Important detail Neil Radio8Z, but what about the field strength of your signal out beyond the wet foliage?
Rich's claim holds that a MW signal will basically ignore the obstruction and pass through it without notable loss.
As for becoming de-tuned, at least we learn from that that the wet foliage is having SOME EFFECT on the MW signal, whereas we've been asked to believe there is no effect.
In the case of outdoor antennas it would be inconvenient and possibly unsafe to go outdoors and retune an antenna during humid conditions.
If Neil's antenna were raised above the wet foliage detuning might not be needed.
Rich's argument that obstructions do not affect field strength is very misleading. It realty all depends on the type of obstruction. Metal and concrete (usually with rebar) will greatly affect field strength. And of course you find those materials in urban settings.
In rural settings, maybe the effect isn't as great, but it's still there.
This is a very bizzare discussion and I'm not sure of th reason for it.
Rich's argument that obstructions do not affect field strength is very misleading. It realty all depends on the type of obstruction. Metal and concrete (usually with rebar) will greatly affect field strength. And of course you find those materials in urban settings.
In rural settings, maybe the effect isn't as great, but it's still there.
This is a very bizzare discussion and I'm not sure of th reason for it.
Rich's argument that obstructions do not affect field strength is very misleading. It realty all depends on the type of obstruction. Metal and concrete (usually with rebar) will greatly affect field strength. And of course you find those materials in urban settings.
In rural settings, maybe the effect isn't as great, but it's still there. I've noted the effect in both types of settings with everything else being equal.
This is a very bizzare discussion and I'm not sure of th reason for it.
Quote Radio8z..
I attribute the measured loss in field strength to the detuning of the antenna due to nearby foliage and not necessarily to absorbed radiation.
I observed the same thing. Tuning was "Funky" with a bunch of trees around but when I set up in an open field, I found a nice peak in tuning and the range was much better..
Get the launch pad good and the signal will boogie. The knife edge effect is really good in MW..
... Rich's argument that obstructions do not affect field strength is very misleading. It realty all depends on the type of obstruction. Metal and concrete (usually with rebar) will greatly affect field strength. And of course you find those materials in urban settings. ...
By the above quote, it would be expected that the field beyond the far side of a tall, reinforced steel building measured from and on a radial path from the transmit site should be greatly reduced compared to the side of the building nearest the transmitter.
To check this theory I measured the fields of a 1530 kHz AM broadcast station on each side of such a structure on a radial path from the transmitter, using a Tecsun PL-310 The results in the graphic below show that no such serious losses are present for this situation -- in fact the measured fields were equal on both sides of the building.
... Everyone knows that AM reception under bridges and overpasses is crushed because of the construction materials surrounding the car. ...
That is true while the MW receiver is located below a bridge or overpass, but a few hundred feet (or less) on either side of that bridge/overpass the field takes the same value as if those structures were not present.
Sorry for all my duplicate posts this morning. The first time I tried to post it the site said it had an unexpected errer, and to try again later. I tried again later several times and got the same error message.
Then I loaded the site again and found that all those posts actually made it to the board.
Hopefully an admin can delete all but the first instance of it. I tried deleting those, but the site said No.
Refer back to Post # 3 and notice that my entry of 5/24 corroborates what Rich just presented: MW Field Attenuation By Overpasses and Tall Buildings.
Indeed my signal at 1680 kHz is present all the way around the building, with the antenna embedded in the rear window.
However, looking more closely, also using a TECSUN PL-310 (a radio recommended by Rich and for which I am grateful), the actual field strength of the signal falls-off (attenuates) with distance from the antenna. The point is, however, the presence of the building does not block the signal from reaching around all the way to the other side.
Also noted is that the metal latice inside the stucco walls surrounding the antenna strongly shifts the capacitance (tuning) of the AMT5000 transmitter and, according to another RF engineer is absorbing some of the available signal strength causing my antenna to be less effective in terms of distance than would be true if the antenna were located in an open field away from the building.
Rich's measured signal at 1530 kHz appears to have been a high power licensed station giving it a massive advantage compared to a micro-signal from a part 15 station.
To keep these technical conversations valid we cannot compare unlike examples.
... Rich's measured signal at 1530 kHz appears to have been a high power licensed station giving it a massive advantage compared to a micro-signal from a part 15 station. To keep these technical conversations valid we cannot compare unlike examples.
My post showing essentially the same field (as reported by a Tecsun PL-310) on two sides of a hospital building was made over a path length change of about 7.7%.
A path length change of 7.7% along the radials having useful fields produced by a typical Part 15 AM system would show similar field changes, when using a Tecsun PL-310. However a 7.7% change in the physical distance from a Part 15 AM system along that radial path would not be long enough to include a tall building.
Carl wrote "Important detail Neil Radio8Z, but what about the field strength of your signal out beyond the wet foliage?"
The far field strength is also reduced but this is due to the change in antenna system resistance (which increased) noted with the presence of the foliage. This might also be due to the absorption by the plants of moisture from the soil which would lower the soil conductivity, but since I have not measured soil conductivity this is speculation.
You are correct that the effect is the same as if it were the plants absorbing the signal but I am not willing to assign this as a cause and attribute decreased field strength to absorption by the foliage. Absorption by the foliage would not appear as increased antenna system resistance.
It has been long observed that waves are not absorbed by matter which is smaller than one wavelength. This is why the electron microscope was developed. It is known that the resolution of a microscope is limited by the wavelength of light and using electrons which have a much smaller wavelength than light gives higher resolution. By the same reasoning, radio waves will not be blocked by objects smaller than one wavelength.
Neil
Neil Radio 8Z posted: "radio waves will not be blocked by objects smaller than one wavelength."
That is very significant to my experimental indoor AM antenna project, because the house is certainly less than one wavelength and the antenna should perform as if it were outdoors.
My statement about objects which are less than a wavelength is oversimplified and may mislead. The effect of objects in the path of a wave is to produce diffraction which is a bending of the wave at the object's edges. When the object is small compared to a wavelength the diffraction effects become pronounced and the bending of the wave at the edges has the effect of illuminating what would otherwise be a shadow of the object. It is due to these "edge effects" that the wave is not blocked.
If a wave source is enclosed within material then the edge effects do not come into play and the wave can be blocked. Placing an antenna within a structure may well result in a blocked or reduced strength signal seen outside the enclosure.
Neil
Everything is True As Is Nothing.
I think John Paul Sartre said that.
(Not J.P.Janzi)
As Carl Blare and ~countless others have experienced, the reception quality of MW signals under a bridge/overpass can be much worse than when such signals are received at a sufficient distance away from those structures.
By the principle of reciprocity, those same effects will apply ~equally to the performance of transmit antennas operating under a bridge/overpass or installed in shielded environments -- compared to MW transmit antennas installed sufficiently outside those environments.
Replying to Rich having said: "By the principle of reciprocity, those same effects will apply ~equally to the performance of transmit antennas operating under a bridge/overpass or installed in shielded environments -- compared to MW transmit antennas installed sufficiently outside those environments."
That's what I thought in the first place.
This thread seems to have branched away from whatever it was about before being about what it's about now.