Posted: 4/6/2012 6:44:02 AM EDT
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I'm setting up a GMRS repeater. Currently planned antenna height is 25' AGL. Antenna is a Comet F72GF tuned to 462MHz with 5.0dB gain. Transmitter will be ~45watts to the antenna through 10' of RG58 (may need to change that). I've done both a Radio-Mobile simulation and Splat! for the same information. I would guess that Radio-Mobile is more accurate since it uses far more detailed information for the transmit station. Can these be relied upon or should I expect drastically lower performance in the field? I have a hard time believing I can get this kind of coverage at only 25' AGL! Granted I'm basing my experience on my time as a "bubblepack pirate"... (before any complaints, I AM getting licensed now; prior to repeater usage) Here are both. ETA: SCALE = Antenna location to "Richmond" is +/- 24 miles. Radio Mobile PDF: http://tinyurl.com/blwngazkit Splat!:  |
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Change to RG8 if you get much longer than 10'.
Don't believe the gain spec's on any antenna, they all lie. However Comet is a very reputable brand and in general works better than others. 10 miles is not totally unreasonable depending on how high the antenna is compared to surrounding terrain, not just it's local height. For round, smooth (billiard ball) Earth, the approximation for distance to the radio horizon in NM is (4/3)*sqrt(2*H) where H is height of the antenna in feet. 25 feet gives 9.4NM or 10.7SM. So your results are not totally crazy, however typically you won't do that well because it's not a round smooth Earth. The results seem to be implying the base of your antenna structure is on a higher piece of land than is average for your area. |
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Gotcha. I think one easy way to think about it is like when you're got a signal strong enough to kerchunk a repeater but not good enough for someone to understand you.
In this case you be able to hear that someone has keyed up at the outer edge of your picture there but probably not able to carry on a conversation with em. |
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I guess I need help understanding what the colors represent. On the Splat! map, I'm assuming reception is represented the orange is GOOD, yellow is OK, green is FAIR, blue is POOR, and purple is BAD/NONE... The Radio-Mobile gives me a dB rating system, but I don't understand it very well... To clarify, these maps are transmission loss maps for the repeater... So I think what I'm looking at are maps predicting reception for field units (with quality antenna) listening to the repeater tx'ing. |
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I guess I need help understanding what the colors represent.
The colors represent the estimated path loss. Take a look at the bottom border of this image: http://www.qsl.net/kd2bd/coverage30ft.png Transmitter gain (dBm) - transmit line loss (dB) + transmit antenna gain (dB) - path loss (dB) + receive antenna gain (dB) - receive line loss (dB) gives you the expected signal strength at the receiver in dBm. Your transmitter gain is 46.5 dBm (10 * log(45 * 1000)). You'll have to look up your line loss in a table or whatever. Your transmit antenna gain is 5 dB. Splat! will give you the path loss between two points if you run a profile plot. Handheld antenna gain is often a negative number. You might use 0 dB as a rough approximation. Your receive line loss is 0 dB for a handheld. For VHF and above: S9 = -93 dBm S8 = -99 dBm S7 = -105 dBm S6 = -111 dBm S5 = -117 dBm S4 = -123 dBm S3 = -129 dBm S2 = -135 dBm S1 = -141 dBm |
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Quoted: I guess I need help understanding what the colors represent. The colors represent the estimated path loss. Take a look at the bottom border of this image: http://www.qsl.net/kd2bd/coverage30ft.png Transmitter gain (dBm) - transmit line loss (dB) + transmit antenna gain (dB) - path loss (dB) + receive antenna gain (dB) - receive line loss (dB) gives you the expected signal strength at the receiver in dBm. Your transmitter gain is 46.5 dBm (10 * log(45 * 1000)). You'll have to look up your line loss in a table or whatever. Your transmit antenna gain is 5 dB. Splat! will give you the path loss between two points if you run a profile plot. Handheld antenna gain is often a negative number. You might use 0 dB as a rough approximation. Your receive line loss is 0 dB for a handheld. For VHF and above: S9 = -93 dBm S8 = -99 dBm S7 = -105 dBm S6 = -111 dBm S5 = -117 dBm S4 = -123 dBm S3 = -129 dBm S2 = -135 dBm S1 = -141 dBm Looking at the signal loss, how do you determine when a radio no longer receives? Is this by looking at the sensitivity of the receiver? If I'm looking at this correctly, I should be in the -40dBm range for the handheld for a large portion of the map... ETA: My rx gear's SINAD is 0.35uV but I don't actually know how to apply this to be useful. |
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Looking at the signal loss, how do you determine when a radio no longer receives? Is this by looking at the sensitivity of the receiver?
Yes. If I'm looking at this correctly, I should be in the -40dBm range for the handheld for a large portion of the map...
In the areas colored red or orange maybe. ETA: My rx gear's SINAD is 0.35uV but I don't actually know how to apply this to be useful.
Okay, 0.35uV is -116 dBm. |
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Quoted: Looking at the signal loss, how do you determine when a radio no longer receives? Is this by looking at the sensitivity of the receiver? Yes. If I'm looking at this correctly, I should be in the -40dBm range for the handheld for a large portion of the map... In the areas colored red or orange maybe. ETA: My rx gear's SINAD is 0.35uV but I don't actually know how to apply this to be useful. Okay, 0.35uV is -116 dBm. So help me add this all together. TX gain 46.5dBm - 0.843dBmm for the 10' of Belden rg58 + 5.5dBm for the antenna gain for a total TX gain of 51.157dBm. Path loss for the light green areas is on the order of -110dBm. TX gain - path loss (51.157-110) = roughly -58.843 signal strength at the RX. RX gear = .35uV or -116dBm + 3.5dBm - ~1dBm line loss antenna gain so the RX'r "should" pick up signals down to -118.5dBm...? IF, big if, I understand, the receive gear should be able to pick up the signal in the green areas of the Splat! map since the signal loss there is taken from the TX strength? |
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So help me add this all together.
TX gain 46.5dBm - 0.843dBmm for the 10' of Belden rg58 + 5.5dBm for the antenna gain for a total TX gain of 51.157dBm. Path loss for the light green areas is on the order of -110dBm. TX gain - path loss (51.157-110) = roughly -58.843 signal strength at the RX. TX gain is 46.5 dBm TX line loss is about 1 dB TX antenna gain is about 5 dB Let's say the path loss is 110 dB That should give you a signal strength of about -59.5 dBm (about 237 μV). RX gear = .35uV or -116dBm + 3.5dBm - ~1dBm line loss antenna gain so the RX'r "should" pick up signals down to -118.5dBm...?
Not sure what you're doing here. Are you figuring in a receive antenna with 3.5 dB gain and a receive line loss of a dB? IF, big if, I understand, the receive gear should be able to pick up the signal in the green areas of the Splat! map since the signal loss there is taken from the TX strength?
It looks like that should be the case. Can you run a path profile to Ashland or Mechanicsville to confirm our interpretation of the color scheme? |
Looks like the path loss is more like 150 dB, good thing we checked. Please disregard the color scheme along the border of the other map. 
TX gain is 46.5 dBm TX line loss is about 1 dB TX antenna gain is about 5 dB Path loss is about 150 dB That should give you a signal strength of about -99.5 dBm. Your handheld should hear your repeater in Mechanicsville, since it's good to -116 dBm or so. How many watts out is your handheld? dBm = 10 * log(watts * 1000). |
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That'd be appreciated.
I have tried using MicroDEM but have to stitch some DEMs together and it looks like the help/tutorials haven't kept up with the software as tabs and things referred to in the help are not in the program. eta: just searched for Radio Mobile online and think I found it but it is offline right now for maintenance. Poop. |
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Handeld is 5 watts out or about 36.8dBm.
So on the handheld's tx, we've got 148dBm path loss minus 36dBm fr a total signal strength of 112 at the repeater. I would say 37 dBm - 148 dB for a signal strength of -111 dBm (just being pedantic  ).
That's on the edge of the rx capability of 120dBm but should still work right?
That's only a margin of 9 dB or so, might be touch and go. |
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Quoted: If you could get your antenna up to 50' or so you'd likely have a noticeable improvement in coverage, just gets you out of the ground clutter and such. Once you get to around 50' or so, you start having to add a lot of height to really see performance gain. According to Splat!, if I boost the antenna to 50' from 25' I gain ~9dBm (from -148 to -139). |