Ham Radio Forum

hello everyone :o
I had constructed the dipole antenna for the frequency about 100 MHz.now my question is which coaxial cable should i use either a 50ohm(RG58 or like) or 75ohm(RG59 or like) and tell me about the construction details of balun (because coaxial is the unbalanced where as the dipole is balance device). I am waiting for the reply.

prashanth

If the dipole elements have small diameters and the dipole is not near other electrical conductors its center impedance will be approximately 72 ohms, so 75-ohm coax such as RG-59 or RJ6 will provide a better impedance match than 50-ohm coax, such as RG-58, will. (RJ6 cox has much less loss at 100 MHz than RG-59. It also is less expensive and is sold in more places.)

You can purchase a commercial 75 to 50 ohm balun transformer or you can make a simple one by winding the antenna-end of transmission line coax into a small coil as shown here http://users.tns.net/~bb/balun.htm.

thank you for your advice ,so i am using RG59 75 ohm coax cable. please tell me about the contruction of 1:1 toroidal balun.

Is your dipole a receiving or a transmitting antenna? If a transmitting antenna, what is the transmitter power?

sir this is the receiving dipole antenna.

You can make a 1:1 impedance-ratio VHF balun for receiving use very easily and with very little cost as follows:

1) Obtain a standard 75 ohm to 300 ohm TV antenna balun. There is no need to buy a new one if you have a used one.

2) Open the plastic case to expose windings on a miniature ferrite toroid core. Most of the plastic cases used for TV antenna baluns snap open easily, but some are glued shut. If yours doesn't snap open, carefully break the plastic.

3) Snip the wires going to the toroid windings with small diagonal cutters and remove all the windings from the core.

4) Obtain a short length (you will need less than 25 cm) of small-gauge enamel-insulated copper wire. The gauge of the wire is irrelevant, except that it must be small enough to make four passes through the center of the toroid core and it must be large enough so it won't break easily.

5) Cut the copper wire in half.

6) Wind two turns through the center of the ferrite core with one piece of wire.

7) Wind two turns through the center of the ferrite core on the opposite side if the core with the other piece of wire.

8) Cut each of the four wires coming from windings to make them each about 1 cm long.

9) Carefully scrape the enamel insulation a short distance back from the end of each wire, so they can be soldered.

10) Connect the two leads coming from one of the two-turn coils to the two balanced antenna terminals. (The connection polarity is irrelevant.)

11) Connect the two leads coming from the other two-turn coil to the unbalanced coax cable, with one lead going to the coax center conductor and the other to the coax shield. (The connection polarity is irrelevant.)

Two-turns probably won't seem like enough for balun transformer primary and secondary windings, but two turns are all that are needed at 100 MHz.

You obviously will need to package the balun in a protective enclosure of some kind to protect it from damage. You may be able to use the original balun plastic case. If not, fashion some other type of protective enclosure.

i had constructed the above said balun and installed it.but my receiver receives electromagnetic interfernce or noise genterated by the home appliances (like motor, tubelight, computer, etc) when the cable is moved here and their, the noise is too high.
i want to know weather the above said balun is current (choke) or voltage balun.
if their is an alternate method about a construction details (or the websites) of toroidal 1:1 choke (current) balun please inform.

The balun described above is a transformer balun. Transformer baluns can be described either as voltage baluns or current baluns, because they are both. However, because they are both voltage and current baluns neither of those terms generally are applied to them. They instead are usually described merely as transformer baluns.

Regardless of the names, a well-constructed transformer balun is an excellent balanced-to-unbalanced coupling device. Most of the baluns that are commonly referred to as current baluns or voltage baluns do not have 1:1 impedance ratios. Where you need a 1:1 impedance-ratio balun, you should use a transformer balun.

thank you for your advice.

I agree with CaveatLector that transformer baluns make excellent balanced to unbalanced coupling devices. A transmission line choke balun like the one described here http://users.tns.net/~bb/balun.htm
can be as good, but not better, at reducing transmission line signal pickup if everything about the choke balun design and construction is perfect, but most choke baluns do not work very well, because they are not perfectly designed and constructed.

Choke baluns do have slightly less signal attenuation, but the difference with a well designed and constructed transformer balun is so small that it can be measured only with laboratory quality test instruments. I don't think you will gain any practical advantage from using a different type of balun.