and complete list of files for this site
. Since 1961,
I have been a radio amateur, and all the time my
interest has been DX on 144 MHz, and in particular equipment
designed for this purpose.
On this site I try to share some of my experiences in designing
and building equipment for VHF long distance communication.
The primary location is at http://www.sm5bsz.com/index.htm
There are two complete mirror sites. One in Europe http://g7rau.demon.co.uk/sm5bsz/index.htm supplied by G7RAU Dave and one in the USA http://nitehawk.com/sm5bsz/index.htm supplied by W6/PA0ZN Rein.
The digital revolution. New possibilities for the experimenter.
In the old days amateurs built their own equipment. Experimenting was a natural part of the hobby - inspired by what others did one had to try to use the parts from one's own "junk box" to do something similar or hopefully better.With the commercial availability of the modern amateur SSB transceiver, the need for experimenting disappeared. It is not easy to design and build equipment that can compete with the commercial units. The best way to get a well performing station has long been to buy a SSB transceiver. Only very few real enthusiasts build their own receivers and transmitters.
Today the situation is becoming different. By use of simple equipment, well suited for home building, the radio signal can be moved into the digital world. Once the signal is available in digital form, a whole new world of experimentation is opened. The computer can do everything we did before using analog circuits. Experimenting with different filter characteristics, AGC (automatic gain control), AFC (automatic frequency control) can be done in software at no cost (except the experimenters time)
The new possibilities in interference reduction, treating different kinds of interference as signals, each one received with a digital receiver that optimises the S/N for the particular interference opens a whole new world of experiments with radio receivers that will allow reception of signals that can not be received at all with a conventional SSB transceiver.
Linrad is a free computer program that works on standard PC computers. (IBM-compatible, x86) Linrad was developed under the Linux operating system but it is available also in a version for Microsoft Windows.
Use this link if you are a newcomer to Linrad: Linrad for newcomers. The other links about Linrad contain very much information and may be hard to understand without having spent quite some time with Linrad.
Linrad receives one or two IF passbands in digital form. Performance is determined by the hardware; the program analyzes whatever data supplied in digital form. Linrad can be used to process the analog audio output from a conventional SSB radio or any other linear receiver with a bandwidth that can be handled by the soundcard of the computer. Linrad can also use two audio channels (stereo) to process the I/Q signal pair produced by a quadrature mixer in a direct conversion radio. In this case four audio channels are needed for 2 IF channels.
Linrad was from start, before year 2000, designed for use with radio A/D converters although such hardware was not yet available at an affordable cost. The first commercial such hardware designed for amateurs was the SDR-14 which appeared in 2004. Digital hardware may not behave like old fashioned analog receivers. Conventional measurements of the intermodulation-free dynamic range, the intercept point, the point of 1 dB compression and other figures of merit may be grossly misleading. This link practical performance observations for the SDR-14 shows that contrary to a conventional receiver that should be operated with low signal levels, a VHF-sampling receiver should be operated at high signal levels.
In 2007 the Perseus appeared. This unit has excellent dynamic range and behaves nearly like an analog receiver although performance figures are much better as compared to normal receivers like IC-7800, Orion and others. Linrad-02.45 and later supports the Perseus hardware, but only under Microsoft Windows. Click here Perseus with Linrad for a more information on this excellent receiver.
When sending digital data via USB or over the network it may be a good idea to truncate the number of bits. Truncation will however introduce quantization noise. This link Bits and Dynamic Range shows the consequences of truncation. At bandwidths used in a wideband SDR (96 kHz and above) 24 bits is more than adequate, truncation is a good way of saving bandwidth in USB links and networks and it also a good way of saving disk space.
With the WSE converters Linrad can be used as a high performance spectrum analyzer. The frequency coverage is limited, amateur bands only and the maximum bandwidth simultaneously present on screen is about 94 kHz. Here are some high resolution spectra that demonstrate the capabilities of WSE/Linrad as a narrowband spectrum analyzer.
It is convenient to have a set of reference files containing the radio signals in digital form on the hard disk. Such files can be used for optimisation of algorithms as well as for comparing different SDR packages. The SDR data file library contains data files with various mixtures of signals and interferences as well as typical loudspeaker output files from Linrad as well as from other SDR packages to the extent others have made them available to me.
Fundamentally there is no difference between analog and digital receivers. Both kinds have the same fundamental problems with dynamic range and spur suppression. All the new methods for combatting interference have their equivalent analog counterparts.
Here is a general discussion on radio receivers which is intended to resolve common misunderstandings and to explain how one can make sure that a receive system is properly optimized. The well known problems of low noise and dynamic range are present in digital as well as analog circuits.
A software defined radio like Linrad, Winrad and many others have a common problem in that the input data stream is not derived from the same crystal oscillator as the loudspeaker output. It is necessary to use a variable resampling rate to accommodate for the frequency deviation from the nominal rate. This link frequency stability of the loudspeaker output in Linrad, Winrad and Perseus shows the frequency stability of the loudspeaker output when a very stable signal is sent into a Perseus HF receiver.
Starting with version 02.36 Linrad has a transmitter as well as a receiver. The Linrad transmitter and receiver are intended to be operated simultaneously and the operator should be able to listen while transmitting in SSB mode because the the transmitter is muted during those short intervals in the speech when the voice level is low. The Linrad transmitter is currently in an early experimental stage.
Antennas
The antenna - the link between the electronics and free space - is of course the most important piece of equipment for the serious VHF amateur.On 144 MHz, maximum gain is often the right criterion for selecting the best antenna. On higher bands clean pattern and/or low losses are more important since, for receive, gain divided by temperature, G/T is the true figure of merit. The optimum antenna is a compromise. High gain is always good, but depending on the local surroundings and the propagation mode, other factors may be more important and lead to trade off some gain in exchange for other good things.
Antennas can be optimized for maximum gain by a least squares fitting of the radiation pattern to a desired radiation pattern (zero in all directions except forward !!) It is trivial to add extra equations in the least squares fit to improve G/T, F/B, efficiency, impedance or anything else that the computer can extract from the antenna model. This optimization method is convergent - there is only one optimum design for each set of design criteria. Theory, software and hints on how to design, build and verify high performance antennas should contain everything you need to make your own optimum design.
The 2SA13 is an antenna designed for general purpose usage as a four-stack of cross polarised antennas on 144 MHz. Look here for all details on the development of the 2SA13 antenna
Polarisation
Polarisation is by convention horizontal for DX work at VHF. In some propagation modes, e.g. aurora and sporadic E, the polarisation plane may twist due to Faraday rotation, and in EME the polarisation plane may twist due to purely geometrical reasons as well. Fast switching of polarisation, as well as different polarisation for transmit and receive is very useful in these propagation modes.Crossed yagi antennas may be used for fast and independent switching of TX and RX polarisation. Look at Electronic Polarisation Control for hints on several ways to make good use of crossed yagi systems. Of course the same methods apply equally well to feed horns or any other antenna with two orthogonal polarisations.
With cross yagi antennas, it is important to make sure that the two orthogonal parts really are orthogonal. Look here for more info and some NEC simulations. How to calibrate an adjustable polarisation antenna
Filters
Particularly for EME, very narrow CW filters may be useful. If you have access to old-fashioned calibrator crystals - use two for this high performance Narrow Filter with 2 * 100 kHz Calibrator X-tals.If you prefer to use more modern technology look at Sliding FFT and DSP Filtering. This section describes part of the system I am currently using for EME and tries to explain why this method is equivalent to the use of several conventional DSP filters at different frequencies - where the computer continuously selects the best one.
In my opinion the best weak signal communication mode is morse telegraphy, so this mode, CW is the only mode I use on VHF. There are different opinions on what is the best way to receive weak CW signals, some use an ordinary SSB type filter, and others use narrow filters of different kinds. Here is my personal experience in listening to weak CW signals and some examples: demonstration with audio and spectrograms how a weak EME signal sounds with different kinds of filtering.
Look here for a description of my old TMS320C25 EME system and some audio file examples of typical EME signals as they reach my head phones.
As an example of my 1998 system using a 200MHz Pentium (with MMX) look at and listen to the signals from EL2RL, a really weak EME signal. Another example of a rally weak signal is 8J1RL
Computers allow all sorts of interesting experiments. It is well known that an EME signal is about 300Hz wide at 10GHz because of the different doppler shifts from different reflection points on the moon. On 144MHz the EME signal is much narrower than one would expect from the frequency ratio. Look here bandwidth measurements of a continuous wave reflected off the moon for an experiment that clearly demonstrates that two different reflection types are present.
Another example of a computer experiment is shown here: 25W emitted from single 10 element yagi detected via EME using only 4x14 elements
PC software project
The very fast development of digital technology has not only made my dedicated hardware (TMS320C25 with 100ns RAM) obsolete. My first generation PC receiver for MS-DOS is also becoming obsolete. It was written using Watcom C and it works only with "old" computers. There is no support for modern screens and the mouse has to be a serial mouse. The MS-DOS package has served well a few years but for the future an environment where the hardware drivers are outside the radio software will prevent the DSP radio from becoming obsolete so quickly.I am currently working on a new DSP radio package. This time the system is designed for flexibility so it can be used for many different combinations of computers, A/D boards and analog radio circuitry. The platform is Linux and the package will typically operate with a 486 computer together with a conventional SSB receiver as the minimum configuration. The current high end operation is with a 4-channel 96kHz A/D board and a Pentium III providing nearly 2 x 90kHz of useful signal bandwidth in a direct conversion configuration (stereo for two antennas).
The LINUX PC-radio for Intel platforms will be continuously upgraded to show various aspects of digital radio processing and how they are implemented in the dsp package. The Linux PC-radio is not designed for VHF weak signal only. It is very flexible and designed to accommodate routines for all radio communication modes on all frequency bands.
Linrad is the new name for the LINUX PC-radio since July 2001.
Noise Blankers
A good noise blanker may improve station performance a lot. It is a good idea to have several noise blankers inserted at different points along the Rx signal path. At times when there is no strong signal present on the band, a wideband noise blanker can do absolutely fantastic things - it may even remove computer spurs!!! (This is experimental, not only a theoretical idea). Look here for hints on Noise blankersPower Amplifiers
To be successful in two way DX communication, you need a good power amplifier. Vacuum tubes are still the natural choice for high power. If you like to design an amplifier of your own, or if you have something commercial to rebuild look here for Building High Power AmplifiersAs amateurs, we often use surplus tubes. They may be brand new, but if they have been stored for many years, they should be reconditioned before use. The vacuum is gradually deteriorating over time if a tube is left on the shelf. Here is a procedure to recondition power tubes that may decrease the risk of arcing due to poor vacuum.
Power amplifiers are often referenced as "linears", meaning linear amplifiers. To my knowledge, few amateurs run their power amplifiers in class C because a class C amplifier is far from linear and can not be used for amplification of SSB signals. In CW-mode the class C amplifier will give the same output as Class AB with much less heating of the anode because of the higher plate efficiency. This helps thermal stability, and reduces irritation among neighbors because of twinkling lights due to mains voltage variations.
It is simple and straight-forward to run an amplifier with variable class biasing by use of a grid resistor. Look here for more information on this and other aspects of power supplies for high power amplifiers.
Electronic antenna relay
PIN diodes can be used as antenna relays at high power levels on VHF. An about $5 device, UM9415, can handle power levels of several kW continuous (key down) carrier. To listen between dots and dashes is useful in contests and during major openings. Look here for details on my High power antenna relay using PIN diodesBy recording the echoes when transmitting dots at meteor scatter speed, aurora, meteors and airplanes can be seen. Look at some typical graphs Aurora and other Echoes With Narrow Band Radar
Dynamic range
For amateurs in densely populated areas dynamic range is probably the most important aspect of the rig. Today, I do not worry since I now live in a rural area, but 15 years ago, the poor design of commercially available transceivers was the dominating problem in my efforts to work distant stations on 144MHz.In order to make my own situation better, I persuaded my neighbor amateurs to allow me to modify their rigs, and the result was a significant improvement in DX possibilities - and a series of articles. I still get questions about these articles now and then, so now they are available here:
Introduction: English / / Deutsch
Modifications for TS700: English / / Deutsch / / Swedish
Modifications for IC211/IC245: English / / Deutsch
Modifications for FT221: English / / Deutsch / / Swedish
Modifications for FT225: Swedish / / English
Keying clicks with FT221 as an example. Perfect high speed ms keying can be done without any keying clicks. Swedish / / English / / Deutsch
AM keying clicks are directly related to the keying envelope waveform. For a good theoretical treatment, look at the article by Kevin Schmidt Spectral Analysis of a CW keying pulse which you can find at the W9CF links page. In case this link has become outdated you can download the article in pdf format from here with the permission from the author: click.pdf (162770 bytes) The article On the Occupied Bandwidth of CW Emissions by Doug Smith which you can find at the KF6DX site is more practical and shows the same thing. In case the link has become outdated, there is a copy here with the author's permission: On the Occupied Bandwidth by KF6DX This article also points out that the ARRL handbook has a treatment of optimum keying waveforms that is limited to envelopes formed by a single RC link. It is unfortunate that the treatment in the ARRL handbook is outdated and misleading. Keying with an exponential waveform belongs in the vacuum tube era 50 years ago when cathode keying was normal - but even then better solutions with LC filters were used.
Keying clicks are often produced by amplitude modulation, morse code is in itself amplitude modulation and it is obvious from the links above how the shape of the RF envelope is related to the frequency spectrum in the case of pure amplitude modulation. In the real world, the transmitter frequency may be disturbed at the moment of key closure or release. If the frequency/phase modulation contains high frequencies, there will be FM keying clicks that are invisible in the RF envelope waveform. Worst case is when a VCO looses locking for a while at keydown, but phase modulation can be caused by many mechanisms. Have a look here Keying clicks in the time domain AM and FM
Amateur transceivers often use ALC to improve the average to peak power ratio by having a short time constant for the ALC. This is discussed to some extent in the KF6DX article. It is not good practise, ALC should be a safety precaution only, a circuit (TGC) that puts the peak power just below the maximum safe level and not an AM modulator that modulates the SSB signal to make the power more constant over time in a millisecond time scale while adding wideband modulation sidebands. Click here for spectra, and a discussion of this problem, the main reason for splatter on the amateur bands. The abominable ALC.
Like around 1970, when new digital technology started to become popular (the frequency synthesizer) the coming years will probably bring many digital solutions that have inadequate dynamic range performance for many situations.
Dynamic range properties are characterized differently and often incompletely, which makes it difficult to compare measurements made at different places. This link Measuring receiver dynamic range suggests how data could be presented in an unambiguous way. The measurements required to make a fair comparison between digital and analog receivers are also discussed. The third order intermodulation performance of a receiver is often fully characterized by IP3 only. The link gives a simple theory with some oscilloscope images showing intermodulation waveforms. There is also a discussion about precision measurements and the small deviations from the simple theory that can be observed in the FT1000D.
Here is performance data of modern transceivers. The link contains dynamic range data for a number of modern amateur transceivers both in receive and transmit mode.
Amateur transceivers often use ALC to improve the average to peak power ratio by having a short time constant for the ALC. This is not good practise, ALC should be a safety precaution only, a circuit that puts the peak power just below the maximum safe level and not an AM modulator that modulates the SSB signal to make the power more constant over time in a millisecond time scale while adding wideband modulation sidebands. Click here for spectra, and a discussion of this problem, the main reason for splatter on the amateur bands. The abominable ALC. There is of course much more severe splatter caused by operator errors, but for properly operated transmitters ALC related splatter is more problematic than splatter generated by amplifier non-linearities, this is due to inadequate design and I hope it will not be like this in new amateur transceivers. The ALC problems are closely related to power regulation. Excessive power output from the IC706MKIIG in low power mode is another problem owners of this rig should be aware of.
It should be clear from the above links that my personal opinion is that transceiver dynamic range characteristics has to be better specified and measured differently from how it has been done traditionally. The below articles that have been published in DUBUS in English and German as well as in CQ VHF or in QEX give detailed descriptions of the problems and how to deal with them.
Receiver Dynamic Range DUBUS 4/2003, pp 9 - 39. Also in DUBUS TECHNIK VI and CQ VHF in two parts. Part 1 Fall 2004 and part 2 Winter 2005.
Transmitter Testing DUBUS 2/2004, pp 9 - 45. Also in CQ VHF in two parts. Part 1 Spring 2005 and part 2 Summer 2005.
Real life dynamic range of modern amateur transceivers DUBUS 2/2005, pp 22 - 37. Also in CQ VHF Fall 2005.
Blocking Dynamic Range in Receivers QEX Mar/Apr 2006, pp 35 - 39.
IMD in Digital Receivers QEX Nov/Dec 2006, pp 18 - 22.
EME signals received with a huge antenna
During the 2001 ARRL EME contest I brought equipment to Tobbe, SM5FRH to make recordings of the EME signals from his array of 32 X-yagis. The recordings cover about 50% of 5 hours time (9 gigabytes). This link ARRL2001 contains information extracted from these recordings as well as links to the raw data (compressed) This material gives a very good picture of 144 MHz EME signals, how their amplitude and polarisation varies with time.The debate about Digital modes vs CW in weak signal communication.
New digital modes can be made much more sensitive than Morse coded CW. This has made it very much easier to complete a DXCC diploma on EME. Some operators who already made DXCC on 144 MHz or above the hard way with Morse coded CW are not very enthusiastic about the situation. A heated debate has been the result.The arguments in the debate are of two kinds. One is about fairness of competition. Bicycle vs Ferrari so to speak. The other kind of arguments in the debate is about integrity and validity of contacts and it is technical in nature.
The first issue, fairness of competition is politics. How to compromise between legitimate but opposite interests between different groups. Politics is best handled with civilized discussions. Here is my contribution: Mixed contests are needed in EME
On technical issues there is right or wrong. Establishing what is true and false is possible and once that has been done no more discussion is needed.
Since I have some limitations in how much time I can spend with amateur radio - and experimenting is my main interest - I do not promise to answer every E-mail that arrives in my mailbox
