Using the excellent Radio Mobile program by Roger VE2DBE, I’ve produced a 144.3MHz coverage map for our current setup at the club-house. For info about the antenna, losses, power etc see the first page.
The report is here.
Strange hot weather but good conditions on 2m 🙂
Monday 5th and Tuesday 6th September very busy on 2m. At times tonight every 10kHz step and often 5kHz step was busy. Great to see/hear. Worked F8IQS in Caen on CW. His signal was easily 599!
Even bumped into another club member, Steve, M6JJV working contest from Berkeley 🙂
Have you ever wondered what it would be like to sit at your antenna and see what it ‘sees’? Well, this evening I attached a camera to my antenna and shot the following short video as the mast was raised and rotated.
The video starts with the antenna pointing at 45 degrees. Once elevated its rotated to the following directions where it stops momentarily: North, West, South, then back around to East, South, back to North then to 45 Degrees before the mst is finally lowered again.
What this little experiment shows is that the horizon looks pretty flat in all directions – no big hills evident. It also gives a good feel for how well the antenna clears the surrounding houses.
So, for a couple of months I’ve had a growing suspicion that I’m not receiving as well as I am transmitting. I first noticed the problem when observing the strength of signals from GB3VHF which after one winter storm seemed much weaker than it had been. However, there seemed to be no impact upon transmit as SWR remained negligible and stations some distance away where still receiving me well even when my transmitter was generating about 0.5W PEP!
Since first noticed I’ve worked many stations and the signal reports always seem to identify that I’m not receiving as well as the remote station. Establishing this pattern has taken some time. It should be said at this point that its the same antenna and feeder used for both transmit and receive.
Its been a wet Winter and not really suitable for removing weather proofing from exterior connections and outdoors work. But now Spring has arrived things are finally starting to get dryer and warmer. Alongside this I’ve taken down the mast and antenna for work to support an new PRO.SIS.TEL 641D rotator. This has enabled a detailed inspection and test of feeder and antenna.
Before starting any work I thought about and identified the following possible causes:
Problem (a) seemed possible but without a second receiver its very hard to be sure. But its a quality Transverter (Kuhne TR144H) and I’d be surprised if that had partially failed – remember signals are only 2 – 3 S points down on what I think they should be.
So, (b) or (c) seemed much more likely.
So with the mast down and everything removed back to a dry, warm environment I removed weather seals – no sign of water or even the slightest amount of moisture ingress. The coax was tested with a DC Ohm meter – no shorts or higher than expected resistances; with an antenna analyser and dummy load – again this looked perfect.
So attention shifted to the antenna feed-point, pictured before it went outside for the winter and before the connections where weather sealed. Note: Grey plastic tape overlays amalgamating tape. Again, there was no signs of moisture ingress into the coax, but there where signs of galvanic corrosion between the aluminium driven element and the stainless steel (A2 grade) machine screws to which the feeder connected. Actually, to be more precise between the aluminium driven element and the flat washers on each machine screw. It was clear that the liquid rubber sealant which I’d applied had not sufficiently covered these connections and thus water had been in contact between the different metals acting as an electrolyte.
To be clear, the machine screws where not showing any significant signs of corrosion, just the washers. So the feed-point was thoroughly cleaned and reassembled without using the washers then liberally coated in liquid rubber to seal against contact with water.
Now, at this point I don’t know if this was the problem, but it does make some sense. Any transmit signal would be of a much higher voltage/current than a received signal so would probably cross a mildly corroded junction with marginal attenuation. But marginal attenuation of a much weaker signal would be more noticeable. That’s my reasoning anyway.
The acid test will be when I erect the mast and antenna again in a few weeks and see if received signals such as GB3VHF have returned to their earlier strength. When doing this I will also use new feeder, so if signals are stronger I’m never going to be absolutely sure the problem was galvanic corrosion. Of course the problem may still be present and it could be another cause such as the receiver!
For information, the antenna is an 8-element LFA Yagi from InnovAntennas.
Over the last few years I have not had to think too hard about ensuring I’m driving my FT-2000 at a good level. i.e. its not being overdriven. It has an Automatic Limit Control (ALC) meter and its been a matter of adjusting the audio amplifier gain such that the ALC triggers occasionally and only on the highest signal peaks. If a good quality transmitted representation of the input signal is to be produced its essential that the equipment stages are not overdriven. I guess we all rely on the ALC and its correct adjustment is second nature, however, take away the ALC function and what do you do? How do you ensure you don’t overdrive the radio or downstream amplifiers? This is the problem I had 🙂
I’m using the FT-2000 as an exciter for my Kuhne Electronic VHF transverter. In this mode the ALC circuitry of the FT-2000 no longer works and the transverter does not output a drive level signal which I could connect to the FT-2000 or to a homebrew ALC meter. But the transverter does have a red ALC LED which illuminates when the drive level becomes excessive.
The signals I’m presenting to the radio are audio and are either voice from the microphone (voice chain) or data from a SignaLink USB audio interface (data chain). Each of these chains has several amplifiers which can be adjusted by an operator:
Ensuring each amplifier is set to a level such that downstream equipment is not overdriven producing distortion and splatter is essential!
So, how did I adjust everything? Well, the transverter documentation says to start by setting the radio to full power (0.1mW when using the transverter connector) in RTTY or CW modes and key the transmitter, then adjust the TX Power control of the transverter to output 20 Watts which should be prior to ALC LED illumination. Having done that using a constant CW modulated signal I had the transverter generating maximum power for a maximum signal drive level from the radio. That set the upper threshold which could not be breached by the voice or data chains without distortion or the transverters ALC activating.
Next I fed the constant CW, RF output of the radio directly into my digital oscilloscope rather than the transverter and noted the signal power. Interestingly, the power levels seemed much higher than the figures stated in the FT-2000 Operating Manual. But from the previous step I knew the transverter was ok with this.
This allowed the accurate identification of the RF power level driving the transverter and thus the baseline levels for the voice and data chains. The next step was to generate a constant RTTY tone using the computer and adjust the amplifiers in the data chain such that the levels identified in the first step where not breached.
Once done, I performed a similar exercise on the voice chain for J3E (SSB). By selecting a cumulative display mode it was possible to record the varying power level over several iterations of a CQ call. As can be seen, most elements of the call resulted in an average level, but interestingly, the ‘G’ or Golf in G0RVM generated significantly more power.
The gain of the voice amplifiers in the voice chain were adjusted so that the upper threshold identified in the first step were not breached. The last step was to adjust amplifier gain for A3E (AM). I had expected this to be very similar to the J3E level but it needed to be slightly higher.
AM is not a mode I use very often, if at all, so I didn’t spend too much time adjusting amplifier gain but I did change the gain from the factory default to ensure that the upper threshold identified by the first step was not breached. It could still go a little higher but it will do…
So, the result of all this testing is that I am now confident that using the voice or data chain’s I’m not going to overdrive the radio or transverter. Downstream of the transverter is an RF power amplifier, but that is specified to generate 1kW for 25 Watts of drive, and as I don’t wish to run it at full power, overdriving should not be an issue.
I’ll just add at this point that the above approach is applicable to amplitude modulated signals only and thus not those that are frequency modulated.
I could not resist the name of this article, it seemed so obvious 🙂
I’ve been in the radio room this afternoon listening to the 2m VHF band, calling CQ on CW and listening to the beacons. The Scottish beacon, GB3ANG in IO86mn near Dundee has been perfectly readable for extended periods which is pretty amazing for a station using a 4 element Yagi and 20 watts.
Back in the 80’s I received my first amateur radio license, a ‘B’ license as it was back then entitling me to use, if I recall correctly, bands above 30 MHz only. We didn’t have 6m and 4m band allocations then so it meant 2m and 70cms were the bands of choice for me and the majority of other ‘B’ license holders.
So I started with 2m and 70cms SSB and my interest was soon peeked by space communications with Oscar 10 etc. Those early Oscar’s were excellent as, if I recall correctly, they where in elliptical orbits which meant that they appeared to hang in the sky for some time, not shooting overhead as do satellites in lower more circular orbits. Their transponders were just like big SSB repeaters in the sky and I worked all over Europe. Happy days. But working those Oscars was not that easy especially at Acquisition of Satellite (AOS) as they were pretty far from the Earth so a good station was needed. My station grew to a 2m, 9 element crossed Yagi; a 70cms, 19 element crossed Yagi, both circularly polarised; azimuth plus elevation rotators and mast head preamplifiers. The radio was a Yaesu FT-736 which I only sold last year…It was a wonderful radio.
Of course with those antenna’s it was perfectly possible to work terrestrial communications too. I made contacts across the UK and Europe using sporadic E and Tropospheric propagation. Those were exciting times indeed as not only was amateur radio new, to me, but it was such fun learning about different types of propagation, satellite communications and having great fun talking to people.
Ok, so by now some of you will be asking – what is the point of this article? Well, bear with me… I learnt Morse code and in the early 90’s received my ‘A’ license. Soon afterwards the world changed as I started doing radio professionally for a few years. This removed all interest to do amateur radio stuff when I got home from work. Anyhow, time passed and I started to operate on HF talking to people all over the world – it was fascinating – I learn’t lots more.
With solar cycle 24 rapidly fading and with the real possibility that cycle 25 may be no better, probably worse, I’ve been looking back and found myself comparing my V/UHF experiences with those of HF.
My conclusion is that HF is fun, its pretty easy to work stations across the UK, Europe and further afield depending on your station. Take a minute and think about that, especially the ‘depending on your station’ bit. Do you have the space to erect, for example, a 9 element Yagi for the 20m band? I’m seriously impressed if you do 😉 I bet most of us are using simple HF antenna’s and we are constrained by space, interference and the cost of erecting a high-performance HF station. But move on up to VHF and things get so much easier. I bet many of us could erect a 9 element Yagi for the 2m band and mount it several wavelengths above ground too…. Such an antenna will cost a tiny fraction of its HF equivalent; its small, light and easily rotated too. Interference does certainly exist above HF but its nowhere near as bad. But, FM repeaters and modes such as D-Star aside, VHF is dead these days I hear you say? I cannot deny that it is much quieter than in the 80’s and 90’s but its still a happening place… Its perfectly possible to work stations using phone and data modes across the UK and into Europe when propagation is flat. All those exotic data modes are present and in active use too, people are exploiting Tropospheric propagation, meteor scatter, satellite communications, auroral propagation to name a few.
So its back to the future for me, the HF antenna is now down and until I receive my new VHF antenna I’ve elevated one of my original 9 element Yagi’s. Already I’m hearing beacons GB3ANG in Scotland, GB3VHF in Kent and GB3SSS in Cornwall. I even worked an old friend towards London using CW (RST:559) and Olivia with 20 watts. Who says VHF is line-of-sight. 🙂
I guess you realise the message by now…. V/UHF is lots of fun, you can work DX, high-performance stations are much more achievable than on HF and you learn lots. Amateur radio is so much more than HF, FM, repeaters, D-Star etc. So, why not explore our higher bands?
In preparation for the pedestrian fox-hunt that happened a couple of weeks ago, I decided to take a look around for some antenna designs that I could build at home. I came across a blog post which showed off a Time Difference of Arrival (TDOA) antenna that someone had designed and built.
How it works
The antenna relies on a timer, in my case a 555 timer, to generate an audible signal. I have mine calibrated to a frequency of 1kHz. This signal is level-shifted so that it goes between -4.5V and 4.5V. The use of a set of diodes causes the circuit to quickly switch between the two dipole antennas mounted to the unit (tuned to 145mHz). When the signal received by the two dipoles is out of phase, the receiver emits a tone. When the RF source is equidistant from both antennas (when you are pointing at the “fox”) the signals are in phase, causing the tone to disappear.
This design is fully functional regardless of how close you are to the source, providing an advantage over other antenna designs. The audible tone also provides a clear indicator of direction. However, the main issue is the fact that you get two readings, at 180° to each other.
The list of components required is quite simple, so I had most of them laying around.
When building the antenna, I used two blog posts for reference. You can find them here(pdf) and here. For some reason I ended up combining both of the designs, using the Bryonics.com design for the dipole circuits, and the 146970.com design for the rest of the circuit. This probably isn’t advisable, but it worked for me. Here is the circuit I used:
You can test the 555 timer circuit by hooking the output pin up to a speaker. You should hear a tone when power is supplied to the circuit. I started by breadboarding the timer circuit, and then moved it to veroboard for the final build. Once the circuit was complete, I zip tied the PTT button down on my PMR446 Walkie-Talkie, plugged the antenna into my Wouxun KG-UVD1P reciever, and tuned into the 446mHz signal. The antenna worked perfectly first time, providing a clear tone that faded out when I pointed the antenna at the radio.
For the physical construction of the antenna, I used some thick, single core, cable to make the dipole antennas, and mounted the whole thing on a 60cm long piece of wood. Lots of glue gun was used as I couldn’t be bothered to find some screws. I also drilled holes in the ends of the wood to poke the coax through. The dipole elements were attached using screws, and can be bent between a folded and deployed position.
I took the antenna out for the pedestrian fox-hunt that happened a few weeks ago. The antenna picked up the first signal very well, and the audio tone allowed me to find the direction of the transmission to quite a high accuracy. Unfortunately for me, I headed off into the wrong direction, as the antenna will give you two readings at 180° to each other. I had it running all evening, without the battery flattening. The RF choke fell off quite quickly, as I had not done a very good job with the soldering. I was also worried that the connection between the RX input coax and the board would break, so I would consider using a small connector in the future (such as an SMA socket). Overall the antenna worked very well, albeit sending us in the wrong direction! In terms of improvements, I would also like to mount the circuit board properly, and put it in an enclosure.
If anyone has any questions, or wants me to bring mine with me on a club night, just let me know.
Over the last two weeks the TSGARC has provided Equipment and personnel to put the Thornbury and District Museum on the Air.
The event was widely supported by Members, the Museum and even the Press.
Two main stations were used both using the Callsign GB4TDM. The majority of contacts were made using VHF 2M with a number of CW and a single SSB contact being made with HF.