I've been doing a bit of research on this topic for a little while.
The RF field strength of a transmitter in the far field decreases linearly as a function of distance. So let's consider BETS-1, which allows 100uv/m at 30 meters. 30 meters is definitely in the far field for an FM broadcast signal. The near field for a quarter wave antenna is generally a wavelength or 2, you then enter a transition zone of a wavelength or two, and finally go into the far field. The wavelength for a 100 Mhz FM signal is approximately 3 meters.
So for the far field; your field strength at 30 meters is 100uv/m, and if you double the distance, you half the field strength. At a distance of 60 meters, your field strength would be 50uv/m. And so on. That's assuming no ground reflections, obstructions, etc. which can scatter your signal, decrease it or even add to it.
In the near field, however, field strength behaves much differently, and can decrease by exponential factors (squared or even cubed) related to distance. Behavior in the transition zone is somewhere in between.
The field strength for a Part15.239 compliant transmitter is 250uv/m at 3 meters, which can be in the near field. However, that field strength will degrade more rapidly in the near and transition fields than that of BETS-1, which is measured in the far field. So even though it appears at first glance that BETS-1 signals are about 4 times stronger than Part15.239, in reality they're much stronger than that in the near field (to get you that 100uv/m measurement).
Interestingly, the near field is not as large for electrically shorter antennas. So for transmitters such as the Whole House 3, whose antenna is a 6 inch piece of wire (wrapped up in a rubber deck cover), 3 meters would likely be in the transition or even the far field. That would mean that a 250uv/m field strength signal measured at that 3 meters would degrade more slowly over distance than a transmitter which had a full size antenna.
What that implies is that not all field strengths are equal, even though they are(?!).
Well if I didn't understand it before I certainly don't understand it now.
Heh. Sometimes I'm too wordy.
I was looking at the way field strength degrades, as it is proportional to distance from the antenna.
The near field of a resonant antenna is 1-2 wavelengths out. Then there's a transition zone of 1-2 wavelengths, and finally, the far zone.
Field strength degrades differently in these 3 zones. In the far zone, it is inversely and linearly proportional to the distance from the antenna. That means if you take a measurement at, say, 30 meters (which is in the far zone for an FM broadcast signal), doubling the distance halves the field strength.
RF behaves much differently in the near zone. Field strength is inversely proportional to the distance from the antenna, but not linear - there are squared and cubed factors. What that means is, if you are in the near zone, and you double the distance from the antenna, the field strength could decrease by 1/4, or even 1/8.
The transition zone is somewhere in the middle, but it is a safe assumption that your field strength will decrease far faster the further you get from the antenna than in the far zone.
What does that all mean in the real world?
Well, compliance measurements in the U.S. for FM signals are taken at a distance of 3 meters. That is likely within the near zone if you are using a resonant antenna (the wavelength of a 100Mhz signal is 3 meters). Your field strength will decrease much more rapidly as a function of distance from the antenna, than, say, if you were taking measurements further out (such as BETS1, which is 30 meters, and in the far zone).
What is interesting is that if your antenna is shortened, then the near zone moves closer to the antenna. But I'll leave it at that.
Continuing on, if you have an electrically short antenna, the near zone shortens as well.
So if you have a transmitter such as, say, the Whole House 3, which uses about a 6 inch wire, the near zone would be much less than 3 meters. The lower the frequency you use, the shorter the antenna is (in terms of resonance) and thus the smaller the near field gets.
It is possible that at a distance of 3 meters, you are in the far zone for the Whole House antenna, and at the very least the transition zone.
Now, assume you have one transmitter with a full length antenna, field strength compliant at 250uv/m at 3 meters. You are likely in the near zone at that 3 meters.
The Whole House 3 field strength is also measured at 250uv/m at 3 meters. But 3 meters is in that antenna's far zone.
Both are the same, right? It's been said in the past that compliance is compliance, and that those 2 transmitters should perform equally. But that is not the case. The field strength of the Whole House 3 will degrade linearly as a function of distance. The field strength of the other transmitter will likely degrade much faster until it hits its far zone. So at a distance of, say, 6 meters, the field strength of the Whole House 3 should be stronger than the other transmitter.
Strange, but true. Equal field strength at a specific distance doesn't necessarily mean that the field strengths will be equal at other distances (all other factors being the same). The antenna near and far fields can make a difference.
According to Ramsey in the manuals of their kits they have a chart that the field strength halves for every doubling of the distance from the antenna.
So going by 250 uV @ 3 meters would be 125 uV at 6 meters and so on.
But we don't know the source of that info. Most accurate is to have an FIM and actually measure field strengths in an open field at distances from an antenna.
But it's hard to wrap my head around having 2 transmitters each operating at 250 uV at 3 meters but one is using a full wave antenna and the other is using an 8 inch antenna and the one using the smaller antenna will actually be better at more distance away?
Strange is an understatement.
So what Artisan is referring to is if I have a wholehouse 3 and the stock antenna of a 6"rubber duck" and then I replace that with a full wave length of wire or even a 1/4 wave length depending on the frequency choice the little rubber duck will actually be better at distances in excess of the reference of 250uV @ 3 meters?
So you will get better range of reception all other things being equal with the smaller 6" wire as opposed to the wire 4 X as long on the same transmitter?
I think I could prove that wrong in a real world test. In fact I know I could.
I would like to see the proof not the theory. I will stand 30 meters from the wholehouse 3 with a FIM. Set the frequency to...lets say...90mhz, or 106mhz doesn't matter. Then see what I get with the 6" antenna and then see what I get with the....never mind the full wave, lets say 1/4 wave length even....and I don't think I will see more field strength in my location with the 6" antenna!
Ok I now see what artisan seems to be saying. If you had a transmitter at a given power and with a 6" antenna you get 250uV at 3 meter and then the same transmitter getting a 250uV at 3 meters with a 30" antenna the shorter antenna in that case would be better at farther ranges. But you would have to reduce the transmitter power to have 250uV at the 3 meters with the longer antenna....right? Maybe I'm still not getting it.
If that's what my understanding of the theory is I think this has no significance in the real world.
All I get from this is a more powerful transmitter can go the same distance as a less powerful one with a shorter antenna? Am I right or wrong?
While I don't know, I don't think he's suggesting the duck antenna gets better range than the full wave antenna. I think it's more a comparisons of what's going on in the first two wavelengths (near -field), and the 5-6 wavelengths (far-feild),.. of the two an... Uh, I don't know what he's saying and am surprised he hasn't answered your questions yet, but I think he's not talking about actual range..
@richpowers Maybe not, to your suggestion he may not be talking about range but I'm trying to understand this in a way that we could relate to better. But seems to me that what goes on in the far field is more important than what goes on in the near field. What is better in the far field should be better in actual real world reception distance as I see it.
I think what Artisan is saying is that if you have 250 uV at 3 meters with a 6" antenna and the same 250 uV at 3 meters with a much longer antenna...a full wave length was the example, the smaller antenna will actually be better in the far field which should mean better reception range.
@richpowers Maybe not, to your suggestion he may not be talking about range but I'm trying to understand this in a way that we could relate to better. ....
Amen to that! I've re-read his post a few times trying to grasp it, but just can't seem to grab hold. I'm surprised he's not come back yet.