Wednesday 17 November 2010

The EFHW - an efficient monoband end-fed half wave for 10m, 20m or any other HF band

Update: A detailed description and construction details are now available for download (PDF).

I like half wave dipoles. They are easy to make and easy to set up. They also perform very well and usually beat a compromise antenna hands down. With 10m about to come alive again as solar cycle 24 gets going what I wanted was a low-angle efficient radiator that could be put up and down in a couple of minutes.

My experience with ground plane verticals has been OK, but they are only as good as the earth beneath them. That is, they really need an extensive array of ground radials to work properly – not easy to put down when you are in a hurry.
I also like to use fibreglass fishing poles as antenna supports. These are available cheaply (I have a 7m version and a 10m version that I bought from Sandpiper at the Leicester rally). The only problem is that they don’t have lateral strength – they are good for supporting verticals, but not so good for half-wave horizontal dipoles.
What I really wanted to do was have a vertical half wave dipole, but the problem is that while the impedance at the centre of a dipole is about 50-75 Ohms, and very easy to match to coax, an end-fed half-wave has a very high impedance indeed, around 3000-4000 Ohms. If you just connect it to your coax or rig you will be disappointed.
Enter the End fed Half Wave as featured in my "Stealth Antennas" book - download the PDF for full details.
Update: now includes full construction details to download (PDF).

Sunday 14 November 2010

NDBs (Non Directional Beacons) in the UK

If you fancy a bit of LF DXing, these frequencies might be worth a try. They are non-directional beacons and are used by aircraft. I searched high and low until I found this list, so thought I would share it. Some may now be off the air.

I can hear my local NWI (Norwich) beacon but that's it. They send their idents in fairly slow Morse code. Have a listen, especially at night

Tuesday 9 November 2010

Understanding LF and HF Propagation ebook

In 2008/2009 Alan Melia G3NYK and I wrote a series of features on understanding LF and HF propagation for the Radio Society of Great Britain's (RSGB) "RadCom" magazine.

My features consisted of a month-by-month look at each HF band in turn, showing the reader the propagation modes behind each band and explaining some of the technicalities of ionospheric propagation.

I looked at the D, E and F layers, Sporadic E, the MUF/LUF, using solar data, propagation programs, NVIS and much more.

Alan then took over and wrote three detailed features on LF propagation. We are told that the features were well received and as a result I have managed to persuade the RSGB to allow me to put them together into a single document, which is now freely available for amateurs worldwide to download.

I hope you enjoy it.

Steve G0KYA

Download the "Understanding LF and HF Propagation" PDF (1mb)