The hazards of LF operating

On the LF reflector there was a salutary message from Mike G3XDV today.  Doing some 136kHz tests overnight his loading coil caught fire melting his flat roof extension!

LF amateur radio can be very dangerous as extremely high voltages and/or high currents can be generated with even quite modest powers because of the electrically short antennas. This is one reason why I stick to QRP(ish) powers on 136 and 500kHz.

See Mike's post:

Damage to the G3XDV loading coil ....and roof! See http://g3xdv.blogspot.com

"I am currently off the air following my loading coil catching fire and damaging the house. I have had to take my mast down to allow access to the builders who are repairing the damage. Fortunately I was insured.

Hopefully I will be able to receive before Christmas and transmit again some time in January.

Details and pictures are on my blog at:http://g3xdv.blogspot.com

Mike, G3XDV"

This picture (linked from Mike's blog) shows the damage.It begs the question of how readily will an insurance company pay up when an amateur installation fails?

More 10m (28MHz) Projects

With the Tenbox coming along just fine - I should have a completed breadboard version on-air this week - I am beginning to think that I could spend nearly all my free time designing different ideas for this, my favourite, band!

Here is just a brief list of some of my 10m ideas (all do-able with some time):

• 10m QRP AM transceiver (Tenbox)
• 10m QRP DSB transceiver
• 10m pocket SSB/CW "holiday" receiver
• 10m mixer-VFO controlled CW transceiver
• 10m compact portable antennas for mobile and hand-portable
• 10m VXO controlled QRP TX
• 10m phasing SSB transceiver (10W)
• 10m beacon RX
• 10m test box (power meter, ATU, SWR bridge, FS/mod meter)
• Modules for 10m rigs (VXO, mixer-VFO, LPF, TX strip, DC RX, AM modulator, audio power amp etc)

Now, as I like to do so many different things in this exciting hobby - including on-air operating some times! - it is unlikely all these will get done anytime soon.

What occurs to me, if I had the time, is that a mini-series in a magazine like PW or RadCom entitled "Ten Projects for Ten Metres" would be a nice idea. 10m is one of the best bands for homebrewing as layout is not too critical, as long as sensible RF rules are followed, antennas are small, superb DX is possible in the better years and Es DX possible for 4-5 months every year, local ranges are useful for nattering across town. All in all, this band has SO much to offer.

A 2m AM calling frequency

At the recent RSGB Spectrum Forum Meeting (Nov 3rd 2012) the ongoing matter of an AM calling/working frequency was raised. At issue is why our national society, which is there to help and support ordinary experimenters like you or I, seems to be totally against putting a 2m AM frequency properly in the UK band plan. 144.55MHz is the frequency of choice.

This is what appeared in the minutes:

"7.7 G-QRP
Report accepted

Listing of an AM centre of activity frequency in the Band Plans
It was noted that this has been raised previously with no success. It was suggested that a “custom and practice” approach would be the only way of moving forward i.e. identify a frequency, use it and make it known."

Excuse me dear Spectrum Forum members, but this is stupid!  You have centre frequencies for all manner of other modes, but not AM. So, why not just print these words in the bandplan? :-

144.55MHz   AM calling frequency and centre of AM activity

I am not one to get easily irritated, but the RSGB stance on 2m AM is beyond belief.

Taxing times

Today I'd planned to finish the breadboard TX of the Tenbox 10m AM transceiver and do some range tests locally. The plan was to put the TX on at home with a PC voice recording and drive around with the FT817 and the Tenbox super-regen to see what sort of range was reliably possible.

Instead, I started to do my annual HM Revenue and Customs tax return, a task I hate doing as so much form filling is needed and lots of bits of paper have to be found in order to fill it all in. I do mine online, so the deadline is the end of Jan 2013. Starting at about 10am it took me until 4pm to complete, not because of the complexity of my affairs particularly, but because I couldn't find all the bits of data I needed. On their website they have a nice picture of a guy smiling away whilst filling in his form online. I spent about 6 hours pulling what little hair I have left out!  A reminder that the deadline for online self-assessment tax returns in the UK is Jan 31st 2013, or you get a fine.

Anyway, job done. Yet again I owe money - why do they never owe me money? - and will pay up in the next few weeks. So, a boring, tedious, annual task done and this week I can return to the interesting amateur radio project stuff.

Simple TRF based AM radio ICs

Many years ago when my sons were little I made them a couple of Medium Wave AM radios using just a ZN414 3-pin TRF receiver IC. It worked very well considering how simple the circuit was using just a small ferrite rod antenna and a crystal earpiece. This IC is no longer available but there are other similar parts available such as the MK484. Although I have not tried one of these as an IF stage I have no doubt they could be useful in simple rigs like the Tenbox currently under development. The advantages over a super-regen would be selectivity. These devices have input impedances of around 4Mohm so if a ceramic filter was to be used ahead of the IC a step-up transformer would be needed to minimise loss. As they only work to around 3-4MHz they would only be suitable as an IF in the Tenbox 10m AM transceiver.

A useful page I've just found on how to get the best from these simple ICs is http://theradioboard.com/best-of-the-best/mcgillis-mk484.htm.  The datasheet for the MK484 is available here.

A page showing the use of the MK484 as an IF stage in a simple 160m RX is  http://www.vk6fh.com/vk6fh/mk484radios.htm . See also http://www.youtube.com/watch?v=qdlXVKOITe4. I am told that a similar design appears in 'Radio Projects for the Amateur' Volume 4 by Drew Diamond VK3XU. 

I'm still likely to stick with a very simple super-regen RX in the Tenbox - they ARE the best in the intended application - but I may make it in a modular form so that different TX strips and RX strips can be used and compared.  I can see a whole series of simple AM designs in the pipeline, HI.

More big wheels and turnstiles

When looking around for possible homebrew designs I saw an article that appeared in QST back in 2008.  Looking at this approach I am not convinced it is easier than the conventional big wheel design though. See http://radio-amador.net/pipermail/cluster/attachments/20100107/a2f81f3e/AntenaHPOD.pdf . 

A design for a 70cm big wheel, designed to be used with a beacon, is available at http://www.qsl.net/dl4mea/antennas/bigw.htm .

The turnstile design on the EA4EOZ website

Another simple approach to a VHF or UHF omni-directional horizontally polarised antenna is the simple turnstile (a pair of phased cross dipoles that produce an almost 360 degree clean radiation pattern with about 0.9dBd loss only). These can be stacked to produce gain, as with the big wheel.

Overall, the conventional big wheel design is my favoured approach for 2m and 70cm. Whether I make my own or buy a Wimo version remains to be seen.

Another ham radio company bites the dust

Via an email from Steve G1KQH I've just heard that Snowdonia Radio Company has ceased trading completely and closed its business. If I understand correctly they closed down last year but had one last try again this year. You may recall that this company produced antennas such as the X80 HF vertical antenna which appears on an earlier blog entry last year I believe. I erected and used one and it worked very well.

This is sad news as it was a small UK company making good products for the amateur market. Clearly the market is not big enough to support the volume of trade they had and all their commercial overheads. One wonders how many other small companies can make a success in our market unless they get major publicity in the big magazines or have a good website that attracts a world-wide audience of potential and actual buyers. I cannot recall SRC advertising too widely: maybe this was their problem - getting a critical mass of customers who told others and came back for more themselves.

This was the message on their website today: 

Due to an ongoing decline in trade Snowdonia Radio Company has stopped trading.
All contact avenues have stopped.
This has been a hard decision as we only reopened in January 2012, but ongoing high costs and a rapid decline in trade have forced our family business to close permanently.
Thank you for the business over the years from Simon and Liz, and we hope to speak to you on air.
Please note that there are some poor quality non SRC antennas available, These antennas look like SRC products but are not. SRC do not make antennas for anyone else......

73,   _._

Big wheel antennas?

www.wimo.com big wheel

When I move to my new (higher) QTH on top of our local East Anglian "hill" next year I'll have to give some thought to what VHF/UHF antennas to erect for horizontal SSB/CW/digital DXing. I could go for a decent set of beams and a rotator or I could go for an alternative approach and erect a stack of big wheel antennas for 2m and 70cm. A single big wheel has a horizontal gain of around 2-3dB over a dipole but a couple will give almost 5dBd, which is similar to an HB9CV beam but without the hassle of a rotator and with almost 360 degree coverage.

For my sort of (occasional) 2m and 70cm DXing the big wheel may be a suitable solution. I have around 6 months to sort this out, so no rush, but I'd value inputs from people on this. Have you used big wheels? How effective were they?

Incidentally, even from my old QTH I've worked all sorts of decent VHF DX with just a halo and a few watts QRP in contests, so a lot depends on how prepared one is to wait for lifts or big contest stations to work. Clearly if the aim is to work 600-700km under flat band conditions then 100W and a largish beam are almost essential. I'm not entirely ruling such a station out of the question, but it would be a big change from my usual QRP, so pretty unlikely.

A photo in QRP Basics (2nd edition)

My wife has bought me a copy of QRP Basics (2nd edition) by G3RJV for Christmas. It came this week so we opened the pack to check all was well, before putting it away for a month.

Imagine our surprise when we spotted a photo in it of me with our grandson (when he was very young) at my operating table.  He is the little lad (now 5) who was tapping out CW on one of my YouTube videos and playing with my audio kit on another. As I never did manage to get either of my two sons interested in ham radio, I'll be lucky to get any of my grandchildren interested, but I shall try.  Regarding the rest of the book, I'll let you know on Christmas day after reading it with a minced pie in hand. As it is written by Rev George Dobbs, I am sure it will be another excellent book about QRP.

10m local range?

With the 10m AM Tenbox design coming along nicely I'm reminded of the fun we had in the early 1980s with 10m FM using converted CB rigs here in the UK. Using a converted CB radio with 4W FM into a vertical half wave antenna I was usually able to make contact with similarly equipped stations up to around 25-30 miles away pretty regularly. Certainly within a 5 mile radius signals were fully quietened, even to mobiles.

So, I am wondering how I'll get on with 0.5W of 29MHz AM? My expectation is that with a similar antenna (1/2 wave vertical - I currently use a horizontal halo) I should be able to cover 3-5 miles without too much problem. I'm wondering what sort of ranges people achieve with AM CB radios (WITHOUT add-on PAs!) where this mode is legal?  Running a few dB more power is no problem if required of the Tenbox design.

When 10m is wide open it is possible to work thousands of miles on FM but I always found that the competition was too great and therefore the mode is not well suited to DXing. The aim of the 29MHz AM Tenbox is just for very local communications and definitely not for DX working. In my mind I see the Tenbox being a modern version of the old Heath HW19 (the Tener) rig - see the image and data on the Rigpix page