# Do you really need a satellite dish to communicate with satellites?

I'm doing a project where I have ground sensors that detect seismic activity and report their GPS coordinates via APRS if seismic activity is detected. I'd like to do the APRS over satellite.

The sensors are small and utilizing a satellite dish for each one would not suit my needs.

There is a product I came across called the Thuraya satsleeve for smartphones that is satellite-enabled and doesn't require a large antenna or dish to make satellite calls and send satellite text messages.

So would a small antenna work for APRS over satellite? If yes what kind of antenna would be appropriate? If no why?

Do you really need a satellite dish to communicate with satellites?

No, if that were true, your phone's GPS would be rather unwieldy, wouldn't it? And satellite phones would be pretty impractical. (Satphones predate modern smartphones solidly.)

All that a satellite dish is an antenna with a very high directive gain. So, if your link budget says you don't need that, you can use a smaller antenna.

No general answer can be given – the amount of directivity you need is defined by the link budget, and the type of antenna you need to achieve that directivity is defined by the wavelength of your transmission.

Considering your previous question, I think I'd recommend looking into what a link budget is. To give you an idea, we can do a quick rough calculation based on purely fictive numbers.

• $$\\text{SNR}_\text{min}\$$: APRS required SNR at receiver: 10 dB
• $$\b\$$: APRS signal bandwidth: 1 kHz
• $$\\text{NF}\$$: Receiver Noise Figure: 4 dB
• $$\N_0\$$: Thermal Noise Density: -174 dBm/Hz (at 20 °C)

Required receive signal power at receiver: (-184 dBm/Hz + 30 dB Hz) + 10 dB = -144 dBm

• Operational Carrier Frequency: 10 GHz
• Transmitter – Satellite Relay free space distance 500 km
• Satellite Relay – Receiver free space distance 500 km

Free space path loss per direction:

\begin{align} \operatorname{FSPL}(\text{dB}) &= 10\log_{10}\left(\left(\frac{4\pi d f}{c}\right)^2\right) \\ &= 20\log_{10}\left(\frac{4\pi d f}{c}\right) \\ &= 20\log_{10}(d) + 20\log_{10}(f) + 20\log_{10}\left(\frac{4\pi}{c}\right) \\ &= 20\log_{10}(500\text{ km}) + 20\log_{10}(10\text{ GHz}) - 147.55\\ &= 20\cdot 5.7 + 20\cdot 10 - 147.55\\ &= 114 + 200 - 147.55\\ &= 366.45 \end{align}

• Satellite Down- and Uplink antenna gains: 13 dB
• Relay amplification on the satellite: 60 dB
• Output power of your transmitter: 60 dBm (that's 1 freaking kilowatt at 10 GHz; that's nation-state level stuff)

Let the ground station antenna gains be undetermined, called $$\G\$$ at this point:

\begin{align} -144\text{ dBm} &\overset!\le 60 \text{ dBm} + G_\text{TX} - \text{FSPL}_\text{uplink} + G_\text{Sat, uplink} + A_{sat} + G_\text{Sat, downlink} - \text{FSPL}_\text{downlink} + G_\text{RX} \\ &=\left(60 + 2\cdot 13 - 2\cdot 366.45 + 60\right)\text{ dBm} + G_\text{TX} + G_\text{RX}\\ &=\left(60 + 26 - 732.9 + 60\right)\text{ dBm} + G_\text{TX} + G_\text{RX} \\ &\approx -600 dBm + G_\text{TX} + G_\text{RX} \\ G_\text{TX} + G_\text{RX} &= 356 \text{ dB} \end{align}

"satellite dish" is not a proper name. Proper name is parabolic antenna. There is nothing special about antennas used for communicating with satellites. It is just antenna for some frequency (or range of frequencies) and with some specified gain.

Satellites are far away so you need antenna with pretty good gain. You don't necessary need a parabolic antenna, but parabolic antennas have pretty good gain so they are very good for communicating with satellites. But you can pick any other type of antenna that fits your specific requirements.

• Not entirely. If two satellites communicate at the same frequency, you need good directionality in addition to good gain, too. (Although I'm not sure if one of these implies the other; may be the case that good gain equals good directionality for passive antennas.) Jul 13 '19 at 11:27
• juhist: gain is defined as directivity, multiplied with efficiency. Since most antennas are pretty efficient, you can usually interchange these terms. Jul 13 '19 at 11:39