Maybe we would get the equivalent of MORE than 10-feet in electrical wavelength?
I said that in jest but the electrical length should be longer. Without looking it up (and I could be wrong) the speed of light is slower in water compared to air by about 30%. Water also has a huge dielectric constant which would increase the capacitance the antenna "sees" to the universe which would reduce the inductance needed in the loading coil but it would probably shunt the signal to ground. Don't know for sure since I haven't really thought it through, but it is fun to kick it around. There probably is a bunch of material from the Navy about underwater antennas if one wanted to pursue this.
If you have some spare time you might like to read about Cherenkov radiation.
Neil
"Water...would probably shunt the signal to ground"
Here's my "ya, but.." portion of the discussion.
The water in my idea is insulated from ground, maybe by a twelve foot rubber base between the giganto-tube and the earth. All the enclosed water would exist in its own universe, connected only by the RF input wire which, of course, would enter the tube the same way as wires enter electron tubes... molded and sealed into the glass.
There are glass maker businesses here in town. I will day dream about talking to them. The resulting structure could be a very attractive yard feature, and in the end, the signal improvement might amount to ...what, inches or feet.
RE: If you take a look at the specs for a top end Marantz home stereo receiver, say, usually sensitivity for a mono FM signal is around 3-5 uv/m with some quieting. AM is generally 10 times that, at 25-35 uv/m.
Receiver sensitivity specs apply to the r-f voltage (in uV) across the antenna input terminals of the receiver. But this is different than the field strength (in uV/m) of the incoming radio wave.
On the AM&FM broadcast bands more field strength is required in the radio wave at a (simple) receive antenna to produce the rated input voltage at the antenna terminals of the receiver. It isn't a 1:1 ratio.
Hopefully this kind of info will not be viewed as unwanted on this list -- but if it is, please advise and I'll not post it.
I agree. Somewhere in a previous thread there were some calculations that boiled down to something over a 2:1 ratio - so if you had a field strength of, say, 70 uv/m, and a 1 meter antenna at the receiving end, you would see a signal strength of around 30 uv or so to the radio. With a better antenna you would have a stronger signal.
The point is, you really can't make blanket (and definitive) pronouncements about range without disclosing all of your assumptions, and those have to include type and sensitivity of receiving equipment and antenna, as well as transmitter info (peak modulation, type of ground, etc.) and some sort of definition of 'useful' signal.
Your statement about maximum range of a typical Part 15 AM station isn't probably too far off (within a factor of, say 2 - that's just what I've seen in the field) but it doesn't adhere to scientific and engineering principles, with all assumptions laid out. And it certainly seems that that's how you're attempting to position it, versus wild, impossible conjecture on the other side.
And again I say, what may seem wild and impossible today might be proven entirely possible tomorrow. History itself proves that statement.
"Receiver sensitivity specs apply to the r-f voltage (in uV) across the antenna input terminals of the receiver."
That's very interesting. Are you sure your talking about a receiver's sensitivity when the front end of a receiver comprises more than just a set of screws or clamp connectors?
If a receiver's sensitivity is based on what's being applied to that receiver at the antenna input point, what happens to that receiver's sensitivity specification on an unused channel?
"But this is different than the field strength (in uV/m) of the incoming radio wave."
That is obvious. Range, obstruction, weather conditions, just to name a few, all play a role in what field strength is present at a given point away from the source, that is all not even mentioning the source site variables but those too play a role...a huge role.
I see a lot of radio receivers out there with a vast range of sensitivities and filtering and such. Why is it so when it is the signal hitting the antenna input terminals that determines the sensitivity? Wouldn't all receivers then have the exact same sensitivity specifications to receive signals capable of reaching that specification at the reception point no matter where that reception point is from the source?
Doesn't make much sense does it.
RFB
I defer to the experts, but I know a mic preamp with variable impedance can change the characteristics of the signal at output. IOW, it becomes more sensitive to frequency and bandwidth Q within the mic's operating envelope.
I would think something similar will occur in a receiver. Inside our local Senior Center, I can't get my cheap little transistor radio to get much of a signal, i.e., it's weak and noisy.
... But when I helped one of the elderly ladies set up her C Crane receiver, I got my signal clear as a bell, plenty loud and no noise, in the exact same room. Checked it again later, with both, and the same thing occurred. It seems to be generally repeatable in that location, except for T-storms and such.
That C Crane does better than most car radios. I bet it does even significantly better if it gets 'accessorized' with C Crane's dual loop external antenna system.
And then there's that ham radio guy's ICOM general-class receiver with its various filters and preamps and longwire antenna that got me at S-8 ... 8-1/2 miles away!! ... And on a day when my car radio was faltering at a little over a mile.
Wild times in the ether ... makes a big difference if your receiver is more like a Porsche 991 than a Ford Fiesta
Why is it so when it is the signal hitting the antenna input terminals that determines the sensitivity?
Thanks for your comments, and to clarify - it is the performance of the receiver (signal/noise ratio etc) for a given r-f input voltage to the receiver from its antenna that sets its sensitivity spec.
The strength of the radio wave arriving at the antenna is subject to a huge number of variables, as pointed out. Also the characteristics of the receive antenna have a large effect. But given all that, some r-f voltage will be present at the r-f input terminals of the receiver. How the receiver performs given that r-f voltage determines its sensitivity spec.
The point is, you really can't make blanket (and definitive) pronouncements about range without disclosing all of your assumptions, and those have to include type and sensitivity of receiving equipment and antenna, as well as transmitter info (peak modulation, type of ground, etc.) and some sort of definition of 'useful' signal.
Good points. I limited my approach to the transmit side, and defined the losses in the loading coil, r-f ground connection and other parameters as shown in my graphic. For those conditions the groundwave field intensity can be calculated for a given distance over a unobstructed, perfect ground plane.
These are all assumptions, but they are defined, and should represent about the upper limit for the field strength at a given distance that can be produced by that system.
The actual field strength at that distance may be quite different, based on path conditions, but it is unlikely to be greater than shown in the analysis. And of course the performance of the receive antenna and receiver, and the local noise level at the receive site determine how useful that signal will be to a listener.
So my graphic cannot be taken (and wasn't intended) to be an absolute analysis of the performance of a complete transmit-receive system. But it is an indicator of the maximum field intensity the transmit system can produce at a given distance, for those defined conditions.
I just entered the YouTube video linked by PhilB and at long last have an idea of what a Potomac FIM 41 is like. It is like a sensitive radio with the world's best S-meter. That would be the thing to have with part 15 work.
Please China. Make an affordable version and ship it over.
Very difficult to find a used one for sale, still expensive. It's been replaced by the digital version (FIM 4100) -- $15 grand if you got da moolah .... I know I sure don't. Overkill for a Part 15 station if you ask me. Like I alluded to ... when I can afford a Porsche 991, maybe I could also afford one of those ... but then I'd just by a real station license, too 😉
First I fainted, now I'm "coming to" or "coming too," which is a strange expression that describes waking up from a faint.
I had no idea any piece of test gear could cost $15k, unless it would be a machine to test jet engines.
From the Potomac point of view, sell one, make the quarter-year.
It would be cheaper to hire a psychic or some kind of mind reader who could detect RF levels using meditation.
What if we all chip in and share a single FIM-digital?
No?
A Tecsun PL-310 and similar ~$50 receivers using a DSP chip such as the si4734 display an r-f voltage present at a sample point within the chip. The linearity of that display with the variation in field intensity of an arriving radiated wave reportedly is quite good, although I haven't measured it myself.
The receiver display does not indicate the arriving field in microvolts/meter. To determine that, the receiver would need to be located in a known field on or close to the frequency of interest, to note what the receiver display showed for those conditions when oriented for maximum indication on the display. Then using a little math, its reading for a Part 15 AM station on/near that frequency could be converted to an approximate field strength in microvolts/meter.
One way of doing this would be to contact the engineer of a local AM station with a directional radiation pattern. They are required to monitor their field strengths at distances close to their antenna site, using accurate test equipment. The station engineer might be willing to tell you that information for a certain location, although it would be a bit of luck if their frequency was very close to one used by a local Part 15 AM operator.
In his blog about many aspects of RF engineering, Dr. Kimber, possibly the world's foremost class E amplifier expert, describes himself not only as a sceptic when it comes to the feasibility of antennas very short compared to wavelength, but an "unbeliever." He says, "There is no radiation without retardation."
Retardation is a time delay, due to the finite speed of an electromagnetic wave along a conductor, that causes phase shift along the length of an antenna structure, and said structure must be large enough for sufficient phase delay to produce a significant amount of radiation resistance at the antenna port.
Enjoying the quote posted by Ermi Roos from Dr. Kimber.
At first impression I felt disappointed by what seemed like Dr. Kimber's very definite stance against short antennas at long wavelengths, but then I started wondering if there might be a second way of reading what he said, but I am not confident enough to attempt describing what I mean.
Ermi, can you put into your own words what Dr. Kimber is telling us?
"describes himself not only as a sceptic when it comes to the feasibility of antennas very short compared to wavelength, but an "unbeliever." He says, "There is no radiation without retardation."
Unfortunately what this Dr. Kimber believes is irrelevant. Right now there are plenty of examples of short antennas operating at long wavelengths that are radiating pretty darn well.
He may be an expert in E class amps, but then again that too could be just something contained within the bubble.
I like how the word "retardation" is being utilized in this case, though misplaced.
RFB
RFB's comment about Dr. Kimber's "unbelief" restores hope for the future of short antennas.
I believe there will be even greater breakthroughs in short antenna efficiency sometime in the future, but I admit that the belief is pure optimism and has no basis in any as yet described technique.