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My first antenna questions ...

Aug 22nd 2014, 06:32

tuulen

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
I am new at this, but have been busy at researching antenna designs and, sure enough, have a number of questions.

For instance, a two-element 1/2 wavelength Off Center Fed (OCF) antenna could be made of a 1/8 wavelength element plus a 3/8 wavelength element (1/8 + 3/8 = 4/8 = 1/2) and at those lengths is said to be balanced at the feed-point in regard to tuning.

Now, if such an OCF antenna has a total length of 1/2 of 80 meters and has open-wire transmission line connected to a trans-match (tuner), then could a 1/8 + 3/8 wavelength OCF antenna work for 80 meters, could that same antenna work for 40 meters, and for 20 meters?

Or, as a 1/4 + 1/4 wavelength 80 meter dipole is known to work for 80 meters, then with the use of a trans-match could that same antenna work for 40 meters, and for 20 meters?

Just to say so, if I need antennas for higher frequencies then I would build them as separate units, but otherwise it would be great if the physically larger antennas for the lower frequencies, as above, could serve more than one band.

.
Aug 22nd 2014, 15:37

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
An OCFD isn't balanced. As a result, no matter how you orient the feedline, the feedline will affect the tuning. This is distinctly different from a center fed dipole--in which bringing down the feedline properly will minimize the tuning interaction.

Typically, one uses an OCFD to eliminate the need for open wire.
Open wire, while an effective way to minimize losses, can create the issue of wide impedance swings that are difficult for a tuner to match without excessive loss or voltage breakdown. So, while open wire can be made to work, you may need to experiment with different line lengths.

At present, nobody has found a good simple all band HF antenna--one that will operate from 80 through 10M with reasonable efficiency on all bands and no need for cut and try. Perhaps the closest is an inverted-L with an autotuner at the feedpoint. But, this antenna needs a radial system with good efficiency, and many hams would prefer to keep all the electronics in the shack.

Zack Lau W1VT
ARRL Senior Lab Engineer

Aug 22nd 2014, 23:50

tuulen

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Yes, I can intuitively picture that an OCF dipole is not balanced, but somebody on a website said that a 1/4 x 3/4 ratio dipole is balanced and so I got to wondering about that, especially for multi-band use. After all, any information published on the Internet is always true, right? However, and with my thanks to you, I have now abandoned that theory and will instead focus only on dipoles having two equal-length elements. And yes, apparently the length and the orientation of transmission line is critical, especially of open-wire line.

I plan to avoid ground radial systems. Rental equipment is available to dig the narrow trenches needed to install such a system, but a ground radial system also helps to turn an antenna into a lightning rod, and then there are the similar hazards of grounded metal towers, too. I plan to avoid operating in lightning conditions and to disconnect any outdoor antennas when not in use, where a double-pole single-throw switch could make that quick and easy.

Thanks!

73, Doug

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