View Full Version : What's the effect of fog on OTA?
01-13-2006, 11:02 AM
What's the effect of fog on OTA? I'm 40 miles out with a DB8 and 125 ft run. Paid someone to put the antenna 10 ft up on the chimney this morning. Fox is 2 degrees off but all other stations are the same direction. However it appeared even though it was on the roof, I couldn't get in ch 64 and 67 in the high 80s like i did in the attic. My guess is the chimney was 5 ft to the right of the original spot inside and i guess that made the difference. But i didnt' want the hassle of a tripod.
Anyway, after over 1 hour of struggling aiming the antenna 1 millimeter at a time, and trying a pre amp from the guy (turns out my regular amp was better anyways), I've now gotten fox to be solid mid 60s and channel 67 and 64 seem to be solid 68ish or 72 ish. If everything stays as is, i'll be thrilled as I can record solid 60 signals. But I was wondering if the dense fog helped or hurt the aiming this morning? If all the channels die tonight after the fog is gone, i'll scream lol.
01-13-2006, 01:12 PM
I'm no expert :( but I can't believe it would have much effect. More solid objects have an effect on OTA reception.
01-13-2006, 01:16 PM
The utility company I used to work for had a private terrestrial microwave system. I don't know what frequency it operated at but heavy fog did adversely affect it.
01-13-2006, 01:16 PM
well day and night has an effect here :) I've already aimed the thing in the AM (when it was in the attic) and had one channel just die at night and the others take a 15 -20 hit on signal.
maybe i live in the bermuda triangle
01-13-2006, 07:05 PM
Water is thicker than air. Is that significant at UHF? Probably not. Fog will cause mild attenuation, but most systems can tolerate that without losing lock. And as long as the fog is uniform (which is kind of the nature of fog, usually) the atenuation should also be uniform, meaning that multipath signals would be attenuated about the same as direct signals, and that means that the multipath interference ratio would stay pretty much the same, again not really affecting reception.
If the wavelength in question is similar to the size of the vapor droplets, the attenuation will be stronger due to the droplets acting like little tuned antennas, absorbing the signal. That explains rain fade in Ku and microwave, which are much more prone to it. I'm told that signals of a much higher frequency, such as sat radio at about 2300 MHz, are also only slightly affected, again because the wavelength is dissimilar from the size of the vapor droplets.
01-14-2006, 05:56 AM
I'll join in with anecdotal evidence.
Normally we receive CBS on frequency 21 and PBS on 56 at ~91 and ~71 *steady* respectively. Yesterday morning the fog was quite dense. 21 was badly locking, blocking and pixellating with 30-70 on the meter and PBS couldn't lock with 0-20. Both were extremely unsteady levels.
All was fine in the clear (if cloudy sky) evening. DirecTV was unaffected throughout.
~30 miles of of fog proved about as effective in blocking UHF as a dense storm cell does to KU.
With our location, foliage in summer etc, satellite reception is more reliable than OTA.
01-14-2006, 09:05 AM
well first night of the antenna on the roof and all is well. the difficult fox station recorded just fine, zero blocking and the signal was only in the low 60s
ch 64 and 67 seem to be cyclical. last night were in the 70s but this morning, ch 67 actually was 58-64..but i watched it live for 5 minutes and it was fine. I guess a stable low signal IS much better than the multipath i had in the attic with the mid-hi 80s signals. Plus i needed the compromise to get the 2 degree off fox station, which is back to 68 signal this AM.
so time of day definite is a big deal at least for me but hopefully they are stable enough to hold for years to come. I've never had all 6 networks in at the same time and am still a bit leary of recording OTA when i wont be home...but hopefully after a week of solid signal i'll be ok
hope so...i can't climb up there to adjust it :)
01-14-2006, 09:17 AM
Glad to hear things are going well newsposter. Something to keep in mind is that in 2009 the broadcast stations in the VHF spectrum that have their current DTV signals in the UHF band will have the option to move their UHF DTV signals down to their VHF channels when analog broadcasting is supposed to cease. It's pretty likely they will do that. If/when they do, it may change the antenna requirements for some of us.
01-14-2006, 10:34 AM
The day after i ordered the antenna, I heard 6 was moving back to 6 instead of staying at 64. but since that's 3 years away, i hope i'll be ok until then. And while the DB8 isn't made for vhf, I did get some signal in when i was testing it on my tv tuner so at a maximum i'd have to add a vhf to the 10 ft pole, or heck, even the attic may work for that and use a jointenna.
If only Fox weren't 2 degrees off i wouldnt have had any problems. And even if this ota 'goes bad' at least later this year i'll be able to get the DVR and heck, if i hate it, i can still use the HDtivo for all channels but fox and just use the HR20 dvr for fox :)
I never actually thought about it that way. of course that assumes the new box isn't more than 10 bucks or so a month. I still can't figure out that pricing in that thread.
01-14-2006, 07:30 PM
...in 2009 the broadcast stations in the VHF spectrum that have their current DTV signals in the UHF band will have the option to move their UHF DTV signals down to their VHF channels when analog broadcasting is supposed to cease. It's pretty likely they will do that. If/when they do, it may change the antenna requirements for some of us.In general, stations originally at 2-6 will opt to keep the new UHF assignment, while stations 7-13 will opt to return. There are individual situations where that does not make sense to do, but for most locations it is technically the best choice.
01-14-2006, 09:50 PM
In general, stations originally at 2-6 will opt to keep the new UHF assignment, while stations 7-13 will opt to return. There are individual situations where that does not make sense to do, but for most locations it is technically the best choice.
Not sure I follow this correctly. Can you elaborate?
01-15-2006, 12:19 AM
Sure. 2-6 are low-V, and have a tendency to be plagued by impulse noise. 8VSB is dithered to tolerate this, but only so much. 7-13 are high-V, and are not so susceptible, plus, they generally have better coverage and lower power requirements (translating into more viewers for less operating overhead) than UHF. The cost of switching is not cheap, but the ROI may be worth it. It usually means changing the exciter and possibly the antenna, but the transmitter itself (the real expensive part) should be futured for this.
As the low-V gets higher in frequency, the effect is less, and if the UHF gets higher in frequency, the effect is possibly more, so if you are on ch 6 and your DT is on ch 67, it might make more sense to return to 6, even as a low-V channel. Terrain/topography and competing signals also enter in to an extent.
01-24-2006, 01:52 PM
Tiny update on my DB8 antenna saga for anyone following along. And he did seal the coax this time after I asked him to. After the guy trying every foot or so on the length of my roof, and even a higher roof, I settled on the original spot over the place in the attic where i got great numbers. I'm definitely shooting between a house and an unseen object in the distance. The installer was pretty flabbergasted how just a foot made a huge difference.
So now that I was 6ft away from the chimney, using a tripod and 5 ft pole instead of a 10ft on the chimney, I'm pretty confident the high winds wont move this mount again. CH 64/67 are in the 80s. Ch42 is a solid 71. Wb 54 is a somewhat solid 68. Of course cbs/upn are 90 since they are in the lower UHF.
In my case, higher wasn't always better. A foot horizontally made the difference between 10-15 on the signal meter. I spent 400 in labor etc getting it this far so hope they kill ch 54wb and move the programs down to 32upn. Then I'd be set. Or if they even put CW on directv I'd be ok if the signal was as good as OTA (not likely though)
I just hope he's not right that when the leaves grow on a tree (200+feet away) that I'll have problems :(
01-24-2006, 07:51 PM
A tree 200 ft away could have a small impact (slightly more-frequent hits than without) but not a significant impact. Jump off that bridge only when you come to it.
01-30-2006, 07:13 AM
another answer: In dense fog (i couldn't see 2 houses down this morning and the fog also permeated the garage at work even..weird), it will KILL a weak signal. Ch 42 , which has been a steady 71 (and even 80s in the mornings), was down to 0-10 this morning. Also, a newly acquired weak PBS channel died. But the other networks were all ok with 71-85 readings.
glad I dont live in London where it's always in a fog :)
01-30-2006, 03:33 PM
glad I dont live in London where it's always in a fog :)
Well the British transmission system is MUCH more robust than ours. Most people in the metro areas can get service with a loop.
01-30-2006, 07:59 PM
One of the many reasons that COFDM is more suited to that area than 8VSB. It is not really more robust, except in the area of multipath rejection, which allows simpler receive antennae. They have fewer regional interference issues, so COFDM can also operate at a higher power there than 8VSB can in the US. 8VSB is more efficient, and has fewer co-channel problems, making it more suitable for the US (although Sinclair would disagree with that).
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