or dual conversion superheterodyne receiver. One with three levels of conversion and three intermediate frequencies is known as a triple conversion superheterodyne receiver. Use of additional conversions improve adjacent channel selectivity and image frequency rejection. First IF stage has a wide separation between the intended signal and the image frequency, enabling RF amplifier to reject the image frequency. Second IF filter has a narrow bandwidth providing good adjacent channel selectivity. Single IF stage in conventional superheterodyne receivers is a tradeoff between

 Activity on LEO satellites in this region has been low after the peak during the ARISS SSTV experiment. Most of the passes of International Space Station were lonely passes. Today morning there was a 50 degree maximum elevation pass of Space Station. I was happy to work Neel early in the morning through the Amateur Radio on International Space Station. I could hear him almost from horizon to horizon, with very good signals in between, without rotating my Moxon Yagi kept at a fixed elevation and azimuth, pointed towards the direction of maximum elevation azimuth.

See also (new post):   Standalone Versions of LMR SDR!  I have been hearing about LMR SDR during discussions among homebrewers for the past few days on 40m. Thought of learning a bit about it and my web searches told me that some of the persons behind the development of LMR SDR are YE3CIF, UT3MK and F5NPV. F5NPV has given detailed descriptions about homebrewing LMR SDR on his webpage, including videos. Even though I did look at the pages of YE3CIF and UT3MK, they seem to be in regional languages. I could not find the expansion of LMR on those pages, though I presume that it would stand for land mobile radio. LMR cables which many of us use were initially designed for land mobile radio. SDR stands for Software Defined Radio like my FT-710, which uses software to do many of the functions done by hardware in the conventional superheterodyne radio receiver . It is interesting to note that LMR SDR covers from 160m to 10m band, that is the whole amateur radio HF bands. There are a few frie

 Even though we have several modes in amateur radio now including the various digital modes and phone modes, still there are a good number of fans for CW or continuous wave operations. If you look at some online forums, there are even youngsters trying to learn CW, even though it is not mandatory to learn for an amateur radio license in many countries. Why is this charm for CW? As CW requires only lower bandwidth than speech, signal to noise ratio is better in the receiver. Human ear can resolve CW or Morse Code signals which are only marginally above the noise level whereas SSB or speech signals at the same signal level cannot be copied well. They say that CW has an advantage of about 20 dB over SSB signals. Narrow band pass filters can improve CW reception. Please note that when such a filter is being used, simultaneous SSB reception will be difficult. CW transmitters are easier to make for a beginner as you need only lesser components. We started by homebrewing low power CW transmit

 Superheterodyne radio receiver, often known in short as a Superhet, was designed to overcome the shortcomings of a tuned radio frequency receiver. It uses a frequency mixer to convert the received radio frequency signal to a fixed and lower intermediate frequency (IF) signal. It is possible to process the fixed IF signal more easily than widely varying carrier frequencies received from the antenna. The superheterodyne process involves tuning the local oscillator to  a frequency which is different from the received signal by as many kilohertz as the intermediate frequency. The process was called superheterodyne because the heterodyne signal or the difference between the two frequencies, was above the audible range. If the difference in frequencies is lower, you will have an audible 'beat' note. That was how we used to receive CW signals in our broadcast receivers by heterodyning with a beat frequency oscillator or BFO. In order to have same IF with varying input signals, the lo

  In the days of analog multimeter, the needle would flick once as the capacitor was being charged and then come back to zero. If the capacitor was leaky, it would continue showing a current through the capacitor. Similarly, with digital multimeter, an initial rising value of resistance is seen and then it goes beyond the range as the capacitor becomes fully charged with no current flowing across it.

 Tried a pass of International Space Station with 50 degree maximum elevation in my region at 8 pm. Could hear my call back on the downlink at least five times. Soon after that I was happy to hear the NA1SS beacon in CW. So there was no problem with my antenna or radio! The pass was at a time when Airnet India was going on, and one regular LEO satellite operator held up as net control on 7150 kHz. Went back and checked into Airnet India and had a few QSOs after that as well on the net frequency when it was cleared for general traffic. I am thinking that a direct NA1SS crew contact with one of the VUs is needed to boost up the LEO satellite activity in this region. I am sure that, if that occurs, there will be a pileup on passes after that, looking for further NA1SS contacts!

 Tuned Radio Frequency Receiver is an old type of radio receiver, also known as TRF receiver in short. It has one or more tuned radio frequency amplifier stages serially amplifying signals received from the antenna. The output from the last RF amplifier is fed to a detector or demodulator, which extracts the audio signal. Simplest form of detector is a diode. Detected signal is further amplified using audio amplifiers. TRF receivers were popular about a century back and was tedious to operate. While tuning a station, each RF amplifier stage had to be tuned separately to the station's frequency. Later gang capacitors with multiple rotors controlled by a single tuning knob was used for tuning. Still it was difficult to achieve accurate tuning of all the RF amplifier stages together. Final output was fed to a loudspeaker or earphone. Basic concept was that each stage would amplify the desired signal while reducing the interfering ones by the tuned circuit . Having multiple stages of

 A new FM satellite launch has been announced on the AMSAT bulletin board. LEO satellite operators have been requested to try receiving the first day telemetry reception from all over the world and especially from Europe and Africa. ASRTU-1 (BJ2CR / RS64S) satellite is planned for a Soyuz 2.1b launch from Vostochny Cosmodrome. Separation is expected on November 5, 2024 at approximately 03:36 UTC. ASRTU-1 has a V/U FM repeater with uplink on 145.875 MHz FM using a 67 Hz CTCSS (PL) tone. Downlink will be on 435.400 MHz FM. Another interesting feature mentioned is an open telecommand system to allow radio amateurs to send commands to control the satellite to take and download images. Image downloads will be on 10.460 GHz.

 Heard VK9CV Cocos Keeling DXpedition 2024 on 24.940 MHz with nice signal. They are listening 5 kHz up. After setting up for split mode for transmitting on 24.945 MHz and receiving on 24.940 MHz and tuning my FT-710 for the frequency using the built-in automatic antenna tuner, I started mentioning my callsign when they were listening. I could get through in my third attempt and I was very happy to work a DXpedition, that too, my very first ever contact on the 12m band. Yesterday I had listened to Burkina Faso DXpedition XT2MD on 12m CW, but only the stations calling them were audible to me. VK9CV Cocos Keeling DXpedition 2024 has listed 9 operators from Czech Republic on their QRZ page . The plan of operation is from 1st November to 15th November 2024. Modes of operation planned are SSB, CW, RTTY and FT8. Bands planned are from 160 to 10m on HF and QO-100  Geostationary Amateur Radio Satellite . Cocos Keeling Islands have an IOTA or Islands on the Air designation of OC-003, of which O

 This is a remote control toy car which works on four AA batteries. The remote control operates in the 27 MHz license free Citizen Band. Todays experiment is not on the remote control function, but another interesting feature which was noted by chance. See that currently there is no battery in this car and the experiment is to be conducted without batteries. When the car is moved forward manually at a good speed, we can see that the headlights light up, with brightness corresponding to the speed! During the process, the motor in the toy car is functioning like a dynamo or generator, producing an electric current which runs in the circuit and lights up the headlights made of light emitting diodes. We know that when a coil of wire in the motor moves in a magnetic field or if the magnet moves relative to the coil, an electric current is generated within the coil. It is that current which lights up the LED headlights.  Now see what happens when the car is run manually in the reverse dire

 When I tuned in to 24.897 MHz a few minutes back, I heard several stations mentioning their callsigns in CW, without giving a full call. Immediately I thought that a DXpedition must be around. Searched for any spots near that frequency on DXWATCH.COM , my favour DX Cluster site. I could easily find that XT2MD was around (24895 up 2). That is a DXPedition to Burkina Faso in West Africa, from 31st October to November 10th, 2024. Incidentally, yesterday I had noted that VU2CPL had worked this station on 10m, FT8. Interesting part of the 12m WARC band here is that the noise level was below S1 and I could hear CW signals below that level! S meter needle was hardly moving when I was listening to the weak CW signals most of the time. There is no other band like that in my radio. I was using my End Fed Half Wave antenna with 10m of wire meant for 20m and above, mounted almost horizontally with 3m long PVC pipes on the parapet of my first floor terrace. The auto tuner on FT-710 was able to

 HS19IARU is the special event station commemorating 19th IARU Region 3 Conference 2024 , to be held in Bangkok, Thailand from November 4-8, 2024. The Radio Amateur Society of Thailand (RAST), has been licensed to operate the special event station from 12 October - 10 November 2024, by  The National Broadcasting and Telecommunications Commission (NBTC), Thailand. International Amateur Radio Union Region 3 conference is for 5 days and participation is expected from 27 Member Societies and 100+ participants. E24PAM, NBTC Commissioner is the Chairman of the ceremony and VE6SH, President, International Amateur Radio Union will give a speech in the conference opening ceremony, along with several other dignitaries. This is the first time I am hearing HS19IARU on CW, on 20m band. Earlier I had listened to the station on SSB mode on 15m . Both times, my calls did not reach them, even though their signals were strong here.  

 In the simplest sense, frequency changer or frequency converter is a device which can change the frequency of an alternating current. It could be changing the frequency of the mains supply to match another grid like changing from a 50 Hz to 60 Hz which is used in another region. Another application is changing the frequency for use in aircraft electrical system for which the standard frequency is 400 Hz. So airports, aircraft hangars and ships need 400 Hz frequency converters. Higher frequency would mean lighter cables and transformers in the aircrafts, compared to standard 50 Hz power supply. Reducing weight of components is an important consideration in the design of aircrafts, as expected.  In radio communications, frequency conversion is an important part of superheterodyne receivers. This is done using a frequency mixer where the received radio signals are mixed with a local oscillator signal to generate two frequencies. Mixing of two radio frequency signals produces one which is