RE: Reply #40
Just to note that the field strength arriving at a VHF-FM band receive antenna mainly depends on:
1) The effective radiated power of the transmit system toward the receive antenna
2) The elevation above the earth of the transmit and receive antennas (Google: Fresnel zones)
3) Terrain elevations/variations along that propagation path
4) Earth conductivity, which affects reflections from the ground plane
5) The presence, location and physical form of obstructing/reflecting structures along, and near that propagation path
6) For long propagation paths: atmospheric conditions
Note that all of the above is separate from the performance of the FM receive antenna system, and of the FM receiver, itself.
The ERP is really the field strength in another form.
It would be interesting to understand just how much the other factors you mention will affect range.
For example, I know, just from experience, that the atmospheric conditions can affect my range 20% or more. It's not a large absolute amount for BETS broadcasting, but in relative terms it's a lot. I would choose a cool, dry day to test to get the maximium range. But at the very least, ensure that the atmospheric conditions are noted during testing, and try to keep them constant when testing each transmitter.
I'm particularly interested in the elevation of the receiving and transmitting antennas, and how roughly (in terms of percentages) that might affect Part 15 or BETS range. I suspect that this is another significant factor.
Obstructions, although significant, you can get around by testing line of sight. I wonder, though if you can mathematically predict anything there (i.e., what would the effect of one house in the path be).
Elevation variations along the propogation path would be interesting, but I would imagine that you could effectively limit the effects of that during testing by choosing a flat testing area. I wonder, though, how significant they would be?
And the same question for ground conductivity - is it really significant (particularly if you have elevated antennas)? It also seems to me that some of these factors interact amongst themselves (i.e., does elevating an antenna reduce the effect of ground conductivity).
Anyway, thanks Rich for that post. It gives us (Tim?) some factors to consider when devising the testing plan. And to explain what actually happened when we get the test results.
By the way, over at HB, they've started a round of FM transmitter testing. I got that from a post that disappeared fairly rapidly (don't know if it's really gone, or just had no responses) - that's why, in light of what we're discussing here, I'm bringing it up.
An oblique comment was made in the post that they didn't get the results they expected, and that they were contacting the manufacturers of those transmitters. It wasn't said whether the range was more than expected, or less than expected (I suspect the former, given their previous statements on Part 15 FM range, but I really don't know).
It will be interesting to compare the results of a shootout here, to those obtained there.
"...The ERP is really the field strength in another form. ,,,"
More ERP results in more field strength, but (other things equal), field strength changes only by the square root of the change in ERP.
Understood. But if you know the field strength, you can calculate the ERP, and vice versa.
It appears that I have successfully ordered a Decade CM-10. Even though the website says they're on vacation for another week, I tried to place an order on the website and couldn't find the actual "order here" page. I sent an email inquiry which was promptly answered with the link to place the order. I went to place the order and it went right to a Paypal payment page, and I was hoping to use a regular credit card. So I sent another email, which was again answered rather promptly that advised me to call their # to place my order with a CC, which I did.
I'm not exactly sure which other transmitters are: Commercially built (not kits), are certified, and readily available for purchase. But I'll see what happens with the Decade first.
Again, for those technically minded my initial plan is to test for legal field strength at 3 meters to first establish the unit is legal. Then, since I have plenty of wide open convenient flat real estate right at hand, I can check field strength at, say 50, 100, 200, feet (and more if warranted). I also plan on seeing how much of a difference polarization can make -- I'll simply turn the transmitter horizontal and vertical and see what different it may make with a horizontal receive antenna. And just for the fun of it, I'll see what I can actually receive on radios from varying distances -- car radio, portable radio, crappy portable radio, good portable radio, etc. I may even put the transmitter in various locations, e.g. in "free space", behind a bush (foilage effect) behind a wet bush (wet foilage effect) in the house in a window, in the house behind a wall, etc. And see what data I can gather.
I have both the Potomac FIM-71 and the Ztechnology R-506. Both units have their own calibrated dipole receiving antennas. The R-506 has the antenna calibration and antenna factor information loaded into it's internal computer. I will also make the necessary adjustments based on frequency. These are the same units used in the field by the FCC, although I won't be using the GPS interface for the R-506, since I know where I am!
Somewhere on this site someone posted a link to a violation report from the FCC where the use of these devices is shown in the evidence report attached. I should have saved that to my computer!
We'll see how it goes from there. I'm guessing since Decade is on vacation it may be a week or so before my unit is shipped -- they may very well be away but still answering emails.
Tim in Bovey
I don't know about Decade being on vacation, as I saw that on the website too. But last week they answered my emails, I placed an order, and I was notified today that it was shipped yesterday and I have the tracing info.
So probably in the next couple weeks I'll have the Decade transmitter to do some testing with. First off, field strength at 3 meters to see how close to the 250uv/m limit it is. Then I'm switching over to feet and doing field strength at 50, 100, 150, 200 etc and also going to check reception at these distances with a large collection of radios. I'll be checking differences in antenna polarization on received signal strength and will also try to do some field strength tests with the transmitter behind foilage, in a window, behind a wall, etc. Just to see what evveft these sorts of things may have.
Luckily I'm in a small town, with nearby wide open spaces, and there are only a few FM stations in the area so finding clear frequencies to work on will be easy.
Still not sure that I myself have a use for said transmitter, but who knows.
Tim in Bovey