[KOC62]

Single channel Preamps

Hi, I'm a casual visitor here. But I have an interest in single channel preamps but didn't see any discussion.
I have asked Tinlee for some info. I already have a new antenna preamp with about 25dB gain. But some days there is a bit annoying sound breakup along with some pixelation on somechannels. Not severe but annoying. The meter says 71% signal strength and 78% signal quality. Usually it sits at 81% signal strength and 100% signal quality.

Anyone remember Robert B. Cooper, President of Community Antenna Development Co.? He wrote an article "All American Sports Amplifer" in the September 1971 issue of Popular Electronics magazine. It deals with selective single channel VHF preamps. I wouldn't mind trying such a device but can't find one anywhere on the internet.

 

[ScaryBob]

There is essentially no such thing. The closest you will find is an amplifier with a narrow bandpass filter. Tin Lee will likely build one to spec. Another option would be to use a separate amp and filter. Separate components will be more flexible if requirements change in future.

If the problems are caused by overloading from strong local channels, a notch filter and/or FM band filter may be more appropriate.

 

[BOUVAL]

A single channel preamp is designed to be used with a channel cut antenna.

It's usually for cable TV headends.
.

 

[wilspin]

Yes, to point the antenna to the NE and to install a pre-amp. Do you know of some other way to adjust a roof mounted antenna? I've tried the method detailed in the guide, but I keep hitting the ground.

Not really what I suggested you try first. You are going to have trouble with that transmitter 4 miles away and a preamp. Reconsider the plan. Also rotors come with controllers.

 

[mrpeter105]

KOC62:

It could also be due to antenna placement. I have 3 tv's in the same room attached to 3 different antenna systems. I notice sometimes that for the same channel that on one tv the signal breaks up but on another the breakup is minimal or nonexistant. I would hookup another tv to a 2nd antenna and monitor the same channel and see if you get the same results.

Also, it could be due to electrical interference. We just replaced the blower on the woodstove. The new one has a relay that turns it on when the temp on the stove gets to a certain point. Whenever it kicks in even the strongest stations go into breakup. The problem with this one is that it'll cycle on/off a few times and I think its faulty. So to check for something like that monitor 2 different channels and see if they breakup at the same time. That assumes that you have a 2nd tv. I also have a converter box that I feed through ch3 into an old 20inch tv. Whenever that relay goes off I can see white line noise on the screen from the interference. It could be something like this although maybe not as bad for you as it is for me.

P.S. As for the signal strength if I see any breakup then I always see some indication on the signal meter. It may be subtle such as a slight knockback on the signal level then recovery (probably due to multipath). Even though you may be at a high signal level that can still cause minor breakup or hiccups that you might notice on the screen. If there is no signal level indication then it could be at the stations end.

 

[wilspin]

better yet why dont you complete the tasks in 'new members read first' post and we can check your TVfool report.

 

[dullgeek]

Also rotors come with controllers.

As mentioned before, a rotor - even with a controller - isn't an option for my TiVo, which (AFAIK) can't control the rotor controller. I'm better off installing a 2nd antenna dedicated to be used by one TiVo while the first antenna is dedicated to the 2nd. Then sharing recordings between the two TiVo's.

I had hoped that I could strengthen the signal along the one cable path that is having problems. But - unless I missed it - I haven't seen any suggestions for how to do that.

better yet why dont you complete the tasks in 'new members read first' post and we can check your TVfool report.

Was this directed at me? Because I've already posted my TVfool report in this thread. Is there something else that I should be doing that I"m not?

Thanks again, all.

 

[ProjectSHO89]

I had hoped that I could strengthen the signal along the one cable path that is having problems. But - unless I missed it - I haven't seen any suggestions for how to do that.

Replace the cable with a temporary run. You may have a bad cable.

 

[wilspin]

As mentioned before, a rotor - even with a controller - isn't an option for my TiVo, which (AFAIK) can't control the rotor controller. I'm better off installing a 2nd antenna dedicated to be used by one TiVo while the first antenna is dedicated to the 2nd. Then sharing recordings between the two TiVo's.

IWas this directed at me? Because I've already posted my TVfool report in this thread. Is there something else that I should be doing that I"m not?

Thanks again, all.

Of course, I forgot U are using Tivo duh. So as I have done for Media Center, multple antennas are the way to go. No, not you for the fool report. looks like the post I reponded to has been removed.

Have you tried swaping the Tivos? Run a temp cable from dmark to upstairs Tivo. Any change?

 

[leszek]

Can someone explain how the maximum dBm values for "two strong stations" in the chart in post#1 were calculated? I get different values for Winegard amps for example. Let's say AP-2870 has a maximum UHF input of 93mV. Max dBmV=20*log(93)=39.37dBmV. Max dBm(@75ohms)=39.37-48.75= -9.38dBm. So I should be able to have two strong stations of -13dBm without overloading the amp, no?

 

[holl_ands]

See "DTV Signal Level Overload Calculator":
http://imageevent.com/holl_ands/files/ota
Note the various conversions between dB variations, derating conversions for different numbers
of strong input signals and the need to convert DTV AVERAGE signal levels to PEAK signal levels
(add 7 dB), such as are used in manufacturer's Preamp specs....which were prepared back when
everything was (very peaky) Analog...and it's the PEAKS that cause the most damage....

Fortunately, Third Order Intermod Noise Levels (which cover THREE adjacent channels) are
reduced 3 dB for every 1 dB reduction in signal level (hence 9 dB reduction for 3 dB input loss).
This is important to know when extrapolating beyond the "manu. Spec Points".

So, needless to say.....it's a bit complicated.....hence the Spread Sheet....

AP-2870 has same UHF specs as AP-4700, which (per manu. specs) has maximized SFDR
when input signal levels are about -22.3 dBm PEAK with TWO strong input signals,
which converts back to -29.3 dBm AVERAGE DTV signal levels.

When the input levels are at the above maximized SFDR signal levels, (Peak/Average) dynamic range
(see spread sheet) is calculated to be 68 dB, whereas the manu. Spec Point is for a much smaller 46 dB
dynamic range. Intermodulation Noise levels are at the Thermal Noise levels when input levels permit
maximized SFDR dynamic range. At higher signal levels, the Intermod Noise will rise on some
(not necessarily all) WEAK channel positions, making them more difficult...or impossible to receive.
The Third Order Intermod Calculator Spread Sheet can help to determine which channels are affected
the most. The trick is to MATCH a Preamp to the expected signal input levels, perhaps adding a few
dB of attenuation on it's input to optimize SFDR (Spurious Free Dynamic Range).

============================================================
ERRATA: Typo in "Manufacturer Specs" section (calculations were correct)
WRONG: "Siv = 93,000 uV (89.5 dBuV) for 19 dB Gain Preamps"
RIGHT: "Siv = 93,000 uV (99.4 dBuV) for 19 dB Gain Preamps"

Uploading a corrected Spread Sheet version now....

 

[Geo35]

Variations between the same preamp model

I have 2 CM7777 preamps that I bought recently(one for me and one for another install at my dad's house). They are not installed outside yet, but will install them in the next few weeks.

In the meantime, I have been playing around with them upstairs in a second floor bedroom where the coax wire comes into my house. I connected the incoming wire to the amp unit, connected the amp output to the power supply with a short piece of coax and then to the TV tuner. I get a decent boost in signal strength percentage on the distant channels with either of the amps.

But I do notice a bit of difference between the 2 amp units. Doing a 1:1 comparison (and using the same power supply), one of them does a little better with fringe signals. One unit does not bring in a couple of fringe channels at all while the second unit brings them in with pixellation. It seems that the second unit has slightly better gain. I realize that there must be some variations in the performance of manufactured products, but is there a simple way that I could test the db gain of the amps with a multimeter or something else? (I don't have a signal meter). The units are rated at 26db gain for UHF...maybe the first one doesn't meet the spec.

 

[ProjectSHO89]

No. There is no way for your to test the gain (or other performance parameters) of a pre-amp with a simple multi-meter.

 

[leszek]

Doing a 1:1 comparison (and using the same power supply), one of them does a little better with fringe signals.

Are you using the same cables (and everything else) and is this repeatable? If so, then one might have a slightly higher noise than the other. TV's have an automatically adjustable gain on input, so in your situation (with no splitters between amp and TV to reduce the amp gains significantly) gain differences would matter very little.

 

[dhd53]

Hi, I am new to this forum and I would like to thank everybody for all the excellent infos I read from this forum. I have installed my 4221HD using the pre-amp CM 7777. It's only 8 feet above ground on the roof of the cabanon. For local stations I got excellent signal, but for transmitters from Burlington, I can have only 45-50% of signal. Without the CM 7777, I couldn't even get those channels. My question is: if I install a 2nd CM 7777 in serie with the first one and yes i can feed power tho the 1st one from the cabanon and then to the 2nd one, can I improve those channels from Burlington? Thanks in advance.

 

[majortom]

Geo,

But I do notice a bit of difference between the 2 amp units.

Maybe confirm they are strapped the same in terms of FM Trap in / out, and / or single / dual input if applicable??

dhd,

if I install a 2nd CM 7777 in serie with the first one and yes i can feed power tho the 1st one from the cabanon and then to the 2nd one, can I improve those channels from Burlington?

No, you will likely wind up destroying any chance of reception for both local and distant stations. way too much gain.

A better strategy would be to check with others in the appropriate reception results thread for your area to see what they are able to do / get with their setup nearest to your area.

 

[Geo35]

Are you using the same cables (and everything else) and is this repeatable? If so, then one might have a slightly higher noise than the other. TV's have an automatically adjustable gain on input, so in your situation (with no splitters between amp and TV to reduce the amp gains significantly) gain differences would matter very little.

Yes, I am using the same cables, power supply, everything...and it is repeatable. The only piece I interchange in my test setup is the amp unit itself.

 

[Geo35]

Geo, Maybe confirm they are strapped the same in terms of FM Trap in / out, and / or single / dual input if applicable??.

Both are setup identically...FM trap switched on and dual input chosen for my single UHF antenna.

 

[Geo35]

...... It's only 8 feet above ground on the roof of the cabanon. For local stations I got excellent signal, but for transmitters from Burlington, I can have only 45-50% of signal. ..

dhd53, your height of only 8 feet sounds too low. Do you have clear line of sight to the transmitters that you are trying to recieve? In my location, I can't get many channels at that height. If possible, 20+ feet is much more desireable.

 

[dhd53]

majortom & Geo35, thanks for your replies. Yes, I have clear line of sight to the transmitters directions, my house is near the street corner. Today I check the signals, I got 66%, it was 50% yesterday. It depends on the weather, yesterday was a rainy day. It's difficult for me to put the antenna in the roof. The only next place is in the attic. I might try to move it this summer. I choose the roof of the cabanon because it was the place where I installed the satellite dish, RG6 cable already there, so it was easy and fast to to remove the dish and replace it by the antenna.