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Recent posts here on Part15.org have touched on this subject without offering any conclusions about it.
Below is a NEC analysis showing one comparison for those setups, for those who may be interested.
The statements in the conclusion there might need some study to understand, accurately.
You can’t tell me that having clearance over nearby houses and other obstructions has no bearing on range!
With part 15AM, obstructions block the signal, there is no question about it! If the obstruction is not present then the signal is not hindered and will travel farther then if the obstruction was there. I’ve experienced such situations too many times.
The presence/effects of obstructions along a propagation path are much less significant to the field intensity received in the MW band than when transmitting in the bands located at~ 30 MHz and above.
Obstructions such as terrain and buildings _can_ affect MW signal strengths, but generally only within a small fraction of a wavelength from/in those obstructions.
The following graphic shows the signal strengths I measured using a Tecsun PL-310 receiver tuned to a local AM broadcast station, for the two paths shown there.
One path was blocked, and one path was clear — yet the signals had the same strength at the same distance from the transmit antenna.
This result may not be intuitive, but it is supported both by theory and measurement.
Let me throw in a few random observations. I suspect the signal blockage may be minimal when working with 1.4 kW as opposed to 100 mW. From a 1.4 kW signal, if the blockage stops 100 mW of the signal, that won’t even be measurable with even the finest field intensity meter and would be within the margin of error of the machine. But if that same obstruction was able to block 100 mW of a 100 mW signal, nothing is getting through.
Example. The property next to our house has a 70 foot long mobile home. It’s sided with aluminum siding. When I first received my Procaster I set it up on a picnic table in the backyard, just freestanding with it’s attached 10 foot antenna sticking straight up, and fed power and music into it and walked around the yard and neighborhood listening. Nothing scientific mind you, just confirming it worked before I mounted it permanently at the third floor attic window. When I walked around the mobile home next door and was around the far side my reception vanished. The signal was not able to penetrate two aluminum sided walls of a mobile home. I wasn’t more than 60 feet from the transmitter, but there was an aluminum clad house between it and me. Now, if this would have had as great of an effect had I moved away from the mobile home but still on the other side, I don’t know. But it was certainly blocking the signal.
I’m sure we can all site examples of where the signal vanishes on AM while driving our cars. Go under a bridge? Signal lost. But again, these are examples at the receiving end. When I pull into my garage the 5,000 watt commercial station I listen to all but vanishes. As a commercial broadcaster I often do “remote” broadcasts. We use a 50 watt FM transmitter to send broadcasts back to the studio, and we generally listen to a radio to pick up our cues off air. Our mall, the hockey arena, and the grocery store are all places where we can’t hear the off air signal on AM. Obviously the signal is being blocked. Again, on the receiving end, but nonetheless, signal is being blocked. So it’s my theory that signal can be blocked at the transmitting end, just when it involves thousands of watts with hundreds of feet of antenna tower, the result is minimal and very limited in size. Our FM transmitter for broadcasting gets out of these buildings just fine, but we have been at marginal locations where moving the transmit antenna next to a window was necessary to get noise free reception back at the studio, which demonstrates FM can easily be blocked by a nearby wall or building.
Further consideration: As part of my engineer duties for our AM station (one tower omnidirectional during the day and three towers directional pattern at night) I’m required to take the Potomac Field Intensity Meter to specified monitor points and measure the signal strength to insure we’re not over the limits for our directional pattern. We did this monthly for decades until the rules changed to “as often as necessary to show compliance” so now I do it quarterly plus whenever I suspect things might be unusual, either technically or because of conditions. I’ve been doing this at several stations for more than 40 years. There are an infinite number of things that happen on the planet than can change these measurements. One example is when management had let pine trees grow willy-nilly on the property where the towers are located. This is a large open field basically in the middle of a forest. If left alone pine trees start springing up inside the fenced area. We used to have a crew go out and remove them as soon as they started growing, maybe reaching a foot or two in height. Hundreds would be removed, piled, and burned. After that station owner passed away and the new owner who was cheap decided not to bother doing this trees worked their way to 6 – 8 feet or more over a few years. I had a fit and told them they had to go — mostly because their roots damage hundreds of ground radials running from the bases of the towers. But then an engineer friend of mine saw the site and went nuts about all the signal we were losing! He claimed that all those trees grabbed some signal and brought it to ground. Made some sense but I was skeptical. We convinced management to have the trees removed (mostly to protect the ground radials) and I went and took field intensity readings before the crew did their work, and the day after, and damned if they hadn’t gone up not only measurably but substantially. TREES at commercial transmitter sites for AM DO make a difference. Now, this engineer, who just retired after being in the industry for well over 50 years told me — when it comes to directional monitoring points for AM stations that were established many decades ago, you’re lucky to get 50% of what you did when the limits were established due to growth of a natural nature (trees, brush, etc) and growth of manmade nature, buildings, towers, utilities, etc. Our directional pattern and monitoring points were established in 1971. I pulled out the old logs from the beginning and checked them at 5 year intervals (all readings at the same points, on the same tower system, same output from the transmitter, same current going to the towers, etc) an they did in fact show a decrease at every 5 year interval. And present readings are about 60% of what they were in 1975. I’ve been doing the readings here since 1988, and it’s one of those gradual things you don’t notice — but when i pulled out my 30 year old readings the difference was amazing! This is a well known phenomenon that is well documented at directional AM stations. Now, how this applies to a 100 mW station with a 10 foot antenna may be nearly impossible to document. Other variables that I had been able to witness first hand include the construction of a metal building not far behind a monitor point but inline with the signal, reflecting signal and increasing the reading. And the same effect of a metal building being put up inline with the signal from the tower and me, causing a reduction in signal strength. This also happened with there was a new water tower built between the tower and the monitor point creating a signal shadow. All real things that I’ve experienced in real life. But that shadow was only measurable in a few feet — moving a few feet either way would move me out of the shadow and readings would go back up!
It is often said that the “elevated power and audio cables to an elevated Part 15 AM transmitter radiate signal….” create increased field strength. Maybe so. And I’ve often been told that that must be the case with my ungrounded, elevated Procaster installation.
My Procaster is mounted at a third floor attic window, mounted to a short section of gray PVC conduit, mounted to mast brackets screwed to the side of my wooden house. There is no ground connected to the rather substantial ground lug on the transmitter, and when I installed it I checked for, and noted no continuity between this ground lug and any of the power or audio cable terminals.
The studio is on the second floor. The power/audio cable (as provided with the transmitter) runs from desk height on the second floor, up through the ceiling into the attic and runs along the attic rafters at probably 5 foot height for about 25 feet, and out the window to the transmitter. Is this a radiator? I’ve never checked. But seeing as how the transmitter passed it’s certification with such a cable connected (It’s in the pictures at the FCC website with the certification paperwork) it would have been a radiator then too.
My one experiment when I was first set up, was to connect a wire from the ground lug and drop it to the ground and connect it to the house ground in the basement. Just to see what happened. I have a spot about 1/2 mile from the house where I take FS readings to keep tabs on things, and when I added this wire I got a substantial jump in field strength. Which showed me the effect of a long ground lead, and also made me believe that the power and audio lines were not providing a ground path, since if they were, adding an actual ground shouldn’t have made much difference. Anyway, I’ve always operated without this ground lead connected. Additionally, even if mine were a ground mounted transmitter, it would still need power and audio and since the studio is on the second floor the cable would be substantially LONGER to reach the transmitter at ground level. So if there was any additional radiation from this cable it would be even more so with a ground mounted installation. No matter where you put it, it’s still going to need power and audio. Unless you have your power supply and audio source buried in the yard and run the cables to the transmitter underground!
Just some real life experiences.
TIB
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<div class=”bbp-reply-content”>Hello, Tim (and all):
Below are some selected clips from your detailed post above, with some comments applying to them:
I suspect the signal blockage may be minimal when working with 1.4 kW as opposed to 100 mW. From a 1.4 kW signal, if the blockage stops 100 mW of the signal, that won’t even be measurable with even the finest field intensity meter and would be within the margin of error of the machine. But if that same obstruction was able to block 100 mW of a 100 mW signal, nothing is getting through.
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The attenuation through/around a path blockage is not a fixed amount, such as “100 mW.” Rather, that attenuation is a fixed percentage of the original signal arriving at the blockage of that path. If the effect of the path blockage is not measurable or important for a 1.4 kW transmit system, that also will be true for transmit systems at every other power level (other things equal).
<div class=”bbp-reply-content”>I’m sure we can all cite examples of where the signal vanishes on AM while driving our cars. Go under a bridge? Signal lost. But again, these are examples at the receiving end. When I pull into my garage the 5,000 watt commercial station I listen to all but vanishes.
Such effects are common for MW reception, but they are localized very near the conductors/structures causing them. Once the receive antenna is moved sufficiently beyond those conductors, the received field increases to about the same value it would have if those conductors/structures were not there.
… Our directional pattern and monitoring points were established in 1971. I pulled out the old logs from the beginning and checked them at 5 year intervals (all readings at the same points, on the same tower system, same output from the transmitter, same current going to the towers, etc) an they did in fact show a decrease at every 5 year interval. And present readings are about 60% of what they were in 1975. I’ve been doing the readings here since 1988, and it’s one of those gradual things you don’t notice — but when i pulled out my 30 year old readings the difference was amazing! This is a well known phenomenon that is well documented at directional AM stations.
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No dispute about that, but then the pattern nulls of a directional MW broadcast array are much more sensitive to re-radiated/blocked fields than those in azimuth directions well removed from those nulls. Some directional AM stations have had to re-locate/re-document monitoring points when re-radiating conductors have been installed close enough to the original monitoring points so that the array appeared to be non-compliant with the station license.
<div class=”bbp-reply-content”>…This also happened with there was a new water tower built between the tower and the monitor point creating a signal shadow. All real things that I’ve experienced in real life. But that shadow was only measurable in a few feet — moving a few feet either way would move me out of the shadow and readings would go back up!
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Those effects really are the result of the net field at that exact receive location resulting from the relative r-f phases of the direct and reflected fields arriving at that location — more than the existence of one or more shadows.—-
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Other comments could be made concerning the Procaster installation, but maybe this is enough for now.Sorry for the garbled format of my post above, but I don’t find any way on this forum to edit it, once posted. Anyone know of a way to do that?
Sorry for the garbled format of my post above, but I don’t find any way on this forum to edit it, once posted. Anyone know of a way to do that?
For a few minutes after you post there will be a “modify” link at the top right of your posted comment window, you click that.
Correction: it doesn’t say “modify” it says “edit”
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I’m sure we can all cite examples of where the signal vanishes on AM while driving our cars. Go under a bridge? Signal lost. But again, these are examples at the receiving end. When I pull into my garage the 5,000 watt commercial station I listen to all but vanishes.
Such effects are common for MW reception, but they are localized very near the conductors/structures causing them. Once the receive antenna is moved sufficiently beyond those conductors, the received field increases to about the same value it would have if those conductors/structures were not there.
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Ok, that’s understandable, but we’re not talking about your average MW signals, we’re talking about very weak part 15 MW signals. By the time the receiver is moved beyond that obstruction it’s very likely (and common) that your going to almost immediately run into another obstruction, IE: the building behind that.. the signal is not going to be returning in most cases unless your in a very sparsely populated area.. in which case there’s not going to be anyone close enough who could tune in anyway.
Height helps to avoid that problem.. If you can see the transmitters antenna then you’re certainly going to be able to tune it in. But if you’re at the same distance away and you can’t see the antenna due to structures in the way, then more often then not you will not be able to tune it in.
I’m not saying you have to be in visual range to pick up a part15AM station, but it does require a rather unobstructed path for the signal to be received.
When the Obstacle and Antenna Are the Same
KDX AM 1680 radiates from a “Wintenna”, simply a metal-window-frame fed by a 3-foot wire at the inside-floor up to the window-frame and wire at the top of the window-frame to the porch roof, totaling 10-feet on the rear stucco wall of our building. By this method we are able to place a signal both indoors and the yard outside from a single antenna.
In previous discussions it has been estimated that the stucco-metal-webwork in the wall absorbs some of the RF energy, but an advantage is that the antenna is on the same vertical plane as the wall eliminating the rear of the building as a reflective surface; the antenna and wall are “in phase”.
What becomes significant in light of this thread is that the signal strength remains very usable around the sides and front of the entire building, most of which has brick walls.
Because of this “surface effect”, as I call it, I visualize the MW signal in this case as being “liquid”, because it “flows” around corners and fills in the opposite (front) of the building. I also estimate that this signal behavior is made possible by the “in-phase” nature of the antenna-in-the-wall.
Needless to say the other examples described in this thread are based on antennas located AWAY from objects, therefore out-of-phase reflections from objects plays a part in results at many points in their overall field.
I can’t comment on the effect of obstructions on licensed radio station signals.
But I can comment on that effect for Part 15 signals.
I also have observed that line of sight makes a huge difference in AM Part 15 range. I once conducted an experiment – I used the same transmitter (Talking House), antenna (wire) and power cord for each.
In both cases, I ran the wire antenna outside the house, but in one instance, elevated that wire antenna so that one end was above the roofline. In the other, it was about a foot lower.
I obtained substantially greater range with the higher antenna.
I don’t know how you would explain it otherwise.
If structures/buildings themselves always caused significant and irreversible losses to the signals radiated by Part 15 AM antenna systems, then that effect also would exist for these signals I measured on each side of a high-rise steel structure (graphic below).
Note that those two signals had identical values at the two locations shown on the graphic below after crossing the building structure along the same radial bearing from the transmit site.
If that transmit site radiated only 1/1,000th of the power it took to generate those two 71 dBµ signals near that hospital building, those signals would drop to 41 dBµ. but still be equal to each other.
@ Rich, If I try to edit a post to correct something for example I get an access denied window and have to fill out my email address and answer a math question and click on the allow icon and it comes back the unchanged so I just say to hell with it! Or if it really matters I delete it and do it again.
This begs the questions:
What would that second reading be if it were measured standing 5 feet away from the building rather than out at the edge of the parking lot?
What would the effect be if that building were plopped down at the transmitter tower site, say 50 or 100 feet from the tower? Would there be no attenuation — readings would compare to those taken at other points around that tower assuming the same distance but with no building?
Is the thought here, then, that if I were to drop my Procaster from it’s 30 foot high mount, straight down to the ground with the same cable feeding power and audio, the same length, but still connected to no intentional ground — that the coverage would be the same? Even though at ground level it would be about 8 feet from a 4 foot high chain link fence, and between a stone and wood 3 story house and a 70 foot long aluminum sided mobile home, and be adjacent to a bunch of 30+ foot high trees on one side? Not to mention a cement block constructed garage? That any change in signal would be basically be too small to even measure in any direction?
Not something I could try, as 6 months out of the year the transmitter would be under 3+ feet of snow!
TIB
RE (from TIB): What would that second reading be if it were measured standing 5 feet away from the building rather than out at the edge of the parking lot?
Definitely it would be less than shown on the left side of that graphic, out in the street running along the east side of that structure — but I didn’t measure signals that close to the building.
The salient point here is that the signal measured on that radial further away from the structure matched the signal arriving via an “unblocked” path at the other side of the structure.
IOW, the affect of the structure on that MW signal does not extend more than a fraction of a wavelength from that structure.
What would the effect be if that building were plopped down at the transmitter tower site, say 50 or 100 feet from the tower? Would there be no attenuation — readings would compare to those taken at other points around that tower assuming the same distance but with no building? …
My earlier graphic (reposted below) shows the effects of a nearby structure on the radiation patterns of the same 3-m monopole when mounted nearby with its base at the surface of the earth, and when vertically elevated so that its base is 5 feet above the top of the structure.
Note that those far-field radiation patterns are practically identical.
Certainly the relative signal levels inside and immediately adjacent to that frame structure would not show that these two radiation patterns were as circular as they are when measured/received several hundred feet or more beyond that structure.
But at every horizontal distance and with other things equal, the relative fields/signals of those two 3-m monopoles for those conditions would be practically the same as each other, regardless of their installation heights next to that structure.
Not too intuitive, I know, but that is the reality for this scenario.
Building Upon the Conclusion
The conclusion offered up to this point is: “There is very little difference between the instrinsic radiation patterns of these two configurations ( ground-mounted vs. elevated MW monopole antennas).”
Accepting this general finding can be useful to part 15 operators faced with the choice of where to locate their transmitters.
Is there enough information within this presentment to extend our conclusion just a little bit further?
Can we say that there is very little difference between indoor transmitter/antenna location vs. outdoor?
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