It's been awhile Gents! Hope all is well in your lives. I have a question for the forum. I once read a thread or two about Mr. Hamilton's ground lead shield. You know the one he designed for the former KENC and others? Tell me. What was the technology/logic behind the shield? How was it supposed to prevent or lessen emissions from a radiating element/ground lead? Was it (the ground lead) wrapped in a thicker, rubber insulation or conduit wrap?
I ask this question because I spoke with a guy last Monday who works for a company that makes "shields" for small electrical components housed within machinery. These shields prevent RF emissions from escaping or entering the components within the units they're designed to protect. How you ask? By simply housing the small radiating components in brass or nickel silver box sheaths, or what his company calls "mini hub protectors".
When I asked him if this method of RF emissions restricting could be used in other applications, he said "Yes". I then informed him about Part 15 AM/FM radio station set ups and their limitations and if he thought they could benefit from using brass or nickel silver shielding methods. He said "I can't see why not?"
Here's what he stated further:
A shield for shielding radio frequency emissions being emitted from a communications antenna can be created. The shield would have a first layer of material having the physical property of generally absorbing radio frequency electromagnetic emissions and a second layer of material having the physical property of generally reflecting radio frequency emissions. The first layer of material is positioned between the second layer of material and the communications antenna. Therefore, the first layer of material absorbs a portion of the radio frequency emissions from the antenna, and the second layer of material reflects back the remaining emissions to the first layer of material. Therefore, the first layer absorbs a further portion of the remaining emissions. A layer of absorbing material is placed between the combined first & second layers and a material that is transparent to radio frequency emissions and through which the communications antenna radiates radio frequency energy. The purpose of the absorbing material between the transparent material and the combined first & second layers is to minimize escape of radio frequency energy along the transparent material. The radio frequency energy could otherwise escaped around the barrier of the first & second layers due to reflection and refraction of radio frequency energy within the body of the transparent material.
What are your thoughts? Could this work to limit field strength emissions from a Part 15 radiating element i.e., an elevated AM ground lead or an FM antenna mast?
This is basically the same thing as what is used on the stealth fighter (F-117) and other military aircraft that absorbs incoming radar energy so that it does not reflect back to the emitting radar dish and do the "blip..blip".
Thing is...that EM energy has to go somewhere. The energy can bounce back and forth for awhile before it finally attenuates to nothing.
From the time of initial EM energy impact on the absorbing material and bounces back and forth between layers, that energy will build up HEAT..like a resistor. This same effect happens on the stealth material layers on the hulls of the F-117's etc.
Of course at the flea power level we are dealing with in Part 15 AM, heat will not be an issue. However there is another point to make.
Shielding a long length of ground "lead" or mast or water tower or whatever is a whole different world than putting circuits inside shield canisters, a technique that is as old as electronics itself.
This shield around the ground "lead" or mast or water tower or whatever will be subject to lightning strikes and static buildup charges, thus will require it to be grounded to the Earth. Since this shield will be within inches if not centimeters of the actual metal structure that is serving as the transmitter ground path, that shield will cause inductive effects on the entire antenna and support structure and quite possibly radiate as well off the shield grounding.
Just my thoughts...but an interesting concept. Perhaps with some additional null-resonant circuitry that is tunable, there may be a method of minimizing any effects from such a shield system that runs from the ground up covering a Part 15 AM transmitter's mounting structure and ground return path to prevent it from radiating.
Adding: While reading my own post here I also realized that even if a shield could be created to prevent the long ground return path from radiating, and while the issue over the "ground lead" thing has yet to be clarified within the rules, it may still not prevent a station from being tagged.
RFB
Thanks for the quick reply. Wow, stealth bomber builder! Now I see why your posts are often so in depth technically and on point. Here's my thinking. I understand how what you've stated could/would apply to an AM operator. But let's examine an FM set up. An operator sets up an elevated antenna say 40 feet, using a 1 watt tx, who wants to limit field his field strength emissions from going beyond 3 meters per FCC rules. As we all know there are no rules limiting power, heigth or ground leads for the FM side of Part 15; only field strength limits which are not 'range' limits. Elevating these antennas can be very beneficial for operators if you do not exceed the field strength limit. Raising the antenna DOES NOT increase the field strength. I've asked an inspector AND done it myself and measured it using a $29 field meter bought brand new from an Ham radio supply store in my region.
If an operator seeks to limit field strength readings by housing the FM antenna mast itself in a sheath made of brass/nickel silver like the ones described earlier to limit said field strength emissions while preserving his antenna's range and grounds it to a rooftop air conditioner unit to prevent lighting strikes, would it work. It (the 19 foot mast) would be housed in a 20 foot brass/nickel silver sheath canister. Would that affect the range of the broadcast signal? Oh, btw the top of the mast would be affixed with a 4 pronged, directional tentacled cap/capacitor for its antenna top. Just a few thoughts. Obviously, experimentation is the mother of progress. Field results and physics anecdotes sometimes yield different results as I've observed on many occasions. What say you, sir? Appreciate any feedback.
"only field strength limits which are not range limits"
They may not be synonomous, but one directly affects the other. In fact, the ONLY way the FCC can limit your Pt. 15 FM's range is with that field strength limit. The only way you can increase your range without increasing the field strength is to elevate your antenna above nearby obstructions. Once you've done that, you reach a point of diminishing returns. Yes, you could put your antenna on a 1,000 foot tower...but then most people wouldn't be able to hear you because you're 1,000 feet away from them before your signal even leaves the antenna!
Since any extra power you put into your antenna needs to be attenuated somehow to meet the FCC's FS limit, why not just reduce your power to the antenna in the first place to stay at the limit? You are gaining NOTHING by having to get rid of the extra power you're putting into the antenna.
Reduce the voltage to the driver in your transmitter. Or reduce power to the entire unit. 1/2 the voltage will result in a 4 times power reduction. (Your 1 watt will become 250mw) Then you can further reduce the power with an attenuator, or multiple power splitters with most of each split going into a dummy load. Eventually your power will be low enough to produce a legal field strength reading, and legally, that's all you can do.
Finally, don't be suprised by how far you have to bring your power down. With a good antenna, it doesn't take much power to exceed the FS limit. I'm running a ground plane antenna here, which is not as efficient as a dipole, and I'm all the way down to 1mw. (one milliwatt! 1/1,000th of a watt!) And I'm still uncomfortable with that! I'm unable to find someone with a meter that will read in uv/m, so I'm guessing. My problem is - that 1mw is still getting out 1/4 mile on a good day...probably still over the FS limit. I'll let you know when I'm able to measure it. Meanwhile, good luck & Happy Broadcasting!
Putting an FM signal to the far distance is greatly aided by setting the antenna high in the air.
The field strength reading made by an official agent is made on the ground.
I think I have made two statements that are absolutely true.
But how these two facts inter-act, which is the heart of your discussion, is very puzzling in my mind, and I cannot visualize whether the height vs. field strength ratio has any "best placement" for ending up with more range and legal field strength, both at the same time.
It seems like the increased field strength high in the air from an elevated antenna would suffer from the fact that there are no listeners up there, and by the time the signal reached the ground at the legal field strength no distance advantage would be gained to listeners on the ground.
On the other hand, listeners on one side of a high rise apartment building might enjoy very good reception.
Fascinating stuff (and proof that Part 15 operators go well beyond just wanting to play Justin Bieber from their iPods to their car radios)!
I would think, though, that unless the ground lead (there's that pesky term again!) were fairly short, the cost of the brass or silver nickel plumbing--if one had to purchase new, would be prohibitively expensive.
And, unless the station were located in a secure area, I would worry about scrap thieves. Of course, I happen to live in an area where bandits steal entire outdoor central air units for the couple o' bucks of copper.
Interesting info, though, and food for thought!
That's the purpose of the question and the gist of the discussion. The effort being made here is to 'keep' the power from the tx-1 watt, 5 watt, even a 10 watt tx since there are no rules prohibiting one from using them for Fm, while at the same time limiting the 'ground level' field strength readings. Dummy loads and attenuation are fine. But I can build an antenna tuned to a set frequency to handle up to 150 watts of power no problem that would effectively handle drift and harmonics issues that would normally result from an operator using power recklessly without regard for the aforementioned issues related to part 15.
The science here or overall question is really related to ground level field strength readings. And if shielding a radiating antenna mast would/could sufficiently prevent emissions while preserving range, which would be the sole purpose of using a more powerful tx. These shields would act much in the same manner as Keith Hamilton's coax shield which was apparently designed to prevent emissions from leaving elevated radiating antenna ground leads for A-m installations. Could not the same technology be used for f-m masts? As you know there no ground lead heigth issues for f-m. So the only radiating elements would have to be its mast-elevated or not in a set up using a more powerful tx while (1,5,10 watts ) and a tuned antenna.
Elevating the F-m antenna while using a more powerful tx is the initiative and it would certainly get the signal out farther. That much is known. The goals in the same test are to limit field strength readings at the same time. Shielding has been used by the military for years to limit emissions from radiating elements i.e., towers, wires etc. Just wanted thoughts on using it in part 15. Happy holidays guys.
Though there is no regulation stating "thou shalt not have in their possession high power FM apparatus", nor is there a regulation stating "thou shalt not fire up ye high power FM apparatus through an antenna", what would be the point in running so much power only to have to attenuate it?
The field strength rule IS specific, which pretty much puts a stopper, in the practical sense, of using power levels that in the end will have to be attenuated by some means, either through in-line attenuators or extremely loss inherent coax or other. No matter, the majority of that RF energy will be "WASTED" because its being attenuated.
Also keep in mind...though there is no rule saying you cannot have a high power FM TX, or run it into a dummy load, or even an antenna, the FCC will still see it as "too big of box for such a little job" and even if you do achieve the 250uV/m @ 3 meters with it, the field agent will most likely frown and either make the suggestion on site to use something less potent or shut it down until you do have an appropriate unit for the job.
Reason for such a decision by a field agent is because you have too much box that could be cranked up, attenuators bypassed and exceed the limits, that kind of thing.
Besides all that, why would you want to run so much power out of a TX, then have to attenuate that power to a level so low to meet the limits, all the while your TX is pumping out that high power, eating up electricity while doing it, and generating operating heat to boot and the attenuators eating away at all that excess power.
Also keep in mind CB field strength meters are NOT accurate for VHF!! There is NO way a CB field strength meter is going to give you precise measurements while attempting to attenuate a 5 watt VHF signal to a 250uV RF field at 3 meters from that antenna. You will need a field strength meter capable of reading well into the uV range and preferably a meter that is tuned to the precise frequency of operation to avoid any readings picked up by other junk.
As one member pointed out, their FM unit is currently operating at a TPO of 1 milliwatt and still concerned the field strength is exceeding 250uV. He may not have any issues at all with the FCC even though the signal is reaching out up to 1/2 mile on a good day. But here is the vital important difference with his setup versus one running watts into attenuators and an antenna.
The inspecting agent will see that setup A (1 milliwatt) is making the effort to stay within the limits and doing so by reasonable and practical ways..IE good engineering practices.
Now this inspecting agent may even suggest a few things to further reduce the field strength if it is too excessive. One thing these agents do not do is squabble over miniscule levels above the limits. They have far bigger fish to fry than to waste their time, your time and the FCC's time and limited budget to go through all the expense and time to file paperwork and occupy the agent and an attorney valuable time plus the costs involved and court time involved. The inspecting agent will measure, suggest a thing or two, and go on their merry way.
But with a setup like you want to do, a 5 or 10 watt TX running through attenuators or the absorber gizmo, that inspecting agent is NOT going to feel comfortable leaving the site knowing there is a high powered unit there that is easy to put all of its power into the antenna by anyone at anytime. It wont matter if you write up a promise note that is signed by you and witness and notarized. To the agent, that is just a piece of paper. The real world does not work inside a piece of paper. And as long as that high power unit is there, the potential for violating the law is there, thus the inspecting agent will either order you to shut if off that instant, perhaps even confiscate the thing and issue you a NOUO, or if your very lucky, the agent will suggest a few things to fix the situation, but no doubt will tell you to shut it off until you do have things in proper prospective for the setup.
I also agree with another post about why do all this extra work just to achieve the 250uV limit with attenuators and shields when the simpler answer, and more practical answer is to simply run a low power TX where the task of meeting 250uV/m @ 3 meters is far more practical as well as satisfying the FCC inspecting agent. Just makes sense all around don't you think?
But as my friend in the FCC once told me, you are free do to what you want, just make sure you are ready to take responsibility if what your doing is breaking the law.
RFB
I agree with your assessment of how a field inspector "might" view a set up where a 100 or 300 mw tx is in use. He might just assume one is trying to be compliant based on the low levels of output being used. That said, in an area of enforcement by the fcc where there is no language in its own rules which define what is deemed to be a "high power" tx per se, how can anyone really know what is offensive to an inspector or not? Is it up to the inspector? If so, it doesn't say so in the rules. We have to live by the rules and so do they. Moreover, is a 200 mw, 1 watt or 3 watt tx a blowtorch? That is half the problem with the rules for Part 15 operations on the Fm side of things. There is no specificity in the language related to power as we all know. If the only limitations are on field strength then why are we programmed to want to only use 1 mw or 100 mw on the Fm side? Fear? Naturally, to me anyway, I'd rather seek a means of satisfying both my goals and the inspector's. The fcc wants compliance and I/we want greater range. Both can be accomplished can they not? And that is the real question!!!!
Sooooo, for experiment purposes only, for now, i think it'd be fruitful to test it and see what can be accomplished. Frankly, I think what is being discussed here is a loophole in the 1934 rules for Part 15. Again it is simple, the goals are more range while staying within the field strength rules. The assumption that an inspector might have a problem with an operator who used the fcc's own rules-or lack thereof to increase his station's range while complying with the only rules that do exist, to me, smacks of something that i care not to broach.
I'd rather be admonished for exercising my rights in life for following following the rules, than to shy away from exercising them altogether for fear of what might occur if i did. The latter seems patently flawed. Its like advising a person who has the right to vote to abstain from doing so because the local bigot sheriff disagrees with the law that gives him the to do so. If the sheriff incarcerates the guy is he justified? Haven't we all been here before as a nation? Oh and by the way, no one has answered the question regarding Hamilton's coax shield and its practicality. Did it work? How?
Hamilton's approach was not a coax shield. It was an RF choke made from a toroid core and wire. This was attached inside the Rangemaster box to the ground lead to prevent that ground path from radiating.
It did its job just fine and prevented the long ground path (water tower) from radiating, drastically reducing the station's range, which is what is to be expected since it was the long ground pathway that was the cause of the signal reaching out so far. It was radiating MORE than the radiating element at the top.
And yes it would be up to the inspecting agent to determine if he could trust you to not bypass all the attenuators and such on your system running a high powered transmitter and go away satisfied that your station won't violate the rules.
Chances are...the inspecting agent will not just walk away even with the signal meeting the 250uV limit. The mere fact there is a high power transmitter there in operation means the potential to violate. The inspecting agent, if he walks away from such a setup, could end up getting into trouble if later down the road your station causes a problem and it's noted in the FCC's records that your station was once visited by an agent and the agent did not take action on that day of the inspection. Opens up a whole nasty can of worms that I know for a fact there is not a single FCC agent out there that is going to take such a risk and let you run a Part 15 station off a high powered TX through a bunch of attenuators.
Experimenting is one thing, broadcasting is another. At most when experimenting, you will have to keep those transmissions intermittent and very short to see if the concept of a shield system would work. But if your caught broadcasting outside of the realm of experimentation...like that of broadcasting beyond the limits for a lengthy time period, you will get slapped with a violation for sure.
Experimentation is the way to discover and I would be the last one to stand in the way of that. But this field of radio and electronics also comes with responsibility and common sense regarding regulations of the EM spectrum.
You may be able to get an experimenter's authorization to test this shield concept idea. I would look into that route instead of just throwing something up hoping no one notices.
RFB
for fm in addition to the f/s rules it does state unit shall use nothing larger than a fixed 20" antenna attached to back of unit. or something to that effect.
it is allowed to have a unique connector to allow for broken antenna to be replaced in the field by end user.
Thanks for the update on Keith's shield and its effectiveness. Experimentation is to discovery what revolution is to change. Okay, enough of my opining for now. Interesting discourse in here for sure. This forum is the best! I'll keep you all apprised of things. Happy Holidays!
Boy, things sure are interesting in here today! Mpmiss, long time no here from. You have raised an interesting set of questions. Boiling it all down it appears as if you may be on to something here. I, like many, have always read the rules for Part 15 FM and interpreted them to mean that an FM signal can't reach more than 3 meters from an antenna. But, as you've pointed out in here before that is not the case. The "3 meter" rule according to the FCC's own website is relegated to 'field strength' only, which is not range. I can understand why you'd question whether or not range can be accomplished without increasing field strength. If it is possible (we need a physicist-someone call Stanton Friedman) then it is a loophole in the FCC's antiquated 1934 matrix of gibberish. The only way to know would be to try it and let us know the results of your testing. You are going to try it aren't ya?!
Hey mpmiss, are you an attorney? No offense, just asking. Your logic kind of reminds me of my old man, who was an entertainment lawyer in L.A. for thirty-five years. He was always finding loopholes in artists ironclad contracts allowing them to leave bogus deals. Merry Christmas everyone!
I thought i would keep my FM signal on 105.3 Mhz inside the mobile home park by having it as low as it is now.
So using my flag pole as an antenna at roughly 4 feet from the ground i figured it would reduce the dreaded picket fencing effect and maybe cover the park more evenly. Well i managed to cover more land but i have also increased my stations coverage to well over 1000 feet.....in some directions.
The local walmart is behind me more or less at 1/3 of mile, on cloudy days i can listen to Rock 105.3 in the super center parking lot. The shopping center next to my by 900 feet has a clear line of sight of my station antenna and depending on how many cars park around my minivan i can listen with very little static.
As cars & trucks move around the parking lot the signal naturally changes.
I have included two pictures of my stations antenna. So even though i am running a Ramsey FM 25 B on it's lowest power setting, my signal still manages to go where ever it pleases.
I think Radio8Z (Neil) has the right idea for limiting his FM field strength by making the antenna too short for the operating frequency.
Here are two pics of the flag pole antenna, don't mind the mud or the bikes lol:
best thing to do is enlist the cooperation of select trailer park residences and install roof mounted (most trailers have aluminum roofs) certified fms running in mono around the trailer park and use barix boxes to link them. or run carrier current am that park should be behind a single large transformer feeding park.