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jheissjr

400MHz and 900MHz for Telemetry

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Choosing between 400MHz and 900MHz, I was wondering which of the two would be better suited for sending data down to the ground for telemetry. I'm sure each frequency has its advantages and disadvantages but I'm not sure what they are. Maybe one is more directional but more susceptible to noise or multipath interference?

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400Mhz: Slightly less multipathing, a bit more effective range per RF watt.

900Mhz: Much smaller antenna (1/2 the size). Commercial RF modems are available on this band.

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I was doing some reading today and found this information which is very good at explaining the benefits and drawbacks of 900MHz vs 2.4GHz.

1. Link Budget

A typical 802.11x link budget is between 90 and 94 dB. This will give an effective range performance of 300-500 feet outdoors and 50-75 feet indoors. In contrast, the Wi.232DTS module link budget is 114dB at the highest RF data rate (152.34kbit/sec) and 122dB at the lower RF data rate (4.8kbit/sec). For every 6dB of link budget improvement, range is effectively doubled. Using this rule of thumb, a Wi.232DTS solution operating at its maximum data rate will operate at 4 times the distance of an 802.11xx solution. In fact, we have had customers report that the Wi.232DTS was communicating at 3+miles in a line-of-sight environment. Likewise, we have had customers report indoor range performance in excess of 1000 feet.

2. Operating Frequency

Higher frequencies are more directional and propagate worse through free space. In general, when the frequency is doubled, range is cut in half (assuming the same transmit power and receive sensitivity). The Wi.232DTS module operates in the unlicensed 902-928MHz band while 802.11x operates in the 2.4GHz unlicensed band. Thus, the Wi.232DTS module gains an additional 6dB in link budget. The Wi.232EUR module is available for European applications, and is functionally identical to the Wi.232DTS module except for the operating frequency (868 MHz) and the number of channels (15).

Additionally, it just so happens that 2.4 GHz is the resonant frequency of water, which is bad for a couple of reasons. This results in two negative effects for 802.11x solutions. First, water significantly attenuates the RF energy, reducing performance in humid environments (e.g., outdoors during a rainstorm) as well as human-worn applications.

Second, microwave ovens emit considerable energy in the 2.4GHz band and are a significant source of interference for 802.11x devices. The reason that the 802.11x specification authors chose 2.4 GHz as an operating frequency band is because it has general worldwide acceptance, allowing one physical product to be sold around the world. The reason that this frequency has worldwide acceptance is obvious, microwave ovens must operate at 2.4 GHz and every uses them. The choice of 2.4 GHz was a marketing choice, not a technological choice (very interesting, I never knew this).

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The technical advantage of the lower frequencies is about as simple as that. The 400Mhz band will essentially have twice the range as the 900Mhz band, assuming all related issues are similar.

However, there are other mitigating issues. For example, the higher frequencies allow for easy installation of idealized antenna designs (they are much shorter). The lower frequencies are closer to common EMI/RFI sources. The lower frequencies have a higher chance of interfering with common R/C signals. Lastly, the local RF environment will often dictate which is best to use (the 400 and 900 Mhz bands are very busy spectrums).

Deciding which is best will depend on your situation and application. Sometimes the best technical solution is not the best practical solution. Personally, I think the 900Mhz band is a good compromise for telemetry (short antennas, cheap legal telemetry components, good range, etc.), but that is just me. :)

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I remember trying to do ATV on 440mhz and it knocked out my 72 mhz RC receiver. I had to make a bandpass filter for my RC receiver to eliminate RFI problems.

How far are you trying to send telemetry? I have some good and very small 2.4ghz modems that are good for at least a mile and with a bigger antenna 2-3 miles.

Matt Klarich

Edited by mklarich

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I am trying to weigh the benefits of using Manchester coding and packet repetition and was wondering what your thoughts are considering how much interference is normally encountered out on the flying field. I am trying to decide on the most robust communication given the data rate tradeoff.

Send one packet: say 30 frames/sec

Send each packet twice (incase first packet has error): 15 frames/sec

Send one packet but use Manchester coding: 15 frames/sec

Send each packet twice using Manchester coding: 7.5 frames/sec

I am not sure what kind of interference to expect (random/burst/prolonged?) and was wondering what your thoughts are on the topic.

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I was looking at some low cost RF transmitters at Radiotronixs but they have relatively low output power. Would it be possible to simply add a RF amp module, something like this, or are using PA modules more involved than just soldering them in? I am interested in increasing the power to around 1 Watt.

Edited by jheissjr

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