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Discussion Starter · #1 ·
After a year of coveting digital TV, I finally have a taste. Unfortunately, getting a reliable signal has been elusive. I would like to detail my setup and situation and see if the forum members can offer any suggestions.


My location is Lat 34.1908455 Long 79.7741165 in Florence SC


The signal I am trying to receive is originating from Columbia, SC. They are broadcasting @ 65 KW from 1000 feet. The tower location is Lat 34 07 07 Long 80 56 12. Their call letters are WRLK-DT. This is the only over the air digital signal that I have a chance at. I am 66.65 miles away.


I am currently using a Dish 6000 receiver and OTA tuner module. My antenna is a CM4251. It has been modified by the addition of 1/4 inch hardware cloth. My preamp is the UHF side of a Titan 7777. It is mounted as close to the bowties as i could get it without being in front of the dish. The whole setup is mounted in a tripod on my rooftop approximately 25-30 feet above the ground. I am using Studio Grade RG6 quad shield with gold plated connectors.


Here is what I have found. I cannot seem to lock on in the daytime. The signal meter bounces around from 0 to 40 to 55 to 0 etc. At night, starting about 30-60 minutes after sunset, I can lock on. Early in the night the signal strength is between 55 and 60. Late at night, I have seen it go as high as 85. Once it locks on, the picture is perfect as you would expect. By contrast, analog channel 35 is being broadcast from the same antenna. The picture is always fairly weak and snowy and seems to be only slightly better at night.


I infer from this that I am right on the edge of the "cliff". I am wondering why the signal is better at night (noise floor?). What else can I do to get the slight improvement in strengh that I need? I have considered height and trying alternate sights on my roof. Is their a better antenna out there? Would using a VSWR meter and trimming the bowtie for lowest VSWR at channel 32 make a difference?


Please help. I am almost there but not quite and it is very frustrating.


Thanks,

Scott
 

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Sounds like you should consider subscribing to ExpressVu for the your primary HD source.


Mike
 

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Discussion Starter · #4 ·
WOW!!


I appreciate the in depth analysis from all the top brains on the forum! I didn't expect such a quick and complete analysis.


Wendell,

Yes there are trees in my neighborhood. Mostly pines which are about80-100 feet high but fortunately not too many to my east. No, my antenna is not above them and I figure height is probably the variable with which I could get the best improvement. The problem is that the 4251 is large and unwieldy (difficult to manage). I might take my antenna to the roof of the hospital (10 stories) one day just to see what happens, but it doesn't help me much here.


MaxHD,

Glad you didn't rain on my celebration http://www.avsforum.com/ubb/smile.gif I guess I got too excited too soon. I am hoping for some serious atmospheric refraction on April 17th for NOVA in HDTV.


dagman,

I actually have been concerned about DST. About 8PM is when I can lock in and that is when the primetime HDTV shows come on. Unfortunately, my watch time will probably be pushed back to 9PM http://www.avsforum.com/ubb/frown.gif


Bob Smith,

How did you calculate 450 feet? What formula do you use? I realize that height is probably most critical at my location. Getting the 4251 up high is an engineering feat, unfortunately. My neighbors already think I am insane. Can you imagine the rumors when I am up on a sissor lift in my front yard with a hugh TV antenna testing reception???!!! Thanks for the suggestion. By the way, there are no mountains in the coastal plain of SC. I live 1 hour from the coast and my house is 140 feet above sea level. There are a few rolling hills in the midlands near Columbia. What is QTH?


Man E,

My lead in goes directly to my tuner cartridge. I talked with Hap Griffin, SCETV's engineer and it is at 65KW and 1000ft. Terrain is flat with a few rolling hills. There is no known source of interference, but I live in the city. I might get a UM. Where and how much?


John,

I am pretty sure it is at about 1000 feet. How do you propose to mount a BUD sideways on my roof? Although in theory it is a great idea, the wind load/neighbors think I am insane factor sort of rules that out. I do think that 4.7 dB of gain would be more than enough to get firmly onto the "cliff."


Mike,

I do get HDTV now via Dish. I just really wanted to try OTA DTV.



Thanks again to everyone.

SCott

 

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Scott, Sorry about using QTH, that's ham radio talk for geographic location. I used a Nomogram derived from the formula in in the Reference Data for Engineers Handbook, 7th edition. There's a lot of good information on the chaper on electromagnetic-wave propagation, as there is in other publications.


Bob

 

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Scott,


Sorry. Thought that since you were able to "get away with using the 4251", that you might be more in a rural area. That, of course, without chcking the map. As for mounting a BUD, I' ve seen some neat mounts to the side of a house. You are beyond the horizon, there, I don't think height will matter much unless you could go way way up as has been suggested by Bob. A neat thing about the BUD is that you could play with tilt. A little upward tilt might be cool. In any case, I am speaking partly in jest on this and partly radically serious.


Regards,


John Stephens
 

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Scott:


That was my point. ExpressVu carries all the US Network HD. Don't know what they will do if the ABC rumor is true though.


Mike
 

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Quote:
I might get a UM. Where and how much?
Here



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Scott,


To calculate the height of your receive antenna to get LOS I used a formula which was posted several months ago.

"Range in miles= Square Root(2*Ht.of broadcast tower in feet) + Square root(2*ht. of receive antenna in feet)"

I always assumed this was correct.I wonder who posted this? Tehehe!!

Anyway,66.65 minus 44.72= 21.93 which calculates out you need to be at 240ft./6"high. A Rohn 45 guyed every 30ft.should do it.Your neighbors probably won't mind if you use their yards for your top set of guys(192ft.radius),especially if you promise to mow their yards this summer!


What about that 68ft. tower you helped put up a while back? Have you tried your STB on it yet? If it's close to you,it might give you some idea if height would help.


Also,John mentioned gain might help.I know in my situation,when I try locking on that 69mi.station and switching back and forth between a Silver Sensor (6db gain)and a U-92 Passive Wave Loop(14db gain) the DTC goes from no signal to lock_22_28etc. I believe you've done everything right and the 4251 is the best or one of the best.However,Ed Hanlon says that a quad stack of U-92's will out-perform it.Of course at $90 apiece plus frt. and stacking hardware this is expensive experimenting!


Another thought.If you have a CATV head-end in Florence,you might look at their tower and see if there's a UHF anntenna pointed towards Columbia.That would be a good example of what's needed for"reliable" reception.


Good Luck!



[This message has been edited by MAX HD (edited 04-01-2001).]
 

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Scott,


Sorry about your bad luck.I noticed that your other post was after 11pm night before last and I wondered if you locked on at that time or earlier in the day.I thought about posting to bring up the subject of enhanced night-time propagation,but I didn't want to spoil your "celebration" so to speak.


I also have a PBS station that is a cliff situation at 69mi. and rarely is watchable unless the signal downward bend is enhanced significantly after dark or some other weather related phenomena is present.


Have you checked out Bill Hepburn's Tropospheric Forecast site? I'm not good at this link thing but I'll try. www.iprimus.ca/~hepburnw/tropo.html Also,what is reception like from other Columbia analog stations during the daytime?


All for now-got to get back to work!
 

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One other option you might try is to purchase a DTC-100 from a local Best Buy or Circuit City (so you can take it back). The DTC-100 supposedly has a little better reception characteristics than the 8VSB cartridge on the 6000. If you do get an improvement, you've got a box you can use for OTA, if not, you can take it back.


Yet another option is the new (?Samsung?) OTA-only HD tuner. This one apparently tests out better than any of the satellite tuner boxes or older OTA-only STBs.


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Scott,


At your location, you are about 15 miles beyond the radio horizon at your antenna height. You would need to raise your antenna to about 450' to be line of sight. What you are receiving is due to refraction. In that zone, height is the most important thing. You might get better reception by either lowering or raising your antenna. The reason things change at night is due to the interface between the atmosphere and the earth at the point refraction occurs. At night, fog density probably increases and causes the signal to "bend" more readily.


I think your first best bet is to try to raise the antenna as high as possible. You might try one of those scissors lift trucks you can rent to find the right "zone". Even if you do find the correct point, this will be variable due to the variability of the diffraction zone.


If you do find an acceptable height, place a second antenna at a lower height for times when things change.


I think you have done everything possible to your antenna, your amplifier has a 2 dB noise figure, and the antenna is the highest UHF gain available. I doubt any work on a better antenna match (swr) would have much effect. Increasing the length of the feed between the amplifier and the antenna by about 6" (a quarter wavelength) would reverse any SWR effects you might have to see if there is any difference, but I doubt you would find any. The SWR between the preamp and your STB is insignificant because reflections will be absorbed rapidly by the cable.


All of this is assuming you have no mountain ranges between you and the transmitter site. If there are mountains >1000', things will be even more difficult. Optimizing the analog signal may or may not be the right thing to do, this will depend upon the analog station's antenna height, and the frequency difference between them. I find different stations at the same height come in at different receive antenna heights at my QTH where I am in a diffraction zone.


Bob Smith

 

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Is the antenna directly plugged into your Dish6000 or is it split to other TVs? I sit about 50-55 miles from all transmitters in my area and had to dedicate an entire antenna to my UM. I use the CM4248 and 7775, similar wiring (~100'), and rotor. My analog signals are strong on my other antennas, but even taking half of that to the UM was not sufficient much of the time - especially during the day. I'm pretty sure that there is an atmospheric issue involved, but don't have an explanation for you.


Check with the station to make sure that they are actually cranking the amount of power that you think they are. There are several stations in the Detroit area that are operating well under their limit.


What about terrain issues between you and the transmitter? It's difficult to move such an antenna around, but have you tried (even up on the roof)? Sometimes only a matter of feet up/down, left/right, or forward/backward can make a difference.


Is there a potential source of interference near you that runs during the day? Long shot.


If it is PBS, you could try a 1M dish and a UM. I know that doesn't solve your terrestrial problem, but might be an option for you.


Good luck!



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Scott,


From your description of your efforts, it appears that you are doing all the right things. Still, as you say, you are right on the reception edge. Very difficult to improve on what you have. To be honest, if the transmitter is truly at 1000', you are well beyond the horizon(it's at 44 miles, check with the station to find the true height ASL). When you are beyond the horizon, there has to be something that bends the radiation in order to get to where you are. One thing that can do this bending is a transition from high refractive index to low refractive index. Note that refractive index is just the square root of dielectric constant. At night, the fog rolls in, the air is "heavy", higher average index than during the day . So more bending of the radation back towards the earth, hence to your site. A notable example of this phenomena is what happens inside a fiber optic cable. These consist of a higher index core with a lower index cladding at the boundary. This high to low interface causes the light to bend back towards the axis and remain confined. Absent this, it would leak right out. So too, with this RF, no high to low, they just sail on off into space. So there may not be much you can do. But if you want to try something radical, here's something radical, if you happen to have an unused BUD dish, mount a double bow tie at the focus and improve your gain in the ratio 10*Log(Area BUD/Area4251). A twelve foot dish would give you about 4.7 dB, improvement on your current performance. This still might not be enough during days, but might get you started earlier at night. It would make an interesting experiment and of course, I'd like to be updated often on your results.


Regards,


John Stephens
 

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Scott,


I knew if I looked around a bit I could find a solution for this "cliff" problem.


These can provide studio quality reception when a standard antenna is receiving a snowy,noisy picture caused by long distance path loss.

www.lindsayelec.com/superzz-uhf.html


For deciding whether you need one,two,or four try this site for a complete path-loss analysis recommendation.

www.biroengineering.com


This probably wouldn't win any points from your neighbors either,but you sure would be the envy of us here on the Forum!!


Good reading material anyway.
 

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Discussion Starter · #18 ·
Once again,

Thanks to everyone for replies.


MaxHD,

I did indeed post the formula you mentioned. I got it out of a RF textbook. Using it, I calculated the same recieve antenna height that you did. What threw me off and the reason I asked about another formula was the 450 foot height above. I think the difference is one is line of sight and the other is radio frequency horizon. The tower I built for a friend is about 60 miles away and thus wouldn't be applicable to my situation. I have considered my own tower, but with closer digital stations coming on line within the next year, it seems that the effort and cost are not justified for a few months of PBS.


As far as the DTC100, as suggested by dkeller, once again this represents an additional equipment purchase which I am not sure I am up to now.


Tell me more about this U-92 passive wave loop antenna. Where can I see one? What is the design? The zigzag antenna you posted looks great except for the HUGE price.


Scott (only Dr. Burns at work http://www.avsforum.com/ubb/smile.gif )
 

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Scott,


Well,I guess if you've checked your e-mail and put a straight-edge on that graph you know that any attempt to improve reception on Ch.32 will be in vain.That free path loss analysis was really amazing.


Kudos to the proffesional on-lookers that pass by this forum!


If you like,you can check out the U-92 at www.antennaperformance.com/new for descriptions and pricing info.There's also a thread I started a while back on this forum.


I'm like you,I'll just have to wait till PBS in Indy or Cinncy come on line sometime in the future.
 

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Discussion Starter · #20 ·
Well...my Saturday project was to mount my antenna on a mast about 15-20 feet above my roofline. It is now about 45 above ground. I have only had a few dropouts. Before now I could not lock on in the daytime, but that has improved significantly.


I recieved this email from an engineer who read this post. He seems to be very knowledgable and scientific in his analysis. However, according to this I should not be able to receive a signal at all. What other explanations could be accounting for my signal?


Hello guys.

I am not a member of the AVS Forum, but I do check it from time to time.

The reason you cannot receive WRLK-DT is that both the line-of-sight and the first Fresnel zone are completely blocked. (After all, the station is

licensed to Columbia, not Florence.)


The line of sight issue is obvious. Blockage of the Fresnel zone hurts you

because radio waves tend to "spread out" as they propagate, and if something like terrain blocks this, it has the same effect as blocking line-of-sight.(BTW, that's a laymans explanation.)


I've attached a diagram which shows the sitation in detail. WRLK is on the

left (at 1036 feet above ground level), and your location is on the right. I assumed a 20' outdoor antenna initially for Scott, but one can scale this wherever you like.


As you can clearly see, the LOS (red line) is blocked at about 28.5 miles

from WRLK by terrain and the 1st Fresnel (blue line) is also blocked at about 24.7 miles. Note that terrain data used comes from USGS 3-second elevation data and does not include trees, buildings, etc... Actual obstruction is probably a bit worse.


Others have already pegged the problem for you Scott. The reason I send this is to help you avoid any unnecessary expenditures. Print the attachment. Then take a ruler or straightedge and draw a line from WRLK to your locale. This line should clear the jagged black lines (which is the actual terrain - the smooth black line is the earth's curvature reference line). Whatever the resulting height above ground level at your locale (right side), that's how high your antenna will need to be. It appears to be a substantial requirement, even more to clear the Fresnel zone.


The Fresnel zone was calculted at TV-32 Frequency: 578 MHz. You do not

absoutely have to clear the 1st Fresnel, but it helps. You should have LOS, or something reasonably close to it. Multipath can also be a problem, but I doubt that is an issue here. At least not yet. Maybe after you raise the antenna?


Anyway, I hope this helps you. I presume you will decide to wait until the

Florence stations construct their digital facilities. Best of luck.


There was a nice graph included. How do I post a .jpg file?


Scott

 
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