One of the most common questions I get asked is how to make a simple, quick, high-gain, VHF antenna for a specific frequency. "Harry, I want an antenna for 146.300MHz. Send me a design and calculated dimensions by e-mail." I really do wish I had the time to calculate and draw for every person. Sorry! But here is the next best thing. The "VPO-le" (pun intended) antenna.
All you need is four small panel pins (nails) and a bit of wood. Hammer a nail into the wood at the top of L1. Hammer another in the measured position for the top of L2. Hammer the other two nails into the wood, in the right place for the cable connection.
Now, take a length of copper wire:
The wood thickness (element space) and all other dimensions are given in the table below. NOTE that the dimensions are adjusted to suit reality and should need no adjustments at all. I calculated the values, built the antenna, then corrected the formulas to make them agree with reality.
I have built models of this antenna for 51MHz, 98MHz, 145MHz and 446MHz. All needed no adjustments whatsoever. The VSWR was well under 2:1 for all models. I did find that the larger antennas were quite large and need to be mounted outdoors, well away from any metal objects that would de-tune them.
Calculate a theoretical 1/4-wave as 75000/MHz = length (mm). I then found that all lengths should be multiplied by 1.028 to get a perfect VSWR (yes! they are LONGER than calculated!). Just enter the frequency in the calculator below and click "Show lengths to calculate all the dimensions. I have even given you the total length of wire you need (but the extra for twisting around nails is not included).
You should be able to build and test this antenna in less than 20 minutes. It has a gain of about +2dBd (+4.5dBi). It is best mounted on a broom handle, or similar. If you want a permanent fixture then use bolts instead of nails, and shove it up a plastic conduit tube. By the way, you can use this antenna with my WBFM Wireless Microphone transmitter (V5) and the TDA7000 receiver. If clear of obstructions, then you should get a range of well over 3Km (2 miles) :-) I extended my V5/TDA7000 to 4.5Km and my 500mW PMR446 radio (TE157) to about 12Km.