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 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. $\endgroup$ – Mike Waters Mar 25 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 at 20:00

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 much higher than 50 ohms.

Phased dipole array

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Mike Waters Mar 27 at 1:09

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