RF Cable Loss on PCB

Question

I'm calculating the link budget for a rocket communication that reaches 3km altitude.

I'm using HopeRF RFM98W for both ends of the system.

• Max RF output: 20dBm
• Antenna Gain: 3dBi
• 3km FSPL at 433MHz: 94,72dB

After the receiver antenna gain we get: 20+3-94,72+3 = -68,72dBm

The RFM98W has a receiver sensitivity of -148dBm. We calculate the link margin:

-68,72-(-148) = 79,28

The link margin seems a little off. In the examples I saw, they get a link margin of around 8dB, and for another RF module I got a link margin of 29,49 dB which seems normal. This module couldn't sometimes connect in a clear line of sight at 500m so I'm curious how I can get a link margin this big.

I think I'm missing the cable RF loss here. I have used this module with an adapter board now I'm switching to custom PCB. I couldn't find any datasheets for the adapter board itself. I would like to calculate cable rf loss due to a 2mm wide 10mm long 0.5mm thick copper wire on a PCB.

• datasheet for the module and antenna: rfm98w
• antenna

Answer 1

An accepted formula for calculating the power needed by a receiver operating at a specific data rate is this: -

Power (dBm) = -154 dBm + 10\$log_{10}(\text{data rate})\$

Formula from this excellent book by Chistopher Haslett: -

The module in question has a data rate throughput as high as 300 kbps so, in that case the receiver input power needed would be: -

Power (dBm) = -154 dBm + 54.77 dB = 99.28 dBm

OK, if your data rate is only low (maybe 1 kbps) the receiver power needed might only be -124 dBm and, if the data rate is only 100 bits per second, the receiver will need a power of -134 dBm.

So, you decide based on your data rate.

Formulas and other link budget information from here.

Don't forget to add another 20 db loss for fade margin.

Answer 2

Indeed you are missing cable loss, but that's going to be a few dB at most, not anywhere near 79 dB.

What you are missing is noise. The receiver sensitivity is the weakest signal that can be received with zero external noise. But unless you have a cryogenic LNA and a highly directive antenna pointed at something very cold, it's going to be external noise, not receiver sensitivity, limiting your performance.

On a cursory read, it looks like table 12 of the datasheet specifies the required SNR at each spreading factor:

You'll have to estimate your environmental noise to calculate the SNR for your link budget.

That said, as Marcus Müller notes in a comment, these modules may not be able to keep up with the doppler change, and the link budget may be the least of your concerns.