This:
You wrote: "It is evident that you have never done or been involved with FCC compliance testing." Reading this literally would indicate that the statements should be summarily disregarded due to incompentence on the part of the poster.
You may not see this as a personal attack and that may not have been your intent, but I see it as personal when one engages in a discussion by questioning the credibility of those participating rather than addressing the issues and technical facts. I trust you understand why I responded as I did.
Neil
That was inappropriate. I aplogize for the comment. I was simply trying to get the readers to look beyond what appears to be impossible and give them the tools that are within the grasp of many. As you recall, I did state that the measurements would not be lab quality but would give the "experimenter" some idea as to what's happening beyond a guess. I also said I expected to catch flak which I certainly did.
You have met with the same success, as many of us on this forum have, with trying to be reasonable with Rich. (etc)
Mr. Johnson, when I read a post here having technically incorrect information in it that could mislead readers into a course of action that clearly could put them at risk.
"-the tone of some of the posts on the forums needs to change. Posts that are positive and encouraging are welcome, no matter what the content or message. Posts that stray too far from the benchmark of positive and encouraging will be considered suspect. If I see a suspect post I will write the author and discuss the matter." -SCWIS
Rich,
I get it. But, I don't think you do. As I have posted here before, you have an exceptional mind and tons of intellect when it comes to the science of radio. I was just hoping you could find some people skills to bring along side your obvious radio skills. I would like you to consider other peoples feelings when you unload on them. And you do unload on them.
I completely support your efforts to keep us on the straight and narrow. Maybe you have heard the old saw, "It isn't what you said that did the damage, but the way you said it." Technical and scienctific correctness does not give anyone license to unload on people, right or wrong. Help us to understand how to translate what you know into a "homebrew" low-buck solution rather than intimating contributors on this forum that don't know as much as you do. I don't care if you are Mr. Know-It-All. The rest of the humans here would like to think you are our friend.
I don't have all the answers. I don't even know most of the questions. But like most folks here, I am trying to learn and grow in the ART of radio. Many of us have not had your advantage of education. And just because a person has complete knowledge of a subject area does not make them a teacher. It takes knowing people to be a great teacher. I know. I have made that same mistake several times. But, I have learned over the years, the student comes first, then comes the learning. Maybe learning a different teaching style would help you share what you know with the rest of us. I enjoy your input, I hope you keep trying.
Marshall Johnson, Sr.
Senior Pastor, President
Rhema Christian Fellowship, Inc.
Rhema Radio - The Word In Worship
AM 1660 - FM 93.5
http://www.rhemaradio.org
wdcx recently mentioned this old thread elsewhere on this Forum. I think that it is worthwhile to take another look at it.
The premise of the proposal in this thread is that a cable meter can give a rough estimate of FM field strength. It is not claimed that the results are a kind that the FCC will accept; but they give the Part 15 FM operator some useful information, anyway. Even the results from a recognized test lab are +/- 4 db, which are not great. Well, the results with this method will be even worse.
The only thing in wdcx's proposal I would change is the pickup antenna. I think that the proposed car radio whip would result in a big underestimate of the field strength, because there would most likely be a big impedance mismatch between the antenna and the meter. A half-wave dipole would be much better. The free-space impedance of the dipole is 73 ohms, which is a good match to the input impedance of the meter, which is 75 ohms. Of course, the dipole will not actually be in "free space," resulting in a lower antenna impedance due to the proximity to ground. This mismatch will reduce accuracy, but the method has already been said to be inaccurate.
It is fortunate that a half-wave dipole in the FM band has an "effective height" of around 1 meter. This causes a field strength of 250 uV/m to produce in the vicinity of 250 uV at the antenna terminals, if they are open-circuited. However, the antenna wll actually be terminated with the 75 ohm input impedance of the meter, which will reduce the voltage applied to the meter to about125 uV. Thus, 6db needs to be added to the field strength reading. I'd add yet another 6 dB to account for other errors.
The reading is likely to be in dBuV or dBmV; so a conversion to uV or mV has to be done.
Agreed.
1. And PLEASE take this with the humor intended... Would one of the brainiacs on here build a small transmitter/ antenna combination meant to be mounted on a pole 3 or 4 wavelengths high, and calibrate it for the uv/m or whatever field strength is allowed on FM. Build a wooden crate to ship it back and forth in. Give it to a volunteer who will accept deposits to insure it's safety and "loan" it out to members of the communuty to run comparison tests to adjust their own equipment.? Please? I know I know there are many variables but it would be a start.
2. OK as promised..Jeeeze this is so simple I can't understand all the problems. Cover your community, promote hope and change, physics challenged green projects and any other gee-whiz thing you want 🙂 note smiley... Mount your part 15 compliant transmitter on a 200 foot tower. yes at 200' you wont reach the ground but.. gather up (oh yes recycle) the old, obsolete TV antennas from around your community (village) and build 25 db gain antennas for everyone to hook to their home FM sets. Give them away based on income lol. Aim those antennas at your tower. You should get several miles legal coverage and at the same time eliminate all competition (pretend your the government). It's not how loud you talk, it's how good the listener hears. Start your programming. The FCC does not limit receiver gain! By the way this would work on AM too but I'm not sure the real estate is available for most people. It would do my old heart good to see antenna pollution on top of homes again. 🙂 Dave
I have proposed a couple times on here the idea of approaching this FS problem from the opposite direction and no one has ever commented on it. A loaner reference setup for those lacking a calibrated field strength meter i.e. everyone. Is it a bad idea? Some simple directions stating how far far away from other objects (note: not how many wavelengths) the reference setup and the eventual installation would have to be to duplicate the original calibrated installation, etc. The installer would install the test setup in his proposed final location (following strict wavelength rules for separation from surrounding objects). Use whatever uncalibrated measuring instrument (ie receiver with meter) he wanted for relative readings all around the test setup, then install his own equipment and "pad" or cut the antenna to equal the test setup. I would propose that since "ultimate" FS readings are the goal, halo antennas be the standard for horizontal and omni direction radiating purposes. You might as well radiate in all directions and no vertically polarized vehicle would be listening anyway. There is the luxury of running as much power into the antenna as you want since it's the output that's measured. With proper directions would this be a closer approximation than any other real world (availability, expense, knowledge, etc.) solution or have I missed something so big as to blow this out of the water? Please don't hold back as I can take it since I am aware that the physics of this governs the response .. Dave
Looks like I'll need to go to FM because I'm moving to an all concrete downtown QTH. Sigh, guess I'll have to put my gizmo, two THs and box full of iron power toroids on ebay 🙁
Be sure to note that the last post before we got stated again was from 2006 - carry on 🙂
The same principles apply to 13.56 MHz. The problem is that a half-wave dipole at 13.56 MHz is HUGE, and and it is difficult to put one in position for field strength measurements. What could be done (if there is enough space) is that a half-wave dipole connected by coax to the signal strength meter can be hung in parallel to the transmitting antenna 30 meters away.
The effective height of a half-wave dipole at 13.56 mHz is about 7 meters. Taking into account that terminating the antenna with the 75-ohm input resistance of the signal strength meter reduces the meter reading by 6 dB compared to the open-circut voltage, the actual field strength in uV/m will be about 1/3.5 times the meter reading in uV.
The measuring dipole will be closer to ground (in wavelengths) than the FM measuring dipoles; so the actual antenna impedance will be appreciably lower than than 75 ohms. This will cause more error. I would recommend adding an additional 10 dB to the field strength readings to account for the reading errors.
Check the signal-strength meter specs to see if the meter's frequency response includes 13.56 MHz. I have seen a minimum frequency of 5 MHz specified. While the measuring antenna has to be a half wavelength long, the transmitting antenna can be a lot shorter.
As was true for FM, the FCC will not accept the results obtained with the signal strength meter. This is especially true at 13.56 MHz, because the FCC requres the use of a loop antenna for field stength measurements at frequencies below 30 MHz. The results will only give the operator some idea about what is going on.
To repeat what I said above, while the pickup antenna has to be a half wavelenth long in order to give this field strength measuring method any validity at all, the transmitting antenna can be considerably shorter.
Good luck getting the cable guy to lend you his signal strength meter!Here is a way you can get your own:
Save enough money to buy one of the two favored FCC certified Part 15 AM transmitters (Rangemaster or ChezRadio). Then, buy an SStran instead, AND a signal strength meter. [Unfortunately, you'll still need to get (or have) an FM or SW transmitter to make use of the signal strength meter in the manner suggested in this thread.]
Since the SStran is a kit, it is not FCC certified, because SStran cannot guarantee the performance of the product after it is constructed at home. However, it will comply with the Part 15 AM rules if built correctly.
You can cheaply roll your own transmitter, too, but you have to be prepared to experiment. The "improved" AM transmitter circuit on techlib.com is called the "Wenzel." It has a transconductance modulator, like in the SStran, and an RF step-up transformer, like in the Rangemaster. If you decide to build this circuit, don't use the potcore recommended by the author, Charles Wenzel. The potcore will give very poor efficiency. Find a large, low-permeability (30 or less), ferrite toroid, and experiment to get the number of primary and secondary turns for best results. I have used the Wenzel (with a better output transformer) myself.
It has been suggested in this or a related thread that compliance should be measured out in the open away from obstructions, and this would be SOP for laboratory grade certification, but for the hobbyist it is the home installation that could come under FCC scrutiny, so I pose the notion that it is the home setup that must pass inspection.