Affect of Nearby Building on Part 15 AM Antennas

[Don2]

Heck, Trees eat my signal up.. And, there isn't any metal in them! ;D

 

[rfry]

Heck, Trees eat my signal up. And, there isn't any metal in them!

For another point of reference:

I have a small, battery-operated, portable radio that provides a readout in "dBu" referenced to the received field. It does not measure true field intensity, but hopefully it does show relative fields (comparisons) with tolerable accuracy. At this point I have not confirmed that, though. Anyway...

Earlier today in daylight conditions, I took it outside and recorded the readings of that relative field indication for two, licensed AM broadcast stations -- first 20 feet from my house in the directions of those stations (which are about 180 degrees apart), and then 20 feet on the opposite side of my house in the directions of those two stations. One station was local, the other was not. In both cases the internal loopstick antenna in the receiver was oriented for maximum indication of field, which was normal (90 degrees) to the directions toward the stations, as expected for clear locations both by theory and practice.

The local station showed of field of about 80 dBu on both sides of my house. The distant station showed a field of about 30 dBu on both sides of my house.

That 50 dB voltage ratio indicates that the local station has about 316 times more field intensity near my house than the distant station has.

But the more important indication in this experiment is that the wires in residential houses along the propagation path cause very little loss to radiated signals in the AM broadcast band -- whether those signals are strong, or weak. Non-metallic trees even less, probably.

This finding also rather well supports my original post in this thread, inasmuch as my house contains a goodly number of vertical and horizontal wires used in a.c. power distribution and also copper water supply/sewer pipes, and an approved "ground connection" for the incoming a.c. service -- which is much more than 2 ohms at its single-wire connection point, no doubt.

RF

 

[Tom Wells]

At 20 feet I would expect little change, what sort of relative readings do these same signals read inside the house?
The influence of coupling/shielding/interaction I suspect will be more evident indoors.
If the signal is diminished it would argue there being "some", incremental absorption.
Maybe we're just over-analyzing a matter of degree.

Tonight, I tried grounding one of several "unused" random "longwire" antennas on my property and found greatly changed coverage.
It wouldn't seem to be a significant portion of the wavelength, but its effect on my 15 was undeniable.

Regarding the directionality question I raised in engineering, while the river seems to have its effect, so does the 100 foot
metal light pole directlly east across the street. On an E-w street due east of my loc, there is an almost perfect null directly "in-line"
with the light pole "reflector", my signal rising either way to N or S of a line running east.

 

[PhilB]

In the building model, current is flowing in all of the wires. They sum at wire 10 and go to ground through the 2 ohm ground resistance in wire 10 to ground

Although I suspect that only 2-3 people are continuing to read this thread any more, I'll add another comment to it.

The currents shown along the metal frame in PhilB's recent posting are a higher percentage of the maximum current on the 3-m whip than my NEC program shows for those conditions (see link below).

There may be some differences with the adaptation made by PhilB for 4NEC2, as in my original NEC input file the maximum current applied to the 3-m whip is 1 ampere (at the center of segment 2), and PhilB's version shows 5.85 amperes. Might there be others?

The link below shows that the currents on all of the conductors in the metal frame except for Wire 10 (with the 2-ohm ground connection) are vanishingly small. Therefore their sum at Wire 10 would not have much affect on the total radiation from that wire.

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

RF

R. Fry:

Come on Rich, let’s focus here. You’re just lobbing another grenade with the antenna current issue. I have been forthcoming with posting my h-field patterns and saying when my results agree with yours (a 3-meter antenna by itself, your original building model, and wire 10 by itself without the other building wires). You have yet to confirm whether your version of NEC results in the same cardioid pattern that I get with my 50k ohm/2 ohm modifications to your original building model. I expect no difference because the actual NEC-2 calculating engine is the same across all the available modeling programs. It was developed under government contracts by Lawrence Livermore Labs, and the NEC-2 calculation engine software is in the public domain.

People are watching this thread, and they want an explanation for what they are observing. Now is not the time for opinions or sidetracking. Let’s see your pattern results for my modified version of your original model before we even begin to pursue possible differences in our modeling programs, or casting doubts on the accuracy of the results.

Take note that one of the changes I made was the number of segments in the 3 meter antenna. I reduced it to 15 segments from your original 25 segments. 4nec2 flagged 25 segments with the following error:

RFry_3m_ant_with_50k_bldg_gnds_and_2ohm_wire10.nec wavelength=176.4 mtr.
Error: Wire 1 (tag 1), seg-len ( (4.723) ) below wlen/1000 (6.943)

It rejects segment lengths less than .001 wavelength. Since your program apparently accepts shorter segments, you might want to run my model both ways and see if there is any difference in the h-field pattern.

Also, FYI, here is what the NEC-2 manual (NEC-2 Manual, Part III: User’s Guide) says about short segments:

”Extremely short segments, less than about 10^-3 Lambda, should also be avoided since the
similarity of the constant and cosine components of the current expansion leads to numerical inaccuracy.”

 

[rfry]

People are watching this thread, and they want an explanation for what they are observing.

I believe they are observing that the directional radiation pattern from your modification of my model (which I also see in my NEC analysis of your modified model) to use 50,000 ohm ground connections except 2 ohms for wire 10 does not represent the real world. In my opinion, any more effort spent on evaluating it is not warranted.

Last October I constructed a NEC model that I believe is much more applicable for this situation (link below). Note that the entire metal frame is elevated 1.5 feet above the earth. This emulates the reality that the vertical conductors in a home or other building do not connect to the earth directly beneath them, even through 50,000 ohms. But the ground conductors of the a-c distribution wires in the building do connect to a common point near the a-c service entrance, and then to an earth ground normally via a buried ground rod.

In this model there is only one attachment of the frame to the earth, to a ground rod under one corner of the frame. The ground connection of the 3-m whip likewise is a (separate) ground rod under the whip.

The r-f loss of both ground rod connections to earth at the operating frequency of 1700 kHz was set at 25 ohms, which should be a more representative value for this situation (rather than your 2 ohms -- which is the typical r-f loss of 120, buried, 1/4-wave radials used by an AM broadcast station).

Even though the 3-m whip is only 6 feet from the metal frame, the h-plane radiation pattern is circular within +/- 0.5 dB of its RMS value.

The more serious affect on the operation of this system is in the fact that its net h-plane gain is approaching 4 dB less than if the metal frame was not present -- which means that over half of the energy radiated by the whip is shunted to the earth by that nearby frame and its ground connection.

http://i62.photobucket.com/albums/h85/rfry-100/Part-15_AM_WhipElevated_Frame.gif

RF

 

[rfry]

At 20 feet I would expect little change, what sort of relative readings do these same signals read inside the house?

I checked the weak station (790 kHz), and waited until a few hours after sunrise so that it should be the groundwave signal, only.

The signal inside the house averaged the same 30 "dBu" as measured 20 feet outside it, but there were variations of +/- 1 dB or so as I walked around in the rooms inside. I didn't try to probe for the wires in the walls to see how much difference it would make if the radio was right next to them.

The house construction is brick over a lumber frame. Plumbing is copper.
//

 

[druidhillsradio]

I would still like to see results when listening to a flea-power Part 15. To me, measuring a received signal at a great distance from the transmitter, is not comparable to measuring the field strength of a 100 mW transmitter with obstacles between the radiator and the receiver. Remember, we are not talking dozens of miles, but in may cases less than 2,500 feet.

Rich, what make/model of receiver are you using?

 

[rfry]

I would still like to see results when listening to a flea-power Part 15.

The FCC propagation charts for the 790 kHz station I was measuring show that it should have a groundwave field of about 0.39 mV/m here over that ~51 mile path. This accounts for the radiated power of the station in this direction, and the ground conductivity, but not for obstructions such as houses -- of which there are many in my own town that lie on the propagation path from the station. If houses along the path caused even 0.5 dB of loss each, it wouldn't take many of them to reduce that 0.39 mV/m field to something below the radio noise floor, and I wouldn't be able to receive it.

That 0.39 mV/m field is less than a Part 15 AM station using an earth-mounted 3-m whip can produce at a distance of about 1/4 mile, so the measurements I took would be applicable to Part 15 AM.

Rich, what make/model of receiver are you using?

A Tecsun PL-310.

 

[druidhillsradio]

"If houses along the path caused even 0.5 dB of loss each, it wouldn't take many of them to reduce that 0.39 mV/m field to something below the radio noise floor, and I wouldn't be able to receive it."

Good Point.

 

[LibertyNT]

All I can say is houses and trees destroy my part 15 AM (which is ground mounted BTW and not because i want it to be, darn HOA rules...)
I think im going to believe Phil on this one. after all he IS a manufacturer of A Part 15 AM Transmitter and knows how it behaves. I use an SSTran and Phils maps model what Mine Does.

 

[rfry]

All I can say is houses and trees destroy my part 15 AM...

What you are experiencing probably is related much more to the percentage of change in the path length needed to cause a significant change in field strength for a very low power Part 15 AM station, compared to that of an AM station radiating higher power.

For example, a functionally legal Part 15 AM station on 1700 kHz might produce an inverse distance field of 280 µV/m at the end of a 400 meter (1/4-mile) path. The field at the end of an 800 meter path would be 1/2 that, 140 µV/m. That 140 µV/m field is getting fairly marginal for useful reception, especially if there is local radio noise and interference at the receive site. Those paths are short enough so that earth conductivity produces little effect in those received values.

But a 1700 kHz AM station radiating higher power might produce a 280 µV/m field at the end of a 16 kilometer path, and a 140 µV/m field 5.9 kilometers further along in that direction. In this case these fields include groundwave propagation losses for an earth conductivity of 5 mS/m.

So in the low power case, the field drops by 50% for a path length change of 402 meters, while in the higher power case the 50% change occurs for a path length change of 5,900 meters.

This may be the reason why houses and trees sometimes are thought to affect low-power AM transmitters more than high-power AM transmitters.

RF