University of Alberta observatory domes

Matching Transformers

"Thank you so much for visiting our class on Friday! The kids loved it...they thought it was pretty cool to meet a "real" Astronomer! Thanks again, Janine"


This section describes the types of cable available for connecting the antenna to your receiver.

Please note that cables are rated according to what is called their "impedance". Impedance is measured in ohms and is a form of resistance to the flow of electrons. Cables and equipment such as receivers and antennas all have impedances. If you attach a 300 ohm cable to a 75 ohm antenna connection an impedance mismatch occurs. Some of the signal energy from the antenna will be reflected back up the wire to the antenna from the join between the two cables and not make it to the receiver. You lose signal as a result. So make sure you get cable with the right impedance rating and also make sure to attach it to the "matched" connection on the receiver. That is, attach a 300 ohm cable to the 300 ohm connection. You can also use matching transformers to achieve the same result.

An antenna that uses a folded dipole for its driven element has an impedance of 300 ohms. A "T" style driven element has an impedance of 75 ohms. The driven element depicted in the illustration above is a T type element. We recommend you use 75 ohm coax cable since it is more resistant to noisy interference in the radio environment.

Coaxial Cable

The correct coaxial cable (often referred to as "coax" - pronounced "co axe") to use for this project has an impedance rating of 75 ohms. Different types of coax are referred to by a numbering system. We recommend you use RG-6. Coax looks like this:

RG-6 cable with attached ground wire (not shown) allows the antenna to be grounded for lightning protection. Other RG-6 cables can be used. RG-59 cable can be used but it tends to be "lossy."

If you have to have a long cable from your antenna to your receiver, coaxial cable has some limitations. It loses signal more quickly than twin lead. If your cable run is long, you may have to use an FM amplifier at the antenna to boost the signal to your receiver. 

Make sure you get 75 ohm cable for your system. There is other coax cable that has an impedance of 50 ohms and needs to be avoided. The cable should be attached to the 75 ohm antenna connection on the rear of the receiver. In some cases (car radios for example) there is only one choice. On table top or home theatre receivers there may be two connections. Connect to the 75 ohm connectors. The braided wire just under the outside insulating skin of the cable should be connected to the ground lug. The centre wire should go to the 75 ohm connector.

To connect to the Chrysler car radios we are recommending for the project, use a Motorola Jack, pictured below, at the end of your cable. This needs to be soldered to the cable end.

Twin Lead

If you use a folded dipole antenna (300 ohm impedance) you should use Twin Lead cable. Folded dipoles can easily be made from Twin Lead as described in the section on folded dipoles. Twin Lead 300 ohm cable looks like this:

Available at The Source, electronic supply stores and many hardware stores. This cable is referred to as Twin Lead, Flat Multiconductor Ribbon, or Twin-Lead Cable.

This wire provides the lowest loss of signal of the various cables that can be used from the antenna to the receiver. It should be attached to the 300 ohm antenna connection on the rear of the receiver if there is one. One wire should go to the ground connection, the other to the 300 ohm connection. It doesn't matter which wire you put on each connection.


Copyright 1999-2015 by Sky Scan, Edmonton, Alberta, Canada. 

We gratefully acknowledge the financial support of the 

Edmonton Centre of the Royal Astronomical Society of Canada, Department of Physics (University of Alberta)

and the

Natural Sciences and Engineering Research Council of Canada

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