The ‘Ugly’ Balun
When you feed a ‘balanced’ antenna, such as a di-pole or inverted-vee, with an ‘un-balanced’ feed cable like 50ohm co-axial cable, you need to use a BALUN to help ensure that RF power is only radiated from the antenna and not the feeder cable as well. When the co-ax radiates power it affects the radiation pattern of the antenna, and can also contribute to TVI (television interference). A BALUN very simply matches the BALanced antenna to the UNbalanced cable.
You can purchase a commercial balun from many sources, or if you need just a 1:1 balun you can very easily construct your own. Note that this design does not change the impedance of the antenna to match it to the cable. If you need to match 50ohm cable to, for example, a 300ohm antenna, then you will need to also connect an impedance matching device that are sometimes incorporated into commerical baluns.
The choke balun presented here should perform well on the HF bands (1.8MHz to 30MHz) and possibly even the 6m band (50MHz to 54MHz), although you may need to shorten the length of the cable a bit if using it only at the higher frequencies.
Construction
To construct a simple co-ax choke balun you will need:
- length of PVC pipe, 4 - 5 inch diameter, approx. 1 foot long;
- 18 to 21 feet of RG213 co-az;
- 2 cable ties
The co-ax is simply single layer close wound on the PVC pipe, with the first and last winding secured to the pipe using a cable tie. The number of turns is not important, just the length of co-ax used. This will work for frequencies between 1.8MHz and 30MHz.
For 6m (50MHz to 54MHz) use a PVC pipe about 3 inch diameter with 5 turns of RG213 co-ax cable.
Placement
Place the balun close to the antenna feed point. You can use either a continuous length of co-ax or place a connector at the bottom so you can plug the cable from the transceiver into the balun.
This may not be the most attractive balun (which is why it is often called the “Ugly” balun) but it is very cheap and easy to make, and that can be very important to some amateurs!