[Andrew K]

I'm already convinced I live in overload territory. I have a channel 49 with a noise margin of +83 dB and a channel 50 with a noise margin of +75 dB and several strong FMs nearby. I'm wondering if there is ANY way to overcome the losses I have without introducing overload with a preamp. That's my only question. The 85' of cable alone introduces around 4 to 5 dB of loss, and that's a lot of loss which roughly equates to a third of what the signal is at the antenna!!

http://www.tvfool.com/?option=com_wrapper&Itemid=29&q=id=f1f09e48805cb5

 

[Digital Rules]

How about this RCA preamp?

I thought the 3414 was working well for you? That thing is nearly imposssible to overload.

I've only had 1 device connected to a 3414 that overloaded, with a full power TV station & 2 FM stations less than 2 miles away. One TUNER was overloading, not the amp. The 3 other sets connected worked fine.

 

[Andrew K]

I thought the 3414 was working well for you? That thing is nearly imposssible to overload.

I've only had 1 device connected to a 3414 that overloaded, with a full power TV station & 2 FM stations less than 2 miles away. One TUNER was overloading, not the amp. The 3 other sets connected worked fine.

I figured the whole point of the CM-3414 was to distribute the signal to 4 TVs without losing any signal, not make up for any losses between the antenna and TV. And right now, I'm wondering if the estimated 85' cable I'm using up to the roof antenna is weakening my signal.

 

[rabbit73]

With such strong signals, why are you worried about the losses?
Are there some weak signals that you want? Which ones?
If that is so, you need to attenuate your 3 strongest signals to reduce the required SFDR.

Some people have had good luck with the Winegard UT-2700 which are less expensive than Tin Lee custom filters, but are now hard to find.
http://www.digitalhome.ca/forum/showpost.php?p=2462425&postcount=4065

 

[Andrew K]

With such strong signals, why are you worried about the losses?
Are there some weak signals that you want? Which ones?
If that is so, you need to attenuate your 3 strongest signals to reduce the required SFDR.
Some people have had good luck with the Winegard UT-2700.

I obviously don't care about amplifying the signals with ridiculously high noise margins. I can receive those without the antenna even attached. I want to optimize my setup so I can pull in those weak stations like WIVM-LD, which right now is just below the decode threshold. And I also DX too, so every bit of signal counts whenever the tropo is up.

 

[DrDon]

Title edited to make more specific. Location added.

 

[rabbit73]

WDLI-TV NM 83.1 dB
WIVM-LD NM 11 dB
83.1 - 11 = 72.1 dB
72.1 +16 dB for WIVM min required SNR = 88.1 dB SFDR needed
Not gonna happen; you need to attenuate your three strongest signals like chinadog did:
his final results
http://www.digitalhome.ca/forum/showpost.php?p=2462425&postcount=4065

background
http://www.digitalhome.ca/forum/showpost.php?p=2440009&postcount=4037

http://www.digitalhome.ca/forum/showpost.php?p=2440049&postcount=4038

http://www.digitalhome.ca/forum/showpost.php?p=2440378&postcount=4039

http://www.digitalhome.ca/forum/showpost.php?p=2440530&postcount=4040

http://www.digitalhome.ca/forum/showpost.php?p=2441330&postcount=4042

 

[Andrew K]

WDLI-TV NM 83.1 dB
WIVM-LD NM 11 dB
83.1 - 11 = 72.1 dB
72.1 +16 dB for WIVM min required SNR = 88.1 dB SFDR needed
Not gonna happen; you need to attenuate your three strongest signals like chinadog did:
http://www.digitalhome.ca/forum/showpost.php?p=2462425&postcount=4065

http://www.digitalhome.ca/forum/showpost.php?p=2440009&postcount=4037

http://www.digitalhome.ca/forum/showpost.php?p=2440049&postcount=4038

http://www.digitalhome.ca/forum/showpost.php?p=2440378&postcount=4039

http://www.digitalhome.ca/forum/showpost.php?p=2440530&postcount=4040

http://www.digitalhome.ca/forum/showpost.php?p=2441330&postcount=4042

Wow, thanks for the information. That may take me a while to sort through. I'm not an expert on this kinda stuff. But from what you're saying, it sounds like I'd need at least two of these tunable attenuators, correct? And I'm also wondering how I'd tune these without having any fancy equipment to test if they're tuned on the right channels. It looks like I'd need to attenuate channels 23, 49, and 50.

 

[Andrew K]

With such strong signals, why are you worried about the losses?
Are there some weak signals that you want? Which ones?
If that is so, you need to attenuate your 3 strongest signals to reduce the required SFDR.

Some people have had good luck with the Winegard UT-2700 which are less expensive than Tin Lee custom filters, but are now hard to find.
http://www.digitalhome.ca/forum/showpost.php?p=2462425&postcount=4065

I was also wondering if inserting just a general 10 dB attenuator somewhere in my setup would help in combination with using a pre-amp. Perhaps inserting it between the mast-mounted pre-amp and power supply??

 

[rabbit73]

I can receive those without the antenna even attached.

It's important that the coax shield is grounded to keep those strong signals from getting into the TV when the coax is connected.
http://www.digitalhome.ca/forum/showpost.php?p=1303730&postcount=126

http://www.digitalhome.ca/forum/showpost.php?p=1304006&postcount=128

 

[rabbit73]

I want to optimize my setup so I can pull in those weak stations like WIVM-LD, which right now is just below the decode threshold. And I also DX too, so every bit of signal counts whenever the tropo is up.

The attenuator before the preamp method that I have tested is the best way to optimize a reception setup at any particular location.

.....When a large SFDR is needed, you might be able to pull it off using a preamp that is resistant to overload with an attenuator at its input, the value determined by trial-and-error. The goal is to use just enough attenuation to reduce the spurious signals without making your desired signal too weak.

If you can't find a preamp that has sufficient SFDR for your location, and you are not able to optimize your preamp with an input attenuator for your signals, then you must resort to more exotic (expensive) measures to receive your weak signals without interference from your very strong local signals.

If you don't have enough signal strength for your weak signal without a preamp, and your strongest signals create IMD in a preamp that wipes out your weakest signal, the attenuator before the preamp allows you to match the SFDR of the preamp to your signals.

I remember a poster who had overload with his preamp at the antenna, but his reception was OK with his preamp at the bottom of his 50 ft coax downlead. This bothered him because he thought his preamp must be near the antenna. He had stumbled on the attenuator technique which gave him about 3 dB of attenuation for UHF. He could have used an antenna or preamp with less gain, but we told him to leave it alone because his setup worked and was cost-effective.

Okay. There is a gap between a preamp with no attenuation and no preamp at all. You could really fine tune the system if you wanted to with 1 dB attenuation steps. Usually I recommend a preamp with no more gain than necessary. Mid way is no preamp but instead use a distribution amp and letting your 100' of RG-6 be the attenuator. Finally if the signals are really strong then no amp of any kind.

 

[rabbit73]

...But from what you're saying, it sounds like I'd need at least two of these tunable attenuators, correct? And I'm also wondering how I'd tune these without having any fancy equipment to test if they're tuned on the right channels. It looks like I'd need to attenuate channels 23, 49, and 50.

It isn't easy to tune them if you can still find them. The Tin Lee filters come pre-tuned.
http://www.tinlee.com/Matv_filters.php
http://www.tinlee.com/MATV-Bandstops.php?active=3#CR7

Here are some old threads about the problem:
Pre-amp overload - notch or YAGI?
http://www.avsforum.com/forum/25-hdtv-technical/574885-pre-amp-overload-notch-yagi.html

Digital Tuners - Overload & Threshold Specs
http://www.avsforum.com/forum/25-hdtv-technical/907551-digital-tuners-overload-threshold-specs.html

Quote from a message from chinadog:

"Thanks for those links, they did make interesting reading. I fiddled with the adjustable traps for quite a bit of time and experienced the "value change in the capacitator" caused by using a metal screwdriver. (kind of like Heisenberg's uncertainty principle) Signals would change when the screwdriver was removed. I then used a screwdriver with a plastic handle ( it may have been de-magnetized) that helped me tune. My instinct was that I needed both traps tuned to the same frequency in order to knock down the interfering frequency by the maximum 15dbs. Visually the two traps are virtually in the same position. My guess is that two "low powered" transmitters on rf 15 and rf17 only two miles way were the culprits, but only a spectrum analyser would know.

I bought a second ut-2700 to see if I can rescue some other weaker channels from higher in the spectrum and cascade the two traps."
End quote

 

[rabbit73]

I was also wondering if inserting just a general 10 dB attenuator somewhere in my setup would help in combination with using a pre-amp. Perhaps inserting it between the mast-mounted pre-amp and power supply??

It should be a variable attenuator with one dB steps. It should be between the antenna and the preamp input because the spurious signals that are a product of intermodulation distortion, that damage your weak signals, are created in the preamp itself.

 

[AntAltMike]

To tune a UT-2700, you HAVE to use a ceramic screwdriver. You can't adjust them precisely enough with even a regular, non-inductive plastic twerker because they have too much flex or "give" in them, and the slot will lurch back and forth past the optimal tuning point. Believe me, I have tuned several hundred of them over the years.

You can get ceramic screwdrivers on eBay from China at reasonable prices if you don't mind waiting two weeks for delivery.

 

[Andrew K]

To tune a UT-2700, you HAVE to use a ceramic screwdriver. You can't adjust them precisely enough with even a regular, non-inductive plastic twerker because they have too much flex or "give" in them, and the slot will lurch back and forth past the optimal tuning point. Believe me, I have tuned several hundred of them over the years.

You can get ceramic screwdrivers on eBay from China at reasonable prices if you don't mind waiting two weeks for delivery.

It should be a variable attenuator with one dB steps. It should be between the antenna and the preamp input because the spurious signals that are a product of intermodulation distortion, that damage your weak signals, are created in the preamp itself.

Thanks for all the help. This is very useful information. I'm a bit pessimistic about using that tunable Winegard attenuator because I actually have one of the Winegard FM notch filters I purchased a couple years ago when I did FM DXing for a hobby, and those filters are really impossible to tune without the proper equipment like a spectrum analyzer, which I don't have. And those Tin Lee filters have got to be expensive. I think I got a quote from them a couple years ago, and was overwhelmed by the cost to notch a few FM stations. Back a couple years ago, I realized that the powerful nearby FMs were ripping apart the FM dial and making weaker stations hard to receive. I noticed it got even worse as I drove by the towers while in my car, so I think the TV frequencies could possibly be suffering the same problem.

But what is odd about my scenario is that I'm able to receive WKBN, WFMJ and WYTV, with WYTV having a noise margin of -17.8 according to that TV fool report. That just doesn't make any sense whatsoever. I get WYTV 24/7, but yet WIVM is very touchy with a positive NM of 11.

By the way, I have a variable attenuator in my basement. I'm wondering if it's worth a try to hook it up before the pre-amp, at the antenna. I have a high, adjustable-gain Radio Shack pre-amp, but I'm worried that it's a piece of crap. I've been reading that pre-amps with low noise figures are the best, but I haven't read anything good about the Radio Shack pre-amps. Perhaps I should try that RCA pre-amp which was recommended above??

 

[AntAltMike]

You don't need a spectrum analyzer to tune a UT-2700. Just buy any old cheap obsolete analog meter on eBay, set it to the center frequency, and tweak away. Tuning a fairly wide UT-2700 to weaken a 6MHz wide ATSC signal does not require the precision of notching down an analog aural carrier.

A variable attenuator is not going to help you much if your problem is signal differential level, and the switchable ones cost a bundle.

Here's a ceramic screwdriver from Hong Kong for $2.99. Select the one that is 4" x 1.3mm blade width.

http://www.ebay.com/itm/1pcs-105mm-...t=LH_DefaultDomain_0&var=&hash=item33931c02ad

 

[rabbit73]

You don't need a spectrum analyzer to tune a UT-2700. Just buy any old cheap obsolete analog meter on eBay, set it to the center frequency, and tweak away.

Good idea; that's the type of meter I started with. Make sure you get one for VHF and UHF, called VU by Sadelco. The VS model covers VHF and Super Band (cable). Don't bid any more than you are willing to lose if they don't allow returns.
http://www.ebay.com/sch/i.html?_from=R40&_trksid=p2050601.m570.l1313.TR6.TRC1.A0.H0.Xsadelco.TRS0&_nkw=sadelco&_sacat=0

The analog meter on the left is the one I started with in 1988:

I also had one like this:

If you can't find a UT-2700, consider a Tin Lee bandstop filter just for your strongest signal. For every dB that you attenuate your strongest signal, you gain 3 dB reduction in spurious signals from IMD. This gives you a net SNR gain of 2 dB for your weakest desired signal.

If you want to try the preamp and attenuator approach, use a CM3410 and a variable attenuator for testing.
antenna > attenuator > CM3410 >

If you find that it helps, you can replace the variable attenuator with inexpensive fixed attenuators, or stack some fixed attenuators for testing.
http://www.mjsales.net/itemsearch.asp?FamilyID=221

 

[holl_ands]

Long ago discontinued W-G UT-2700 contains TWO NOTCH FILTERS, designed to be tuned to the relatively narrowband Video and Audio Carriers in an old NTSC Analog signal. They are NOT very effective at attenuating DTV signals, which are ONLY significantly attenuated close to the two Notch frequencies (much less than 1 MHz bandwidth) and continue to "leak" thru at all other frequencies in the 5.8 MHz signal bandwidth.

A Notch Filter can generate 40+ dB of Attenuation....but ONLY over a very narrow bandwidth, such as is shown in Figure 8 for Tin-Lee CR7E Single Notch Filter for Cable Ch78:
http://www.tinlee.com/Graph_NotchTraps.php?active=#CR7E-999

What you NEED are FOUR Tin-lee CR-7 SINGLE CHANNEL ELIMINATOR Filters in series for Ch49, 50, 23 and 24....such as is shown in Figure 7. To bring the Strong signals to about the same level as your other signals, you NEED about 30-40 dB....which means the 55 dB attenuation shown in Figure 7 is TOO MUCH. So discuss your requirements with Tin-Lee.

Since you're probably going to need a Custom Filter anyway, you could alternatively order TWO CUSTOM FILTERS, one that covers ALL of Ch49-50 and the other that covers ALL of Ch23-24. A Filter for Two Adjacent Channels would extend over twice the Bandwidth and should be easier to build with ONLY 30-40 dB of attenuation.

 

[Andrew K]

Long ago discontinued W-G UT-2700 contains TWO NOTCH FILTERS, designed to be tuned to the relatively narrowband Video and Audio Carriers in an old NTSC Analog signal. They are NOT very effective at attenuating DTV signals, which are ONLY attenuated close to the two Notch frequencies (much less than 1 MHz bandwidth) and continue to "leak" thru at all other frequencies in the 5.8 MHz signal bandwidth.

A Notch Filter can generate 40+ dB of Attenuation....but ONLY over a very narrow bandwidth, such as is shown in Figure 8 for Tin-Lee CR7E Single Notch Filter for Cable Ch78:
http://www.tinlee.com/Graph_NotchTraps.php?active=#CR7E-999

What you NEED are FOUR CR-7 SINGLE CHANNEL ELIMINATOR Filters for Ch49, 50, 23 and 24....such as is shown in Figure 7. To bring the Strong signals to about the same level as your other signals, you NEED about 30-40 dB....which means the 55 dB attenuation shown in Figure 7 is TOO MUCH. So discuss your requirements with Tin-Lee.

Since you're probably going to need a Custom Filter anyway, you could alternatively order TWO CUSTOM FILTERS, one that covers ALL of Ch49-50 and the other that covers ALL of Ch23-24. A Filter for Two Adjacent Filters would extend over twice the Bandwidth and should be easier to build with ONLY 30-40 dB of attenuation.

That's perfect, and that's exactly what I was thinking I would need, but I had no clue as to how much attenuation would be required. I sent Tin Lee an email last night asking for a quote to attenuate channels 23, 49, and 50, but I agree that I would now also need to include channel 24 as it is also a strong signal on my TVs. I'll see if they email me back, and when they do, I'll explain my requirements which you mentioned.

Is there anything wrong with attenuating the strong signals too much? That wouldn't seem like a problem to me. I'm hoping the quote they give me won't give me sticker shock. Any idea of how much they would charge for something like that? I'm willing to pay a decent amount, but I don't want to go broke either.

 

[rabbit73]

I'll see if they email me back, and when they do, I'll explain my requirements which you mentioned.

Is there anything wrong with attenuating the strong signals too much?

Talk to their engineer by phone and email your tvfool report.

The bandstop filters that provide a lot of attenuation in the stopband usually have greater insertion loss in the passband which will make your weak signals weaker than necessary.