O.T.A. television yagi question, - Page 1
Forum section: electronics / rf-microwave

author: CaptDon
author_rank: Super Contributor
timestamp: January 03, 2025, 02:38:49 am
content: 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!!

author: antenna
author_rank: Frequent Contributor
timestamp: January 03, 2025, 05:21:48 am
content: My best guess, adding ice/water is like adding a dielectric coating, and dielectrics decrease the velocity factor making the elements appear longer electrically.

author: Muttley Snickers
author_rank: Unknown
timestamp: January 03, 2025, 06:03:59 am
content: Quote from: CaptDon on January 03, 2025, 02:38:49 amHave 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.

author: CaptDon
author_rank: Super Contributor
timestamp: January 03, 2025, 01:36:59 pm
content: 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!!

author: bostonman
author_rank: Super Contributor
timestamp: January 03, 2025, 02:17:16 pm
content: QuoteI'm curious, what did you use to measure this?

author: CaptDon
author_rank: Super Contributor
timestamp: January 03, 2025, 08:30:34 pm
content: It's in the above post, My TinySA (that is a device and brand name I guess) which is very close to 'right on the money' at 512MHz from a calibrated signal generator. My sig gen has Motorola's name on it but it is actually a Wavetek 3000. The Wavetek's levels were verified within .5dB across its usable frequency range. Mine stays locked from 400KHz to 530MHZ although the published range is 1MHz to 512MHz. The TinySA is very nice!! It works way better than I expected for such a budget price!

author: geggi1
author_rank: Frequent Contributor
timestamp: January 04, 2025, 04:46:10 pm
content: Remember some of these antennas do have a integrated pre-amp and require a 5V between center conductor end screen to work properly.If you are using a set top box they usually have integrated power injector that can be enabled in the on screen configuration.If you are using the TV set you must check for the actual model.

author: CaptDon
author_rank: Super Contributor
timestamp: January 04, 2025, 08:23:37 pm
content: Antenna is totally passive non-amplified. I do have a mast mounted 15dB preamp with the proper power supply unit in my basement where the RG-6 enters. I haven't measured the elements yet to determine their optimum frequency. When springtime comes and I can play around outdoors with this thing I'd like to see if I can optimize it for the weak station. I have plenty of margin on the strong signals. I may be looking for a resonance at the weak channel frequency by adjusting element length. I have found in some cases a small resonant antenna outperforms a larger non-resonant antenna.

author: geggi1
author_rank: Frequent Contributor
timestamp: January 04, 2025, 09:18:40 pm
content: If you place the preamp at the antenna you will get better S/N relation because you will amplify less of the noise and more of the signa.

author: coppercone2
author_rank: Super Contributor
timestamp: January 04, 2025, 09:24:22 pm
content: i think you need to cool the antenna with a low temperature coolant and put a mister infront of it in the summer to find out

author: CaptDon
author_rank: Super Contributor
timestamp: January 05, 2025, 12:59:25 am
content: Preamp is 6 inches from antenna and all feedlines are RG-6 low loss foam from antenna all the way to the T.V. The weak station is generally good during the winter when no leaves on the trees and as mentioned snow cover on the ground does increase signal strength by about 1 to 1.5 dB, BUT when the elements have snow build up resting on them (recently nearly three inches) the gain on the weak station increases by nearly 3dB!! I guess the strong stations also see the increase but I didn't specifically look at them. I am surprised to see pixelation when the signal is in the -79 to -82 dB range measured with my TinySA who's scale is dBm @ 50 ohms. I have one adapter in line, it is F female to SMA male to mate with the TinySA.

author: pienari
author_rank: Regular Contributor
timestamp: January 18, 2025, 08:04:48 pm
content: Hi.Can you take some measurements?.Lenght of the reflectorFolded dipole.1 director