Efficient electrically short antennas

Way back in my early professional life I was responsible for the design and implementation of a tiny 70cm antenna that had to fit inside the Pye PF8 UHF handportable transceiver. The mechanical design team had left virtually no space for the plate antenna we had intended, so yours truly had to come up with a solution that both fitted and worked efficiently. I managed it by changing the plate antenna to a skeleton plate antenna. In the end it worked very well but the tuning was VERY sharp. Reference here should be made the Wheeler and the Chu limit that defines the bandwidth that an efficient electrically short antenna can achieve.

Basically the thing to remember is this: the smaller the antenna is the higher the Q, and the narrower the bandwidth is, for efficient operation. This is why magnetic loops can work well if designed with a very very high unloaded Q and why they have a very narrow bandwidth if working efficiently. It also explains why a short loaded antenna will have a narrower bandwidth if working efficiently.  Dr Underhill has argued that a small magnetic loop actually violates the Chu limit.

There is a good analysis of small antennas at http://www.m0rzf.talktalk.net/RobCentral/E.S.Antenna.html which goes into some of the maths and looks at various electrically small antennas. Some are "snake oil" of course, but some well designed ones do work with the caveat of narrow bandwidth e.g. well designed magnetic loops.

What is the interest in all this for me today? Well, I am still thinking about HF operation at the new QTH and how to meet my goals of a small, low visual impact antenna, that works well. One thing in my favour is I tend to operate on quite small parts of the HF bands e.g. around the WSPR/PSK frequencies or around the QRP frequencies. As long as I am  prepared to stay close to certain frequencies like 28.1246MHz I should be able to design a very small efficient antenna. I already know that a magentic loop antenna works very well indeed and would certainly meet my goals.

Gardening at the new QTH

This evening I've been busy laying some additional turf in the back garden at our new (soon) QTH. About 5sq m of additional lawn was laid with about the same again to be added in the coming weeks. The bundalow is still a bit of a mess with lots of building work ongoing. The new back garden is around 70 feet wide, so an end-fed 1/4 wave for 80m should be possible. Tomorrow evening I hope to plant a new tree in the lawn to provide some screening from the neighbours.  There is a lot of gardening to do to get the place looking as we would like, but it is coming on OK.

I am still deciding what to do about amateur radio and antennas at the new QTH. I dislike large and unsightly antennas, so whatever I do it is likely to be something stealth like. Whatever, it will be a whole new set of challenges to enjoy. At least I shall have a purpose built shack in which I can both operate and build, unlike the present QTH, with a nice view down the length of the back garden.

Incidentally I bought the turf out in the fens - much cheaper than the garden centre. It did involve driving my car into the middle of a large 100 acre field to collect it though (see RHS).

The Low Power SPRATbook.

http://www.rsgbshop.org/acatalog/

Today, my new paper logbook (yes I still use a paper logbook!) and a new book arrived from the RSGB. The book was the Low Power SPRATbook, recently published by the RSGB and edited by Steve Telenius-Lowe 9M6DXX. As I want to de-clutter and possibly clear the book shelves of my old paper SPRAT copies (I still have them on DVD), this book was ideal as it contains a very good selection of the best of SPRAT articles from 1974-2012. If you have not read SPRAT and seen the excellent articles each quarter, then I recommend this book. I challenge you to buy it and not be inspired to build something.

Perhaps I should also mention that my 14 component 80m CW transceiver is one of the articles. But don't let that put you off! The circuit works well and G3XIZ worked GM with his version (despite running only about 18mW) and was copied on Switzerland.

PY1RO on 6m CW - real DX reception!

The magic band (6m) is full of surprises. I came into the shack this evening after gardening most of the day up at our new bungalow to hear some signals on 6m WSPR. Rather than decode, I tuned down to the DX portion of the band to hear PY1RO (GG87lb Brazil) coming through at 559 on CW on 50.108MHz at a distance of 9369km. I was copying him on the V2000 vertical and my FT817 with a long lossy feeder. A few minutes later and he had faded out completely!  Although I called him a couple of times, I did not break the pile-up, not that I was really expecting to, HI. Still, it was good to hear my best DX on the band in some years.  At this time of year the propagation is more likely to be multi-hop Es rather than TEP I believe. Here is a video of PY1RO on 6m (not this occasion though).

G4HJW "Finningley" Optical Transceiver kit

The G4HJW optical transceiver kit

Bernie G4HJW is well known for his innovative work on microwaves and optical comms. Last year he designed a neat, part SMA, optical transceiver kit to build at the Finningley round table meeting. The transceiver is capable of very good results and several (around 70) have been built and used very successfully. I believe the best DX QSOs at 481THz have been over 60km using these in 100mm optics.

Exactly when I'll get my kit built I'm not sure but it will be good to have one of these available for optical line-of-sight tests in the autumn. Once assembled, I shall be looking for some 2-way optical QSOs beyond the 10km speech contact I achieved with my own kit last summer. In East Anglia, the issue is finding some hills to allow long line-of-sight paths.

Bernie is, I understand, considering putting together a further batch of these kits, which make a good introduction to nanowave communications. In addition to these electrical kits, all that is needed is a microphone, headset and some simple optics than can be built for a few pounds.

See http://www.earf.co.uk/nanotrx.htm for more details

Yaesu price drop

http://www.universal-radio.com/

In this month's Practical Wireless , received today, I notice that some of the UK dealers have reduced the price of the Yaesu FT-450D and the FT-950 radios. The FT817ND is still around £534, i.e. no drop (yet). At last, we are beginning to see some of the exchange rate savings being passed on to customers. Thank you dealers. Please may we have even more savings? The exchange rate has moved FAR more in our favour than the price changes would suggest.

Playing with a ferrite rod at VLF

Just for amusement, but with a slight hope it might work, I tried resonating a LW ferrite rod coil (about 3.5mH) at 8.97kHz to see how it might perform as a miniature portable receiving loop in my earth-mode tests.

To test other antennas and VLF preamps, I first connect my 8.97kHz 5W transmitter into a resistive dummy load and check that I can detect the signal strongly some distance away locally (about 10m away only). The emissions from the cables are such that this gives me around 30-40dB S/N on Spectran with the usual settings on my 80cm loop. Switching over to the ferrite rod RX antenna it was hard to tell if a signal was there at all.

So, that's one experiment I'll close and report as a failure. Had it worked, even 10s of dB down, it might have made a magnetic field antenna that could have been deployed mobile. You may recall I tried /M on 8.97kHz a few months ago with my 80cm loop strapped behind the car, until someone pointed out this probably would have invalidated my car insurance and I stopped.  I had vague ideas of dropping the VLF resonated ferrite rod close to the ground behind, somehow fixed from the rear bumper.

Ho hum, another idea bites the dust.

Shortwave Radio Archive

Just spotted on the Southgate site that K4SWL is creating a website to archive shortwave broadcast  recordings before these disappear. Already many SW BC stations have closed at least parts of their services. The HF broadcast bands are nothing like they were even 20 years ago. See http://shortwavearchive.com/ . 

My shortwave radio experience started over 50 years ago listening to SW broadcast stations on a simple crystal set in my bedroom. As much as I hated the propaganda from the communist broadcasters like Radio Moscow and Radio Sofia, Bulgaria, I do miss them and their evocative interval signals.

See also https://sites.google.com/site/g3xbmqrp3/hf/interval and take a nostalgic trip down memory lane.

Good results with new VLF earth-mode RX system

Today I carried out a test with my new simpler RX system for VLF/LF earth-mode "through the ground" communications, testing with a 5W TX into 20m spaced earth electrodes at 8.976kHz.

The new preamp is just a single MPF102 impedance converter feeding into a small,low cost, external USB sound card (£3) that feeds my baby Asus X101CH netbook (£172) running Spectran software to display the received QRSS3 signal. The external soundcard was only needed as I didn't have a 4-pin jack to use the audio-in socket on the Asus.

Tests were carried out at the usual test sites at 1.6, 3.6 and 6km as well as one further (unsuccessful) test at 6.2km.

In all locations, decent signals were copied, even at 6km. The little Asus netbook has an SD card running Readyboost and behaved perfectly well during the tests. Now, the whole system is very simple to deploy in the field to take measurements. The preamp is only tuned by the loop and its resonating capacitors (a capacitance decade box), so the very same preamp can be used right across the VLF and LF spectrum with a suitable receiver.

Today the weather was dry and has been for around a week now. I believe signals by earth-mode are stronger after a period of dry weather as the soil conductivity is lower. Results are as good, or better, than I have ever experienced before with earth-mode at 8.976kHz.

The next test will be to repeat this test at around 1kHz and possibly lower frequencies.I also want to try WSPR and repeat the tests at much higher LF frequencies around 72-73kHz. For the latter test, I need to build a simple down converter to audio.

YouKits TJ2B SSB handheld kits on eBay

The TJ2B 4-band SSB HF handheld is available direct from China for around £176 as a kit with all the SMA components already mounted on the PCBs. See eBay item number 271195721443.