I did not claim that reciprocity failure was responsible for field strength measurement error. I merely noted that reciprocity failure was not detected during numerous measurements of transmitting and receiving gain in an antenna lab over many years. Reciprocity failure became apparent only because of theoretical calculations. The 6 dB reciprocity failure over a ground plane was not noticed because of the normal variation that occurs in field strength measurements.
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Buried Radial Length Same as Monopole Height?
- Thread starter rfry
- Start date
Ermi Roos ink=topic=167593.msg1443051#msg1443051date=1273521167 said:... Reciprocity failure became apparent only because of theoretical calculations. The 6 dB reciprocity failure over a ground plane was not noticed because of the normal variation that occurs in field strength measurements.
Theoretical calculations need to be supported by real-world experience, if such theory is valid.
But that is not the case even as shown in Mr Roos' earlier posts here, "Trainotti had been the head of the Antennas and Propagation Division of CITEFA for 40 years, and he had made numerous measurements of both the receiving and transmitting gains of numerous antenna designs. ... He told me that he never noticed any difference between the transmitting and receiving gain of any antenna."
Neither has it been shown in the work of Brown, Lewis & Epstein in their classic 1937 I.R.E. paper, the paper of duTreil, Lundin & Rackley that I linked to earlier, or in the multiple thousands of other measurements made of AM broadcast station field intensities going back over 70 years.
It would be interesting to learn anyone's explanation for the reason why this theoretical 6 dB reduction in the gain of a receiving antenna over a ground plane was not detected and recognized under these circumstances, inasmuch as calibrated, medium-wave field intensity meters are specified for much better measurement accuracy than would be required to detect and prove a reciprocity failure of 6 dB.
RF
R. Fry said:Theoretical calculations need to be supported by real-world experience, if such theory is valid.
Isn't that essentially what we've ALL been Telling YOU about these part 15 systems?
I have to say that, although Ermi's discussions about his reciprocity calculations are interesting from an obscure technical view, they have zero relevance to the world of Part 15, which is the typical subject area here on Community Radio.
We deal with people listening on radios with ferrite rod antennas, little loop antennas stuffed behind a hifi receiver, or 3 ft antennas sticking up from a car. Reciprocity is irrelevant here.
We deal with people listening on radios with ferrite rod antennas, little loop antennas stuffed behind a hifi receiver, or 3 ft antennas sticking up from a car. Reciprocity is irrelevant here.
"Isn't that essentially what we've ALL been Telling YOU about these part 15 systems?"
hahahahahahahah....That's a good one! ;D
hahahahahahahah....That's a good one! ;D
LibertyNT said:Isn't that essentially what we've ALL been Telling YOU about these part 15 systems?R. Fry said:Theoretical calculations need to be supported by real-world experience, if such theory is valid.
The real-world experience that should concern unlicensed operators claiming to be compliant with "Part 15" AM and FM is the real-world experience of people who receive NOUOs from the FCC for operating non-compliant systems.
Physics and real-world data from many other sources can show why those non-compliant systems receive NOUOs, and therefore how such NOUOs may be avoided -- should people wish to do so.
RF
I am puzzled by PhilB's hostility to my comments about reciprocity. I have noticed that he tends to be a bit of a bully on this and other discussion boards. Reciprocity applies to all of the types of antennas he mentioned in his post.
System gain calculations are just as relevant to Part 15 as to any other form of radio communications. Without taking reciprocity into consideration, system gain of a Part 15 transmitter and receiver system would be overerestimated by 6 dB. It happens that reciprocity failure was rediscovered as a result of a discussion on a Part 15 website, and the theory will be presented to a wide antennas and propagation audience in two upcoming IEEE papers.
System gain calculations are just as relevant to Part 15 as to any other form of radio communications. Without taking reciprocity into consideration, system gain of a Part 15 transmitter and receiver system would be overerestimated by 6 dB. It happens that reciprocity failure was rediscovered as a result of a discussion on a Part 15 website, and the theory will be presented to a wide antennas and propagation audience in two upcoming IEEE papers.
C
carlvanorden
Guest
Once again, since I "invented" the use of ground radials on part 15am antenna systems, I'll chime in.
Sure, if you can run a mile of copper wire for each radial, in theory, it would be a good thing. But, 100mW just isn't worth it, and the signal won't travel far. So, I decided 8 or 9 years ago (against the better judgement of the engineers who post here, who at the time, said that ground systems were illegal) that just running 10 feet, and about 10 of them would be practical for the use of stablizing the signal and mostly for the use of lightning protection. Of course there was all the talk about grounding out at an outlet or around some water pipe which may or may not help you at all for protection of your transmitter.
A short story: when I first started to experiment with the AMT 1000 (sstran) I lived in a metal, mobile home, which sat on what basically was very rocky, clayey soil. Not a lot of conductivity there, however I was right next to a nice sized river.
And, in a river bed, where all the land around my station was above where my antenna was. I like the antenna mounted in the ground, so my station was literally 20 or more feet below the average terrain. The use of ground radials (not chicken wire) was my choice to just 'see' if it helped.
Ironically, my third or forth experiment ended up with an antenna system that seemed quite directional. But, it was weird; I put the antenna 'in front' of my mobile home, with 10-10 foot long radials surrounding it. Surprisingly, my signal was strongest behind my home, meaning the signal may have been strenghtened BY the mobile home. And that is not where I wanted the signal to go since no one lived behind me! I settled on a site behind the mobile home, and again, ironically this was a lower patch of ground which collected and retained rain water. Again, only about 10 feet away from the home, I spread out my radials, and behold!...I had major signal in front of the home in the direction of living beings. ....and very little signal behind the home.
AM radio is fickle, but I learned a lot just by experimenting with different things even if just for an afternoon.
Part 15am is '@#$%'ed" at night, no matter what frequency you use, but, during the day, you can still get a nice signal provided you put a few wires 1/2 inch into the soil, hopefully into wet soil *, and put it in the right place.
*I had a customer several years ago ask me if I thought that laying the radials in a shallow ditch, layered with styrofoam insulation pellets (when it rains these would retain moisture) would work. What a fantastic idea!...especially in dry areas and areas of poor conductivity. Never did it, because I do have a bit of water where I live, almost constantly, so that works for me. But, wow. Simple idea, and it works. Again, the most important thing, safety is at hand here, to make sure you are grounded out well, so no one gets hurt. Cause, it can happen. I was working on my antenna (cause I'm stubborn) and a storm came up and I did get a little shock...I'm lucky. Maybe it wouldn't have came out that way. So be careful, stay legal and you will have the time of your life if you love legal low power broadcasting like I do.
Sure, if you can run a mile of copper wire for each radial, in theory, it would be a good thing. But, 100mW just isn't worth it, and the signal won't travel far. So, I decided 8 or 9 years ago (against the better judgement of the engineers who post here, who at the time, said that ground systems were illegal) that just running 10 feet, and about 10 of them would be practical for the use of stablizing the signal and mostly for the use of lightning protection. Of course there was all the talk about grounding out at an outlet or around some water pipe which may or may not help you at all for protection of your transmitter.
A short story: when I first started to experiment with the AMT 1000 (sstran) I lived in a metal, mobile home, which sat on what basically was very rocky, clayey soil. Not a lot of conductivity there, however I was right next to a nice sized river.
And, in a river bed, where all the land around my station was above where my antenna was. I like the antenna mounted in the ground, so my station was literally 20 or more feet below the average terrain. The use of ground radials (not chicken wire) was my choice to just 'see' if it helped.
Ironically, my third or forth experiment ended up with an antenna system that seemed quite directional. But, it was weird; I put the antenna 'in front' of my mobile home, with 10-10 foot long radials surrounding it. Surprisingly, my signal was strongest behind my home, meaning the signal may have been strenghtened BY the mobile home. And that is not where I wanted the signal to go since no one lived behind me! I settled on a site behind the mobile home, and again, ironically this was a lower patch of ground which collected and retained rain water. Again, only about 10 feet away from the home, I spread out my radials, and behold!...I had major signal in front of the home in the direction of living beings. ....and very little signal behind the home.
AM radio is fickle, but I learned a lot just by experimenting with different things even if just for an afternoon.
Part 15am is '@#$%'ed" at night, no matter what frequency you use, but, during the day, you can still get a nice signal provided you put a few wires 1/2 inch into the soil, hopefully into wet soil *, and put it in the right place.
*I had a customer several years ago ask me if I thought that laying the radials in a shallow ditch, layered with styrofoam insulation pellets (when it rains these would retain moisture) would work. What a fantastic idea!...especially in dry areas and areas of poor conductivity. Never did it, because I do have a bit of water where I live, almost constantly, so that works for me. But, wow. Simple idea, and it works. Again, the most important thing, safety is at hand here, to make sure you are grounded out well, so no one gets hurt. Cause, it can happen. I was working on my antenna (cause I'm stubborn) and a storm came up and I did get a little shock...I'm lucky. Maybe it wouldn't have came out that way. So be careful, stay legal and you will have the time of your life if you love legal low power broadcasting like I do.
Here is a pre-publication copy of an IEEE paper on reciprocity failure, which is expected to be published in the IEEE Transactions on Broadcasting in September, 2010. This paper, and another one that will follow, resulted from a discussion on a part15.us between Richard Fry and me. The authors mention me on the first page. Prof. Trainotti and his graduate students are in the process of preparing the second paper (which is about horizontally-polarized antennas) for publication.
http://svn2.assembla.com/svn/tesis_gfigueroa/paper_BTS_09_143/paper/paper.pdf
http://svn2.assembla.com/svn/tesis_gfigueroa/paper_BTS_09_143/paper/paper.pdf
In case my link to the IEEE paper in my previous post does not work, please google the title of the paper (with quotation marks), which is "Vertically Polarized Dipoles and Monopoles, Directivity, Effective Height and Antenna Factor". Adding +PDF (no quotation marks) will narrow the Google results to the pdf file that I linked.
http://svn2.assembla.com/svn/tesis_gfigueroa/paper_BTS-09-143/paper/paper.pdf
Here is the corrected link to the paper mentioned in my previous two posts in this thread. I apologize for any inconvenience. The paper has 34 pages and uses 3.5 MB of storage space. Prof. Trainotti is now a Fellow of the IEEE, and so his IEEE member rank will probably be corrected at the time of publication.
Here is the corrected link to the paper mentioned in my previous two posts in this thread. I apologize for any inconvenience. The paper has 34 pages and uses 3.5 MB of storage space. Prof. Trainotti is now a Fellow of the IEEE, and so his IEEE member rank will probably be corrected at the time of publication.
Ermi, I am sure this will be a very interesting read. Thanks for posting it!
During recent experiments at my lakeside location, I did not have good results. I tried siting a Rangemaster transmitter about 25' from the shore with 8 radials. Six were 25' long and two were 20' long. This was dictated by the space available. Experimentally I tried extending one of the radials to a full quarter wavelength (150 feet) laying on the bottom of the lake. This appeared to make a very slight improvement. In any case, reception was not very usable beyond a few hundred feet.
Then I moved the transmitter out to the end of my dock, which has three steel sections each 10' long. Each section is supported by two aluminum posts. I would estimate that this was about 10 dB better than the onshore location. To this, I added the single quarter wave radial laying on the lake bottom, and again observed a slight improvement. The signal was audible around most of the lake, but near the noise level on a GE superadio III. The maximum distance would be about 1 mile. It could not be heard in an area of the lake that is shielded by a bluff between my house and that area.
One day there was a light rain shower and I happened to take a drive into town that day. On my way home, I was surprised to hear the signal almost a mile further than before. I assume that this is due to improved ground conductivity after the rain. Within a few hours everything dried out, and the range returned to the normal distance.
I measured 27 dBu on my Tecsun radio at a distance of 500 feet from the Rangemaster while it was mounted on the dock, at a frequency of 1610 kHz. At the same location I also measured a local station on 1520 kHz about 8 miles away that has a power of 400 watts. It produced a reading of 34 dBu. A second station on 1660 kHz that has a power of 10 kW produced a reading of 30 dBu. It is about 30 miles away. The ground in this area is very sandy and I assume that the conductivity is poor.
This would tend to confirm my suspicion that low ground conductivity does indeed affect the coverage of a Part 15 station at relatively short distances (i.e. 1-2 miles).
During recent experiments at my lakeside location, I did not have good results. I tried siting a Rangemaster transmitter about 25' from the shore with 8 radials. Six were 25' long and two were 20' long. This was dictated by the space available. Experimentally I tried extending one of the radials to a full quarter wavelength (150 feet) laying on the bottom of the lake. This appeared to make a very slight improvement. In any case, reception was not very usable beyond a few hundred feet.
Then I moved the transmitter out to the end of my dock, which has three steel sections each 10' long. Each section is supported by two aluminum posts. I would estimate that this was about 10 dB better than the onshore location. To this, I added the single quarter wave radial laying on the lake bottom, and again observed a slight improvement. The signal was audible around most of the lake, but near the noise level on a GE superadio III. The maximum distance would be about 1 mile. It could not be heard in an area of the lake that is shielded by a bluff between my house and that area.
One day there was a light rain shower and I happened to take a drive into town that day. On my way home, I was surprised to hear the signal almost a mile further than before. I assume that this is due to improved ground conductivity after the rain. Within a few hours everything dried out, and the range returned to the normal distance.
I measured 27 dBu on my Tecsun radio at a distance of 500 feet from the Rangemaster while it was mounted on the dock, at a frequency of 1610 kHz. At the same location I also measured a local station on 1520 kHz about 8 miles away that has a power of 400 watts. It produced a reading of 34 dBu. A second station on 1660 kHz that has a power of 10 kW produced a reading of 30 dBu. It is about 30 miles away. The ground in this area is very sandy and I assume that the conductivity is poor.
This would tend to confirm my suspicion that low ground conductivity does indeed affect the coverage of a Part 15 station at relatively short distances (i.e. 1-2 miles).
audioguy said:This would tend to confirm my suspicion that low ground conductivity does indeed affect the coverage of a Part 15 station at relatively short distances (i.e. 1-2 miles).
Interesting reports, audioguy, which lead me to go back to the worksheets I used to develop the paper linked below.
Considering the coverage circle plotted in green in that paper, the distance to the 150 µV/m field shown at 0.4 miles there would decrease to 0.266 miles for an earth conductivity of 1 mS/m (other things equal). This distance is based on the FCC groundwave propagation charts for that field, conductivity, frequency, and radiated power.
If your normal conductivity was 1 mS/m but after a rain it improved to 5 mS/m, that would increase the distance to the 150 µV/m contour by about 50%.
So theory tends to support your experience.
http://i62.photobucket.com/albums/h85/rfry-100/150_microvolt_per_meterRadius_Part_.gif
RF
I have a couple of small corrections to make to the above report, after using the Google Maps Distance Calculator to make more precise measurements. The distances are still somewhat imprecise because all of the features cannot be seen clearly on the satellite imagery available.
The distance from the transmitter to the measuring point should be 608 feet rather than 500 feet. The distance from the measuring point to the station operating on 1520 kHz should be 9.3 miles rather than 8 miles. The distance to the station operating on 1660 kHz is correct as shown (30 miles).
Next time, weather permitting, I will take additional readings at other points around the lake and report on the results. I would also like to try a capacity hat antenna in place of the CB whip. This should be very interesting if I can get the materials together in time. The hat will consist of 6 radial spokes each 1 meter long. The vertical antenna span will be shortened such that the hypotenuse of the triangle formed by the vertical and horizontal spans does not exceed 2.5m.
The distance from the transmitter to the measuring point should be 608 feet rather than 500 feet. The distance from the measuring point to the station operating on 1520 kHz should be 9.3 miles rather than 8 miles. The distance to the station operating on 1660 kHz is correct as shown (30 miles).
Next time, weather permitting, I will take additional readings at other points around the lake and report on the results. I would also like to try a capacity hat antenna in place of the CB whip. This should be very interesting if I can get the materials together in time. The hat will consist of 6 radial spokes each 1 meter long. The vertical antenna span will be shortened such that the hypotenuse of the triangle formed by the vertical and horizontal spans does not exceed 2.5m.
About 20 years ago I had a chance to operate a part 15 test station on 1625 khz from an abandoned AM broadcast site. I utilized a home built solid state CW transmitter connected to a 3 meter mast with a 1 meter diameter top hat. The transmitter/antenna sat on top of the concrete base of the former AM tower and used the existing ground system of 120 radials, approx. 160' long. At the time I was interested in propagation and not programming so it was operated as a CW beacon transmitting morse code. I received reception reports from five states with 100mW of input power! This was in CT where the states are pretty close together and the use of a narrow "modulation" such as CW allows extremely narrow receive bandwidths to be used so coverage with AM would have been much, much less but there was no doubt in my mind that the extensive ground system helped a great deal.
TPO said:About 20 years ago I had a chance to operate a part 15 test station on 1625 khz from an abandoned AM broadcast site. I utilized a home built solid state CW transmitter connected to a 3 meter mast with a 1 meter diameter top hat. The transmitter/antenna sat on top of the concrete base of the former AM tower and used the existing ground system of 120 radials, approx. 160' long. At the time I was interested in propagation and not programming so it was operated as a CW beacon transmitting morse code. I received reception reports from five states with 100mW of input power! This was in CT where the states are pretty close together and the use of a narrow "modulation" such as CW allows extremely narrow receive bandwidths to be used so coverage with AM would have been much, much less but there was no doubt in my mind that the extensive ground system helped a great deal.
Plus the fact that CW puts ALL the energy into ONE frequency, not dividing it into sidebands.
Much easier to detect with a BFO and there can be no issues with "low modulation".
TPO, that is the Part 15 experimenter's DREAM SETUP! Did you attempt any AM test transmissions at all? I have found that a fully modulated test tone is audible for a significantly greater distance than normal programming, particularly in the presence of co-channel interference, because the human ear is capable of acting as a very selective filter.
I would have loved to have been there when you ran those tests!
I would have loved to have been there when you ran those tests!
When I first put the digtal reverb into my airchain, one day It had somehow picked up enough rf to begin feeding back on one audio frequency, maybe about 1000 hz. I was checking on the signal as I was on the way home from work one morning, and I began to hear the "single note fully modulated" at just about one mile, where normal modulation isn't audible until about half a mile. I was aghast, and hung a bunch of snap-on ferrite beads on the wiring as soon as I got home.
TPO said:About 20 years ago I had a chance to operate a part 15 test station on 1625 khz from an abandoned AM broadcast site.etc
Modeling that radiator setup in NEC using a 100% efficient transmitter (100 mW output power), a 5-ohm coil loss, and a zero-ohm connection to a perfect ground plane shows that the radius to the 150 µV/m groundwave field would be about 4.8 km (~3 miles).
So even with these unrealistic parameters the useful service area for AM broadcasting probably would not be all that is expected/hoped for. The useful service area would be considerably smaller when using practical values for the tx output power, r-f ground connection, and ground conductivity.
The EZNEC file leading to this conclusion is available to anyone -- just PM me with an e-mail address to send it to.
Also just to note that the average power of the carrier of an AM transmitter remains constant with modulation.
If the transmitter uses high-level modulation then the additional power present in the AM sidebands is provided by the modulator. This design permits the final r-f stage to operate with an efficiency of 70% or better.
If the transmitter uses low-level modulation, then the output stage of the transmitter must be linear, and operated at a carrier power level that permits it to amplify the AM sidebands without exceeding its ratings. Efficiency of this design is around 30-35%, typically.
RF
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