AmateurRadio.com

I have been browsing for information about various circuits recently. Two books that are often recommended are “Solid State Design for the Radio Amateur” (which is now quite old) and its successor “Experimental Methods in RF Design”. I decided to get a copy of the latter and was gobsmacked to find on Amazon.co.uk that only used copies are available, priced from £557.14. No, that’s not a misprint, it’s about 800 bucks in real money.

I clicked over to Amazon.com and found the same book listed as available new for a slightly more reasonable (joke) $600.00, or I could buy a used one for a whisker under $500.00. Who are these sellers kidding? Do people really pay $600 for a book that was published by the ARRL in 2003 with a cover price of about $40? I wish I’d invested in a pile of them – they would have been a better return on my investment than my shareholdings (sick joke.)

I browsed down the Amazon.com page to see the usual stuff Amazon tells you about a book and found that 37% of people (rich or insane people, presumably) buy the item featured on the page, while 36% buy “Experimental Mathods (sic – not a joke) in RF Design” for a mere $42.70. That’s 36% who can’t spell, I guess, but I can live with a misprint in the title if it saves me $550.

But seriously, what’s going on here? Is “Experimental Mathods” a pirate copy using a mis-spelt title to avoid copyright infringement? Is a joke being played on somebody? If I buy it will I receive something other than what I expected, like searching for “mammaries” instead of “memories” on Google? I’m off to confused.com.

I’m preparing to build my 40m version of Roger G3XBM’s XBM80-2 QRPP transceiver into a little box but before I start I have one thing more to do. I have to design an output filter for the transmitter.

Roger didn’t bother. Comments from him suggest that he was aiming for a low parts count and assuming that any filtering would be provided by an external ATU. However Alan VK2ZAY comments that the output was more like a square wave. The second harmonic of my 7.030MHz signal would be on 14.060MHz, also a QRP frequency and I can’t be sure that my antenna won’t radiate this too. So I think an output filter is a good idea.

Alan used a two-stage pi network, each stage consisting of two 820pF capacitors and a 2.2uF inductor. I built this up on my breadboard, terminated the end with a 50 ohm load (actually 47 ohms, the nearest resistor value I had) and checked it with my SWR analyzer. The picture above shows the result.

Out of interest I also checked the response of a single stage of filtering using the same values, shown in the next picture. You can see that the cut-off frequency is a lot shallower than when two stages of filtering are used, so clearly attenuation of harmonics would not be as effective.

My problem is that I need a filter for 40m not 80m. I tried searching the web to see if I could find some online design tools, and I did here, here and here. However when I fed the design parameters for 80m into the calculators the results they came up with were a lot different from each other and also from the values VK2ZAY used.

In the end I decided to use trial and error. I figured that since the frequency is higher the values I would need to use would be lower. The next lower value inductor I had in my parts box was 1uH, so I don’t have a lot of choice in the matter. I substituted that for the 2.2uH. The result shown by the SWR analyzer was promising, but there was a big hump below the cut-off frequency and the SWR at 7.030MHz was a bit on the high side. So I then tried lower values of capacitors. The lower I went, the shallower the cut-off but the lower the hump as well.

In the end, I settled on 680pF capacitors with the 1uH inductor. There is still quite a steep hump below the cut-off frequency but I’m only concerned with the performance at 7.030MHz where the SWR is 1.2:1, which I don’t think is too bad. The cut-off curve of this one stage filter seems as steep as the two-stage one VK2ZAY used on 80m so I think one stage of filtering will do.

After I’d written the above, the thought occurred to me to try the low pass filter in the circuit while it was on the breadboard. I’m glad I did, because I’d have been disappointed to find after soldering everything into place that the output power had fallen from 100mW to 25mW. After a bit of experimentation, it appears that connecting the low pass filter directly to the collector of the first transistor as VK2ZAY did in his version of the transceiver loads the circuit and reduces both transmit power and receive sensitivity. The solution seems to be to couple the transistor to the filter using a small value capacitor – I found 180pF gave the best results. This removes the need for the DC blocking capacitor on the output. The power is still down quite a lot, but part of this may be due to all the harmonic energy which is now not making it through to the power meter.

My QRP crystals finally arrived. Murphy worked overtime on this eBay purchase: the first batch apparently got lost in the post and the replacements were sent by mistake to another customer with the same surname. But I have at last been able to try out the little QRP CW transceiver built to G3XBM’s design on the QRP working frequency.

The transceiver is still on the breadboard as I was waiting to try it on both 40m and 80m before deciding which band to make it up for. Certain components need to be optimized for the band in use, and even the crystal in use, since to save space I’ve hand picked a fixed capacitor to set the transmit frequency instead of using a trimmer. I need also to decide how much power output I want because that is governed by other components. I have had over 200mW out of it but I’m not sure if a 2N3904 without a heat sink could really handle that, so at the moment it is set up to give 100mW output when powered by a 9V rechargeable PP3 battery.

I’m leaning towards 40m as this would make a nice little fun portable rig and the antenna being half the size of 80m would make things much easier. 40m generally has more activity during the daytime, too. I have never found 80m an easy band to work with limited antennas and low power, so it has always surprised me that the lower band is so popular with QRPers.

Of course, it being the weekend there is a contest on today, so not much chance of being heard with 100mW. Even if someone did hear me and reply, it’s doubtful if I could pick them out using a receiver with no selectivity whatever! This tiny, simple transceiver is amazingly sensitive, as you can hear in this 1 minute audio clip recorded on 40m this afternoon using my MFJ magnetic loop for an antenna. How many different stations can you copy?

This evening I may put the 80m crystal in and see what it sounds like on that band. Unfortunately the little receiver has no tools for fighting the terrible QRN that comes on during the evening when all the neighbourhood plasma TVs are switched on, and sometimes 80m is all but unusable.

Hot from the K3UK Sked page. “Breaking news from ARRL. They concur that ROS is a spread spectrum mode and as such is not allowed by the FCC on bands below 222 MHz.”

I had just been giving ROS another try with my K3 and 40W this time, and managed to complete contacts on 20m with Alabama and Washington State. Neither of these are locations I would expect to work under current conditions using another mode. As soon as the announcement was posted on the site it all went quiet.

My sympathies to my fellow digimode enthusiasts in the USA. It has always struck me as ironic that hams in the land of the free have to operate under more rigid controls than those of us in “socialist” Europe.

Attempts to use the newly developed ROS weak signal mode are resulting in more chaos and frustration this morning as an increasing number of people pile on to the same frequencies. And attempts to decide frequencies to use with the mode seem to indicate total ignorance of other band users and modes. The latest list of recommended frequencies includes:

7.040 – which is going to interfere with the now long-established WSPR weak signal mode
10.140 – which will also interfere with WSPR’s established frequency on that band
14.100.9 – which does not avoid the long established 24/7 packet network which has already complained about interference by this mode, and is also within the range reserved for beacons.

There are a total of three frequencies suggested for 30m, in ignorance of the fact that 30m is supposed to be used only for narrow band modes.

Someone responded to my earlier comment that a body like the IARU should decide which modes can work where, by saying that the IARU does not recommend frequencies for specific modes as that would interfere with experimentation. However I still think they should. When left to individual amateurs you simply get a fight between one group of people who don’t have a clue and another group, users of existing modes, who have strong views about where they should NOT go. Without some overseeing arbitrator this will just end up as a mode war.

I think it is arguable that there just isn’t room for a 2.2KHz wide weak signal mode on the HF bands at all. But if true, who is going to make that decision?

Late this afternoon I noticed that the ROS website had been updated with a new frequency suggestion for 20m 1 baud operation of 14.105, so I decided to have one more try.

For a while I seemed to have the frequency to myself. However I posted that I was calling there on the K3UK digital sked page. Shortly after, WB2YDS posted that he copied my CQ. I didn’t copy anything from him, but I called again and the second time I got his report, though not perfect copy as the meter in the ROS program showed he was 30db down in the noise.

I sent a report, which I know via the sked page he received, but unfortunately a few seconds after he started KB1PVH started calling CQ and the program started decoding his CQ call instead. With two stations on the frequency it was hopeless, and soon after that Olga called me for dinner and that was that.

Normally an almost-QSO wouldn’t rate a mention, except that I was using 5W from the FT-817 to a dipole, and WB2YDS was also running 5W to a long wire. I don’t think I have ever worked across the Atlantic QRP to QRP before, and indeed I still haven’t, but I nearly did, which shows what the ROS 1 baud mode is capable of if you are lucky enough to have a clear frequency for the duration of the contact.

Unfortunately the software has a number of issues that need to be addressed before it can be considered suitable for general use, one of which is the ability to lock on to the replies to you and ignore anyone else who comes up on the frequency.

Another problem is that ROS is still a mode without a home, and at 2.2KHz wide it needs quite a big home and no-one seems to want to make it welcome. There have already been complaints that by settling on 14.101MHz it is disrupting a long established packet network, and while I’m typing this someone seems to be jamming the 40m frequency 7.053MHz with some sort of digital idle signal.

As I said in an earlier post, the issues involved in releasing something like this to the ham populace at large haven’t been thought through. Perhaps there needs to be an overseeing body like the IARU that decrees what modes can be used and where, so there can be no arguments. It’s a pity that the use of a mode with such promise is being thwarted by so many difficulties, but practical reality often stops you doing what you want to do and ham radio is not immune to this.

I got fed up rather quickly with the new digital mode ROS. On 20m it is a zoo, with everyone calling on top of everyone else and very little being worked. I tried it on the advertised frequency on 10m and got nothing, despite the fact that DX is being heard via WSPR on that band.

What this proves, I think, is that it isn’t enough to be clever enough to come up with a new super duper digital mode. That might be fine if you keep it between you and your fellow experimenters. But if you release it to the masses you need to have a plan for how it will be used given the expected number of users and how you will educate people on how to use it.

If you develop a weak signal mode you have to take account of the fact that a lot of people don’t have a QRP mentality. They can’t see the point in trying to make a contact with low power when they can simply crank the power up. And if they do that, they miss the whole point of the mode and ruin it for everyone else. If a mode cannot be used on a particular band for its intended purpose because of overcrowding or interference perhaps it would be best if it was not used at all.

Instead I decided to use my FT-817 lash-up to try the APRS Messenger APRS-over-PSK63 software instead. Whilst doing that I noticed on the waterfall a strange signal almost spot on the 10.149.70 frequency used for APRS over PSK63. It looks like an upside down three pronged fork but comes in two sizes, one wide and one narrow. It starts sounding like a single tone, and then widens to a chord of three distinct tones. But what is it, why is it on that frequency, and are my PSK63 beacons interfering with it?