I saw a post on Facebook recently that stated that field strength decreases inversely and exponentially as the distance from the antenna increases.
Our intrepid @mark refuted that claim, and a few days ago that guy came back in Bill DeFelice style, claiming the inverse square law of physics applies here, and also stating his extensive background in radio and physics.
Well, that guy is just plain wrong. He's right in that he claimed that the inverse square law with "intensities" is universal in physics. However, it's power that follows the inverse square law here.
The electric field strength in the far field of an antenna is relatively predictable, at least in a vacuum in space, and is inversely and LINEARLY proportional to the distance from the antenna.
In the near field, calculating it is very complex, and there are cube and quadruple exponential factors involved.
For FM, the near field ends, and the far field begins, very close to the antenna; at 3 meters, which is where certification is measured, you are in the far field.
The guy then went on to quote that FM signals could cover the nation if the linear inverse law held; that's actually quite true, at least in space, with absolutely no obstructions. However, on Earth, you have reflections and scattering from a myriad of sources, not the least of which is the ground, buildings, even particles in the air.
The more distance the field potentially travels, the more obstructions.
That's why, even though theory states that you can get 1000 feet or more from a Part 15 compliant FM signal, in most cases you're lucky to get 200 feet. Unless, of course, you're totally in the open, with highly sensitive receivers and antennas.
I guess HB isn't the only source of misinformation.
This from Ramsey FM 25B manual
https://www.manualslib.com/manual/748862/Ramsey-Electronics-Fm25b.html?page=27#manual
I looked it up with AI on Google and it said what the poster said on Facebook with references so who's right?
If the signal goes down by 1/4 for every doubling lets take our BETS and assume 1000 uV at 3 meters. If he's right by the time you get to 30 meters the signal would be long gone on the best of receivers.
Going by Ramsey's chart the field strength would be close to the 100uV at 30 meters but based on Tim's actual field tests he demonstrated that to still have a signal strength of 100uV at 3 meters would need maybe 3 or 4 times that at 3 meters and I remember Rich with an illustration showed something like close to 2000 uV at 3 meters would get you near 100uV at 30 meters. But looking it up even though Ramsey shows it halving for every doubling maybe the poster at Facebook could be right, again the best way to find this out is I have to, with Gerry at Procaster, take the FIM and the transmitter out to the schoolyard or the park. Measure an actual reading at 100uV at 30 meters then move the transmitter up to 3 meters and now see what you get. The FIM would stay in the exact spot. Then we will know by actual measuring what is what and we can see exactly how the math works in the real world. But I do know that it takes more than 1000uV at 3 meters to have a 100uV field strength at 30 meters.
I am sending him an email now!
I looked it up with AI on Google and it said what the poster said on Facebook with references so who's right?
Just something to keep in mind, Google AI references that it provides often mean almost nothing, it might cite it got confirmation from 4 sites, or 8 site or whatever - but all that means is that those sites have a general topic of interest in reference to your question, but if you actually go to each of those individual links often you'll find that one even address the specific subject. You can go to every individual link it provides and sometimes you'll discover that none of them actually confirm whatever summary conclusions Google AI gives you.
I'm not saying that's the case here, in this case it provided you the Ramsey manual, which is presumably accurate, but more often than not it seems Google AI just goes along with the general consensus of social media -- which can be vastly different than the facts.
Yeah, I use it all the time, but if it's some kind of nitty-gritty facts I'm looking for, I certainly don't trust it, in fact I know it's skewed.
@richpowers What ever it is it's "killing me" to know what really is and that is to actually do real world measurements. I have contacted Gerry and hopefully we are going to take the FIM and a transmitter out to see the measurements at distances from the transmitter at 30 meters and 3 meters outside in the real world.
I just found this on Ramsey next page after the chart of the field strength at distances
The poster on Facebook is wrong and I am right. The power is quartered but the field strength is halved for every doubling of the distance.
From Ramsey.....
Yeah, I had that erroneous AI reference as well. The Facebook poster probably did also. Another search interestingly enough came back with the correct answer by the AI.
A while ago I did some research, going directly to engineering sites (as well as amateur radio sites) and antenna theory sites. That's where I found the correct information.
Within the near field (a couple of wavelengths from the antenna), there are all sorts of crazy things going on, and field strength is not linearly inversely proportional to distance. I'm sure that's why they measure the FM field strength at 3 meters - well into the far field, where it is somewhat predictable. I say somewhat, because your field strength will be degraded by reflections (off the ground, buildings, particles in the atmosphere) causing nulling (but sometimes also being additive if you're lucky). Some materials will also absorb energy, and that will degrade field strength.
So, your field strength equation in the far field will not be 1/distance but rather dependent on a location factor. That location factor really indicates installation factors; if your antenna is higher, then your ground reflections are lessened. Are you in open space or are there buildings nearby? What are the building materials? Is it a hot humid day, or a cool, crisp one (the scattering due to humidity is lessened for the latter situation).
In Canada, we measure FM field strength at 30 meters (legal is 100uv/m). In outer space, with a perfect (almost) vacuum, you would need 4 times the Part 15 field strength at 3 meters to obtain that 100uv/m at 30 meters. But because of those pesky reflections, you need more. Probably a lot more.
You could minimize the effect of reflections by putting both the transmitting and receiving antenna at height, but you'll still get waves bouncing off the ground (the transmitting antenna is omnidirectional) and interfering.
I too am looking forward to the real world results from @mark's testing with Gerry.
Reading over my post, I should clarify that quadrupling (say) the distance from the antenna means the field strength will be decreased 1/4 as per theory, and a bit more as there is loss from reflections. The field strength does NOT decrease by 1/16, as that Facebook poster states.
The loss is still linear, not exponential.
The OP encourage me to test field strength vs. distance myself in an antenna chamber with concrete floor (to include effects of ground).
I posted this on the FB thread as well saying...
"Your post got me wondering. Hence I decided to make a measurement of V/m in our test chamber of a 200 MHz signal with a calibrated field strength meter with E field probe vs. distance from the antenna. The antenna is a dual polarity log periodic antenna and I ran the test twice... once for each polarity and graphed the results. Under 12 feet we were in the near-field of the antenna, but 12 and up things smoothed out. By the way our chamber has a floor that emulates roadway (it's a vehicle test chamber) so "ground" effects are part of the measurement. The vertical polarized RF was affected by the floor more than the horizontal. Regardless, beyond 12 feet the trend downwards in V/m vs. distance is linear indicating field values drop off at a 1/d rate."
I will add, however, the simulated electric field strengths of an MF vertical antenna (The WWV 2.5 MHz in this example) has the following trend from 50-1000 meters away from the antenna at 1 m AGL...
That's obviously not a 1/d line. This likely has more to do with this being a ground wave propagation than my chamber measurements that are much closer to free space.
Also, at 2.5Mhz, the wavelength is about 120 meters. You're still in the near field until 300-400 meters or so (the lower the frequency, the farther out the near field extends).
The exponential degradation of the field strength seems to be mostly in the near field, which makes sense.
And a big thank you to @John for the tests that he ran in the real world (as opposed to theory or simulation).
The OP encourage me to test field strength vs. distance myself in an antenna chamber with concrete floor (to include effects of ground).... .. By the way our chamber has a floor that emulates roadway (it's a vehicle test chamber) so "ground" effects are part of the measurement....
Excuse my ignorance please, not intending to derail your discussions with a stupid question, but how exactly does a concrete floor play into all this? As well as the reference to roadways.. I'm not grasping what's going on here.
@artisan-radio Good point. Let's examine the same 2.5 MHz WWV antenna beyond 600m...
This has the behavior of surface waves.
I think the point of all this is the defense of field value 1/d behavior in free space that the OP must be referring to... and to keep in mind where this behavior deviates from the ideal.
The OP encourage me to test field strength vs. distance myself in an antenna chamber with concrete floor (to include effects of ground).... .. By the way our chamber has a floor that emulates roadway (it's a vehicle test chamber) so "ground" effects are part of the measurement....
Excuse my ignorance please, not intending to derail your discussions with a stupid question, but how exactly does a concrete floor play into all this? As well as the reference to roadways.. I'm not grasping what's going on here.
My experiment was created to confirm and defend your original assertion the field values degrade 1/d in free space. I mentioned the concrete floor, a permanent feature in our chamber, merely to be honest about all the factors and conditions. Ideally I would have done this entirely in free space, but this was the easy test I attempted in short time. The source antenna was set to about head height as was the handheld E field probe as we took measurements so you can say this approximated free space. Monday I'm going to repeat this at twice the distance just to make sure the 1/d continues.
Terrestrial AM broadcast folks are used to surface wave E field falloff that's very different from the free space 1/d so they might be mistakenly applying that universally to all radio propagation behaviors. That's just a guess though.
Anyway, this topic is a good one and I enjoyed doing the experiment.
The OP encourage me to test field strength vs. distance myself in an antenna chamber with concrete floor (to include effects of ground).... .. By the way our chamber has a floor that emulates roadway (it's a vehicle test chamber) so "ground" effects are part of the measurement....
Excuse my ignorance please, not intending to derail your discussions with a stupid question, but how exactly does a concrete floor play into all this? As well as the reference to roadways.. I'm not grasping what's going on here.
My experiment was created to confirm and defend your original assertion the field values degrade 1/d in free space. I ....
I actually wasn't involved with the discussion (but still interested), so you are not confirming any assertions of mine, but I was thrown off because (and my understanding is only redundemtry) I didn't think ground wave had much of any relation to FM propagation via ground wave - which is why I questioned what it had to do with anything regard your observations. Your input on the matter is definitely interesting - I wasn't trying to imply otherwise, I was only trying to comprehend what significance the concrete floor played into it.