Mram,
Thanks for sharing your project and the link to the picures and description are very nice.
There is certainly a lot to be gained by experimentation and it is nice that you shared yours with us.
Mram,
Thanks for sharing your project and the link to the picures and description are very nice.
There is certainly a lot to be gained by experimentation and it is nice that you shared yours with us.
As we experiment and build and test antennas, it seems there is someting missing though. I recently reported on my design for a colinear antenna but I apologetically could not report on its performance since I know of no basis for comparison to others or a workable practical field strength measurement.
You did report a 1/4 mile good signal range with yours which is probably significant. But since you mentiioned the Thomas Edison approach I began to ponder this.
I do not mean to detract from what you did and I think you should keep experimenting, but Mr. Edison did not do things the way many have been led to believe. When he developed the electric light bulb, he tried many filaments, many vacuuums, many currents and voltages. He was happy when one of his bulbs burned for 2 days….this was success since previously his filaments burned for seconds. According to my readings, he knew what materials were possilbe filaments and which were’t and began to make selections based on the theories of vaporization, combustion, and electron emission.
The key feature from Mr. Edison is that he compared one design against another and decided which was better based on experimental data. But his experiments showed that carbonized cotton did not work as well as other materials He knew this because he did experiments with different materials and by measurement, some were better than others. He was guided by experience and theory of materials.
From these data he was guided to the the best electric incandescent bulb that he could produce. It became commercially viable, yet it was later vastly inproved by researchers at General Electric who discovered how to draw tungsten to a filament which was far superior to Edison’s carbon threads. Had Edison not laid the groundwork, GE would not have perfected it.
Edison was not a person who just tried things until they worked, his experiments were based upon known science and he knew where he was going. A common misunderstanding of Edison is that he just threw things together, stirred the parts on the table with a soldering iron, and chose those which worked. If you visit the recreation of Menlo Park N.J. at the Ford Dearborn Museum in MI you will get a sense of the orginization and science he applied to his research and that he had funding, a machine shop, glass blowers, and a large scientific staff at his command..
So, taking some guidance from Edison, you have devised an antenna system and have published it here for us to review. You claim, and I do not doubt that it works. The problem with all of us who post our ideas is that we have yet to develop an objective way to compare our results. Edison measured the time that his lamp filaments burned objectively, yet we only subjectively measure the performance of our antennas. I would like to compare my antenna to yours so we need to come up with a way to objectivaly accomplish this. I am open to ideas.
Richard Fry has posted here the theortetical maximum field strengths for a base coil loaded antenna working against buried ground radials in normally conductive soil. Here is the target. We need to develop a repeatable means of measuring our antenna design performance against a standard whereby we can judge, as did Edison, which one works better than another. It is highly unlikely that any of us can exceed the established theoretical maximum performance predicted by the theory as presented by Mr. Fry but wouldn’t it be fun if we did? We don’t need to have FCC accurate measurements of field strength, we just need to come up with some way to have repeatable and reproducable readins.
Your contribution to the body of knowledge is important and apprecitated. You have designed and built a workable antenna which others can use, however we still do not know if it is optimum as measured against the theoretical objective. Keep your antenna set up until we can agree on how to compare it with other designs and with the theoretical ideal.
We can learn for Mr. Edison, and we can learn from theory.
Neil
More on antenna performance measurement
Lee posted the following on another thread and I copied it here so I could reply.
[quote]… I dont think there will ever be any scientific way of comparing antenna performance at two different locations,there are way too many variables! If you had set up an antenna test range and brought various antennae there to test, that would be scientific,but not very practical. So I guess the best we can use for comparison is the effective range, again there are many variables. So the best we can do is judge the final result,tweaking and experimenting and checking the range for relative improvement. This is just a hobby after all, most cannot afford any decent test gear, so we just tune for maximum smoke and keep on playing. I am looking forward to hear how it works out in the end, let us know!
Regards,Lee
WILCOM LABS – January 9, 2007 – 15:06
[/quote]
Your mention of the difficulty in obtaining antenna test range measurements is correct as is your comment about different locations affecting the antenna performance. What I have in mind is not strictly scientific but rather a means where experimenters can get a basis for comparison other than subjectively stating range.
For example, if we could agree on a cheap receiver model which is readily available and which could be easily tapped so the AGC voltage can be monitored with a $5 DVM then we could have a basis for comparison not only between experimenters but for testing and optimizing our own antennas. I did this in 1959 with my part 15 AM system using a modified car radio so this is not advanced technology.
For example, if I reported my signal at 100 feet produced an AGC voltage of .7 volts on a GE model xyz oriented for maximum voltage, that could be replicated by someone else with their antenna and a comparison could be made. It would be even more reliable if I could report I changed something on my antenna and the voltage went up to .9 volts. I would know for sure that it is an improvement.
Granted, there are confounding factors, but I think this would be better than just saying “my signal goes 1/2 mile”.
Neil
Its all relative
Its all relative,but only to that particular station and would not have any meaningful correlation to another station. You can take measurement “x” and tweak your system relative to that reading and improve it but even with identical receiver/meter or whatever you use,its still an uncalibrated reference with no meaning or basis in science. Even the differences between the same model radios are too great to be reliable. You cannot change that,unless maybe you are Mr. Einstein. I agree,however,that we need a way to do this. Maybe as a group we could purchase a quality calibrated field strength meter and reference antenna and pass it around. I think the different frequencies used will nuke that idea as the instrument probably has a limited frequency range,the antenna will definately be frequency specific. But dont stop searching for the holy grail,we will all cheer you on!
Until that happens,the best comparison will be the range,after all,that is what is sought after and has meaning we can compare somewhat. I will tell you that I use a fixed reference antenna here to do field strength monitoring with and it makes a fine tuning aid for relative readings but is not consistent over time or with weather and/or atmospheric changes.
Regards,Lee
Excellent Idea
Lee,
The limitations you mentioned are well know here and I agree, but you have hit on what I think is an excellent idea. I would not hesitate to spend the shipping cost to pass such a homebrew meter to the next user.
As you mentioned, each user can have his own FSM such as I have here, but the problem is how does it compare with those that others use. If we all used the same meter then at least the relative readings could be useful.
If we consider the frequency of use regarding a new antenna installation, and not just for fine tuning, it looks as if your idea of a pass around meter is workable.
It need not be calibrated to an absolute standard;, the important point is that it could be used as a basis of comparison for assessing one design vs. another.
I will be willing to design or collaborate on such a device if there is enough interest.
So we all understand what is being proposed, we can make available a device which will indicate a relative field strength on AM and it will be shipped upon request to any who wish to compare their antennas with others.
Please, if you are interested and think this is worth pursuing, post your comments.
The real test of technical advancement comes from comparative measurements between different designs and I think this approach may advance the art.
Neil
Available units???
Are there any available units that are sensitive enough for our needs? I will start searching my library for plans and specs….we could build a couple of identical units and calibrate them together and double the coverage and availability. I have a ton of parts(actually,several tons!!!)available to the project as well as a lab calibrated source. This will be a small and lightweight meter,cheap to ship! Anyone have schematics/plans?
Share them now,pleeze….
Any thoughts on one for FM?
Regards,Lee
Field strength meter project
Lee,
I also have parts, assembly equipment, and test equipment and I have some ideas we might discuss. I invite you and others who are interested to email me at:
[email protected]
and we can go from there. It would be nice to do this on this board, but I think to get things going PMs are a bit better.
Neil
Field strength meter project
Engineering calculations show that a ground-mounted, base-loaded, resonant, 3-meter vertical monopole with 80 mW of applied transmitter power and an excellent r-f ground system, and operating at about 1600 kHz can produce groundwave fields of several millivolts per meter over clear paths within 500 feet of the antenna site. Those field strengths are fairly substantial. It wouldn’t take a highly sensitive field strength meter to detect them, but the meter would need to be tunable to the measured frequency in order to reject local AM stations from the measurement.
The bad news is that the measured field strength will be highly dependent on the quality of the r-f ground, the r-f power that the transmitter can deliver to the feedpoint of the antenna system, the operating frequency, and the installation/propagation environment (overhead wires etc). It also will be highly affected by the existence and configuration of any radiating ground conductor/flagpole/mast/billboard steel, etc, which consists of the entire conducting path to the r-f ground — not just the short wire at the transmitter often called and considered to be the entire “ground lead.”
So the measured field strength will tell much more about the installation and operating conditions of a 3-m, loaded, resonant monopole system than about the radiating performance of that monopole, itself.
I don’t want to discourage anyone from proceeding in the project, but to provide some realistic information that might be useful during its development and use.
//
FSM limitations
Rich is correct regarding potential problems with comparing field strength measurements at location A with those of B, as well as separating the effects of the antenna from the rest of the factors. The differences in transmitters and operator skill in tuning also come into play.
It is certainly necessary to know the limitations of any measurement and it may be that this in not worth pursuing. Or, if we can get a pass around meter set up and gain some experience, we can either refine the data analysis to account for covariants or we may conclude it is not worthwhile.
If, as I envision, the meter will be based on a cheap pocket radio, then at the least we might be able to provide a simple design which interested hobbiests could build for themselves and use for their own relative field strength measurements to assess changes they make to their stations.
I worked in research and we had two sayings which I present here just for entertainment.
“With the data you do not have we will establish what you do not know”.
“No data is bad, bad data is worse”.
Keep thinking about the project.
Neil
Nothing to fear but fear itself……
So far,from what I see being used in the profession,here is the basic breakdown. Reference antenna,nothing special except it has been calibrated(well,correction factored)for the intended range of frequencies of interest on an antenna range with NIST traceable equipment(THE SCIENCE!).A selective receiver tunable for the range of interest and metered in volts or db. Most any receiver will do and a quick calibration on any good sig gen will do. Next we will need a couple of attenuators,easy.With this equipment,real F/S readings are possible!!!
NOW,the antenna and its correction/freq table is what will be hard and expensive as it requires the services of that antenna range.Would someone voulunteer to contact a few sources to see what it costs for this service? I would start with TX/RX,Decibel,Celwave or whatever they are named this week. I know one of them has an antenna range in Angola,NY as I used them before to proof a Bogner antenna pattern. Beyond that we’d be guessing. As my grandpap would say”good work aint cheap and cheap work aint good!” Your comments guys??? Regards,Lee
Nothing to fear, but to be understood…
[quote=WILCOM LABS]So far,from what I see being used in the profession,here is the basic breakdown. Reference antenna,nothing special except it has been calibrated(well,correction factored)for the intended range of frequencies of interest on an antenna range with NIST traceable equipment(THE SCIENCE!).A selective receiver tunable for the range of interest and metered in volts or db. Most any receiver will do and a quick calibration on any good sig gen will do. Next we will need a couple of attenuators,easy.With this equipment,real F/S readings are possible!!![/quote]
Below are some earlier comments of mine on this topic. They also apply here, even though they address VHF measurements. They are relevant to MW also when using a meter designed and calibrated to measure conducted voltages.
________
The problem with a cable TV meter is that it is not calibrated to measure radiated fields. The meter scale on it will be calibrated either in units of voltage (mV, dBu, etc), or power (mW, dBm, etc). None of those units is a measure of field strength, such as µV/m. These units are not interchangeable.
Certainly if you connect a cable meter input to some kind of an antenna, the meter may show a reading. But it won’t be valid, because the efficiency of that receiving antenna at converting the arriving field strength into whatever the cable meter reads is unknown.
This is why true field strength meters include a receiving antenna that has been calibrated along with the meter in a standard field, and that antenna must be used with that meter in order to provide accurate measurements of field strength.
I think part of the confusion here is that r-f fields commonly are referred to in terms of mV, uV, or dBu. But the accurate forms of those terms are mV/m, µV/m and dBµV/m. A free space field has to be related to the distance across which that value exists — that’s why the “m” (for meter) needs to be there.
The “dBu” term, for example, may also appear as a measuring unit on the cable meter, although in that case it means decibels with respect to 1 microvolt (should really be dBµV).
But because of this, it is a common error to consider that radiated field strengths and conducted voltages are measured in the same units. They are not.
(added later) I just did some calculations for the 100 MHz field strength that a typical, matched 1/2-wave dipole would need to see in order to produce a reading of 250 µV on a cable meter.
If the cable meter reads 250 µV in its 75 ohm environment, that means that a power of 0.0033… mW (-24.7 dBmW) exists in the measuring circuits of the meter (P = E^2/R).
A matched, 1/2-wave dipole would need to be immersed in a radiated field of over 32,700 mV/m in order to produce the amount of power needed by the cable meter to show a reading of 250 µV.
Of course, that is nowhere even remotely close to being an accurate measure of that field.
//
Exactly
You are exactly right,the antenna (and meter) need calibrated to field strength readings in uv/m on an antenna range to be accurate. Actually,they dont change anything,rather they supply you with a chart to convert the reading to what is measured on their range. Once that is established,there is a formula to scale it to any frequency of interest. This is probably beyond the scope of our little project due to costs. So the best we could do is build a relative reading system and pass it around. If we build more than one and match them well,we could build a relative database that may be of some use. Maybe someone at some point might have a chance to compare our results to a calibrated unit or get one of them range tested and validate our data. I dont have any other hope for any kind of sanity in measurements,do you agree??? Regards,Lee
Validating Field Strength Measurements
[quote=Wilcom Labs]…So the best we could do is build a relative reading system and pass it around. If we build more than one and match them well,we could build a relative database that may be of some use. Maybe someone at some point might have a chance to compare our results to a calibrated unit or get one of them range tested and validate our data. I dont have any other hope for any kind of sanity in measurements, do you agree???[/quote]
My thought is that even if everybody had a professional quality field strength meter with NIST accuracy/traceability, there still would be no way of separating the intrinsic performance of the 3-meter radiator from the effects of its installation and the ability of the tx to drive it with a known power level — as I described in my earlier post.
Those are the factors that to a great extent determine the field strength that will be generated by that system — and not the electrical qualities of the 3-m, resonant radiator, itself.
//
Yeah,but
Yeah,but isnt that what we want,the whole picture? Again,all results will be relative to the particular installation. THAT IS WHAT THE FCC WILL MEASURE! Einstein couldnt change that and neither can I. So I guess I am done with this idea and done with this thread. Its been real…….
Regards,Lee
Still..
Well, still it’s not a bad idea. If one takes the original idea of some sort of an inexpensive reciever and meter, it’s true that the readings taken with it will always reflect more than *just* the antenna’s operation. And that the readout may not be verifiably translated exactly so such a simple tool could replace something like a precision calibrated FSM.
But that doesn’t make it useless or anything. Just useful within a reasonable expectation of it’s capability.
Ok, say person X builds one. They use it to make some kind aof a measure of their system’s output (“system” including antenna, transmitter, soil conductivity, etc). Say at the sime they have no buried radials. They add 8 buried radials to their system and see a significant increase on the readout of the tool at the same distance. That *is* useful and interesting information. If they pass it to person Y who conducts a similar experiment and sees an increase of some degree when they add 8 buried radials (it probably won’t be exactly the same increase because soil conductivity, transmitter and the surrounding terrain will be different), then there is data which begins to suggest that at least 8 buried radials are a good idea in most cases.
(Note: I intentionally picked buried radials since there seems to be a general agreement that for the 3M vertical antenna they are helpful. This was done to avoid possible sidetracking as to whether something less generally accepted here was useful or not when that is not the point of this example)
Enough people do that with the same piece of test equipment, and while you might not be able to say exactly how much it will help, it can become a recommended procedure to try for someone trying to improve their range. It doesn’t *have* to translate exactly into mv/M or db or whatever a professional calibrated FSM reads out in to see if there is an increase or decrease when an experiment is attempted.
Similarly, if X measures with that piece of equipment at a distance of 10 M and gets a certain reading from their system, and Y measures at 10 M from Y’s system and gets a significantly higher reading, then it’s worthwhile to compare notes.
Now, if there were a couple such test rigs that could be sent around when someone does a new installation or tries a new project, we might collect some data which would be useful in eventually developing some theories to try on what may be useful and what variables (of those which can be changed, like number of buried radials) to try when someone is trying to get better range.
Alternatively, with a simple test jig like we’re discussing, another feasible way to do it would be for one person to build two and calibrate them to read at least very close to each other, then put one away as the “standard”. Anyone who wants one could build it since the parts are inexpensive and send it to the person who keeps the “standard” with return postage and maybe a couple bucks for the person’s bother so the one they built could be calibrated to be close to the “standard”.
Then readings taken from different stations might begin to suggest what sort of things work best. Sure, it’s still pretty relative and not comparable to what a meter costing thousands could do. But it’s achievable and practical enough and could result in usable data. Maybe not data precise enough to set any new lab standards, but enough to be of use and interest to many of the hobbyists here.
Kind of like back in high school or college chem or physics lab. Sure, there are such things as precision scales, and some schools have one. But the old triple balance beam is still a very useful tool and accurate enough for student experiments to be meaningful if they’re checked once in a while with a known weight.
So then if someone tries a new sort of antenna as an experiment and gets what appears to be better performance, it’d be interesting and might suggest that others try and see if it works for them since there’s at least some indication it might work better.
I think that’s what was originally being proposed?
One can’t expect 10$ or so of gear to do the job of a piece of gear costing thousands, it’s just not made to the same sort of specs or quality. But it could be good enough to see how X’s system is doing at 10 M from the antenna compared to Y’s system. Or how antenna A does vs antenna B at X’s station.
Useful enough to be of interest at least to some.
Daniel
Generate a calibrated signal?
How about instead of a calibrated receiver a calibrated signal source representing the 15.209 field strength and a step attenuater was what was passed around? With this set up db’s could be used for comparision and that would be more appropriate for discussion. The difficult part would be getting the generator calibrated and keeping the surroundings from changing it’s output. If this were possible then each person could calibrate there own receiver/antenna combination. This is probably very do-able for FM. Just a thought. 73’s
Meter
Ok,I will give this project a chance,been real sick and not myself lately. Can we agree on a slick and cheap meter or two or three and get them out in the field? Anyone have any schematics or build plans for a meter? I will continue my search here as well… Regards,Lee
Potomac Instruments FIM-41
You want to know accurately what your field strength is? Here is what you need:
http://www.pi-usa.com/fim2241/fim2241g
Unfortunately the current price is over $10K, but perhaps they can be found for less in the used equipment market.
WEAK-AM
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