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I see on the internet that a radio amateur made bowtie antennas for transmitting and receiving on the 2 meter and 10 meter bands. They produced very good SWR bandwidth when fed with 50-$\Omega$ coax. Why, then, are bow tie antennas for UHF TV connected to 75-$\Omega$ coax through a 300-$\Omega$:75-$\Omega$ stepdown balun?

Commercial 4-bay bow tie antennas connect the feedpoints of the bowties with ~14ga solid conductors and feed the antenna through a 300-$\Omega$:75-$\Omega$ stepdown balun. If the impedance of a single bow tie is close to 50-$\Omega$ or 75-$\Omega$, then connecting four of them with half wave wires should result in an impedance around 12.5-$\Omega$ or 18.75-$\Omega$, which is a very poor match to the 300-$\Omega$ input impedance of the balun. This situation would be even worse for an 8-bay antenna. How do these antennas work?

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    $\begingroup$ Please tell us where you found the amateur and commercial antennas so we can address your specific questions. $\endgroup$
    – Brian K1LI
    Mar 25, 2019 at 12:36
  • $\begingroup$ Hello Robert and welcome to the site! I believe that I know the answer, but I would first like to see what these look like. Please provide either photos or links so that we can help you. $\endgroup$ Mar 25, 2019 at 14:31
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    $\begingroup$ I looked at a couple of commercial UHF TV 4-bay bowtie antennas. Are you sure the feedline is connected to a transformer? It looks like it might be a splitter/combiner. $\endgroup$
    – Brian K1LI
    Mar 25, 2019 at 20:00

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A single bowtie dipole does have an impedance of about 50 ohms. However, stacking multiple dipoles with a properly-designed phasing system can change that.

Here's a TV antenna that I made and use, with four phased bowties. Notice the phasing lines running between them. At the feedpoint where the 4:1 balun is, the impedance is about 300 ohms.

Phased dipole array

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ Mar 27, 2019 at 1:09
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A bowtie is in fact a full wave dipole so it's impedance is very high which is why they require a 4 to 1 balun.

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  • $\begingroup$ Hello, and welcome to the site! Center-fed full-wave dipoles do indeed have a high feedpoint impedance; but I always assumed that they were just broadband half-wavelength dipoles. Also, the pattern of a full-wavelength dipole would have a few more nulls and lobes. $\endgroup$ Aug 20, 2019 at 15:56

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