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

WISPY 10m WSPR beacon video

A few people have requested a video of WISPY, my QRP WSPR beacon for 28.1246MHz. This video shows the TX beacon only. I have a companion RX breadboard that works well and at some point these will be combined into a small 10m WSPR transceiver.

Tenbox 10m AM RX video

It is hard to contain my enthusiasm for simple transceivers and super-regen receivers! This video shows just how well the RX breadboard for the little Tenbox 10m AM transceiver works. 1uV (-107dBm) is a very good signal and it will still detect a well modulated AM signal at -120dBm. Selectivity is not good with this type of receiver, which is the main drawback. However for its intended application - a simple transceiver for local nattering across town - it should be fine. Some people use super-regens as the IF stage of AM receivers. If a filter is added ahead of the super-regen IF this overcomes the selectivity issue but makes the RX design more complicated.

Tenbox 10m AM rig - a bit more

Although I had little time today, I experimented with adding varicap tuning to the Tenbox AM transceiver's RX section. Only problem was I had no varicaps, so tried using a few different diodes and a transistor as a varicap. 1N4148 and a Germanium diode reverse biased hardly produced any change in capacitance going from 2-12V. I know a 1N4007 works quite well but could not find one. Then I used a 2N3904's base-collector junction and got a few pF of change. In the end I just did as I did in the Sixbox and used a fixed capacitor in series with a 365pF polyvaricon tuning capacitor. This moves the RX frequency as much as I need at around 2MHz maximum.

2 countries worked on 500kHz with JT9-1 mode

Well, the JT9-1 test QSO with G3ZJO was a total success and completed in around 10 minutes this evening. Later, I called CQ on JT9-1 and got a reply from OR7T in Belgium who is 334km away. We worked, exchanging reports and 73s but I am not quite "all with it" with the sequencing of reports, R, RRR etc in JT mode exchanges.I am learning, HI.

G3ZJO in QSO with G3XBM this evening on 500kHz

The WSJT digital mode JT9-x is certainly an interesting mode. There are a few bugs in the code still, and more bugs in the operator using it (me!) but I am sure to use this mode more in future on 472kHz and 136kHz.