August 2016 – owenduffy.net

A certain formula for antenna system Q

A correspondent questioned the writings of an online expert who opined whilst discussing loaded monopole antennas:

… there is a formula circulating the Internet which states that antenna Q is equal to 360 times the frequency in MHz, divided by the 2:1 VSWR bandwidth in kHz. One has to assume they mean antenna system Q, but that’s not a given. While this formula might give you a comparison between antenna A and antenna B (all else being equal), the actual Q of the antenna (system or otherwise) requires a textbook-full of formulas, and a lot more information than just the 2:1 bandwidth! Fact is, this formula is no more specific than the number of DX contacts a specific antenna garnered.

The formula given is:

Q=360*fc/B(VSWR=2) where fc is the centre frequency.
Continue reading A certain formula for antenna system Q

Last update: 30th August, 2016, 9:09 AM

Tuning combiner lines

A common method of combining two 50Ω antennas to a single 50Ω feed is using a quarter wave transformer using 75Ω line from the common feed point to each antenna.

A recent posting to one of the ham fora raises the posters problems with making this really simple feed system work.

Above is his measured input characteristic with good 50Ω loads on each leg. Reading a hundred posts, it seems that he attributes this to legs of 0.167m length of RG11. The problem is that RG11 as most of us know it has a solid PE dielectric giving it a vf=0.66 and that 0.167m is 63° at 207MHz… so why the response above. Continue reading Tuning combiner lines

Last update: 1st September, 2016, 6:35 AM

Treatment of the -ve DC return path for transceivers in mobile installations

A correspondent wrote with questions on the -ve return connection in a mobile installation of a typical ham transceiver. He was confused by the advice on an online expert who opined…

If instead, you decide to connect the negative lead to the nearest chassis ground point (seat support, trunk brace, etc.), there will be a difference in resistance between any of these points and the battery’s chassis ground. A differential of three to five ohms is not uncommon. Whether this causes a ground loop to occur is moot, the resulting voltage drop under load is not.

A resistance of 3-5Ω from any part of a metal car body to the terminal clamped to the battery -ve terminal is way above anything I have observed, and would seem to be sign of a fault rather than not uncommon. Continue reading Treatment of the -ve DC return path for transceivers in mobile installations

Last update: 23rd August, 2016, 3:48 PM

Fan controlled by humidity sensor – pre implementation data gathering

This article documents measurements of temperature and relative humidity (RH) over 10 days prior to implementing the fan solution to provide a baseline for designing the Fan controlled by humidity sensor.

A RC-4HC datalogger was used to collect temperature and RH measurements over 10 days which included a range of late winter weather, some rain, some fine clear days. The logger was located at 1.5m above floor level in the shed in a relatively clear spot in the middle of the area..

Above is a chart of temperature and RH. The daily RH peaks are typically between 08:00 and 09:30, and pretty much coincide with the minimum observed temperature. There is clearly a lag from outside temperature which would tend to be minimum a few hours earlier, and a lower minimum (there were plenty of frosty days in the sample set). Continue reading Fan controlled by humidity sensor – pre implementation data gathering

Last update: 22nd August, 2016, 5:43 PM

Antenna analysers – the basics – QST Aug 2016

I was browsing Joe Hallas’ award winning article Antenna analysers – the basics in Aug 2016 QST when I saw some welcome news.

In the notes to this graphic, Joe tells us that you can display any “individual plot” in Antscope. I would dearly like to NOT display the |Z| curve which is apparently there as a concession to the ham audience that doesn’t understand the complex nature of Z and the need to see it in two dimensions. Continue reading Antenna analysers – the basics – QST Aug 2016

Last update: 19th August, 2016, 8:28 AM

RC electric drive noise

The website Mini Quad Test Bench by Ryan Harrell contains a growing set of test results for motors and ESCs.

In respect of ESCs, it appears to use BLHeli unless the ESC is incompatible in which case the proprietary firmware is used.

The configurations are targeted mainly at the mini quad racer part of the market, and the requirements for different size copters and different applications varies significantly.

The site contains an online editable graph facility that allows customisation of the graph.

Above is a graph of current vs time for a uniform ramp servo input, and I have disabled most of the ESC types for clarity, selecting just the first two on the list and one which I use (F-30A). Continue reading RC electric drive noise

Last update: 15th August, 2016, 6:58 PM

New hosting service – 2016/08/10

owenduffy.net has been moved to a new HSP.

Though for the most part it should be transparent to visitors (if I have done my job correctly), the underlying IP addresses have changed and some software that caches IP addresses as part of their optimisation may need to be forced to refresh from the current site.

Further, the level of some key server software is inevitably different and although some considerable time has been spent on compatibility testing, there may be some defects that come to light in the coming days.

Hopefully we will put behind us the service experienced with A Small Orange, one of the EIG group companies.

Last update: 10th August, 2016, 2:26 PM

Apologies for service disruption

Some changes were made by others that disrupted the site availability.

As a result, I am moving the site to another HSP, and that may result in some issues over the next week. It is quite a complex site and exposed to the way in which apache and PHP evolve (without 100% backwards compatibility).

My apologies.

Last update: 10th August, 2016, 1:00 PM

Time to get some experience with 32bit microcontrollers – some progress

At Time to get some experience with 32bit microcontrollers I wrote that after almost 50 years working with 8bit microcomputers and microcontrollers, I thought it about time to get my hands dirty on some 32bit microcontrollers.

This article reports some work in the STM stream.

I abondoned the ST Discovery board in favour of a low cost basic ST32F103 development board costing about $4 on eBay. The chip is a 32bit microcontroller clocked at 72MHz.

As a learning vehicle, I decided to implement the functionality contained in Arduino thermistor thermometer – a tutorial.

The development environment is Eclipse with the GNU ARM toolchain, a debugger probe, the basic development board, and a 1602 LCD display with I2C backpack and an inexpensive CP2102 USB RS232-TTL interface.

Above is the working trial. The GDB debugger allows On Chip Debugging (OCD).
Continue reading Time to get some experience with 32bit microcontrollers – some progress

Last update: 8th August, 2016, 6:10 PM

Exploiting your antenna analyser #23

Seeing recent discussion by online experts insisting that power relays are not suitable to RF prompts an interesting and relevant application of a good antenna analyser.

Above is a sweep of an A/B changeover relay intended for HF application at up to 100W and lowish VSWR. The sweep is actually from 1 to 61MHz to be confident that there is not poor behaviour just outside of the HF range that might present on another implementation of the same design. Continue reading Exploiting your antenna analyser #23

Last update: 3rd August, 2016, 2:46 PM