owenduffy.net – Page 44 – Mostly electronics…

Post mortem review of a 144MHz combiner / splitter

This article is a post mortem review of a 144MHz splitter combiner that was made using RG6 coax. It is post mortem (ie post death) because the combiner was stored outdoors without checking that the connectors were protected from weather.

The combiner was used successfully for over 10 years on a 144MHz four over four antenna system (above) without any maintenance problems.

Above is a close up of the Tee point of the network. The coax cables are protected by HDPE sleeving to reduce the chance of damage at the hands of Sulphur Crested Cockatoos, in the event there was no damage.
Continue reading Post mortem review of a 144MHz combiner / splitter

Last update: 3rd September, 2016, 4:17 PM

Transmitter pulse generator for SSB RF PA adjustment

This article describes a pulse generator for adjustment of SSB RF power amplifiers.

The need

Valve RF power amplifiers usually use high voltage power supplies with poor regulation, and typically the voltage may sag by 10% or more on full power CW output, whilst on SSB telephony the voltage may sag a quarter of that.

The effect is that finding PA loading conditions for maximum power output on a key down CW signal optimises the loading for conditions that are significantly different to SSB telephony and not only is the maximum power output likely to be lower for key down CW, but it will be lower when used for SSB telephony than if it were adjusted using a drive that created full output power without sagging the power supply more than speech would.

Additionally, RF PAs intended for the amateur market cannot sustain key down CW for very long before overheating and sustaining damage forcing very short adjustment sessions. Adjustment at continuous maximum power puts great demands on a dummy load if one is being used.

So, to solve these problems, there are three objective:

create a drive / load scenario that is similar to SSB telephony conditions;
operate at reduced duty cycle to reduce internal heating of valves and power supply;
reduce the average dissipation requirements of a dummy load.

Continue reading Transmitter pulse generator for SSB RF PA adjustment

Last update: 1st September, 2016, 12:58 PM

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