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Help With Weak Upper-VHF Digital?

post #1 of 60
Thread Starter 
Hello:

I'm a college student at the University of Virginia who lives in on-grounds housing (an apartment) but uses OTA for digital TV in my room. Many here probably already know my name.

I'm attempting to receive an upper-VHF digital with an indoor antenna. I know this is a bad idea, but I literally have no outdoor access. I've been receiving the three UHF digitals with my Silver Sensor for a year and a half but would like the alternate PBS programming.

http://www.rabbitears.info/market.php?mktid=218

Ignore W58DK, as that signal does not make it here.

I have set up my rabbit ears at 29.4 inches per ear, to make the channel 11 wavelength of 58¾ inches wide, and with this I can receive the three UHF signals, but only brief blips of signal on channel 11 that don't break 18% on my DViCO (which needs 60% for a successful decode).

WVIR-DT 32 1000 kW
WHTJ-DT 46 165 kW
WAHU-LD 40 9.5 kW
W50CM-D 11 0.4 kW

You can see what I'm up against. I'm doubting that bringing my CM7777 up here is a good idea.

Does anyone have any thoughts on what I could do to bring in this signal?

Thanks.

- Trip
post #2 of 60
Try using a UHF & VHF signal combiner (UVSJ). Connect the Silver Sensor to the UHF input and your VHF antenna to the VHF port. This will keep the VHF antenna adjustments from affecting your UHF reception, with very little signal loss.

I doubt if the preamp will help because it will increase the noise as well as the signal, with no improvement in the signal-to-noise ratio. It is worth a try as a last resort. What you need to do is find a "hot spot" for your VHF antenna.

If adjustments to the rabbit ears isn't enough, you next step is to make a folded dipole for CH 11 out of 300 ohm twin lead and feed it with a 300 to 75 ohm balun. The formula for the length in inches is 5540 divided by the frequency in MHz: 5540/201 = 27.5 inches for the total length of the half-wave T top of the folded dipole. You could also cut down an FM twin lead folded dipole. The ends of the T need to be shorted together.

If that's not enough, your next step is a VHF Bowtie antenna, which is a full-wavelength dipole with wiskers 27.5 inches on each side: >< (Caution: the ends of the whiskers are sharp! You can put wire nuts on the ends.) When you are looking for a "hot spot" also keep in mind that the presence of your body near the antenna will affect the reception. A reflector behind the antenna will reduce that problem.

Come back and let us know how you are doing; we can take you to the next level for an indoor VHF antenna if necessary.
post #3 of 60
Thread Starter 
But see, I can't get the VHF antenna to receive the signal at all. Even ignoring its reception of the UHFs with the rabbit ears, I can't get more than 18% signal on channel 11.

- Trip
post #4 of 60
The next step up is an antenna with more gain. Would you consider making a VHF 2 Bay bowtie antenna with reflector or buying a ready made VHF-Hi antenna?

Because of the "cliff-effect" of digital signals you either get it or you don't. There is no inbetween with snow like analog. At some point, with an increasing signal coming from the VHF antenna, you should get a lock on CH11. The location of the antenna is even more important than the gain of the antenna itself.
post #5 of 60
Thread Starter 
I'm going home for a week on Monday, maybe I'll make a bowtie while I'm home. What materials should I use for it?

- Trip
post #6 of 60
For UHF antennas I have been using number 8 aluminum ground wire (1/8" dia) for the elements and number 10 aluminum wire for the phasing lines for more than one bay. My local BigLots has inexpensive rabbit ears (what else would I think of?) for sale for $2.99. You could use the 32" telescopic rods for VHF bowtie elements. The length would be adjustable and the tip has protection on the end.
Falcon_77 likes to use galvanzed steel wire (for UHF & VHF) from Lowes (in section for suspended ceiling parts); mclapp likes to use copper wire. Please visit the How to build a UHF antenna... thread for ideas about suitable materials; mclapp would have good ideas for your situation. You can send him a PM to get his attention to this thread and your situation.

There is also a How to build an indoor VHF antenna thread:
http://www.avsforum.com/avs-vb/showthread.php?t=1024739
post #7 of 60
I suggest that you connect only the VHF antenna when trying different CH11 antennas and looking for the best location for the antenna, and then add your UHF antenna with a UVSJ. The VHF antenna might need to be in another location since WVPT is 43.9 miles away at an azimuth of 282 degrees VS 4.2 miles at 149 degrees for your other stations. It is also much weaker than your other stations with a NM of only 12. This is based on tvfool using zip code 22904 because I don't know the address of your apartment:
Attachment 124952
Attachment 124953
LL
LL
post #8 of 60
Thread Starter 
TV Fool is wrong because what I'm trying to receive is not in the FCC database (it's licensed "experimental") and TV Fool is limited the same way my own site (RabbitEars) is, by the accuracy of the FCC database. I hacked around it on my own site, but I figure it's too much trouble to bother Andy with trying to get it figured out. WVPT-DT has a 0.4 kW on-channel repeater on Carter's Mountain in Charlottesville, same location as W50CM and all of the other Charlottesville TV stations.

All I'm interested in is a VHF antenna that will bring in the station. I'll send Falcon_77 an e-mail (I talk to him almost every other day anyway) and see what he thinks.

Thanks!

- Trip
post #9 of 60
Oops! OK, I see it now in your data base under Technical Data and Screencaps: 1066' 0.4 kW DA Carter's Mountain.

Falcon_77 is the right person to ask; Mike is usually up to date on that type of information.

If you find that your preamp helps with W50CM before adding the UHF antenna with a UVSJ, it should be set to only amplify VHF because of possible overload from the strong UHF signals. The setup would be:

CH11 antenna to preamp set for VHF only.
Preamp out, through longest cable for this run, to power injector/power supply.
Output of power injector to VHF input of UVSJ.
Output of UHF antenna to UHF input of UVSJ.
Output of UVSJ to set.

There is still the possibility of overload from the very strong UHF signals entering the VHF antenna. This might require a preamp that can better handle an overload. A cheaper solution would be to try a second UVSJ used as a filter to keep the UHF signals from entering the VHF preamp:

CH11 antenna to VHF input of UVSJ.
75ohm termination cap on UHF input of UVSJ.
Output of UVSJ to VHF input of preamp.
The rest is the same as above.

There is another possibility of VHF overload from a nearby strong FM station which might be helped by the FM trap.

Is there an attic that you can use?
post #10 of 60
Here is a link to the 2-bay upper VHF bow-tie that I made. I ended up shortening the elements to about 30" to a side, but it is still a wide antenna and a bit awkward to handle. However, it worked quite well in the attic.

http://www.avsforum.com/avs-vb/showt...9#post13789319

It's easy to make and if you are unable to buy something like a Y5-7-13 and suspend it from your dorm room ceiling, it might be worth a try.

http://www.solidsignal.com/prod_disp...p?PROD=Y5-7-13

Are you on the ground floor?

I would try the CM7777 in addition to a better antenna, but would bypass it for UHF (combine the VHF and UHF sides after the PIM with a UVSJ as suggested by rabbit73).

As there are many FM stations nearby, per FM Fool, I would also suggest using a HLSJ to filter out below the upper VHF band in addition to the FM trap on the 7777.

Even with all this, it might be a long-shot, but 18% indicates that there may be some hope. Are you sure you aren't receiving WVPT itself? It appears to be within reasonable range (+2dB NM on TV Fool for a general area plot for UVA).
post #11 of 60
Thread Starter 
Quote:
Originally Posted by Falcon_77 View Post

Here is a link to the 2-bay upper VHF bow-tie that I made. I ended up shortening the elements to about 30" to a side, but it is still a wide antenna and a bit awkward to handle. However, it worked quite well in the attic.

http://www.avsforum.com/avs-vb/showt...9#post13789319

Interesting, I'll be considering it.

Quote:


It's easy to make and if you are unable to buy something like a Y5-7-13 and suspend it from your dorm room ceiling, it might be worth a try.

http://www.solidsignal.com/prod_disp...p?PROD=Y5-7-13

I thought about it, I'm just not sure how much I want to spend, and I'm not quite sure what my roommate would say about an antenna duct taped to the ceiling. I also don't know how much weight duct tape would bear.

Quote:


Are you on the ground floor?

Yes.

Quote:


I would try the CM7777 in addition to a better antenna, but would bypass it for UHF (combine the VHF and UHF sides after the PIM with a UVSJ as suggested by rabbit73).

I'll be quite honest, the UHFs are so strong here that I doubt I need a separate UHF antenna.

I'm worried that putting the 7777 in the line with 5000 kW worth of WVIR analog and 1000 kW worth of WVIR digital will make my tuner or my preamp (or both) deaf. WVIR is strong enough that I can see the analog with no antenna plugged in.

Quote:


As there are many FM stations nearby, per FM Fool, I would also suggest using a HLSJ to filter out below the upper VHF band in addition to the FM trap on the 7777.

My FM reception makes me think FMs aren't much of a concern. I have a really hard time decoding WVTW 88.5's digital radio signal here, and that's a B1-class signal on Carter's Mountain.

Outside, yeah, the FMs are pretty vicious. But 95.1 is the only one that's not on Carter's and I have a hill blocking me from it that really cuts it down, and the rest on Carter's other than 8 kW WWWV are all power limited due to their height and don't overpower me in my room. Outside, definitely, but not in here.

Quote:


Even with all this, it might be a long-shot, but 18% indicates that there may be some hope. Are you sure you aren't receiving WVPT itself? It appears to be within reasonable range (+2dB NM on TV Fool for a general area plot for UVA).

I'm 100% sure. There's a line of mountains in the way and even sitting on top of said mountains with direct line of sight to WVPT-DT with my rabbit ears I just barely managed to lock it after fidgeting with the antenna for 5 minutes. There are a few people with outdoor antennas who can see the channel 11 signal from Carter's that could not see the signal before the Carter's booster went on the air.

18% is the minimum signal level my DViCO will register. The WinTV-D has shown me exactly one blip at 5 dB, but the carrier is almost always present.

- Trip
post #12 of 60
I built a hybrid single bowtie for my sons dorm room. Where he goes to school they have both VHF-HI and UHF stations.
It's an upscaled closed end uhf bowtie type antenna only with the closed end folded back much more than usual and a straight wire connecting the center of the foldback to the feed point, it looks like some sort of a modern art butterfly.

I don't have the dimensions or a picture of it, I made it real quick one day to take to him and didn't write down the dimensions. He will be coming home for thanksgiving and I will have him bring it so I can take a picture and write the dimensions down. I do have the computer model of it somewhere, if I find it I will post the dimensions. It's no super antenna barely better than rabbit ears gain wise but the reception pattern is a little different and it works for both VHF-HI and UHF.

I've also had good luck with UHF 4 bays on VHF-hi, they will have positive gain on a few VHF-HI channels if sized correctly. The gain will be within 1 or 2 db of a 2 bay VHF-HI antenna for the channel it's cut for plus you get UHF too.
post #13 of 60
Quote:
Originally Posted by Trip in VA View Post

I'll be quite honest, the UHFs are so strong here that I doubt I need a separate UHF antenna.

I'm worried that putting the 7777 in the line with 5000 kW worth of WVIR analog and 1000 kW worth of WVIR digital will make my tuner or my preamp (or both) deaf. WVIR is strong enough that I can see the analog with no antenna plugged in.

I was thinking of capping the UHF input of the 7777 with a 75 Ohm terminator and just using the VHF side (with a HLSJ before it). A UVSJ would provide for additional attenuation if used before the 7777 as well.

I have had some luck with the CM distribution amps in strong signal areas, so perhaps that would be another option. With short cable runs, only about 7-8dB of gain is useful, if the amp has a lower noise figure than the tuner.

It probably doesn't do much good to imagine how easy reception would be if they were on UHF. Ground floor antennas are at a significant disadvantage for VHF.
post #14 of 60
Thread Starter 
Quote:
Originally Posted by mclapp View Post

I built a hybrid single bowtie for my sons dorm room. Where he goes to school they have both VHF-HI and UHF stations.
It's an upscaled closed end uhf bowtie type antenna only with the closed end folded back much more than usual and a straight wire connecting the center of the foldback to the feed point, it looks like some sort of a modern art butterfly.

I don't have the dimensions or a picture of it, I made it real quick one day to take to him and didn't write down the dimensions. He will be coming home for thanksgiving and I will have him bring it so I can take a picture and write the dimensions down. I do have the computer model of it somewhere, if I find it I will post the dimensions. It's no super antenna barely better than rabbit ears gain wise but the reception pattern is a little different and it works for both VHF-HI and UHF.

I've also had good luck with UHF 4 bays on VHF-hi, they will have positive gain on a few VHF-HI channels if sized correctly. The gain will be within 1 or 2 db of a 2 bay VHF-HI antenna for the channel it's cut for plus you get UHF too.

Thanks! I'd very much like to see the data on it. I'm thinking now that I won't try to do anything until Christmas break, when I might make myself an antenna. I got a suggestion on the Harrisonburg local thread to try a Radio Shack antenna so I'll give that a shot just to see if it helps (it may not, I find reception in this room is lousy at best) and then probably return it.

Quote:
Originally Posted by Falcon_77 View Post

I was thinking of capping the UHF input of the 7777 with a 75 Ohm terminator and just using the VHF side (with a HLSJ before it). A UVSJ would provide for additional attenuation if used before the 7777 as well.

I have had some luck with the CM distribution amps in strong signal areas, so perhaps that would be another option. With short cable runs, only about 7-8dB of gain is useful, if the amp has a lower noise figure than the tuner.

It probably doesn't do much good to imagine how easy reception would be if they were on UHF. Ground floor antennas are at a significant disadvantage for VHF.

I'll see what I can do.

Today I passed my ham test, thus making me a Tech now. I borrowed the UVA club's VP's HT for the week and find that this room is terrible. It can't hear much of anything in here. Where signals come to die?

- Trip
post #15 of 60
Congratulations on passing your ham test. I made my first crystal set when I was 8 and have been a ham since the early 1950s; it has given me a lot of pleasure. I'm a W4, ex-W2, ex-DL4.
Quote:
Originally Posted by Trip View Post

I'll be quite honest, the UHFs are so strong here that I doubt I need a separate UHF antenna.

I agree. You should first try just the VHF antenna optimized for CH11 in the best "hot spot" that you can find. If you could borrow a TV signal level meter (SLM) it would help you find that spot. Sencore says you need to keep the digital signal level between -15dBmV and +15dBmV:
http://www.sencore.com/uploads/files...veGoodHDTV.pdf
I have used my old NTSC SLM to measure a marginal signal that I have (13.1 on RF41) that is too weak for a lock but I can still get a reading of about -27dBmV. It has an audio feature: an NTSC video carrier is a buzzing sound; the digital signals sound like white noise:
http://www.avsforum.com/avs-vb/showthread.php?t=1084674
I tried to use my CM7777 preamp to bring it up to lock, but it didn't help:
http://www.avsforum.com/avs-vb/showp...6&postcount=90

Quote:


I also don't know how much weight duct tape would bear.

Many people have used a floor-to-ceiling spring loaded pole (like for a pole lamp) to safely support an antenna---no tape, no screws, no nails, no holes.

Quote:


I'm not quite sure what my roommate would say about an antenna duct taped to the ceiling.

Your roommate might be more tolerant of your antenna experiments if your roommate derived some benefit from them.

My wife tolerates aluminum tubing around the apartment that I use for HF antenna experiments because she would rather have me play with antennas than play with other women. She has encouraged me to do UHF bowtie antenna experiments to be able to receive digital TV when the cable is out and to receive news bulletins during a power failure or weather emergency.

I know that you said that you couldn't have an outdoor antenna, but would it be possible to make a temporary set-up of an outdoor antenna for CH11 to see if you could get a lock on it? If you could measure the CH11 signal level outside, you would know if it is even possible to do what you want. If you can't borrow a signal level meter, and you have enough signal for a lock, you could at least use an attenuator (as in the Kelvin link below) to see how much margin you had before dropout.

Quote:


My FM reception makes me think FMs aren't much of a concern. I have a really hard time decoding WVTW 88.5's digital radio signal here, and that's a B1-class signal on Carter's Mountain.
Outside, yeah, the FMs are pretty vicious. But 95.1 is the only one that's not on Carter's and I have a hill blocking me from it that really cuts it down, and the rest on Carter's other than 8 kW WWWV are all power limited due to their height and don't overpower me in my room. Outside, definitely, but not in here.
I borrowed the UVA club's VP's HT for the week and find that this room is terrible. It can't hear much of anything in here. Where signals come to die?

Your remarks remind me of my situation. When I retired I bought an ICOM 735 and set-up an indoor folded dipole for 15 meters. I was trying to have a CW QSO with a JA, but it was't going very well. I thought that something was wrong with my receiver; it wasn't very sensitive and I was getting low S-meter readings. I then set-up a second identical antenna outside with an A-B switch. Wow! What a difference! After a lot of investigation I discovered that the problem was the aluminum foil vapor barrier on the insulation in the outer walls. I was living in a shielded enclosure; can you say Faraday cage?

The aluminum foil also makes it difficult to use a cell phone inside. It also affects TV reception. We have been able to use a battery operated 5" B&W analog TV inside to receive news bulletins during a power failure, but it's no go for digital. I was able to get permission from the landlord (If it looks "nice" and is not obvious from the steet) and built an enclosure for an outside antenna:
Attachment 125095
Attachment 125096
Some of the signal is lost (about 40%) on its way through the holes in the decorative block wall, but there is still enough left to get the latest local emergency news bulletins on channels 3.1 and 10.1, which is important to me and my wife.

The optimum location for the antenna was determined by measurements with a signal level meter, because sometimes the best location is counterintuitive. In this case, higher was not better.
When I made measurements with the antenna in front of and then behind the block wall I found that the wall causes a loss of about 2 dB on UHF. The loss for VHF channel 13 is about 5 dB and for channel 10 about 7 dB. It seems that the holes in the blocks are acting like a high pass filter! The readings for 10 and 13 were made with the above 4-bay Channel Master 3021/4221 UHF antenna. I plan to repeat it with a channel 13 antenna, because digital channel 13.1, which is now on UHF channel 41, will move to VHF 13 on February 17, 2009.

Do you have a window that faces the transmitter? I don't, and wasn't able to get a lock on any channel with an antenna in the window because it's 90 degrees off from the ideal direction. Some people can't even use a window if it has low-e glass, which will block RF. We have had several reports of the problem on this forum. One student couldn't figure out why he couldn't get a signal inside his dorm window, but he could outside:
http://www.avsforum.com/avs-vb/showthread.php?t=1064806

I know that this post is pretty long but old hams help new hams, that's the tradition.
LL
LL
post #16 of 60
I keep thinking about your situation. It presents an interesting challenge and a chance to learn. My tech goal for 2008 is to learn more about DTV; the AVS Forum has been a big help with that.

We have considered the antenna and its location, but your CH11 success might also depend upon the capabilities of your reception equipment, which involves several factors; the first is:

Weak Signal Capability
This is not just tuner sensitivity, but also the capability to handle multipath problems at the same time. Both will cause an increase in bit error.
I haven't had any experience with your FusionHDTV5 USB (nano?), so I have to depend upon other people for a ranking:
Quote:
Originally Posted by Larry Kenney View Post

Finally, in regards to the sensitivity of the Fusion 7 tuner, I'd place it at 8 on a scale of 1 to 10. Comparing the receivers I have, here's how I'd rate them:
Insignia D to A converter 10
Sony XBR4 9
Fusion 7 USB 8
Dish ViP622 DVR 8
Sony XBR 2 CRT 5
MyHD MDP-130 4
Hi-Pix DTV-200 3
Hauppauge Win-TV 950 3

Maybe you could borrow a FusionHDTV7 USB to see how it compares with your 5.

For MY marginal signal, my tentative ranking (they are very close), best first, is:

APEX DT502 (from BestBuy; extra label on top of box said "gen5")
Zinwell ZAT-970A
Audiovox 8" TV PLV16081 (used by itself; I also used it as a monitor for the CECBs)
Zenith DTT900

I bought the 970A when I was learning about BER because it has 2 signal bars. The quality bar is related to BER. I bought the 502 because it also has 2 signal bars and systems2000 gave it a high ranking.

When I was testing the DT502 with my CM4221 antenna I got (for 13.1 on RF41):
Signal Quality 60%
Signal Strength 55%

I had aimed the antenna with my SLM, but when I rotated the 4221 slightly to the right I got:
Signal Quality 100%
Signal Strength 56%
Note the BIG change in signal quality with only a slight change in signal strength.

The 970A, after antenna rotation was:
Signal Quality 95%
Signal Strength 40%

The Zenith appears to combine signal quality and signal strength in one bar. It increased slightly when I rotated the antenna, but it's not as sensitive as the 2-bar box quality bar.

It seems that the signal quality indication is a more sensitive aiming tool than signal strength, because it shows the increase in BER from multipath reflections. In my situation the BER is affected by the weak signal, the fixed multipath reflections, and the changing multipath reflections. My antenna is aimed across a well-traveled road, so I get reflections from cars. (This is an example of the need for the new ATSC M/H standard.) When the quality went up to 100%, the car reflections were less of a problem. My stronger signals maintain a good lock inspite of the cars.

Quote:


Last night, I spent two hours fighting with my Silver Sensor to the point I wanted to throw it out of the window.

It gets WAHU-LD and WHTJ-DT just fine, but WVIR-DT is so powerful that it just reflects off of everything and insists on breaking up no matter how I position it. I was trying to watch NBC29's HD news last night, which I don't usually do due to having too much other stuff to do when I had this happen. I started fidgeting with it during Dr Phil (for the whole hour, figured I'd start fighting with it early) then flipped away to Judge Judy, and then back to NBC29's news where I continued to fight with it.
So now I'm thinking I need a larger indoor antenna. The Silver Sensor is supposed to be pretty good with multipath, but the position that gets me WHTJ and WAHU best is 80 degrees or so off the direction of Carter's Mountain. It must be too much for it here.

I think a CECB with 2 signal bars would be a big help when aiming your Silver Sensor.
I will have more to say about multipath in another post.
post #17 of 60
Thread Starter 
The Fusion5 is better than the Fusion7. I say this because I have a Pinnacle receiver that I believe has the exact same chip as the Fusion7 and it's an inferior receiver to the Fusion on weak signals, though I don't know how they compare on multipath/strong signals. At home I live in the middle of nowhere, where weak signals are the only problem--multipath doesn't enter into it--so that's what I'm most familiar with.

I'm trying to get another DTT900, as they're what my parents have at home, but getting another coupon is turning out to be a pain. I don't really want to spend $60 on one when I should be able to get away with spending $20. We'll see what happens.

- Trip
post #18 of 60
post #19 of 60
Thread Starter 
I brought my 7777 up here with me and gave it a shot. Fed the rabbit ears through the combined input and into the tuner.

My WinTV-D actually gets better reception! I didn't expect that; it locked WAHU-LD 40 solidly for the first time. The WinTV-D doesn't handle multipath very well at all, so I'm not quite sure what this means.

Anyway, even with the amp, the signals remain the same. 32/40/46 are all solid with maxed out signal, 11 gives the occasional blip. Amp didn't change anything on the good tuners (the Pinnacle and the Fusion5). I'm tempted to drag my gear outside and try it all out, but it's raining right now, so I can't. Maybe another day.

- Trip
post #20 of 60
Thanks for the progress update, Trip.

I have a few more ideas for inexpensive solutions. I'll post them before they evaporate when I have thought them through. You can come back when you have time.

If I don't hear from you, have a good holiday break.

73,
rabbit
post #21 of 60
Quote:
Originally Posted by rabbit73 View Post

... After a lot of investigation I discovered that the problem was the aluminum foil vapor barrier on the insulation in the outer walls. I was living in shielded enclosure; can you say Faraday cage?

Trip, if the exterior walls have a foil-backed radiant barrier insulation, this is probably your worst reception enemy. Another problem may be low-e glass in the windows. Low-e glass has a thin metallic layer to reflect heat. It's also a great RF attenuator. If the building is newer construction or has energy-efficiency retrofits, the odds are high it has one or both of these products.
post #22 of 60
Thread Starter 
This building has been here since the 70's. Some of the buildings here in this group of apartments have been renovated but not this one. Either way, the walls are all cinderblock, I don't think that changes in renovation, though I haven't been in one of the renovated apartments to know.

- Trip
post #23 of 60
Well, it's probably just typical indoor low power VHF reception problems.
post #24 of 60
Thread Starter 
At 0.4 kW, I wouldn't be surprised.

- Trip
post #25 of 60
When I lived in Little Rock, I made a cut-to-channel low vhf dipole antenna that worked well in the attic, when combined with a CM 7777 preamp. At the time, the ch 5 transmitter was ~25 miles away and about 1kW ERP.

The antenna was two pieces of heavy gauge wire of the appropriate lengths, and a balun, screwed to a small block of 2x4 wood.

http://i36.tinypic.com/2r6efmg.png
post #26 of 60
Starting again at square one, the most important factor is:
Dynamic Range Capability

Quote:


WVIR-DT 32 1000 kW
WHTJ-DT 46 165 kW
WAHU-LD 40 9.5 kW
W50CM-D 11 0.4 kW

Using the power levels given above, the difference between WVIR-DT and WAHU-LD is about 20dB. If you add to that the minimum SNR as per Doug Lung of about 15 dB and place WAHU-LD 15dB above the noise floor, the tuner is required to handle an input range of at least 35dB. Being able to decode WAHU-LD without overload from WVIR-DT is a lot to ask of the tuner.

The difference between WVIR-DT and W50CM-D is about 34dB. If you add the minimum SNR of 15dB, the required input range is at least 49dB, which is probably too much for any tuner.

holl_ands has a few helpful words from other threads:
Quote:
Originally Posted by holl_ands View Post

The very tough spec for the CECB boxes (NTIA Coupon) allow a STRONG undesired signal
to be no higher than 20 dB above the desired signal and 33 dB for weaker undesired signals.
Your situation is outside the CECB performance capability....and most DTVs are WORSE...

Quote:
Originally Posted by holl_ands View Post

Preventing Preamp "Overload" really means maximizing the Spurious Free Dynamic Range (SFDR),
so you can receive the weakest signals possible without intermods wiping it out.

Back to Trip:
Quote:


I'm worried that putting the 7777 in the line with 5000 kW worth of WVIR analog and 1000 kW worth of WVIR digital will make my tuner or my preamp (or both) deaf. WVIR is strong enough that I can see the analog with no antenna plugged in.

I have set up my rabbit ears at 29.4 inches per ear, to make the channel 11 wavelength of 58¾ inches wide, and with this I can receive the three UHF signals, but only brief blips of signal on channel 11 that don't break 18% on my DViCO (which needs 60% for a successful decode).

It gets WAHU-LD and WHTJ-DT just fine, but WVIR-DT is so powerful that it just reflects off of everything and insists on breaking up no matter how I position it.

So now I'm thinking I need a larger indoor antenna. The Silver Sensor is supposed to be pretty good with multipath, but the position that gets me WHTJ and WAHU best is 80 degrees or so off the direction of Carter's Mountain. It must be too much for it here.

Anyway, even with the amp, the signals remain the same. 32/40/46 are all solid with maxed out signal, 11 gives the occasional blip. Amp didn't change anything on the good tuners (the Pinnacle and the Fusion 5).

Your remarks support my analysis using power levels. If you have to point your UHF antenna 80 degrees off the Mountain, you must be overloading the tuner with WVIR. Yes, you can reduce the WVIR signal level by aiming away from the transmitting tower but this is usually not a good idea because it increases the multipath reflections.

Your task is to reduce the dynamic range of the signals presented to the tuner. You can do this by using a VHF antenna designed to collect as much signal as possible for CH11 to bring up the lower end of the range. Next you have to reduce the signals from WVIR to prevent tuner overload; it can't decode anything when overloaded.

WVIR analog on CH29 is most likely also causing tuner overload with its -7.3dBm signal, but this will no longer be a problem in feb:
Attachment 125807


TRY THIS Trip:
The next setup that I suggest would use two UVSJs back-to-back, to pass the CH11 signal with as little attenuation as possible, and pass the UHF signals with only enough attenuation to keep WVIR from overloading the tuner, so as not to harm the WAHU-LD signal any more than necessary. Using only a VHF antenna, as you want, might also give some UHF attenuation:

1. CH11 antenna to common port of first UVSJ
2. VHF port of first UVSJ to VHF port of second UVSJ (a VHF preamp and FM trap could be inserted in this VHF line to help bring up CH11)
3. UHF port of first UVSJ to attenuator and then to UHF port of second UVSJ
4. Common port of second UVSJ to tuner

I think that the VHF passband insertion loss of two UVSJs in series will probably not harm the CH11 signal. In 2001 I measured the passband and stop band losses of a CM 0549 and a RS 15-1237 UVSJ using my SLM and steady UHF & VHF cable signals fed into the commom port. The CH13 insertion loss for the CM was 0.4dB and for the RS 0dB (too little to measure); it was 0dB for CH3 for both. The CH15 UHF signal (cable channel 66) out of the VHF port was down about 38dB for one and 36dB for the other.

The trick will be to select the correct attenuation in the UHF line to bring WVIR below the overload point while keeping WAHU at least 15dB above the noise floor. It would be easy with a spectrum analyzer, but you'll probably have to use the empirical (trial-and-error) approach like I do. If you had a CECB with 2 bars you could use the signal quality bar to help; it should go down at either extreme because of the increasing BER.
Here is another APEX DT502 user: www.avsforum.com/avs-vb/showthread.php?p=15259991post15259991
Perhaps you can use the diagnostics screen SNR and data error rate of your WinTV-D.

If you find that the difference in level between WVIR and WAHU is still too much for the tuner to handle with a resistive attenuator, it would be necessary to use a frequency selective device like a CH32 bandstop filter in the UHF line to bring down WVIR without out affecting WAHU:

http://www.tinlee.com/MATV-Bandstops.php?active=3
http://www.tinlee.com/MATV-Bandstops...e=3#ASSISTANCE

A good bandstop filter is about $100.

I'm working on another inexpensive solution.
LL
post #27 of 60
Multipath & a narrow bandwidth UHF Antenna

If the previous solution doesn't work because WVIR digital on CH32 is still causing overload, I would like you to consider using a VHF & a UHF antenna, even though you prefer just one VHF antenna. The advantages would be:

1. Each antenna could be adjusted for the best aim to reduce multipath reflections, which might not be the same for VHF & UHF.
2. You are presently using a Silver Sensor LPDA which is a broadband antenna. A narrow bandwidth UHF antenna could be designed to favor CH40 & CH46 and give less gain for CH32, which would almost do what a bandstop filter would do at a lower cost.

The setup, using one UVSJ, would be:

1. CH11 antenna to VHF port of UVSJ
2. Narrow bandwidth UHF antenna to attenuator and then to UHF port of UVSJ
3. Common port of UVSJ to tuner

If you need some amplification for CH11 the setup, using 2 UVSJs, would be:

1. CH11 antenna to VHF port of first UVSJ
2. 75ohm termination cap on UHF port of first UVSJ
3. Common port of first UVSJ to VHF preamp and FM filter and then to VHF port of second UVSJ
4. Narrow bandwidth UHF antenna to attenuator and then to UHF port of second UVSJ
5. Common port of second UVSJ to tuner

The UHF antennas that come to mind are a yagi, log-yagi, and stacked bi-squares. The yagi, which is very narrow-band to start with, can be broadbanded by lowering the Q with larger diameter elements and tapering the reflector and director element lengths. The log-yagi is a hybrid that has a greater bandwidth than the yagi but not as great as the LPDA.

WA5VJB "cheap yagis" look like this (more sharp wires--I've scratched myself many times on the whiskers of 4-bay antennas changing baluns):

http://www.fredspinner.com/W0FMS/CheapYagi/vjbcy.html
http://www.qsl.net/ve3cvg/antennas/c...gis/index.html
http://www.wa5vjb.com/yagi-pdf/cheapyagi.pdf
http://www.wa5vjb.com/references/Che...ennas-LEOs.pdf
http://www.popular-communications.co...asWeb92708.pdf (wide-band for UHF DTV)
http://portal.ucpel.tche.br/py3vhq/h...nas/uhf-sw.pdf

A quad is a closed loop 1/4 wave on each side; a bi-square is 1/2 wave on each side and open on the top and fed at the bottom when configured as a diamond for horizontal polarization (p 151, fig 7.16):
http://books.google.com/books?id=HQC...um=9&ct=result

The stacked bi-square antenna is what the Hoverman was derived from using broadbanding techniques, which our north-of-the border friend Autofils (of Gray-Hoverman fame) has been optimizing. The latest are extended and narrowband versions:

http://www.digitalhome.ca/forum/showthread.php?t=96624
http://www.jedsoft.org/fun/antennas/dtv/xgh.html
http://www.jedsoft.org/fun/antennas/dtv/gh.html (narrow-band 3/4 down the page)

This is the Hoverman that Falcon_77 made:
http://www.avsforum.com/avs-vb/attac...9&d=1210737926
http://www.avsforum.com/avs-vb/showp...&postcount=137

You wouldn't need many bi-squares, perhaps only 2, and you probably wouldn't need the broadbanding "ears" at the top and bottom unless CH46 needed some help, just the bi-squares cut for CH40 and some collinear reflector pairs.

If I have the time, I want to test my theory with the UHF channels that I have from Norfolk: 16, 29, 31, 38, 40, & 46; probably using 31, 40, & 46 to simulate your situation. It's not clear to me what would happen to your CH32 signal if it were on the steep-slope skirt of the gain curve of a narrow-band antenna optimized for CH40; the gain across the CH32 6MHz channel would not be level.

For your VHF antenna, a bowtie is one wavelength wide, but a yagi is only 1/2 wavelength wide. I'm going to build a CH13 yagi because 13.1 on RF41 is moving to 13, and my antenna enclosure is only 36 inches wide.
post #28 of 60
You may be doomed. Are there any strong FM transmitters on Carter's Mountain that are in the 99 to 102 MHz range? If so, they may develop a harmonic that overwhelms the anemic digital signal.

As I recall, an 8VSB waveform can withstand an on-channel analog peak signal that is 2dB stronger than the digital signal, so there is some hope. Can you borrow a spectrum analyzer from the Engineering Department to see if there is a harmonic spike on your downlead?

In another thread currently running, I mentioned horizontal stacking as a way to cancel undesired FM signals, but that won't work when the undesired FM signal and the desired highband signal come from the same place.

Some time ago, holl_ands showed plans for developing some kind of vertical stack where you place one antenna 1/4 wavelength if the undesired signal behind the other and then do some fudging with cable lengths. I think he was trying to help someone in Baltimore get a weak Philadelphia station in the presence of a stronger channel 45 or 46 signal from nearby. That kind of a stack may represent the most effective remedy for your situation.
post #29 of 60
Thread Starter 
Actually, there is nothing on Carter's in that range. There's nothing at all local in that range except for:

99.7 WCYK is a B-class station but is way up near Afton Mountain, signal's prone to static in my apartment.
101.9 WHTE is an A-class station from Ruckersville, which has a translator in town because their signal can be poor.

The most powerful FM station on Carter's is WWWV-FM, a B-class station on 97.5 that does 8 kW.

- Trip
post #30 of 60
Thread Starter 
Rabbit73:

Thanks for these ideas. I will definitely have to check everything out, however with finals approaching (first one is less than four days away) and then me going home, I won't have a chance to try anything until mid-January most likely.

- Trip
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