-
Posts
-
Total posts : 45366davec asks: “Is there any interest in an FM RF power meter?”
My answer is YES.
Total posts : 45366Here is a link to the paper, Prediction of the ‘useable’ coverage of FM radio services, by Ofcom. The paper details FM receiver sensitivity, antenna gain, field strength and so on. 6.1.3 Summary posits the limit of useful coverage for a typical portable radio to be 30 dBuV/m; This is 31.6 uV/m. Given 250 uV/m at 3 meters, 31.6 uV/m is reached at 24 meters.
As pointed out in the paper, mono allows a 14-20 dB reduction in signal strength (see 2.4 and others). So, transmitting mono we can potentially gain 5X to 10X the range. This is 400-800 ft to a portable radio.
https://www.ofcom.org.uk/__data/assets/pdf_file/0013/54310/annex-f.pdf
I like the C.Crane (Digital FM Transmitter 2) calibration method proposed by Thelegacy. C. Crane claims a 40′ or 70′ of range.
Total posts : 45366… 6.1.3 Summary posits the limit of useful coverage for a typical portable radio to be 30 dBuV/m; This is 31.6 uV/m. Given 250 uV/m at 3 meters, 31.6 uV/m is reached at 24 meters. …
In free space, far-fields along a single ray are related to each other by 1/r, e.g., doubling the path length reduces the field to ½ of its starting value. For free space conditions the fields in the above quote are correct. But this relation isn’t necessarily true for propagation paths near the earth.
For example — at an elevation of 2 meters above the earth, the NEC4 graphic in Reply 11 shows a field of about 40 µV/m at a path distance of 3 meters, and about 21 µV/m at a distance of 30 meters.
If the 1/r (inverse distance) rule applied for this path, the field at 30 meters would be 3m/30m = 0.1 x 40 µV/m = 4 µV/m, not the 21 µV/m shown there.
A person adjusting their real world Part 15 FM transmit system based on the 1/r values of its field at some distance could (likely) be using data not fully understood.
Total posts : 45366My analysis is for a Tecson-like radio with the whip antenna vertically oriented. To confirm this a NEC simulation of a ground plane 16′ above ground is used with the vertical field strength component measured 5′ above ground. The field strength follows 1/r within 1 dB starting at 100′ from the antenna. This models someone walking around with the radio, or driving a vehicle.
Now move the field strength measurement point 16′ above ground and we see the field strength dropping odd at 1/r from 3 meters. Taking the 3 meter to 24 meter points, the 1/r formula gives 31uV/m at 24 meters while NEC says 33uV/m (this good agreement).
Conclusion: the 1/r rule is valid for vertically polarized antennas.
As you point out, things can be different with horizontal polarization.
Total posts : 45366Member Rich tells: “A person adjusting their real world Part 15 FM transmit system based on the 1/r values of its field at some distance could (likely) be using data not fully understood.”
That statement is true and reflects the obvious reason why we are exploring the present course of inquiry.
For me, my only Part 15 FM system is the real world kind.
Total posts : 45366My analysis is for a Tecson-like radio with the whip antenna vertically oriented. In this case field strength closely follows 1/r (within 1 dB).
My NEC simulation is with a ground plane 16′ above ground and the vertical field strength component is measured at 5′ above ground.
That may be true (I haven’t modeled the setup described in the 2nd paragraph of the clip), but would you expect the Tecsun radio used as described in your paragraph 1 would show a ~ 1/r rate for the fields/conditions in my NEC model, and other configurations?
Total posts : 45366Rich, your analysis for horizontal polarization is an eye opener that shows a serious disadvantage for horizontally polarized FM antennas. The field strength of the horizontal component falls off not at 1/r but at 1/2r.
For two crossed dipoles (one horizontal and one vertical centered 16′ above ground) fed with 1 amp of RF current each at 90 MHz, NEC shows the following field strengths in V/m:
DX VERT HORIZ RATIO
100′ 3.2 1.6 2:1
200′ 1.8 0.42 4:1
400′ 0.92 0.11 8:1
800′ 0.46 0.027 17:1
With what you’ve pointed out, my antenna plan for maximum FM range (using mono) has changed from circular polarization to vertical polarization. While the vertical component might be usable to 800′ the horizontal would be good only to 200′.
Total posts : 45366… Conclusion: the 1/r rule is valid for vertically polarized antennas.
Sorry, Sir, but that conclusion is not supported by the fields shown in my NEC4 model.
At an elevation of 2 meters above the earth, the NEC4 graphic in Reply 11 shows a field of about 40 µV/m at a path distance of 3 meters, and about 21 µV/m at a distance of 30 meters.
That is a field change of about 0.5X for a path length change of 10X — which is not the result of a 1/r decay rate.
Rich, your analysis for horizontal polarization is an eye opener that shows a serious disadvantage for horizontally polarized FM antennas.
The fields in my NEC4 analysis were (and are) vertically polarized, as was shown in the “bullets” at the top of the graphic in Reply 11.
Total posts : 45366I believe the discrepancy is caused by your “3 meter” measurement being 3 meters from the vertical axis of the vertically polarized dipole and not 3 meters from the antenna itself. For the close-in measurements I mentioned in post 19 that the measurement height was raised to the antenna height. It seems that in your model the actual antenna-to-measurement distance, combined with being off the maximum of the dipole lobe, adversely affected the 3 meter measurement.
I was mistaken on your post #11 polarization. But, my error seems to be serendiptous in that a disadvantage of horizontal polariztion has been found. My aim is to obtain maximum AM and FM range within the limits of Part 15.
Total posts : 45366… For the close-in measurements I mentioned in post 19 that the measurement height was raised to the antenna height. …
Thanks for the comment.
The basis of my model was that the vertical, dipole transmit antenna had a radiation center located 5 meters above a flat Earth ground plane having a conductivity of 5 mS/m, and for the other conditions shown in the bulleted list of that graphic.
The goal was to learn the fields that are present at various horizontal distances along, and vertical distances above the earth from the vertical axis of the dipole (not its radiation center).
It is pretty clear that those fields don’t always follow a 1/r decay rate.
Total posts : 45366In post 22 I wrote 1/2r but that should be 1/(r^2). This is what my particular NEC simulation showed for field decay using horizontal polarzation.
Rich, I agree that measuring field strength at any point other than 3 meters from the antenna is going to introduce propagation errors that are difficult to de-embed from the measurement. Driving a dipole with 11nW might be the most accurate, and simple, way to achieve a field strenth of 250 uV/m at 3 meters. Of course this won’t work for transmitters that don’t have a coaxial RF output connector.
So, after discarding horizontal polarization and FM stereo in favor of vertical polarization and FM mono I ran NEC simulations to try to maximize FM range. Based on the paper mentioned earlier, my field strength target was 6 uV/m at 6′ above average GND. Two antennas were tried, a vertical dipole and stacked vertical dipoles 1 wavelength apart. There was no real difference in range. Vertical dipole heights of 15, 30, 60 and 100 ft were tried with 6 uV/m occuring at these distances:
Height DX
15′ 240′
30′ 300′
60′ 370′
100′ 380′
The 100′ height resulted in low field strength from the base of the mast out to 380 ft. The 60′ height is closer to optimum.
Hi I know this is a really old subject but here is a copy of a chart that coverts what you see on the radio over to the correct measurements.
Hope this helps out anyone who trying to use these radios for testing.
- You must be logged in to reply to this topic.
