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Please why is the transmit power of a microwave positive and the received power -ve?

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ovide

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I am a new engineer in Microwave transmission where I do microwave deployment (installation, commissioning, service configuration, and maintenance). I just observed that Microwave transmit power is positive while the received power is negative. Honestly, I am curious to know the reason.

Also, remember that wave theory in Physics told us that frequency and wavelength are inversely proportional. With this in mind, in practice, low frequency microwave radios incorporated with an antenna of diameter d tend to travel/cover far distances as compared to a high frequency microwave radio using the same antenna with diameter d. Please, I do not know why. Any help?

Thanks and patiently waiting for replies.
 

drkirkby

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I've never come across negative power. If you put negative power into a resistor, does it cool down?

Or are you talking about power levels measured in dBm? In that case, anything above 1 mW will be a positive number of dBm and anything below 1 mW will be a negative number of dBm. Transmitters are normally > 1 mW output, and receivers normally don't get signals larger than 1 mW.

I'm guessing you are thinking of dBm, but if not, I've no idea what you mean.

Range vs frequency depends on many things, which gets fairly complicated if not a line of sight path. Assuming a line of sight path, free space path loss increses as the square of frequency. However, for a given size of dish antenna, the capture area is independant of frequency, but the gain increases as the square of frequency, so the two effect cancel.

But there are lots of other effects to take into account. Loss of coax increases with frequency. At microwave frequencies, even short lengths can have significant loss. Exactly how loss of coax depends on frequency is not easy to predict. Copper loss increases with frquency due to skin depth, and the equation for that is easy to show. But dielectric loss also increases with frequency. But how much depends on the dielectric.

It seems like you need to find a basic book on RF.

Dave

Dave
 
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Syncopator

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To state drkirkby's answer a little differently ...

Anything written as, for example 4dBm, 18dBm or whatever, means so many dB greater than 1mW. Those two examples, 4dB greater, and 18dB greater, translate to 2.4mW and 63mW respectively.

Anything written as, for example, -8dBm, or -13dBm, means so many dB less than 1mW. These two translating to 158µW and 50μW respectively.

And 0dBm is 1mW. It is neither greater than nor less than 1mW.

Similarly for watts. xdBW is xdB greater than 1W. -xdBW is xdB less than 1W. 0dBW is 1W.
 

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ovide

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drkirkby, Pls I like to ask if you have a basic book on RF, honestly, I need it.
Also, pls how do you mean that the two effects cancel?
Finally, the equation relating copper loss and/or dielectric loss with increase in frequency would be highly appreciated. Thanks.

- - - Updated - - -

Thank you Syncopator
Your post is really helpful.
But please, I like to ask how you came about getting the power rating/value (in mW) from the dBm value. Is there a formula for it. Please do you mind sharing it with me?
Thanks.
 

drkirkby

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drkirkby, Pls I like to ask if you have a basic book on RF, honestly, I need it.
Also, pls how do you mean that the two effects cancel?
Finally, the equation relating copper loss and/or dielectric loss with increase in frequency would be highly appreciated. Thanks.

- - - Updated - - -

Thank you Syncopator
Your post is really helpful.
But please, I like to ask how you came about getting the power rating/value (in mW) from the dBm value. Is there a formula for it. Please do you mind sharing it with me?
Thanks.
RF book. One would need to know more about your background, eduction level, what you need to know. Some of the amateur radio books might be helpful. The ARRL have an antennas handbook, which might be useful for this sort of thing. I'd be tempted to look on Amazon, and find the books which allow you to preview the content (see at least the contents, index etc) and see how relevant it looks. Professional RF books are quite expensive, but those aimed at the ham market are less so, and might do what you want. It's hard to say.

For dBm see http://en.wikipedia.org/wiki/DBm

If the frequency is doubled, the free space path loss

http://en.wikipedia.org/wiki/Free-space_path_loss

will go up by a factor of 4. However, the gain of an aperture antenna

http://en.wikipedia.org/wiki/Antenna_aperture#Aperture_and_gain

like a dish will also go up by a factor of 4. So the overall path loss remains the same, as the two effects cancel. Take a look at http://en.wikipedia.org/wiki/Link_budget and also http://www.antenna-theory.com/
 
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