b1gmoose
09-21-08, 10:43 AM
So I've been reviewing the official AVS Antenna Topic sticky and have come accross some big words and some other things I would like some clarification on.
Maybe we can create a sticky for the math behind selecting an antenna & pre-amp?
There are a few pre-amps out there with various levels of gain and noise levels.
Take the CM 7777 pre-amp. It is a decent consumer grade pre-amp at a good price. It works in quite a few areas, but not all.
But apparently, it has a problem with overloading and intermodulation distortion (one of the big words).
I have googled a little, and have sort of figured it out.
But other things like harmonics and how to calculate those would be nice. Like for identifying FM problems as well.
Or beamwidth of an antenna and associated gain and FB ratios. Or maybe maximum dbmv output of a pre-amp and what that means to the receiver.
I'll also include a copy of my FMfool.com report to see if I have any problems coming from there.
But could somebody write a guide for the average OTA person explaining some of the terms?
TVFool.com gives a reasonable picture of what signal levels are expected at the input location.
We can use my TVFool map as a guide for some of the math.
WVTB-DT on UHF 18, my signal is -78.4. However, it says I have 1-edge reception, really, I have LOS. I live right next to a river the drains from Burke Mt. So 15 miles up a river valley from 500’ where I am to 2000’ where the antenna is.
I have a 91XG at 25’ AGL with the CM7777 pre-amp and 95’ or RG11 into the house. Without the pre-amp, WVTB-DT is 95% signal when pointed right at the transmitter. With the pre-amp, no matter what direction the antenna is facing, I have no less than 85% signal.
Right now, I can receive analog station CIVSTV on UHF 24. It is very snowy, but it does come in at 65.7 miles with 2Edge and a RX dbm of -135.8.
WCAX-DT, WVNY-DT, WPTZ-DT, WETK-DT and WFFF-DT are all located on Mt. Mansfield at 4000’. I have a hill that is 1500’ tall about ½ mile away blocking my view.
I can receive WCAX-DT at 80% signal with some pixels every 15-30 minutes and WETK-DT at 75% with not drop-outs. All other channels are no higher than 50% and I am not able to get a lock on them.
What math is involved in determining when a pre-amp is overloaded, what channels are causing the overload, and how do I calculate the intermodulation distortion?
Once those numbers are calculated, how do I determine what size notch filter is needed to block out a specific channel? 30db, 50db, etc.
I know WVTB-DT is the likely culprit and I can receive that PBS station from Mt. Mansfield on WETK-DT, so a notch filter won’t prevent me from receiving PBS programming.
My plan right now is to:
Increase the height of my mast to 25’. I will use a rotator support bearing so the mast head can extend 6’ above the rotator. Total height to top will be about 30’ after it is sunk into the ground a foot or so and guy wired at 10’ and 25’.
Antennas will be 2x 91XG horizontally stacked. Spacing will be determined on a 10’ pole at ground level. It’ll be easier to make adjustments there instead of after it’s up on the tower.
3’ below the 91XG’s will be either 1, or 2 Winegard YA-1713 VHF highband Yagi’s.
I will use a CM7777 pre-amp and on the UHF input, I’ll probably use a TinLee bandstop notch filter tuned for UHF CH18.
All coax on the tower will be Commscope RG11 2285K series with digicon connectors. I have a 100’ run of ½” coax hard-line that I scrounged that will run to the house.
Once in the house, it will be 25’ of the same RG11 to the Zenith DTT900 converter box & the TV.
If this is confusing to you, please let me know. Sometimes I ramble on. But if you can help me out, and maybe we can make a sticky that will cover things other than just antenna selection, but the math behind selecting the correct equipment and determining system losses.
Thanks,
~ryan
Maybe we can create a sticky for the math behind selecting an antenna & pre-amp?
There are a few pre-amps out there with various levels of gain and noise levels.
Take the CM 7777 pre-amp. It is a decent consumer grade pre-amp at a good price. It works in quite a few areas, but not all.
But apparently, it has a problem with overloading and intermodulation distortion (one of the big words).
I have googled a little, and have sort of figured it out.
But other things like harmonics and how to calculate those would be nice. Like for identifying FM problems as well.
Or beamwidth of an antenna and associated gain and FB ratios. Or maybe maximum dbmv output of a pre-amp and what that means to the receiver.
I'll also include a copy of my FMfool.com report to see if I have any problems coming from there.
But could somebody write a guide for the average OTA person explaining some of the terms?
TVFool.com gives a reasonable picture of what signal levels are expected at the input location.
We can use my TVFool map as a guide for some of the math.
WVTB-DT on UHF 18, my signal is -78.4. However, it says I have 1-edge reception, really, I have LOS. I live right next to a river the drains from Burke Mt. So 15 miles up a river valley from 500’ where I am to 2000’ where the antenna is.
I have a 91XG at 25’ AGL with the CM7777 pre-amp and 95’ or RG11 into the house. Without the pre-amp, WVTB-DT is 95% signal when pointed right at the transmitter. With the pre-amp, no matter what direction the antenna is facing, I have no less than 85% signal.
Right now, I can receive analog station CIVSTV on UHF 24. It is very snowy, but it does come in at 65.7 miles with 2Edge and a RX dbm of -135.8.
WCAX-DT, WVNY-DT, WPTZ-DT, WETK-DT and WFFF-DT are all located on Mt. Mansfield at 4000’. I have a hill that is 1500’ tall about ½ mile away blocking my view.
I can receive WCAX-DT at 80% signal with some pixels every 15-30 minutes and WETK-DT at 75% with not drop-outs. All other channels are no higher than 50% and I am not able to get a lock on them.
What math is involved in determining when a pre-amp is overloaded, what channels are causing the overload, and how do I calculate the intermodulation distortion?
Once those numbers are calculated, how do I determine what size notch filter is needed to block out a specific channel? 30db, 50db, etc.
I know WVTB-DT is the likely culprit and I can receive that PBS station from Mt. Mansfield on WETK-DT, so a notch filter won’t prevent me from receiving PBS programming.
My plan right now is to:
Increase the height of my mast to 25’. I will use a rotator support bearing so the mast head can extend 6’ above the rotator. Total height to top will be about 30’ after it is sunk into the ground a foot or so and guy wired at 10’ and 25’.
Antennas will be 2x 91XG horizontally stacked. Spacing will be determined on a 10’ pole at ground level. It’ll be easier to make adjustments there instead of after it’s up on the tower.
3’ below the 91XG’s will be either 1, or 2 Winegard YA-1713 VHF highband Yagi’s.
I will use a CM7777 pre-amp and on the UHF input, I’ll probably use a TinLee bandstop notch filter tuned for UHF CH18.
All coax on the tower will be Commscope RG11 2285K series with digicon connectors. I have a 100’ run of ½” coax hard-line that I scrounged that will run to the house.
Once in the house, it will be 25’ of the same RG11 to the Zenith DTT900 converter box & the TV.
If this is confusing to you, please let me know. Sometimes I ramble on. But if you can help me out, and maybe we can make a sticky that will cover things other than just antenna selection, but the math behind selecting the correct equipment and determining system losses.
Thanks,
~ryan