Introduction
When I got my Hi-Fi, they supplied me with a piece of wire which plugs into its antenna socket. It might pick up the radio stations whose transmitters are almost in my back garden, but it didn’t show what the radio was really capable of. Antenna designs in books and magazines were all based on dipoles, which I have never found to work well in areas with high RF levels, so I designed this antenna. It could be made for many parts of the radio spectrum, but this one is designed for the FM band (VHF band II to use the correct terminology).
Like all good designs, this antenna started life as a piece of wire tied around a shelf, and was held in place with some carefully placed pencils! It soon proved itself as a DXing antenna after picking up stations from all over Europe during sporadic e openings (my location being London), including Spain, Italy, France, and some Scandinavian stations too! It also has no trouble picking up tens of London pirate radio stations.
Requirements
The following items are required to build the antenna, and should be available from any good electronics shop.
- 300 Ω to 75 Ω balun
- 75 Ω coax plug (×2)
- RG59U or similar 75 Ω coax (length to suit, but preferably no longer than necessary)
- solder tags (optional, but good for presentation)
- insulated wire (any reasonable insulated wire—length is described next)
- two lengths of wood (length described later)
Preparing The Components
Wire Length
The length of the wire making up the antenna is critical. If it is too long or too short, the antenna won’t be tuned for the frequency you want to use it on. The length I have used is equal to one wavelength at the frequency I wanted to use it on (It is actually slightly shorter for reasons I won’t go into, but it can be regarded as one wavelength for all practical purposes). This length can be calculated by the formula:
L = 294 ÷ f
Where L is the length of wire needed in metres, and f is the frequency in MHz
i.e. (for 98.0 MHz FM) L = 294 ÷ 98 = 3 m
Wood Length
The length of the two pieces of wood are calculated by:
W = L ÷ (2 × √2)
Where L is the length of wire a W is the length of wood.
i.e. W = 3 ÷ (2 × √2) = 1.06 m (approx)
The Coax
The length of the coaxial cable isn’t quite so critical, but shouldn’t be much longer than required, because the signal loss is proportional to cable length.
When connecting the coax to the coax plugs, the core of the cable should go to the centre pin of the plug, and the foil screen of the cable should connect to the outer of the coax plug. Connection is normally done via a simple screw fitting.
Constructing The Antenna
Ideally the wire loop should be circular, but such a design is not easy to acheive. The improvement in performance would probably not be that great anyway, so a square was used instead.
Your two pieces of wood, length W, are connected in a “+” shape. The wire is attached to each of the tips, and each end of the wire connects to a screw terminal on your balun. It doesn’t matter which wire goes on which terminal, but it makes sense to connect it to the nearest one. You may want to put some solder tags on the ends of your wire to make it easier to screw the wires in.
Now all you need to do is connect your coax to the balun at one end, and your Hi-Fi at the other end.
Using The Antenna
I could give you loads of laws of physics on how to get the best results from this antenna, however the best method is trial and error. Try it in different parts of the room, and rotate it. Don’t forget that there are three different directions in which you can rotate the antenna.