Active RX loops

Some months ago I swapped my little used X80 multi-band vertical for a Wellgood RX active loop. This is essentially the same as the Wellbrook RX active loop.

I just use mine stuck to the shack window. It cannot even be turned! It has been used from 2200m to 15m. It may even work at 10m, but I have not tried it. The convenience of not having to swap or tune antennas is great. As far as I can tell it is every bit as good as my alternative antennas.

More loop experiments at 500 and 136kHz

Today I converted my earth electrode antenna with elevated feed wires into a full "in the air" loop for both 500kHz and 136kHz. As might be expected, tuning is now very sharp as there is no longer a 50 ohm earth resistance in circuit dampening the Q. At first I found matching and tuning impossible until I decided to match the loop with a step-down transformer (3.5:1 on a 3C90 core) and separately do the tuning. This works a treat and it is now a simple matter to tune the loop either for 500kHz or 136/137kHz.

First reports on 136kHz with the QRSS3 beacon suggest the signal is about 6dB stronger, which is a great result. The loop is wound with quite thin wire (1-2mm diameter), so a higher efficiency is possible with thicker wire, but with even sharper tuning.

LATER: Attracting any attention is turning out to be a serious issue with real QRP on 136kHz. Even with alerts on my blog, the LF-reflector and the GQRP Yahoo groups it's hard to get that many people taking a listen/look. With uW ERPs I'm beginning to think that I'm never going to be able to get much further than my present best DX of 62km.

Useful multi-band HF vertical

The Snowdonia Radio Company is offering the SRC X80, a 5.8m long multi-band HF vertical matched via a 9:1 unun matching box. This sells for just £47, which is excellent value compared with similar offerings from the Japanese manufacturers. Such Rybakov antennas offer a decent match, easily brought to 1:1 VSWR using an auto ATU, on the higher HF bands. On the higher bands performance is quite acceptable considering the compromise in size. In the latest edition of PW Magazine G6MXL reports working plenty of DX with 50-100W using one.

Antenna feeders

Today I have to move my two main antenna feeders which wind their way untidily through the house from the back of the house, where they enter the building, to my shack in a front bedroom. This means getting up into the small loft space to run the cables across and down. Some years ago I had a bent end fed antenna on 10m squeezed up there and it managed to get to South America on QRP SSB. A small ground plane for 10m (with a capacity top hat to shorten the vertical section) would probably perform quite effectively.

UPDATE: Job done. It took about 1 hour and everything is fine.

Super-efficient VHF antenna?

See http://www.nist.gov/public_affairs/techbeat/tb2010_0126.htm#antenna

"The National Institute of Standards and Technology (NIST) and partners from industry and academia have designed and tested experimental antennas that are highly efficient and yet a fraction of the size of standard antenna systems with comparable properties. .....NIST engineers are working with scientists from the University of Arizona (Tucson) and Boeing Research & Technology (Seattle, Wash.) to design antennas incorporating metamaterials—materials engineered with novel, often microscopic, structures to produce unusual properties. The new antennas radiate as much as 95 percent of an input radio signal and yet defy normal design parameters."

Antenna measurements at 500kHz

Matching my antenna at 500kHz has been a bit hit and miss: my technique, if you can call it that, has been to match the antenna "by ear" on RX, adjusting taps on the ferrite rod ATU inductor, to give me the best S-meter reading on the intermod product on 504kHz. Then, with a 3dB pad between the PA and ATU, carefully tweek the tap settings on the ATU for maximum antenna current measured using a toroid current transformer. The 3dB pad is to prevent damage to the PA during the adjustment process. Up to now this has been OK, but I could really do with a better way of measuring the antenna parameters. I'd really like to get an antenna analyser, but cannot justify the expense. I need to check out some other simple, low cost, techniques.

More on the active LF antenna

Yesterday I used a PA0RDT designed active antenna (but with MPF102 and 2N3904 devices and a 9V supply from a PP3 battery)to listen successfully on both 500kHz and 136kHz. This is a bit better picture of the antenna. Although it is recommended it is mounted well in the clear and away from the house (feeding the power up the coax), I just stuck mine onto my double glazed window - it still worked and heard stations on both LF bands. This is the ENTIRE antenna - no wire is needed down the garden.

In the last few days SM6BHZ has been heard in central Bejing China using a version of this antenna.

136kHz WSPR on an active antenna

Just tried a PA0RDT designed active antenna (but just used an MPF102 and 2N3904 instead as these devices were to hand) with 1m of wire, later just a small piece of copper laminate (see picture), indoors taped to the window and have just spotted M0BMU running 200mW ERP on 137.45kHz WSPR. Amazing that this can work so well. The antenna looks useful for VLF through to low HF. See also
http://carconline.blogspot.com/2009/05/pa0rdt-active-antenna.html and http://carconline.blogspot.com/2009/05/pa0rdt-active-antenna-continued.html