75 ohm Coax verses 50 ohm Coax

To Use or Not to Use – That is the Question.

by Steve Finch, AIØW

(07/07/05)

Like many hams I subscribe to ham radio online newsgroups. Yahoo groups, for example, has groups that center on almost any ham radio operating interest, technical topic, or specific radio. One such group is the “amateur repairs” group. This group focuses on a variety of technical topics and you never know when something interesting arrives in your email. An interesting question arrived last week.

HI ALL, A question, I want to use the 100 percent shielded satellite dish wire,(rg-6,) with a 50 ohm radio, so short of using an antenna matcher, does anyone sell a converter to use on hf? I have seen them for vhf/uhf but that was a number of years back, where can I buy one? or does anyone have any for sale? this is a serious inquiry. thanks ahead of time... ANY positive input would be appreciated greatly..”

Most responses were along the lines of “just use it as it is.” Probably not bad advice as most antennas are not 50 ohms anyway so a mismatch occurs even if you use 50 ohm coax. But I began thinking, RG-6 (75 ohm coax) is less expensive than RG-8 (50 ohm coax) and is designed for higher frequencies so the loss seems to be less. There must be a way to homebrew a broadband transformer to match a 50 ohm coax to a 75 ohm coax. Besides, I have nearly 100 ft. of RG-59 (75 ohm) coax in my “junque” box that I would like to use.

I have a fondness for books of all kinds and ham radio books are no exception. From what I have spent on ham radio books, I could have a new FT-897! I remembered a book I bought several years ago called,” Transmission Line Transformers,” by Jerry Sevick, W2FMI. It was published in 1987 and cost $10 in hard cover (Try to find a new ham radio book in hard cover for $10 now!!) Jerry’s book goes into the design of ununs and baluns and other impedance transformers. I also have his follow-on book, “Understanding, Building, and Using Ununs and Baluns,” published in 2003 as a paperback for $20. (Times have changed!!) For those of you who have not heard about “ununs,” they are impedance transformers to match an unbalanced transmission line of one impedance to another unbalanced transmission line impedance. Thus, coax of 75 ohms in and 50 ohms out would be a perfect use for an “unun.”

Sure enough on page 6-18 of the ” Transmission Line Transformers” book and again in Chapter 13 of the “Understanding, Building, and Using Ununs and Baluns,” W2FMI provides an easy to duplicate design of a 50 to 75 ohm broadband unun. This transformer uses insulated (enamel or plastic coated) wire. For low power (200 watts or less), 16 gauge wire is adequate.

The unun consists of four windings of a five-wire bundle, on an FT-140-61 1.4” OD toroid. Each winding has the five-wire bundle laying flat. The diagram below shows the how to connect the windings for three configurations.

Source: page 92 and 94, Understanding, Building and Using Baluns and Ununs.

The third design is great for general use as both a coax matching transformer and to feed vertical. The theoretical impedance of a vertical is 37.5 ohms (1/2 of a dipole antenna which has a theoretical impedance of 75 ohms.) I have decided to make unun “C.” When its done, I’ll bring it to a Club meeting.

Until then – happy hamming.

References:

Transmission Line Transformers, 1st edition. Jerry Sevick, W2FMI, American Radio League,
Newington, CT, 1987.

 

Understanding, Building, and Using Baluns and Ununs, 1st edition, Jerry Sevick, W2FMI,
CQ Communications, Inc., Hicksville, NY, 2003.


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