Electronics > RF, Microwave, Ham Radio

O.T.A. television yagi question,

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CaptDon:
I'll try to get some physical measurements but here is the scenario. The yagi has one reflector, the so called driven element which is actually the receive element is a 300 ohm loop with a 300 to 75 balun and then there are 10 director elements all of which are the same length and spacing. (I would have suspected a log periodic type length for bandwidth). When the antenna has snow laying on the elements (recently up to 3 inches of snow accumulated on top of the elements) the gain goes up by at least 3db or more. With that information what can you determine about the element length? Does it suggest they are too long or perhaps too short? Fortunately, all of the stations I want to receive are close together in frequency such as 27, 28 and 29 being the real channel frequency not the digital station numbers. If I recall they were around 540MHz. Thanks!!

antenna:
My best guess, adding ice/water is like adding a dielectric coating, and dielectrics decrease the velocity factor making the elements appear longer electrically.

Muttley Snickers:

--- Quote from: CaptDon on January 03, 2025, 02:38:49 am ---I'll try to get some physical measurements but here is the scenario. The yagi has one reflector, the so called driven element which is actually the receive element is a 300 ohm loop with a 300 to 75 balun and then there are 10 director elements all of which are the same length and spacing. (I would have suspected a log periodic type length for bandwidth). When the antenna has snow laying on the elements (recently up to 3 inches of snow accumulated on top of the elements) the gain goes up by at least 3db or more. With that information what can you determine about the element length? Does it suggest they are too long or perhaps too short? Fortunately, all of the stations I want to receive are close together in frequency such as 27, 28 and 29 being the real channel frequency not the digital station numbers. If I recall they were around 540MHz. Thanks!!

--- End quote ---

Have you tested or recorded these UHF signals with a field strength meter or equivalent device? My best guess is the signal variation might have more to do with snow in the region and perhaps the local topography than element length.

For example, if there were pine trees towards the direction of the signal source then these may also influence your reception as they are notorious for signal attenuation particularly when damp or wet, additional factors such as these should also be considered.

CaptDon:
I had considered snow cover on the ground as helping increase the signal strength somewhat and I believe it does since it also helps with my 20 meter band 'hamstick' in the center of my house roof which is metal covered. However, I looked at the real signal strength of the UHF OTA antenna before and after knocking snow off the antenna using my TinySA and looking specifically at the weaker of the three signals it went from -72 with snow on the elements to around -75 to -76 with clean elements. Sadly, in the -76 to -80 measured at the television I start to get a lot of pixelation. Also sadly, the weaker station is carrying 6 channels 35-1 through 35-6 where as my other stations only carry 4. I wonder if carrying 6 channels instead of 4 makes the available signal recovery of any one channel more difficult? I do have a 15dB decent quality preamp at the antenna but it does not include FM and 5G internal traps. The feedline is low loss RG-6 50 feet long. Reception in the winter is better since I live in a wooded residential area. I'd have to go with 80 feet of tower to be able to look over the tree tops. Not really practical and sadly would be a HUGE lightning magnet!!

bostonman:

--- Quote ---the gain goes up by at least 3db or more
--- End quote ---


I'm curious, what did you use to measure this?

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