Affect of Nearby Building on Part 15 AM Antennas

[rfry]

A popular concept about the reason for the improved coverage from "elevated" Part 15 AM systems is that the 3-meter whip has better clearance over nearby houses and other obstructions.

However the dimensions of even large houses and buildings are rather small compared to a wavelength in the AM broadcast band, and such r-f radiation is not much affected by them.

The NEC study linked below compares the radiation patterns of a 3-m whip mounted with its base at earth level to one elevated 5 feet above the top of a nearby metal frame (other things equal). Their performance is very nearly the same.

http://i62.photobucket.com/albums/h85/rfry-100/Affect_of_Building_On_Radiation.gif

RF

 

[LibertyNT]

Well Fry this chart is somewhat true. But not entirely.
The effect of Buildings on Part 15 systems of any kind is rather obvious and very noticeable. The effect gets worse as the signal decreases. The result is usually an unwanted null in the direction of the building regarding the antenna. The Null can vary as the size of the building, interior walls, ect. At such low power levels Line of sight is kind of the key. This is more so true when the antenna is against a wall or inside of the house.

This is more so true for Part 15 FM. But still eats up on The AM portion as well.

 

[rfry]

Well Fry this chart is somewhat true. The effect gets worse as the signal decreases. The effect ... is usually an unwanted null in the direction of the building regarding the antenna. ... Line of sight is kind of the key. ...

Thanks for your reply. But physics shows that line-of-sight paths are far more important at VHF and above, where structures such as those in the NEC model linked above are much more effective obstructors and re-radiators than they are at medium-wave frequencies.

RF

 

[audioguy]

Unfortunately, I can't agree with Mr. Fry's conclusions in this case. The problem is not with the idea of modeling per se, it has more to do with the fact that the model does not capture all of the real world problems because it is too simplistic.

Any ham with more than a few license renewals under his belt is well aware that you can't mount a vertical antenna-- on any frequency-- near a building and expect to get out. There are wires in the walls that resonate and soak up power, as well as lossy building materials. No one wanting to construct an efficient antenna system would ever choose such a location. Because these factors are not known with any degree of precision, they can't be included in a model. The only way to ascertain the performance of an antenna sited in this manner would be to perform in situ field strength measurements. If anyone would like to loan me an FIM-41, I would be happy to take measurements.

There are other reasons why ground mounting is impractical or impossible for many users. This is why we are contemplating filing a petition with the Commission to clarify these issues.

 

[Tom Wells]

I find it has to do with the issues audioguy raises. Just any building isn't the problem, it's the amount and type of metal in the building.
All metal electrical conduits, water pipes and wiring absorb some energy, as does metal siding, metal framing, and metal beams.
In Chicago many old buildings have expanded metal lath behind the plaster, which also absorbs signal.
I have one outside parking space beside the garage, a stucco covered 1930's garage, with lots of steel lath imbedded in it.
When I park a car there, even though the antenna is still in the "free and clear", it's about 3 feet from the stucco wall, and
AM signals (all of them) are reduced by a noticable amount.
On the other hand, there's a place about 2 blocks away, between 2 large hospital buildings, where the signal is stronger
than it is a block closer with "line of sight". Next to the parking garage of the hospital, ALL of my signal is blocked/absorbed shielded.

 

[rfry]

Unfortunately, I can't agree with Mr. Fry's conclusions in this case. The problem is not with the idea of modeling per se, it has more to do with the fact that the model does not capture all of the real world problems because it is too simplistic.

To clarify: the NEC model doesn't show that mounting a 3-meter AM transmit antenna near a grounded, conducting frame does not affect the radiation patterns of the two mounting configurations for the antenna. In fact it does. The same 3-m antennas would have a few decibels more peak gain if that structure was not present, and the two h-plane patterns then would be the same, and exactly circular.

What the NEC model does show is that the effect on those patterns is very nearly the same whether the antenna is mounted with its base at the earth, or its base elevated 5 feet above the roof of the structure (other things equal). This illustrates that line-of-sight paths are not very important in the AM broadcast band, as they are at VHF and above -- which was the concept being studied.

The NEC study shows the h-plane radiation envelopes with only one nearby structure. As the groundwave signal propagates it encounters other structures at other distances and compass directions, and conductors such as the "ground" leads on power grid utility poles, metal flagpoles, highway lighting poles, cell towers, overhead wires etc. Such other conductors can affect the original radiation envelope randomly, to produce areas of better and worse coverage than the ~circular NEC plots suggest.

In summary, the NEC study shows that buildings and other conductors have nearly the same affect on radiation whether the base of the 3-m radiator is near the earth, or elevated -- not that the groundwave coverage of Part 15 AM stations is almost uniform in all directions.

RF

 

[druidhillsradio]

Hmmmmm. No effect huh? Close placement of metal, hands, etc., has a significant effect of the measurements while attempting to tune the transmitter. Placement alongside a tower, not at the top, skews the pattern in real-world observations.

 

[WCWalker]

Unfortunately, I can't agree with Mr. Fry's conclusions in this case. The problem is not with the idea of modeling per se, it has more to do with the fact that the model does not capture all of the real world problems because it is too simplistic.

To clarify: the NEC model doesn't show that mounting a 3-meter AM transmit antenna near a grounded, conducting frame does not affect the radiation patterns of the two mounting configurations for the antenna. In fact it does. The same 3-m antennas would have a few decibels more peak gain if that structure was not present, and the two h-plane patterns then would be the same, and exactly circular.

What the NEC model does show is that the effect on those patterns is very nearly the same whether the antenna is mounted with its base at the earth, or its base elevated 5 feet above the roof of the structure (other things equal). This illustrates that line-of-sight paths are not very important in the AM broadcast band, as they are at VHF and above -- which was the concept being studied.

The NEC study shows the h-plane radiation envelopes with only one nearby structure. As the groundwave signal propagates it encounters other structures at other distances and compass directions, and conductors such as the "ground" leads on power grid utility poles, metal flagpoles, highway lighting poles, cell towers, overhead wires etc. Such other conductors can affect the original radiation envelope randomly, to produce areas of better and worse coverage than the ~circular NEC plots suggest.

In summary, the NEC study shows that buildings and other conductors have nearly the same affect on radiation whether the base of the 3-m radiator is near the earth, or elevated -- not that the groundwave coverage of Part 15 AM stations is almost uniform in all directions.

RF

This issue where you keep trying to dictate to Part 15 AM broadcasters what is and is not fact or real is tantamount to a green, recent college grad Lieutenant in the Army being put in charge of a seasoned combat patrol that has real combat experience. College Boy can't make the right decisions about how to engage the enemy because the book does not cover every real world situation. We respect your experience with licensed broadcast stations and applications that require that kind of experience. But it does not fly in Part 15 AM broadcasting.

 

[rfry]

Hmmmmm. No effect huh?

My posts don't show that there was no affect on the patterns. They show that the structure in the study setup made little difference in the patterns of a nearby 3-m radiator at earth level, and when elevated 5 feet above the elevation of that structure.

Close placement of metal, hands, etc., has a significant effect of the measurements while attempting to tune the transmitter.

Agree, but such effects were not modeled (or eliminated) by the NEC study. Twenty feet is not particularly "close" as far as tuning/matching is concerned.

Placement alongside a tower, not at the top, skews the pattern in real-world observations.

That may be someone's belief/expectation. But physics shows that, while doing so can affect the feedpoint impedance of the Part 15 AM antenna system, it won't change the circularity of the h-plane radiation pattern "launched" by that antenna configuration.

Even in the case of the NEC model of this thread, the two radiation patterns have very good h-plane circularity even though the re-radiating structure is at least 20 feet from the 3-m whip.

To Wm Walker: The same physics applying to commercial AM broadcasting applies equally to Part 15 AM systems.

RF

 

[WCWalker]

The laws of physics are not always accurately interpreted by Man. Man has not conquered or fully begun to understand the universe as a whole, let alone just one small aspect of it in the form of physics.

More proof. I belong to a HDTV forum that discusses the technical side of reception and occasionally transmission of HDTV signals. People will pull a TV Fool report for their location and determine what kind of antenna, cable, amp if any is needed etc. to pick up the signals that the database report claims can be received. More often than not, people have reported reliable and regular reception of stations that are 2 edge path or even tropo path at great distances. These are stations that are not even showing in a light red category of weak deep fringe reception either. These are stations that are showing in gray and should not be receivable.

Example:

A signal showing a negative DB in Noise Margin such as minus 23 should not be receivable at least not reliably and on a regular basis. Yet, people are reporting that they get good reception of such signals at great distances.

TV Fool, unlike Antennaweb.org uses real FCC data.

http://www.tvfool.com/?option=com_content&task=view&id=57

 

[Tom Wells]

William, I sure-gosh don't get why you believe Mr Fry is here to be a spoiler. ???

Incorporate your knowledge, experience and that of others if you would best understand any technology.
I've been at this for 35 years, have a degree with much RF engineering, and continue to learn from my own part 15 AM,
and I very much respect and appreciate Rich's expertise and input. And I learn from his input.
If you find inconsistencies in your experiences and that of others, investigate further.
I believe Rich wants our pt 15 signals to be a strong as possible, but give us the best understanding of mw behavior,
groundwave propogation, and behavior of rf currents in conductors.
I have never found any difference in the behavior of my flea power AM vs a big signal. Except no chance of skywave. :-\
Weird strong/weak spots, power line current cancellation spots, "funny" lobes where it sounds oddly phased, all the same as big signals.

All obstructions absorb some RF energy. Free and clear is better, especially if it just so happens to provide a long conductor
somewhere back to the true RF ground. While ground mounting should NOT become the only option, I think the point he is trying
to make is that a good ground (or "lower" level) placement may have advantages over the possible cancellations of the elevated
3m radiator combined with the incidental radition off a feedline or ground.
In radio there's theory, and how it actually works out. It's not an argument, it's a best attempt to model a solution and predictable
behavior. Yes, of course every single detail and flagpole and building and butterfly makes some difference and changes RF behavior.
This does not make theory OR practice superior one to the other, but always, one supports the other in part.
The laws of physics are immutable, but haven't been defined in expressions for RF currents which would include every butterfly.
So we use formulae with just a few terms to simplify. Naturally, if we havent any knowledge of a huge ground conductivity anomoly in our area, the model will not give us the right answer, but it is not wrong. It has no "term" in an equation to account for the anomoly.
This is why AMs do (or did) extensive proofs, and often showed results a bit different from what was calculated.


See the thread in engineering where I find that the nearby (300 feet) river seems to have a directional effect to my 1620 AM.
Others have also seen ground conductivity anomalies from rivers.

Just...these things are matter for dicsussion and consideration, not rancor.
Engineers MUST see beyond things that divide people by opinion and transmit as much truthful knowledge as possible.

 

[WCWalker]

Tom.

WVRM aka Village Radio AM 1620 in Montclair, NJ actually skipped on at least one occasion in the 1990's. I read a story about how it had been heard hundreds of miles away from Montclair despite the fact they were using a USI Trans AM 100 100 mw transmitter that had recently been inspected by the FCC.

I have friends that are also broadcast engineers or have decades of experience with broadcasting. They just don't see eye to eye with what Rich Fry purports to be fact on these boards.

I am very familiar with how water and certain mineral elements in the soil can improve the signal of AM station regardless of size. I did read the string you mentioned on the Engineering board but I did not chime in because I could not really add anything that had not been said. Example: AM signals in central NY can really carry because there are large salt deposits in the ground not far below the surface.

 

[rfry]

...People will pull a TV Fool report for their location and determine what kind of antenna, cable, amp if any is needed etc. to pick up the signals that the database report claims can be received. More often than not, people have reported reliable and regular reception of stations that are 2 edge path or even tropo path at great distances. These are stations that are not even showing in a light red category of weak deep fringe reception either. These are stations that are showing in gray and should not be receivable.

Yes, I agree that it is difficult to nearly impossible to accurately calculate a field intensity at a given elevation above a given geographic point. I expect the TV Fool results also show that there should be a very useful signal at some locations where reception hardly ever is possible. For analog TV and FM, the FCC addresses this reality by publishing statistical values based on a field intensity that should exist at a fixed elevation at X % of locations, Y % of the time.

However the NEC study and my comments in this thread have been confined to the radiation actually launched by the 3-m whip and nearby structure, not on the amount of it that would be received at any particular height and location.

Note that broadcast consultants now use NEC to design the antenna arrays of licensed, directional AM stations. After installation of the array, these patterns must be measured and documented by the station/consultant, and then approved by the FCC. The use of NEC has made this whole process a lot easier, and has proven the accuracy of this technique.

RF

 

[Hamilton]

I usually don’t post here much, am pretty busy, but I will say that I believe Rfry does not have any Part 15 design or field experience, his experience and background is from the full power broadcast industry. Also he has not ever dealt day to day with the FCC in Part 15 matters that I am aware of. He will say it doesn’t matter, but on the finer points especially, it really does.

 

[rfry]

... I believe Rfry does not have any Part 15 design or field experience, his experience and background is from the full power broadcast industry.

Electrical components and circuits don't behave any differently when used in transmitters and r-f systems for "the full power broadcast industry," or in those used for Part 15 systems.

Also he has not ever dealt day to day with the FCC in Part 15 matters that I am aware of. He will say it doesn’t matter, but on the finer points especially, it really does.

My comments about Part 15 are limited to topics that can be demonstrated and proven scientifically. Unlike some posters may do, I don't write that I know what the FCC did or probably will do on a Part 15 field inspection, and offer specific analysis, or advice about that based on hearsay and/or opinion.

RF

 

[Hamilton]

“Unlike some posters may do, I don't write that I know what the FCC did or probably will do on a Part 15 field inspection”

True because you are just not involved with the Part 15 industry,

“and offer specific analysis, or advice about that based on hearsay and/or opinion.”

Rich, I see you as a (Board Bully) you beat up those who don’t see things exactly your way! People who totally agree with you, or just do not have any tech knowledge, you are fine with, but those who should disagree, you “pull the spin tricks out”.

 

[rfry]

“People who totally agree with you, or just do not have any tech knowledge, you are fine with, but those who should disagree, you “pull the spin tricks out”.

There is no "spin" in physical law. It is described by equations.

//

 

[edarmsttrong]

Yikes...this IS true! But, from my practical observations, objects, etc. can affect Part 15 AM antennas in adverse ways. Keep the radiator at least 20 feet away from ANY objects, keep your ground short, and all is good....word up.

 

[druidhillsradio]

"There is no "spin" in physical law. It is described by equations."

Can you say "Free Space", Open Area Test Site?

 

[rfry]

Can you say "Free Space", Open Area Test Site?

The only objects in the NEC study are the 3-meter whip (at two heights above the earth) and the wire model of the building -- the radiation patterns of which configurations are shown.

These models have no other obstructions in their useful propagation paths, e.g., they are in the center of an open area test site on the surface of the earth.

Free space conditions do not exist for antennas that are not actually in free space. These are mounted near the earth.
//