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I've been researching Yagi-Uda antennas and it's not clear what the frequency response of the antenna is. Some people are saying it will only receive signals that are 1% away from the frequency it's designed for. Others say "few percent", still others say 10%, 15%, 20%. What the heck? Finally, the last page I read said it really only matters if you want to receive a frequency higher than the frequency the antenna is designed for--it does well for frequencies below, so you should design the antenna for the highest frequency you want to receive. Time for a stack exchange question to help clear things up...

Assuming all else being equal, when you want to receive multiple UHF stations, do you pick a design frequency in the center, or the highest frequency, or something else?

UPDATE: I just found this. It seems to imply the frequency response curve of the antenna is asymmetrical (it says "A UHF Yagi today is designed for channel 69. If you see an old Yagi, it might be intended for channel 82". Now look at frequency response curve for it (I'll paste it below as well)--it falls off sharply at higher frequencies. Update: I was looking at the wrong curve (Yagi/Corner Reflector). The Yagi curve does fall off sharply at higher frequencies, but it falls off at lower frequencies too (just not as quick). IF this graph is correct, I don't think the asymmetry explains why I see so many different bandwidth numbers.

enter image description here

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  • \$\begingroup\$ For that sort of application, you want a wideband antenna, which is why you usually see log-periodic, corner reflectors or dishes. \$\endgroup\$
    – Dave Tweed
    Nov 15, 2021 at 2:01

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Assuming all else being equal, when you want to receive multiple UHF stations, do you pick a design frequency in the center, or the highest frequency, or something else?

The choice would hinge more on the type of antenna than on the frequency.

A Yagi-Uda antenna, with a bandwidth of about 10%, would not be suitable for multiple UHF stations.

enter image description here

A Log-periodic array, with a bandwidth of about 50 % would be more suitable for that.

enter image description here

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    \$\begingroup\$ Yes, a log periodic has much greater bandwidth, but a Yagi-Uda might be appropriate if the UHF frequencies are close enough to each other. That's one of the things I was researching--trying to establish the antenna's bandwidth and I saw the wide variation in answers. When I saw the one about an asymmetrical frequency response, I thought that might account for the variation in postings I read--MAYBE you can receive 20% below the design freq., but only 5% above it. That's why I'm asking this question. \$\endgroup\$ Nov 15, 2021 at 15:06
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    \$\begingroup\$ I ran out of room in previous comment, so I had to cut it down...I will probably use a log periodic antenna, but want to still understand the variation in Yagi-Uda bandwidth I see. \$\endgroup\$ Nov 15, 2021 at 15:27
  • \$\begingroup\$ Hi Adirondack Jim, Many thanks for your comments. You may find this paper quite informative. ijseas.com/volume2/v2i4/ijseas20160450.pdf \$\endgroup\$
    – vu2nan
    Nov 16, 2021 at 3:25
  • \$\begingroup\$ Thanks for the paper! It reinforces what the person who answered below is saying. Adding a reflector makes a big difference in bandwidth. I also couldn't help notice what the corner reflector did for the Yagi in the diagram I later found & attached. I'll be looking into all this more closely (I played with a log periodic antenna calculator and very early estimates say I'll need a 40+ inch boom which I'd like to avoid). \$\endgroup\$ Nov 17, 2021 at 0:14
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A Yagi-Uda antenna has a relatively narrow bandwidth. It's very narrow when made with thin wire elements, a few percent, but the bandwidth can be pushed up a little by using large diameter elements.

You design the centre frequency of the antenna to be the centre frequency of the range of signals you want to receive.

You need a log-periodic antenna to get more than 10% bandwidth.

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  • \$\begingroup\$ Thanks. I think you're saying the frequency response of the antenna is symmetrical about the "design frequency", so you pick the center (and that posting that says it's asymmetrical was wrong)? \$\endgroup\$ Nov 15, 2021 at 15:07
  • \$\begingroup\$ I did learn the bandwidth can increase if you use larger diameter elements. Could that account for the variations in bandwidth I see getting posted? Do you know how much extra bandwidth you could get with larger diameter elements (is it the 10% you mention in the last sentence)? \$\endgroup\$ Nov 15, 2021 at 15:11
  • \$\begingroup\$ @AdirondackJim Varying element diameter would easily make the difference between a few % and 10%. You can't get more than 10% with a Yagi, and need to go to different structures. A narrowband antenna like a Yagi has a frequency response that is essentially symmetrical. You could get some more complex designs that can be mutliband, so look very asymmetrical. \$\endgroup\$
    – Neil_UK
    Nov 15, 2021 at 15:39
  • \$\begingroup\$ Thanks! I'm starting to understand why I see variations. 10 percent won't be large enough for my needs (but 20% might, which is why I was diving into this). Interesting stuff. \$\endgroup\$ Nov 15, 2021 at 16:05
  • \$\begingroup\$ I'm going to bump this answer up because it explains why I see most of the variability in answers. I'm going to assume the 20% I saw is either wrong, or possibly that person assumed the antenna had a corner reflector (it also explains why someone said to design for the highest frequency). \$\endgroup\$ Nov 17, 2021 at 0:16

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