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Discussion Starter · #1 ·
I live just 3/4 of a mile from Sutro Tower in San Francisco where 11 digital TV stations and 5 FM stations transmit from. Because of the high level of RF here, preamps get completely overloaded. I haven't found any that work.


I have an Antenna Craft Y-10-7-13 and a Channel Master 4228 with a rotor on my roof, about 35 feet above ground. I get the stations from Sutro with the antennas pointing in just about any direction, and I receive the stations transmitting from Mt. San Bruno, 5 miles to the south, Mt. Diablo 30 miles to the east, the hills above Fremont, CA about 35 miles to the southeast, and the two to the north from Novato and Cotati, without any problem by pointing the antennas in the right direction.


The Walnut Grove transmitter site for the Sacramento/Stockton stations is 62 miles to the northeast over a 1000 foot ridge that's about 15 miles away. I get one of the stations from there, KMAX transmitting on channel 21, about 99% of the time with about a 20 dB SNR. However, the other stations, both VHF and UHF, average about 12.5 to 14.5 dB SNR most of the time. Occasionally conditions will improve enough for a few of them to produce a picture. KVIE on 9, KNTV on 10, KOVR on 25 and KQCA on 46 can sometimes be solid for several hours at a time; other times none of them produce a picture.


My question is: Are there any better antennas, any amp or preamp or any other method any of you know of that will work in this high RF environment here that will increase those Walnut Grove stations 3 or 4 dB? That's all I need... just 3 or 4 dB... and I'll have reception most of the time from these stations. My RG6 coax is only 55 feet long and there are no splitters, switches or anything in line between the coupler at the antenna and the input to the TV.


Any suggestions will be greatly appreciated.


Larry

SF
 

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Quote:
Originally Posted by Larry Kenney /forum/post/16989244


I live just 3/4 of a mile from Sutro Tower in San Francisco where 11 digital TV stations and 5 FM stations transmit from. Because of the high level of RF here, preamps get completely overloaded. I haven't found any that work.


I have an Antenna Craft Y-10-7-13 and a Channel Master 4228 with a rotor on my roof, about 35 feet above ground. I get the stations from Sutro with the antennas pointing in just about any direction, and I receive the stations transmitting from Mt. San Bruno, 5 miles to the south, Mt. Diablo 30 miles to the east, the hills above Fremont, CA about 35 miles to the southeast, and the two to the north from Novato and Cotati, without any problem by pointing the antennas in the right direction.


The Walnut Grove transmitter site for the Sacramento/Stockton stations is 62 miles to the northeast over a 1000 foot ridge that's about 15 miles away. I get one of the stations from there, KMAX transmitting on channel 21, about 99% of the time with about a 20 dB SNR. However, the other stations, both VHF and UHF, average about 12.5 to 14.5 dB SNR most of the time. Occasionally conditions will improve enough for a few of them to produce a picture. KVIE on 9, KNTV on 10, KOVR on 25 and KQCA on 46 can sometimes be solid for several hours at a time; other times none of them produce a picture.


My question is: Are there any better antennas, any amp or preamp or any other method any of you know of that will work in this high RF environment here that will increase those Walnut Grove stations 3 or 4 dB? That's all I need... just 3 or 4 dB... and I'll have reception most of the time from these stations. My RG6 coax is only 55 feet long and there are no splitters, switches or anything in line between the coupler at the antenna and the input to the TV.


Any suggestions will be greatly appreciated.


Larry

SF

Larry, given what looks to make Walnut Grove off of the side of the antenna for you near the Sutro tower, I am wondering if a 91XG with either no pre-amp or Winegard HDP-269 (that may overload too) or a good low noise distribution amp would be the ticket.

The VHF stations are prolly worse off from the huge RF footprint of the Sutro tower site then the UHFs, at least from my experience around tower sites. I would recommend first trying a good FM trap and get rid of the FM overload from the nearby transmitter site. The FM harmonics are usually right in the middle of VHF-HI and engineers here on AVS have had good results in improving VHF-HI results with FM traps. (Winegard makes a few 20+dB models)
 

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Quote:
Originally Posted by Larry Kenney /forum/post/16989244


I live just 3/4 of a mile from Sutro Tower in San Francisco where 11 digital TV stations and 5 FM stations transmit from. Because of the high level of RF here, preamps get completely overloaded. I haven't found any that work.

I don't think that you will find a preamp that will work. Your best hope is to configure a pair of antennas positioned to add Walnut Grove and null Sutro. You'd gain 2 db from Sacramento and loose 10 db from SF.


The exact azimuths to both locations may suggest a convenient design. tvfool will provide that info. Is your data available?


The other choice is a sharp tuned single channel filter before a preamp. You'd need one filter and one preamp for each Sacramento station.
 

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How much trouble are you willing to go to with this? To avoid cross modulation a preamp must be able handle the sum of all the signals being passed through it at once. A pre-amp with lower gain will obviously reduce the problem but probably not enough to help you. Limiting how many signals must be passed at the same time would help. This could be accomplished via separate pre-amps for UHF and VHF with filters to be sure no UHF is fed to the VHF pre-amp and vise versa. As previously mentioned filtering out FM signals and orienting your antenna to null Sutro would also help.
 

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Quote:
Originally Posted by Tower Guy /forum/post/16990443


The exact azimuths to both locations may suggest a convenient design. tvfool will provide that info. Is your data available?

I guessed 21st street.

http://www.tvfool.com/?option=com_wr...23c557ad40b645


The azimuth difference between Sutro and Walnut Grove is 211 degrees. If this is correct you are a candidate for stagger stacking.

www.anarc.org/wtfda/stagger.pdf


The stacking offset and cable lengths can be optimized for 211 degrees as well as the upward tilt to the top of the Sutro structure.
 

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Discussion Starter · #6 ·
Thanks for all of your input! I did have an FM trap on the VHF side, but it didn't make any difference on the signal strength of the Walnut Grove signals, so I took it out.


That's not good news about the preamps, but it's pretty much what I've found. None will work here.


Tower Guy, you hit our address right on target. (How'd you guess so closely?) Sutro is almost directly west of us (267 degrees) and Walnut Grove is at 55 - 57 degrees. I'll read Bill Thompson's article. A quick look at it seems to say that I need to get more antennas stacked together to increase the gain. I'll study up on the stacking offset and cable lengths you mentioned. I'll have to consider whether the expense and work involved is worth what I'll be getting as a result.


Thanks again, Guys!


If anyone else has anything to offer I'm open to any suggestions you have.


Larry

SF
 

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How bad do you want the 55-57 degree channels, which ones, and how much can you afford to pay?


You can most economically add ABC-10 and PBS-9 by passing them through and affordable bandpass filter. I just had a batch of ten 7-9 filters and a batch of ten 11-13 filters made up for, I think, $37.50 each. They are tuned, paired cylindrical filters like the cable companies use for tier traps, and they have an out-of band rejection depth of about 50dB, and an in-band insertion loss of maybe 3 dB. If you had to, you could add in an off-the-shelf channel 7 notch filter for $12.50, but I doubt you'd need it.


CBS-25 might be do-able. Your TV fool chart shows its field strength to be -87 dBm versus a channel 27 coming from the other direction which is -44 dBm. You can buy a cable TV channel 76 cylindrical bandpass filter for I think about $65. It probably will only roll channel 27 off by about ten dB, but your antenna's front to back ratio will surely knock it down by another 20 dB.


Unfortunately, NBC-35 and Fox-40 will be very, very difficult. Your chart shows them to be at -92 and -91 dBm respectively, whereas channel 33 is at -7dBm and 38 and 39 are at -5 dBm.


You can forget Fox 40. since there is nothing you can do to adequately suppress an adjacent signal that is 86 dB stronger, but if you wanted to take a shot at NBC-35, you could start with a 1/4 wave staggered stack tuned to reject 38/39 and have someone like Tin Lee make you a reasonably priced ($110, last I checked) channel 35 filter, but it, too, would only reduce 33 by less than ten dB. You could reduce 33 a little more with a $200+ Blonder Tongue MWT-U, and then see if you an get away with some preamplification before using cruder cable TV lowpass and highpass tier traps before preamplifying a second time. Basically, taking a good shot at NBC-35 can run you $500 to $1,000 and a lot of time, and you'd really benefit from using a spectrum analyzer if and when you try to do it.
 

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This is probably not practical at all, but I thought I'd add it to the discussion. For weak signals in severe RF environments, the common amateur radio practice is to use a cavity preamp. Basically what AntAltMike is suggesting with the cylindrical filters, but when the cavity is part of the preamp input circuit, the losses are much lower.


Here's some photos of what it looks like.

http://www.hb9bbd.ch/article.php3?key=34


Finding something tuned to UHF TV channels seems impossible. You would have to build it yourself. Something like the one pictured above requires some real machining skill in addition to the electronic design itself.


Ron
 

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Quote:
Originally Posted by dr1394 /forum/post/16992937


This is probably not practical at all, but I thought I'd add it to the discussion. Basically what AntAltMike is suggesting with the cylindrical filters.


Finding something tuned to UHF TV channels seems impossible. You would have to build it yourself. Something like the one pictured above requires some real machining skill in addition to the electronic design itself.


Ron

It is practical. I've done it myself. The bandwidth of TV suggests 2 or more poles in the filter. I've built a 3 pole for channel 35 UHF TV and a 2 pole for 90.3 FM. For the UHF filter I used 3/8" OD copper inside of a soldered together PC board box. For the FM filter I used a section of WR-229 rigid copper waveguide for the box with the 3/8" copper tubing for the tuned elements.


Here is the design software on-line. http://www.wa4dsy.net/cgi-bin/idbpf


AntAltMikes pre-built VHF filter should work too.
 

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Quote:
Originally Posted by Tower Guy /forum/post/16993743


It is practical. I've done it myself. The bandwidth of TV suggests 2 or more poles in the filter. I've built a 3 pole for channel 35 UHF TV and a 2 pole for 90.3 FM. For the UHF filter I used 3/8" OD copper inside of a soldered together PC board box. For the FM filter I used a section of WR-229 rigid copper waveguide for the box with the 3/8" copper tubing for the tuned elements.


Here is the design software on-line. http://www.wa4dsy.net/cgi-bin/idbpf


AntAltMikes pre-built VHF filter should work too.

The author of the original 1985 Ham Radio article that the WA4DSY program is based on is one of my old ham buddies, Jerry Hinshaw N6JH. I'll never forget the time we rented a Lincoln Continental and took it up to a mountain in Ventura County to do some VHF contesting. I'm sure that car was never the same after that weekend.


Here's an article for a tunable interdigital filter that I thought would make a great UHF TV preselector if it could be scaled down in frequency.

http://www.eecs.umich.edu/rebeiz/Cur..._Filter_00.pdf


Ron
 

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Quote:
Originally Posted by dr1394 /forum/post/16992937


This is probably not practical at all, but I thought I'd add it to the discussion. For weak signals in severe RF environments, the common amateur radio practice is to use a cavity preamp. Basically what AntAltMike is suggesting with the cylindrical filters, but when the cavity is part of the preamp input circuit, the losses are much lower.


Here's some photos of what it looks like.

http://www.hb9bbd.ch/article.php3?key=34


Finding something tuned to UHF TV channels seems impossible. You would have to build it yourself. Something like the one pictured above requires some real machining skill in addition to the electronic design itself.


Ron

Wow, talk about a project, with the level of quality and precision of NASA in the '60's. If I were looking to get a better mouse trap, this would be the place to have it done.
 

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Quote:
Originally Posted by Larry Kenney /forum/post/16992132


Thanks for all of your input! I did have an FM trap on the VHF side, but it didn't make any difference on the signal strength of the Walnut Grove signals, so I took it out.


That's not good news about the preamps, but it's pretty much what I've found. None will work here.


Tower Guy, you hit our address right on target. (How'd you guess so closely?) Sutro is almost directly west of us (267 degrees) and Walnut Grove is at 55 - 57 degrees. I'll read Bill Thompson's article. A quick look at it seems to say that I need to get more antennas stacked together to increase the gain. I'll study up on the stacking offset and cable lengths you mentioned. I'll have to consider whether the expense and work involved is worth what I'll be getting as a result.


Thanks again, Guys!


If anyone else has anything to offer I'm open to any suggestions you have.


Larry

SF

Some relatively inexpensive things to try:

1. Try making custom baluns http://www.kyes.com/antenna/balun.html

2. Replace RG6 with RG11 using 360 degree compression fittings

3. You did not mention what antenna coupler you are using. If it is not already, then find one with the lowest loss.

4.If possible try vertically tilting the antennas 1-2 degrees.
 

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Discussion Starter · #14 ·
AntAltMike = How bad do you want the 55-57 degree channels, which ones, and how much can you afford to pay?


Not a huge amount. It's mainly for DXing, not for viewing purposes.


AntAltMike = You can most economically add ABC-10 and PBS-9 by passing them through and affordable bandpass filter.


That might be worth trying.


AntAltMike = CBS-25 might be do-able. Your TV fool chart shows its field strength to be -87 dBm versus a channel 27 coming from the other direction which is -44 dBm.


With the 4228 pointed toward Walnut Grove, channel 27 drops out to "No Signal", so I don't think that channel is a problem. Adjacent channels don't seem to bother that much. I'm able to get channel 46 from Walnut Grove fairly often and we have 43, 44 and 45 on Sutro. Before the transition, I got 55 from Walnut Grove with stations on 56 and 57 on Sutro.


Channel 40 is definitely hopeless. There's a low power station on 40 just five miles away.


Larry

SF
 

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Discussion Starter · #15 ·
Looking back over all of the suggestions, KD0GTI mentioned the 91XG. As far as I can tell, it and the 4228 have about the same gain, but I don't know about the off-axis signal rejection of the two. The 4228 does pick up signals off the side at about 50 degrees on either side of the center.


Seeing that a pre-amp won't work, is there a possibility that a low noise distribution amp would work in my environment?


I don't want to spend a lot of money on one specific channel, so the cavity preamp is probably impractical and the tuned balun is out, but I do like two of TV Trey's ideas... using RG11 and tilting the antenna. How much difference would 55 feet of RG11 make over 55 feet of RG6 quad shield? Have any of you tried the tilting idea? What were your results?


Again, thanks for the suggestions.


Larry

SF
 

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Quote:
Originally Posted by Larry Kenney /forum/post/17003626


How much difference would 55 feet of RG11 make over 55 feet of RG6 quad shield? Have any of you tried the tilting idea? What were your results?


Again, thanks for the suggestions.


Larry

SF

for Ch 25 about 1 dB.
 

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Quote:
Originally Posted by Larry Kenney /forum/post/17003626


Looking back over all of the suggestions, KD0GTI mentioned the 91XG. As far as I can tell, it and the 4228 have about the same gain, but I don't know about the off-axis signal rejection of the two. ......


Again, thanks for the suggestions.


Larry

SF

Larry,


I can give a real-world example that illustrates your question.


Here's my TVfool: http://www.tvfool.com/?option=com_wr...23c57a39a09397


My major stations are actually worse than the plot would indicate as my house hugs the back side of a nasty hill, is surrounded by hills and the TVfool algorithm doesn't have enough precision to account for the sharpness of my terrain . On the other hand, that channel 14 signal from 10.3 miles away and 40 degrees off-axis is piped into the valley directly.


I have a 91XG with 4th boom section up in the air with a mast-mounted pre-amp and can just barely get that weak PBS signal although with severe multi-path.


That channel 14 signal is still the strongest signal on my antenna system by at least 20-30dB, even at 40 degrees off to the side. I don't get overload from the pre-amp, fortunately.


Just an illustration...


BTW, this replaced an original-design 4228 which wasn't able to get that PBS signal.
 

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Quote:
Originally Posted by Larry Kenney /forum/post/17003626


How much difference would 55 feet of RG11 make over 55 feet of RG6 quad shield? Have any of you tried the tilting idea?

I predict no difference between RG-6 and RG-11. It's a matter of the excessive strength of the SF stations, not the loss of the feedlline.


Tilting can work, but more to find a weak signal from a ridge line, not to reduce interference.
 

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You can get "luckier" at rejecting off-axis signals with the 8-bay bowtie, because those signals will hit each of the two, four bay stacks out of phase with each other, but to most effectively mitigate the strongest stations that way, you'd have to have a spectrum analyzer always available.
 

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Discussion Starter · #20 ·
This is an indication of how close I am to getting the stations from Walnut Grove. For the past two days I've been getting solid signals from KVIE 6 (RF9), KXTV 10, KOVR 13 (RF25), KMAX 31 (RF21) and KQCA 58 (RF46). I've seen a bit of pixelation, but overall the signals have been plenty good to watch entire programs. Conditions must be better than average for this to happen. Wish they were this way all the time.


Larry

SF
 
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