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 grohgregDunno. Ask The Chief
join:2001-07-05
Dawson Springs, KY
4 edits | reply to joebob42
Re: Minimum cable length said by joebob42 :
The transmitter in the TRIA is a fixed gain device, with a maximum output power of (let's assume) 4 watts. Converting to dB, 4 W = 6 dBW = 36 dBm. If we assume 62 dB gain, this means that to drive the transmitter to saturation we need (36-62=) -26 dBm input to the transmitter. If we need only 1 watt (0 dBW or 30 dBm) out of the transmitter, the input level would be -32 dBm. So the transmit power is controlled directly by the output level from the modem.
OK. It's becoming clear that somebody's feeding at least part of this stuff to you, since till now you didn't seem to realize that there is a thousandfold difference between a milliwatt (dBm) and a watt (dBW). In the professional world, dBm is usually specified. When an unqualified "dB" is used, it's usually assumed dBW. Hence, a 62dB transmitter the size of Delaware. The SurfBeam specs list the modem input/output in dBm. To avoid confusion you must carry the same unit of measurement forward in the discussion - and in the calculations. Forget about the 4 watts, and start with the modem; known output -35 to -5 dBm. Then figure 6.3dB loss over 75' of CS5729. Then add your 62dB "transmitter gain", then add the known antenna G/T of 15.4
-35 - 6.3 + 62 + 15.4 = 36.1 dBm or ~4 watts (EIRP). Discounting the antenna gain, that's about 0.117 watt. So far so good.
-5 - 6.3 + 62 + 15.4 = 66.1 dBm or 4074 watts (EIRP). Discounting antenna gain, that's still ~117 watts out of the transmitter.
See the problem I have with your 62 db gain figure?
said by joebob42 :
The Transmiiter needs to be able to draw enough power to produce it's 4 watts. If the voltage is too low (or the cable is too long) it won't be able to hit its max output.
Nor do you seem to have a real good handle on the difference between voltage and current. When you say "draw enough power", that means the transmitter drawing current from the modem. Consider voltage as little more than the delivery vehicle for current. But - voltage changes CAN be used as the trigger to change polarity, change frequency, change output power. Hence the confusion over fixed versus ramped.
//greg//
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| said by grohgreg :
When an unqualified "dB" is used, it's usually assumed dBW.
I'm sorry, I'll have to disagree with you here. In 25 years of satellite engineering, I've never seen this assumption. If it's "dB", then it's either gain/loss, or a ratio (like C/N). Otherwise, a 10 dB attenuator would reduce the signal by 10 Watts, which doesn't make sense... I would be interested if you could point me to an example of this usage. And I certanly know the difference between dBm and dBW (it's 30 dB  )
said by grohgreg :
Nor do you seem to have a real good handle on the difference between voltage and current.
I've got a very clear handle on this; though I did blur the destinction in my response. Sorry for that.
The key is that the voltage is not used to control the TRIA in any way. | |
grohgregDunno. Ask The Chief
join:2001-07-05
Dawson Springs, KY | said by joebob42 :
I would be interested if you could point me to an example of this usage. No problem: see »www.atis.org/tg2k/_dbw.html. Note the definition; "dB referenced to one watt". So when you say "dB", I see an all-encompassing term. Generic, if you will. True, "dB" expresses a "ratio", but relative to what? It could be dBA, dBC, dBV, dBm, dBi, dB0, dBW et cetera.
To see what I'm talking about - the difference between a simple "dB", and the specific "dBm" - see »www.isa.org/Content/ContentGroup···_dBm.htm. dBm is specific to miliwatts. When you're talking negative power levels (-5 to -35 for example), it's absolutely imperative to keep the discussion in dBm.
//greg//
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| reply to grohgreg
said by grohgreg :
Then add your 62dB "transmitter gain", then add the known antenna G/T of 15.4
I'm sorry, it looks like it's my turn to call this gibberish.
First off, G/T is a measure of receive performance, you can't add it to the transmit side like you're trying to do.
Second, the 3 watt spec (sorry, I had the 4 watts wrong) is for the output of the transmitter, and has nothing to do with the antenna it's connected to. The Transciever spec (referenced earlier) says that the typical maximum output power (P1dB) is 34.8 dBm, or 4.8 dBW, or 3 watts. So your examples become:
-35 dBm - 6.3 + 62 = 20.7 dBm, or about a 10th of a watt.
-5 dBm - 6.3 + 62 = 50.7 dBm, which is limited to 34.8 dBm by the transmitter capabilities.
Now, if you want to talk about EIRP, figure the transmit gain is about 45 dBi. (Again, I haven't seen the specs for WB's particualr dish, 45 is a spec for a Patriot Ka band dish of similar size.) This is added to the above results, and do give what seem rediculously large numbers. But, EIRP is how much power you would need if you had an isotropic antenna (non-directional), not a dish.
Does the 62 dB figure make more sense now? (Actually 48 to 63, as it turns out I was picing the high end.) | |
| reply to grohgreg
said by grohgreg :
Note the definition; "dB referenced to one watt".
This is the definition for dBW. dB is always a ratio. Nothing here says that "dB" can be assumed to be dBW. If you're talking about the gain of a device, it's gain in dB is derived from the ratio of its output power to its input power, at its normal operating point.
Your second reference states quite clearly: Use dB when expressing the ratio between two power values. Use dBm when expressing an absolute value of power. My usage is consistant with that. So, I still don't see why you assume when I write dB, I really mean dBW. | |
| reply to joebob42
said by joebob42 :
figure the transmit gain is about 45 dBi.
To correct myself, this should be 39.5 dBi for the WB dish. | | |
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grohgregDunno. Ask The Chief
join:2001-07-05
Dawson Springs, KY
2 edits | reply to joebob42
said by joebob42 :
First off, G/T is a measure of receive performance, you can't add it to the transmit side like you're trying to do. Granted, earth terminal G/T is the figure of merit at the receiver antenna. The standard formula is "G/T = Receive Antenna gain – 10 log(system noise temperature). Bear with me here, cuz I can't edit subscript into the next few sentences. But consider that since this same antenna transmits as well, the Gt (gain-transmit) is also related to G/T. And since gain is a function of frequency, Gt is always better than Gr (on the same antenna). I will concede though, that using 15.4 to represent antenna gain in the previous example was redundant. Rather than grab the convenient G/T value, I should have crunched the numbers.
But no, a transmitter gain of 62dB didn't compute initially, until you qualified it as dBm. 62 dBm equates to 32 dBW. I originally interpreted your post as meaning 62 dBW. Big difference. Anyway, we agree that
-35 dBm - 6.3 + 62 = 20.7 dBm, and
-5 dBm - 6.3 + 62 = 50.7 dBm.
But to arrive at EIRP, you must add Gt. Using your numbers:
-35 dBm - 6.3 + 62 = 20.7 + 39.5 = EIRP 60.2 dBm (30.2 dBW)
-5 dBm - 6.3 + 62 = 50.7 + 39.5 = EIRP 90.2 dBm (60.2 dBwW)
ViaSat data rates the 62cm configuration at EIRP 48 to 58 dBW. So now we're pretty close at the top end. I'm sure the tenths will be variable relative to cable loss. Not sure what's going on at the bottom though (30.2).
Backing up a bit, we tentatively agree that -5 dBm - 6.3 + 62 = 50.7. And I'm guessing you just picked 34.8 dBm because it conveniently equates to 3 watts. What I'm still missing is how the 50.7 becomes "limited to 34.8 dBm by the transmitter capabilities".
//greg//
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| said by grohgreg :
But no, a transmitter gain of 62dB didn't compute initially, until you qualified it as dBm.
I still don't think we're quite on the same page, but the end result may still be the same.
I never said the transmitter gain was 62 dBm. dBm only makes sense in terms of transmit power.
said by ghrogreg :
And I'm guessing you just picked 34.8 dBm because it conveniently equates to 3 watts.
No, I picked 34.8 dBm direct from the U.S.Monolithics datasheet (»www.usmonolithics.com/products/c···1024.pdf), as the Output Power at the 1dB compression point. From the same datasheet:
Small Signal Transmit Gain (over freq. and temp.):
Min: 48 dB
Typical: 56 dB
Max: 63 dB
Note the units: power as dBm, gain as dB. As it turns out, when I guessed 62 dB for the gain, I was high, but still within the spec.
For more on how RF amplifiers are rated, and the meaning of P1dB, take a look at »www.evaluationengineering.com/ar···iers.htm .
Disclaimer... I hadn't seen the spec sheets when we started this discussion... but now we have more information... | |
| reply to grohgreg
said by ghrogreg :
Rather than grab the convenient G/T value, I should have crunched the numbers.
Thanks for clarifying that.
It looks like the antenna datasheet is at »raven.co.uk/satellite-products/d···heet.pdf
Though they show Gt at 14.25 GHz, rather than at 30 GHz, so I'm not sure... | |
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