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Discussion Starter · #1 ·
A couple of months ago y'all helped me pick the right equipment to put up for fringe reception 60 miles from most of the towers. The vertically-stacked 91XG and YA-1713 are working very well, even with no amplification at all...usually.


This morning we had what I assume were atmospheric conditions that caused severe freezing and pixelating on some of those channels. What usually came in at 75-80% signal strength turned into 30s and 40s. Within a couple hours it was back to normal signal levels. But it reminded me that I wanted to try a CM7777 preamp I found ridiculously cheap a couple weeks ago.


When I first installed it using combined UHF/VHF, I got no signal anywhere. Suspecting overloading on one channel (27, about 8 miles away), I changed the settings to separate UHF/VHF and only connected the VHF. The idea was to make sure this equipment worked and give me better reason to suspect overloading. I did notice considerably better VHF signal strength on 13 with the preamp in place.


One last test had me connect the UHF as a separate lead into the preamp. The VHF was still strong, but the UHF had mostly really low readings in the 30s and 40s and some I was getting before had no signal. The one nearby station had its own signal strength drop into the 50s. This test (with separate VHF/UHF inputs) was better than when I used combined input for UHF/VHF through a UVSJ into the preamp, but still pretty bad.


My guess is overloading on 27, for which I'm thinking a jointenna on 27 would be the next thing to try. I'd take the UHF from the 91XG into the jointenna to attenuate 27 and then into the UHF input to the CM7777. Does it sound like I'm on the right track here? TVFool info is below (current, not post-transition), and note that the programming on 21 and 27 are identical:

 

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Discussion Starter · #2 ·
Update: No matter what I tried, the 7777 I got couldn't be made to work. Even with a jointenna on 27, I think it was just too strong and overloaded the tuner. Unfortunately, even with a very directional UHF antenna like a 91XG, the directions are too close between KXAM and the Austin stations (128 versus 119/120), so that complicated things.


I was able to test out an HDP-269, knowing its reputation for being very tolerant of potential overload conditions. And it works very well, even with no attenuation of RF channel 27 -- KNVA (the weakest of the stations from the Austin towers) was coming in with signal strength in the high 50s just before adding the HDP-269, and now it's in the mid 70s. I think the signal is probably strong enough now to split three ways into the house.


So add another data point for the HDP-269 as a good choice for someone facing likely overload issues.
 

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Rick, what is the cost of the CE filter. He has a huge signal from on channel 21. I got a channel deletion filter from Tinlee that has almost 40 dB of attenuation on channel 17.

John
 

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The 7777 overloads quite easily. I wouldn't recommend it with signals above -70dBm or so.


Notch filters tend to be expensive, and the one I had made for 17/26 didn't help during my last trip (to Mystic), but I wasn't overloading then anyway. The leaves on the trees must have made up the difference. The 7777 was definitely overloading during the winter, with -57dBm signals, less affected by trees.


Thanks for the report on the HDP-269.
 

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Discussion Starter · #5 ·

Quote:
Originally Posted by Falcon_77 /forum/post/14460983


The 7777 overloads quite easily. I wouldn't recommend it with signals above -70dBm or so.

Yeah, I had a chance to pick one up real cheap (I could probably sell it for at least what I paid for it), so I figured I'd try it. So much of this stuff is trial and error. I suspected I wouldn't be able to stop the overloading of RF 27.


Interestingly enough (or is it by design?), the HDP-269 really only has significant impact on the signal strength of the weaker signals. Anything that was coming in with signal strength in the 80s and higher wasn't really increased much. But the two signals (KNVA and KTBC) which were weaker show gains of about 15-20 points on the signal strength meter.


The last interesting thing is that KXAM (the nearby "pest" on 27 causing overload concerns) seems to be *weaker* than it was before! It used to show up in the low 90s, and now down to low to mid 80s. And as far as NBC affiliates go now, the much more distant KXAN is now stronger than the very local KXAM! Maybe that's how this preamp deals with overload so well -- knocking the potentially overpowering signals down a bit before strengthening the weaker ones?
 

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Quote:
Originally Posted by ziggy29 /forum/post/14462172


The last interesting thing is that KXAM (the nearby "pest" on 27 causing overload concerns) seems to be *weaker* than it was before! It used to show up in the low 90s, and now down to low to mid 80s. And as far as NBC affiliates go now, the much more distant KXAN is now stronger than the very local KXAM! Maybe that's how this preamp deals with overload so well -- knocking the potentially overpowering signals down a bit before strengthening the weaker ones?

The pre-amp isn't that smart. It's just an analog component, no fancy processing like that built in. What you're seeing could be due to a number of possible causes, incuding


1) The pre-amp is overloading, but the overload doesn't affect 21 much. This can be modeled but I don't have the tools handy


2) The pre-amp is not in fact overloading, but the television's own tuner is. Since the television's tuner can filter out channels far away from the ones it is tuned to, the strong signal on 27 wouldn't necessarily damage 21.
 

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The signal strength percentage number developed by the receiver is not a measure of actual signal strength. It is derived from some secret algorithm that relies heavily on how much error correction is taking place.


Amplified signals develop all kinds of intermodulation byproducts that fall variously throughout the broadcast spectrum, and when you are unfortunate enough to have a significant intermodulation byproduct fall on a weak channel, it beats up that channel. Also, There is on-channel intermodulation distortion that sometimes will make your strongest channel degrade first , rather than your weakest.


Have you tried connecting JUST the UHF antenna to the UHF input on the 7777 and leaving the VHF antenna off completely? Even though the UHF and VHF have separate amplifier circuits, they draw offf the same power supply, and a strong FM station could gobble up enough amplifier power to adversely affect your amplified UHF channels.
 
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