|Originally posted by holl_ands
Bob: Thanks for the valuable "real world" measurement re attic loss.
I'm surprised as how much more you measured for your attic compared to the statistical numbers found for a large number of lower, indoor locations in the above ATSC Field Test Report.
Could you remark on construction materials, esp roof and proximity of foil backed insulation either in the line of fire or below the antenna as well as any other nearby metal objects?
Also, how did you measure signal strength?
I'm surprised by how well the mast mounted CM4221 4-Bay performed compared to ANY of the presumably higher gain models....and how poorly the CM4248 Yagi performed both outside and in the attic.
And why did the Scala 1469 Log Periodic have 7+ dB less attenuation at mid to high frequencies in the attic than ALL of the other antennas, but was right in the middle of the pack outdoors? Very strange....
The roof is standard asphalt shingles, over 15# roofing felt, over 1/2" plywood. The metal flashing at the edge of the roof is minimal, maybe a 2"x1" angle, just enough to cover the edge of the plywood. (No ice on the gulf coast of Texas.)
Texas attics in general (and mine specifically) have lots of metal foil covered flexible ducts for the AC. In mine, there is also a hot water heater and two combination furnace/AC evaporator/fan units up there. All of these were behind the antennas during testing.
Testing was done with a spectrum analyzer using the Max Hold function. The data was allowed to build for three minutes and then downloaded into Excel on the laptop. I also used Excel to extract the analog channels visual carrier peak value and the level at the middle of the digital channels from the 4001 data points. The extracted data shows up as the data-points on the graph.
As the Scala truly is flat (+/- 0.2 dB) across the UHF band, it was declared the graphs base line when it was outside on the mast. Then, each antenna's performance was compared to the Scala by subtraction. (Subtracting logarithms is really division, hence it gives us the Relative Gain Ratio of the antennas.) So comparing the 4228 to the 4248 is a real comparison of the relative gain between them that was experienced in this one location. The method is not as clean and scientific as I would like but I think that it does show the relative gain of the antennas outside.
In the attic, all bets are off. 1st, there is the consequences of the attic refections punching through the back-side of the antennas, either adding to or subtracting from the signal coming into the front. 2nd, attic placement turns out to be as important as what antenna is purchased to put up there. (see the attachment to this post.)
I've got some more data taken from another attic but I have not had time to extract it. So all I can say at this point is that that attic was more transparent to UHF than mine was. It also had a sizable window, perhaps 3'x6', in the general direction of the antenna farm. We were close to downtown, well into the city, with lots of taller buildings all around.
What started me on this adventure was just plain curiosity. I had heard so many glowing reports about the CM7777 and how much it improved reception, that I just couldn't believe what I was reading. My previous experience with preamps has always been a disappointment. While they may have reduced the amount of 'snow', they always added noise (grain) and intermodualtion products (IM) that I couldn't stand to watch.
I had folks 10 to 15 miles from our transmitter reporting significant improvements in both analog and digital reception. I knew from experience that any preamp should be in serious overload, causing all sorts of IM, killing any chance of watching anything. Particularly in Houston, with 17 TV stations and 34 transmitters filling its front end with RF energy. It was when I tested the antennas in my attic that the proverbial light went off. The reason any preamp works for them is that there is a 20 dB attenuator in front of the preamp called 'The Attic'.
By the way, an indoor study done in the 60's at 3500 locations, where they measured the signal levels right at the owners TV set on ch-31, showed a median of 17.5 dB loss for wood frame and 26.9 dB loss for brick construction .
testing attic antenna placement.pdf 201.1630859375k . file