Signal analysis

[scott784]

I've got a HD 9095P UHF Yagi Style HDTV antenna with a HDP 269 12db amplifier. I'm getting good results with this antenna. However, I was curious about something.

One of my desired (out of town stations) is WAXN Channel 64 (Charlotte NC). I can get this station from one of the translators, 38 miles from my home but I cannot receive the main signal from the tower directly out of Charlotte. Interestingly enough, I did a rescan of my stations last night and the main (HD) broadcast out of Charlotte came in perfectly. However, now it is gone, again. So I am back to watching this station (WAXN) through their translator tower only (38 miles away versus the main tower at 61.5 miles away). I would prefer to receive the station through the main tower in Charlotte (on Vir. Channel 64-1) because that broadcast is in HD versus viewing the translator/repeater channel on Vir. Channel 64-7 which is in 480i only.

If you look on my tvfool report, you see that the translator station for WAXN is in yellow and the NM(db) are 17.2 Note: this is for the translator station which broadcasts in 480i only, which is located 38 miles away from my home.

However, my report also suggests that my outdoor antenna should be able to get WAXN (Virt Channel 64-1) from the main transmitter in Charlotte. See this highlighted in red/pink on my report.

Anyhow, I was just curious about this situation. I got this main station, once last night in HD but not again. The main transmitter shows 4.9 NM(db). That is not much to work with. However, I do have the preamp on my antenna. The entire length of the coax from outside to the livingroom is under 40 feet. There are no splitters or anything else that degrades the signal. Of course, I know the obvious day to day things that impact signal strength--especially when it gets weak.

But just a couple of questions about this situation. First, had I gone with a higher amplifier, is it logical to assume I would have received the main signal from this station out of Charlotte versus relying on the repeater signal tower only? Second, because I was able to receive the 'main' signal from Charlotte once on this station, and now I am back to not getting the main signal......is it reasonable to assume that I am losing about 5 NM(db) on all my channels? Maybe, any station listed (for anyone) at around 5 NM(db) is just too little to realistically work with? Thanks for any replies.

Again, I was just curious about this matter.

http://www.tvfool.com/?option=com_wr...e07c93c07c6564

[Dave Loudin]

Note the path of WAXN's main ch. 50 broadcast: 2-edge. This means that the signal has to scatter over two obstacles to get to you. Click on that listing in the table of your report, and you'll see what I mean (transmitter's on the left, you're on the far right at the bottom.) While propagation via direct line of site is reasonable stable, each scatter adds some variance to the predicted receive strength. Additionally, atmospheric effects will occasionally enhance the bending of signals beyond the horizon. That's why you'll see ch. 50 only sporadically.

The only sure-fire way to improve the situation is more antenna gain or a higher mounting position. More amplification won't help.

[No static at all]

Good suggestions Dave,

Here is what I would do if in your shoes.

1. Try raising/lowering the antenna in small increments to find a better sweet spot for RF50. As little as 3 inches can make a difference with problem stations, especially when you have diffraction issues.

2. Tilt the front of the antenna up slightly if possible.

3. Raise the antenna 5-10 feet more if possible & then try fine tuning height in small increments if you still find RF 50 unreliable.

4. Replace the 9095 with a higher gain antenna if all else fails.

Just keep in mind every time you change height, signal strength on other stations may decrease, so re-test all stations once 50 is providing satisfactory reception. By the way, are you using a rotator?

[scott784]

I had a rotator installed with the brand new installation of my outdoor antenna a month ago I thought I would have a need for it due to my true locals which are about 30 miles away to the southeast (versus southwest for Charlotte). As it's turned out, I rarely use the rotator except with one of my local stations which operates at low power and sometimes will pixilate if the boom of my directional antenna is not pointing to the southeast. However, again, for the most part, I just leave the rotator alone (pointed south to southwest for Charlotte stations) and this highly directional antenna seems to pick up most everything else off the side (without the antenna being pointed to my true locals).

With this being an eve mount 30 feet off the ground, it is very difficult for me to get up there. (It clears the A part of my roofline but sits right on the edge of the roof up on a tall mast.

I guess my biggest question now is the following. Is 5 NM(db) of gain too little to work with? I thought, at least theoretically, that anything above 0 was okay and that 5 or 10(db) would definnitely clean things up.

In may case, I get all of the stations in green and yellow but only one station in the red/pink section of my tvfool.com report. Maybe, it's a lost cause to get WAXN Virt Channel 64-1. There is a distance factor there of over 60 miles. However, I am not having any problems grabbing the other Charlotte networks, which are the same distance in mileage and some of them even a bit further than WAXN. Of course, I know some of these stations operate at lower power.

But again, when a tvfool report shows a station at 5NM(db), is it normally a lost cause in terms of trying to grab the station? I am thinking the answer is generally yes. Thanks again.

Thanks for your feedback.
Scott

[John Candle]

Read and understand this about , Real Digital Tv Channels , Virtual Digital Tv Channels , Analog Tv Channels , http://forum.tvfool.com/showthread.php?t=695 . . UHF 50 also has adjacent channel and co-channel interference . .

[Dave Loudin]

Scott, what I was driving at was the 5dB NM for 50 is probably a bit optimistic for two reasons. First, all propagation paths have variability. Line-of-sight paths have the least, so the received signal strength doesn't change much moment to moment. When a VHF or UHF signal has to be bent over an obstruction, the actual scattering mechanism is less stable (air density, terrain roughness will now have a greater impact.) Thus the received signal strength will vary more moment to moment and day to day. Add another scatter point in the path and there will be even more variability.

Propagation predictions use statistics to account for this behavior, and there is a statistic attached to the NM prediction. I don't know if it is 50% or 90%, but there will be chunks of time where the predicted signal strength will be far less than what a 5 NM implies. To account for this, we normally recommend a design target of 10 to 15 NM for LOS and 1-edge and 15 to 20 NM for 2-edge paths to account for the variability.

[No static at all]

Originally Posted by scott784

But again, when a tvfool report shows a station at 5NM(db), is it normally a lost cause in terms of trying to grab the station?

A noise margin of 5db is doable in some cases, but it is so close to the digital cliff, reliability is unpredictable. As John pointed out, you have an adjacent channel issue with a much stronger RF 51 in the same direction which isn't helping either. I personally find about a 1-3 db noise margin as the drop off point for an un-amplified 91XG.

[Tower Guy]

Originally Posted by No static at all

As John pointed out, you have an adjacent channel issue with a much stronger RF 51 in the same direction which isn't helping either.

I think that's the real problem. In order to pick up WAXN you need to attenuate channel 51.

This could be done using a sharp tuned channel 50 bandpass filter or stacking two identical antennas side by side with a critical spacing of 3.5' to null the signal from 83° off axis.

[scott784]

I appreciate all of your feedback. Yes, I can see that my local CBS affiliate WFMY on RF51 would deffinitely be a factor here. This channel shows 56.5 NM (DB) on my tvfool report. So obviously, it is a much stronger signal at 30 miles away to the SE versus the main signal for WAXN which is over 60 miles away to the S-SW. Topping that off, since my report shows I've only got 4.9 NM(db) to work with on the main signal for WAXN (in the first place), it makes things bad all the way around.

As Tower Guy posted, I guess WFMY RF51 could be attenuated. (I am not sure how that works but could find out).

[John Candle]

http://www.tinlee.com has filters

[scott784]

Thanks for the link, John. I'll check it out.