Calendar of events

Unfortunately this webpage is sometimes a bit out of date, due to a lack of time by the webmaster.

The club has two official meeting nights per month (2nd and 4th Mondays), but there are some more informal meeting too. The meetings are:

Events planned for 2016

Note, TBC indicates the exact date of the talk is to be confirmed.

Some information about previous events may be found here. In many cases, clicking on any image will bring a higher resolution one.

Previous events in 2016

Impedance measurements by Dr. David Kirkby G8WRB (25/01/2016)

David discussed various methods of measuring impedance, using commerical instruments, rather than ham-grade items, which claim to be instruments, but which rarely igve any specifications on their uncertainty. The model numbers linked are models for which David's company (Kirkby Microwave Ltd) owns. However, due to the size, weight, value etc, only the HP 8753ES was bought along to the talk. The methods discussed were

Roger Jones, one of the club members, bought along an unknown antenna for testing. Stephen Hedgecock (M0SHQ) bought along a homemade 2m/70cm antenna built for satellite work. The 2 m resonance was clear to see, but on 70 cm the resonate frequency was rather unstable. This installability is confirmed in computer models using MMANA-GAL, because one of the dimenstions is very critical. Two eements are spaced about 16 mm apart, and slight changes causes significant deteriotion of performance. A commerical antenna, which is sold as a log-periodic had somewhat better characteristics, although it looks nothing like a log periodic, and it is almost certainly mis-named.

Since David did not bring along a computer with a GPIB controller, those wanting data had to photograph the LCD screen of the HP 8753ES.

Virtual Computers by Dr. David Kirkby G8WRB (11/01/2016)

David gave a talk about the advantages of virtual computers, which are usually called virtual machines, which allow more than one operating system to simultanously run on the one computer. There are various pieces of virtualulation software available, but David Chose to install VirtualBox on his machine. David bought along his own computer, which is a Sun Ultra 27 running OpenSolaris. He showed how its possible to have other operating systems running at the same time, as any can be used with the click of a mouse. The machines are not dual-booted.

In the first screenshot

Host operating system

we see that the host computer is called "hawk", and the host operating system is running a 2010 copy of OpenSolaris. David is not one of those people that always wants to have the latest, and takes the attitude "if it is not broken, do not fix it". OpenSolaris is good for David's personal needs, as well as his business (Kirkby Microwave Ltd), but OpenSolaris incapable of running many programs that are sometimes needed.

In the second screenshot

VirtualBox installed

it can be seen that VirtualBox is installed. VirtualBox is available to run on a variety of operating systems. For this installation, a version for OpenSolaris needed to be installed. It can be seen there are 15 guest machines installed. These 15 machines run either one of 5 different guest operating systems

In principle all 15 guest machines could be run simultaneously, although even though the Sun Ultra 27 host has 24 GB of RAM, that would be incapable of running 15 machines well all at the same time. If each guest was configured to use the minimum amount of memory needed for that system then it would be possible to run all 15 at the same time, but performance of both the guests and the host would be poor.

In the final screenshot VirtualBox installed it can be seen that the host system is still running OpenSolaris, but there is also a copy of Mint Linux running, as well as an XP installation which was allocated 2 GB of RAM and a Windows 7 installation which was allocated 4 GB of RAM. The amount of RAM a system uses is set before the system boots, so depending on what one intending doing with a particular virtual machine on a particular day, it could be increased. One of the Windows 7 or 64-bit XP machines might be increased upto about 18 GB, if it was the only system running. That would leave 24-18= 6 GB for the host system, which would be enough to run it confortably.

David showed how a guest is instlalled, by starting an installation of XP, but since it takes some time to complete the installation, this was not completed as part of the talk.

Previous events in 2015

Electrostatic discharge by Maurice Summer (26/11/2015)

Maurce gave a talk and demonstration of the static electricity which can be generated so easily - with sellotape being one of the worst culprits. The subject of latent damage was discussed, where damage is caused to a part, which is not detectable when tested, but causes premature failure later.

Power supplies (part 2) by Patrick Hingham (G8JLM) (23/11/2015)

Patrick gave a second in a multi-part series on DC power suppies. This time he concentrated on the shunt regulated power supply, where the current drawn from the power source (Psource)is constant. It is made up of two components.

  • Power used by the load, Pload
  • Power internally disipated in the power supply Pinternal
At all times,Psource = Pload + Pinternal. This means there's the DC output from the power supply keeps a more stable voltage as the load current is changed, since despite changed in load current, the power used by the power supply is constant. Pat shown giving the second part of his talk

Power supplies (part 1) by Patrick Hingham (G8JLM) (26/10/2015)

Patrick gave a talk about DC power supplies. Each member of the audience was asked to name one parameter for the specification of a DC power supply. The first person got had it easy, mentioning DC output voltage. Numerous parameters were discussed including

  • DC output voltage
  • DC output current - both continuous and short periods.
  • Short circuit protection
  • Mean time between failure (MTBF)
  • AC input voltage range
  • AC input frequency
  • Whether the common two wires were used for the connections, or whether 4 wires were used for remote sensing purposes.
  • How to configure two DC power supplies for redundancy, so the load still gets power even if one of the power supplies fails.

Although the talk was not aimed to show how to design power supplies, it was mentioned that large power supplies generate a lot of heat in the semiconductors, and that heatsinking them properly does not mean putting in lots of heat sink compound, but the bare minimum. There was quite a lot of audience participation.

Pat shown giving his talk

The next photo shows how 4-wire sensing may be used to keep the voltage at a load constant, irrespective of the voltage drop across the cables feeding the load.

4 wire sensing. Pat showed how 4 decoupling capacitors may be used in a power supply in such a way that either an open or short circuit failure of any one capacitor does not result in total loss of capacitance.

Fail safe decoupling , and was a very successful talk. Pat will be delivering the second part of his talk in the near future.

Annual General Meeting (13/10/2015)

The society's AGM was held on Monday 13th October 2015. Minutes should have been circulated to all members.

Noise in RF pre-amplifiers (28/09/2015)

Dr. David Kirkby (G8WRB) gave a talk about noise figure, with a particular emphasis on RF pre-amplifiers. David explained the concepts of noise figure, and demonstrated measuring the noise figure of two RF amplifiers with an HP 8970A noise figure meter. The two amplifiers used were both made by Minicircuits - one being designed for low noise operation, and the other high power. David explained that using the Y-factor method, which is the most common method of noise figure measurement, two different levels of noise are required. Although these can in principle be obtained from resistors immersed in liquids of different temperatures, which gives a calibrated results, in practice a semiconductor noise source is often preferable, as it just requires 28 V to switch the noise source on/off, which is a lot more convenient than using liquid nitrogen, ice or boiling water.

For accurate results, the connector on the noise source must mate directly to the connector at the input of the amplifier. This was not the case for the amplifiers used, so the results could not be expected to be very accurate, but these components were sufficient to illustrate the principle.

In the first figure, we see a Minicircuits ZHL-2-8 medium power amplifier being measured at 432 MHz. This amplifier was designed for use of powers up to 29 dBm (almost one Watt). Generally amplifiers designed for low noise have very low output powers, so one would not expect an amplifier of this power to be particularly low noise. The HP 8970A indicated a gain of 29.55 dB and noise figure of 6.38 dB. (Click the image for a higher resolution version.) This amplifier was designed for use of powers up to 29 dBm (almost one Watt).

Power amplifier has higher noise figure

In the next figure we see a Minicircuits ZFL-1000LN+, which has a significantly lower noise figure (3.23 dB), but is designed for much lower powers (2 mW) than the medium power amplifiers. Again, the gain and noise figures indicated on the HP 8970A noise figure meter should only be used as a guide, due to the fact the noise source did not have the right connector for the amplifier.

Low noise amplifier

HF Antennas and MMANA by Don Wilks G3VCG (15/9/2015)

Don gave a talks about HF antennas. The emphasis was mainly on using wire antennas, which have several advantages over using beams at HF.

  • They are much cheaper to construct, so don't require getting out a credit card.
  • They don't require the expense of a large tower, with the planning permission issues such towers have. The antenna can be supported with bamboo canes and pieces of string.
  • In the event such an antenna did fall down in high winds, it would not do as much damage as a tower mounted antenna would. In fact, in most cases wire antennas would cause no damage at all.
  • They can be erected in restricted spaces.
  • They can be taken portable quite easily.
  • One learns more experimenting with wire antennas, than one ever would erecting a large commercial beam antenna.

In addition to handing out some notes on HF antennas Don made use of the club's whiteboard to illustrate the points.

Don

Don covered a wide range of antenna types including

  • λ/2 dipole.
  • 3λ/2 dipole.
  • 5λ/2 dipole.
  • End fed Zepp.
  • Long wire fed at λ/4 or λ/2.
  • Electrically short antennas with bottom loading by a coil.
  • Electrically short antennas with capacitive top loading.
  • Electrically short antennas with both top and bottom loading.
  • Horizontal end-fed array with λ/8 spacing.
  • Half square.
  • Lazy H.
  • Slot antenna (Don also has a webpage about the HF Skeleton Slot antenna he developed)

There was various descriptions about methods to lower the radiation pattern of antennas, to work longer distances, as well as the construction of tuning units, using ferrite cores, variable capacitors and fixed inductors. Overall the talk was very well received.

Despite the title, there were no simulations shown from MMANA (now renamed MMANA-GAL), but the use of the MMANA-GAL software was covered in previous talk a couple of months earlier by Don.

Non-ideal Components by Dr. David Kirkby G8WRB (24/8/2015)

"Dr. Dave", as he is known in the club, bought along a bench-top LCR meter, to show how electronic components don't always behave as you expect. For example, an inductor will become a capacitor above a certain frequency! The frequencies used for the demonstration were all below 1 MHz, showing that care is needed in the selection of components at even modest frequencies. David also showed how a standard diode may be used as a variable capacitor.

The Hewlett Packard 4284A precision LCR meter shown below (click image for a higher resolution version), was used for the demonstration.

4284A

The HP 4284A LCR meter covers the frequency range 20 Hz to 1 MHz. This was calibrated only a week earlier by Keysight, and given its basic uncertainty is 0.05 %, it is a much better than typical junk LCR meters from China, where random number generators are used to generate the specifications! David bought a number of fixtures for the LCR meter, including some he had made, and some made by HP, Agilent and Keysight. All have 4 BNC plugs, for a true 4-wire Kelvin connection to the LCR meter.

The first photograph shows 3 commercial fixtures for the LCR meter.

4284A

The next photograph shows a home-made fixture, which has two banana sockets with a standard 3/4" (19.05 mm) spacing, for connection to devices with banana plugs.

4284A

David

  1. Demonstrated a 33 mH inductor was no longer inductive above 400 kHz, but was acting as a capacitor due to the self-resonate frequency
  2. Asked the audience to guess how much capacitance would be needed to resonate his homebrew 1.5 x 1.0 m loop antenna

    4284A

    on top-band, and after getting answers showed that actually the self-capacitance of the 19 turns of 2.5 mm2 wire was making the antenna resonate at 320 kHz, so no amount of parallel capacitance would resonate the loop on top-band. The antenna was designed to pick up Loran-C signals at 100 kHz for a Stanford Research FS700 frequency standard. Since Loran-C signals spread over a large spectrum, so an antenna tuned for resonance at 100 kHz would not work.

    Stanford Research FS700

  3. Showed how applying reverse bias on a standard rectifier diode (1000 V, 6 A) the diode behaved as a variable capacitor, with capacitance changing from around 400 pF with no bias to about 200 pF as up to 2 V of reverse bias was applied. Due to limitations of the LCR meter, a larger bias could not be applied. Patrick Higham (G8JLM) pointed out the importance of this effect in almost any modern radio, as they all use a frequency synthesizer, which itself needs a voltage controlled oscillator (VCO), which would use a varicap diode. Following a recent eBay purchase of an Agilent 16065A 200 V DC External Voltage Bias Fixture, maybe David will at a later date talk more about varicap diodes, when their characteristics could be investigated more.

Amateur Satellites by Stephen Hedgecock M0SHQ (27/7/2015)

Stephen bought along various antennas for 2m/70 cm, including one he had made, and one he had purchased. An attempt was made to contact the International Space Station, but was unfortunately unsuccessful.

Antenna towards ISS Satellite software Several of us

HF antenna modelling using MMANA-GAL by Don Wilks G3VCG (22/6/2015)

Don, one of the club's antenna experts, gave a demonstration on the use of the free antenna analysis software MMANA-GAL for the analysis of HF antennas. Even though some other members of the club had used MMANA-GAL before, they too learned from Don's talk. You can find some plots Don produced with MMANA-GAL on his webpage about the HF Skeleton Slot antenna he developed.

Entry to the RSGB May 2 m contest

The club entered the 24 hour 2 m contest, which started on Saturday and finished Sunday, being active for almost all of the 24 hours. Despite only having a 25 W transceiver on the day, we were placed third in our category, where other entrants would have had considerably more power. Unfortunately, although at a fixed site, we were in a /P division, as our license had not been changed to reflect our new address. Our license has since been updated to our new address

Hunter SDR receiver by Graham Leggett G7JYD (13/4/2015)

Graham demonstrated the Hunter Software defined radio (SDR) receiver he had built. Graham is working on the construction of a SDR transceiver, which he will no doubt demonstrate once it is completed.

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