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Old 27-Mar-2015, 3:55 PM   #1
ADTech
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A Building Can Ruin Your Whole Day!

Note: I originally wrote this last fall for internal use, but it's relevant to a wider audience, so I'm posting it here also. This experiment took place in the St Louis, Missouri vicinity, the precise coordinates are included in the first photo. Sorry, but the conversion of the photos to jpegs for uploading to the site lost quite a bit of resolution of some of the finer details of the photos.

Early last spring, I stopped in Kirkwood at a location to take some signal readings of our local TV signals from a point where I could see a number of the local broadcast towers. The location was on South Lindberg (US Hwy 67), just north of I-44, in the parking lot of the Burlington Coat Factory store, across Lindberg Blvd from the Walmart Super Center.

Overhead view of parking spot.



I snapped a quick photo out my car window facing east. A number of the local TV station towers were visible as they were less than 5 miles away. I’ve highlighted the locations of the towers in the photo.



Reception forecast for test location, provided by TVFool.



I set up the Rigol spectrum analyzer, running off a battery pack/inverter, to use a prototype of the Eclipse which was affixed to inside of the left rear car window so that the local signals could be viewed.



One thing immediately obvious is that KMOV’s million watt signal is nearly gone and shows evidence of severe multi-path (about 10 dB differential from min to max within the channel’s bandwidth and is down an estimated 20-30 dB from where it was expected to be. In fact, it’s displayed power level is about the same as a 2.0 kW channel 36 signal (KPNT) that transmits from KPLR’s tower. Looking at the photos on the first page of this report, it can be seen that the KMOV broadcast tower is directly behind the Best Western hotel next to the Chili’s restaurant. The building is six stories tall and is of concrete and steel construction.
Several additional observations:
1. TVFool calculated Rx signal powers for the top four stations on the list of between -11 and -15 dBm. Actual signal power measurements were substantially lower, in the neighborhood of -27 to -42 dBm with a difference of up to 25 dBm below the forecast levels, presumably because the main lobe of the broadcast signals was well overhead.
2. KMOV’s signal, when measured at the location behind the building, was some 40 dB below its forecast level.
3. Stations KNLC and KETC are visible, but their signal was coming from the edge of the antenna and on the other side of a shopping center building.
4. WBRU, operating on channel 47, is barely visible as a small bump in the noise between the signals for KTVI and KUMO-LD (which transmits from same tower as KPLR).

By the way, here’s what happens when a “fringe” area preamplifier gets installed.



It took about 16 dB of attenuation at the pre-amp’s input to cool off the intermodulation distortion that was visible above the analyzer's noise floor.


Edit 3/30/15:

Several notes:

1. The combination of bandwidth, MLP usage, and attenuator settings resulted in a displayed noise floor of only -65 to -70 dBm. It is possible that, with different settings, that the displayed noise floor might be pushed down (up to) another 20 dB. It is necessary to keep the signal level below -10 dBM at the Rigol's mixer input to avoid having it add its own internal IMD tot he display.

2. The majority of St Louis' FM stations transmit from those same towers as several of the TV stations. My test setup that day did not include any additional precautions, except using the UHF-only design of the Eclipse, to reduce signal powers below the displayed band. Therefore, there is a distinct possibility that FM broadcasts *might* have contributed to displayed images of distorted signals from some of the amps. My tests that day focused on the UHF band and harmonics up to 1.5 GHz. If I repeat any of those experiments in the future, I'll include FM signal levels as well as investigating the effect with FM attenuated, either with an external UVSJ or FM trap.
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Last edited by ADTech; 30-Mar-2015 at 1:16 PM.
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Old 28-Mar-2015, 12:47 AM   #2
Jason l
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Yes buildings can be worse obstacle then long distance between transmitters and receivers. What kind of fringe amp were you using?Looks like it might be good for someone in a fringe area like myself
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Old 28-Mar-2015, 5:52 PM   #3
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This is an excellent illustration of the damage that can be done by an over-driven amplifier... Looking at the first spectrum analyzer photo, you see the KNLC signal peaking above the noise floor by at least 10 dB... In the second analyzer photo, the added noise and distortion products of the 'fringe amplifier' come close to completely obscuring all of the KNLC signal. KMOV and other signals illustrate the same problem.

Amplification is not a cure all.

Thanks for posting this ADTech
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Old 28-Mar-2015, 11:43 PM   #4
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Amplifying antenna is A MUST DO! For me and only the high gain brings me my Chanel at 74 miles away uhf I will absolutely not get it without. I use s winegard 8275 28 db gain uhf 29 db vhf on my system it works GREAT! Where channel master Titan 7777 barely got my 68mile uhf. And antenna craft 30 db gain works on 68mile uhf and pixelated 74 mile uhf. I don't need amp for 62 mile vhf,But it doesn't hurt any other Chanel's So I'm more curious Fringe amplifier that he did use. Charts are cool to. I'd rather try it for myself to see if it works. Senses charts were done in view of towers in a parking lot it's not very REAL LIFE situation I'm nowhere close to seeing a tower. So I think it would be cool to know which fringe amp he used. And yes amplification is not a cure-all for everybody but it is a absolute cure-all for me in my extra TV stations that I get to watch. My Winegard that I use is no longer produced and I have contacted Winegard to let them know that old high gain should be put on the market for people like me because their new boost XT was just like not using an amp at all on my antenna. So yes without that old amp I'm down about 9 channels. Yes no cure-all just yes to gain more stations. So Dtech For some normal people and some normal situations where the Towers are not visible; THANK you for your charts but please and tell us the amps used if you can recall
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Old 29-Mar-2015, 10:55 PM   #5
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ADTech:

Thanks for posting your test results, and adding your observations to help us understand the results. I always wondered what the noise floor would look like when damaged by spurious signals from IMD, but a SA wasn't in my budget.

The closest I could get was using my Sadelco DisplayMax 800 in the single channel scanning mode to approximate what a SA does to scan a channel with a signal and its adjacent unused channel to measure SNR.

Best regards,
rabbit
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Last edited by rabbit73; 29-Mar-2015 at 11:04 PM.
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Old 30-Mar-2015, 1:17 PM   #6
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I added several notes to the initial post.

Rabbit73,

For less than $20, an SDR dongle can get you started with an inexpensive, albeit slow, SA. http://www.edn.com/electronics-blogs...analyzer---SDR

Quote:
Amplifying antenna is A MUST DO!
And, for many other people, it's a DON'T. There is math involved in these decisions if they are to be made in a considered manner. Otherwise, it's about the same as throwing you-know-what against a wall to see what sticks. Sometimes it does, sometimes it doesn't.

The "fringe" area amp I used was one of the discontinued Winegards, I forget which, either an AP8700 or AP8800. Both overloaded easily.
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Last edited by ADTech; 30-Mar-2015 at 1:24 PM.
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Old 30-Mar-2015, 3:03 PM   #7
Jason l
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Lol yup my math was make antenna hook it up see free Channel's. Buy a preamp for half the price of ONE cable bill. Try it and say wow! Even more free channels.Thanks the winegard is my best channel getter. ��
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Old 20-Apr-2015, 9:59 PM   #8
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Inexpensive Software Defined Radio Spectrum Analyzer < $10.00

Rabbit73,

I purchased three of these (SDR’s) directly from China and find them very useful. All of the spectrum analyzer graphics I post are captured through one of my SDR’s using a free program called RTLSDR Scanner. They only have ~50 dB dynamic range, but are capable of seeing signals down to ~-114 dBm. I wrote about my initial experiences here:
http://www.highdefforum.com/local-hd...r-10-00-a.html
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