Low VHF Antenna ideas

Most of the antennas I use are home built and very simple to hook up. Having a tiny garden and questioning neighbours has meant that for me the antenna of compromise has always been the simple but effective loft mounted dipole!. I have several built from the remains of old Band 1 (48-62mhz) TV antennas. A quiet word in the ear of the local rubbish tip man brought an arm full of fun stuff to cut up and play with!. Isnt it just amazing what some folks will throw away!. Each of the dipoles consist of a central plastic connection box with brass terminals and half-inch aluminium tubing. Cut them down to size using simple equations and they work just great mounted either vertically or horizomtally. I use two for scanning cut to 35 and 164mhz, and both seem to work pretty good. The 35 MHz one is incredible for US Fire traffic at 33mhz- doing well right up to 50 mhz, while the 164 version regularly pulls in tropo signals on VHF High from 150 to 200 miles away.

Recently I have started using a short Long Wire antenna for Low VHF reception fed into a magnetic longwire balun which drops the impedance of the wire to 50 ohms and gives a good match into coax cable. It is cut to be a half wave at 42.5mhz and therefore is short enough to hang around my curtain rail! The results are astounding; some signals around 42 MHz on the Long wire are simply not there on the dipole . The Long Wire is constructed out of cheap speaker cable and mounted horizontally. The length is 3.47 metres or 11.56 feet.

Not an awful lot of Antennas are commercially available to cover Low VHF. I have however found one and it seems to work very well right across 30- 50 mhz. It is the Sirio Tornado, made in Italy. It's a 5/8th wave vertical tunable between 42 and 50 mhz. Tuned to 42mhz it has a length of 14.5 feet with four horizontal groundplane. This is (so far) the best antenna I have come across for Low VHF. Another antenna which worked well for me was a 5/8th wave base CB antenna tuned to 30 mhz with a length of 20.5 feet. The results were almost comparable to the Tornado, and these have the advantage of being cheap and readily available second hand.
Building Dipoles

Ok heres how to build a half wave dipole. It can be built out of anything handy such as wire or coax, or you can even get 1/2 inch aluminium tubing from your local hardware supplier. Locally I can get this tubing for £0.60 per metre. The length ( A as in the diagram below) is 492 divided by the frequency you want it resonant at. For example a dipole cut for the middle of the USA 33mhz fire allocation will be 492/33.7 = 14.599 feet multiply this by 30 to get centimetres and we have a total length of 437.98. So each side of the dipole will be 218cms or 2.18 metres.

Take some coax (either 50 or 75 ohm will give a good enough match) connect the inner conductor to one leg of the dipole and the outer braid to the other and your antenna is ready to go. The simple equation 492/frequency = Total Length in Feet works for any frequency. A dipole has a figure of 8 pick up pattern when horizontal, so align its length in the direction you wish the most signal from. When vertical a dipole picks up equally from all directions.
Quad Loop Construction

The quad loop is a square of wire the total length of which is a full wave at the frequency you build it for. I have had very good results with a loop cut for 42 mhz. Heres how to work out the length- each side ( A in the above diagram) is 76.5 divided by the frequency of interest = length in metres. For example 76.5 divided by 33.7 = 2.27 metres per side. To find out the total length of wire needed for a quad loop you divide 306 by the frequency for an answer in metres.

The loop is fed with regular 75 ohm TV coax cable and how it works depends on where you feed in the coax. For Horizontal pick up feed in the bottom as in the diagram. For a mixture of polarisation feed in a corner (point C). For vertical pick up feed at point B. On Low VHF Long distance signals usually arrive horizontally while locally radiated ones arrive vertical.

Mount the loop away from metal objects which will interfere with its performance and remember that like a dipole the loop has a figure of 8 pick up pattern. My loop is in the loft suspended from rafters with nylon string.

Essentially the key to good results with antennas is experimentation, there is nothing better than coming up with something that works well at little or no cost!. Use your web browser wisely and search out some of the many great antenna designs available online. Try anything that comes into your head- but remember to abide by the golden rule -STAY AWAY FROM OVERHEAD POWERLINES!- the only shock you want is the strength of the incoming signals!.