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Why is 72 Ohm Impedance center feed point desireable?

Oct 2nd 2013, 21:10

xof7fox

Joined: Jul 7th 2012, 16:27
Total Topics: 0
Total Posts: 0
Hello - I have read more than one ARRL article that report that the typical 40 M coax single wire center feed dipole in theoretical 'free space' can be expected to provide a 72-73 Ohm Impedance at the center feed point. My confusion has to do with wondering why I would want that if I should be matching the center feed point with the 50 Ohm Impedance signal coming at it from my Transceiver. In other words, if there were some other wire design/composition that would offer 50 Ohms at the center feed point would that not be more desireable? Thx,
Fred K4XXK
Oct 3rd 2013, 02:21

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

We use half wave dipoles at HF because they work very well, with minimal effort. The feedpoint impedance is "close enough." It is generally better to optimize an antenna for the best signal possible at HF, rather than a good impedance match. It is hardly unusual to run amplifiers that double the cost of an amateur station on HF--they only make the signal stronger on transmit.

At VHF, where signal strength isn't quite as important, you do see beam designs that are compromises to get a match close to 50 ohms. Amplifiers are relatively rare on VHF compared to HF.

If you calculate the loss caused by the mismatch with typical station components, you will see that the loss caused by a small mismatch on HF is often negligible, while the same mismatch loss can become significant on VHF.

To answer the question, the 73 ohm impedance isn't really desirable, but it happens to be what you get when you calculate impedance of one of the most effective antennas with the math skills available to highly skilled electrical engineering students. You can actually lower it to 50 ohms by bending the wires or introducing ground, but very few students can handle that level of complexity. The theoretical impedance of a thin dipole in free space is 73 ohms.

You should keep in mind we don't all think alike--there are different approaches to understanding and solving problems. The typical engineering approach is only works well for a small fraction of the population--those who have enough math to understand 2nd year calculus. I'd recommend reading Antennas by John D. Kraus, W8JK, if you want to go this way. The early editions can be purchased inexpensively on the used book market.

Zack Lau W1VT
Oct 3rd 2013, 22:21

xof7fox

Joined: Jul 7th 2012, 16:27
Total Topics: 0
Total Posts: 0
Thank you for your time, your thoughts and your guidance Zack.
Am heading to Amazon for Antennas by Jack D. Kraus.

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