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Old 7-Feb-2017, 6:54 PM   #13
rabbit73
Retired A/V Tech
 
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,171
Quote:
Originally Posted by WIRELESS ENGINEER View Post
probably what you should be using is a ch2- 51 antenna system
Correct; his stacked yagis certainly don't sound like they would be any good for VHF-Low.
Quote:
Originally Posted by davodavo View Post
I live in a deep fringe area (literally, no transmitters of any kind above 1 kW within 30 miles) and have a nice setup (stacked yagis with a CM7777 amp)......Nominally, they are full-VHF band...but none of the elements is longer than 40 cm so it's not going to have a bunch of gain down at 80 or even 60 MHz...but it won't have zero gain. With the NM of the repeaters (and a couple of them are LOS) I'm not understanding why I don't see their signal.
Quote:
Originally Posted by WIRELESS ENGINEER View Post
the fact is that your system. noise figure is set by the TV tuner which are ALOT higher than the routine 3 or so db that most preamps have
Not true. The system noise figure is set primarily by the preamp at the head end of the system.

The calculation of the System Noise Figure is done with the Friis Cascaded Noise Figure Formula.
https://en.wikipedia.org/wiki/Noise_figure

I use this calculator:
http://imageevent.com/holl_ands/files/ota

COMPARE System Noise Figures
With and Without Preamps


It's now listed as File Number 8, but might change if more files are added.
http://imageevent.com/holl_ands/file...=0&w=1&s=0&z=4

Here is the calculation I did for the Noise Margin diagram in post #6:



The NF of the tuner doesn't matter as much when using a preamp because its noise is added AFTER the Noise Floor has been amplified, so it is buried in the amplified noise.
Quote:
it like most of these made up specs I see these days is just another of many sales gimmicks
Yup, and it's getting worse.
Attached Images
File Type: png SysNFDX.PNG (34.0 KB, 1508 views)
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Last edited by rabbit73; 7-Feb-2017 at 7:30 PM.
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