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AREDN hit the 2019-2023 General Class Question Pool, Yea!

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AE6XE's picture
AREDN hit the 2019-2023 General Class Question Pool, Yea!

G1E08 (B) [97.313(j)]
When using modified commercial Wi-Fi equipment to construct an Amateur Radio Emergency Data Network (AREDN), what is the maximum allowed PEP transmitter output power?
A. 100 milliwatts
B. 10 watts
C. 100 watts
D. 1500 watts

Part 97.313(j): No station may transmit with a transmitter output exceeding 10 W PEP when the station is transmitting a SS emission type.

Ah'o... AREDN devices or commercial Wi-Fi equipment predominately do not transmit spread spectrum signals.

The following standards are in use:

802.11b: Direct Sequence Spread Spectrum in 22MHz BW
802.11a: OFDM -- 64 modulated carrier waves in 20MHz BW
802.11g: OFDM -- 64 modulated carrier waves in 20MHz BW
802.11n: OFDM -- 64 modulated carrier waves in 20MHz BW
802.11ac: OFDM (future use by AREDN)

What is the part97 power limit of a Spread Spectrum signal in these bands = 10 watts
What is the part97 power limit of OFDM or modulated carrier waves in these bands = 1500 watts

What is used on these devices?

A) on 2GHz and 900MHz devices, 802.11b, 802.11g, and 802.11n are used.
B) on 5GHz and 3GHz device, 802.11a and 802.11n are used.

What does this mean? Predominately, AREDN devices do not use Spread Spectrum (SS) modulation (802.11b). AREDN 3GHz and 5GHz devices only use 802.11a and 802.11n or OFDM with 1500 watt limit. On 2GHz and 900MHz SS is generally only used for beacons, broadcast packets, and management frames. All the data ends up using the higher rates in 802.11n. Any given transmitted frame could be using a different modulation scheme, rate, and error correction code than the prior transmitted frame.

While there are 2 answers to this question, the more correct, or more prominently in use, would be 'D'. But this misses the intention of a question for 97.313(j)...




So... Can I upload AREDN firmware to my microwave oven? ;)

AA7AU's picture
It depends ...

Only if it has 64MB of RAM in the device, otherwise you can only use it as an AP.

- Don - AA7AU

al0y's picture
Probably not. Let me explain:

Probably not. Let me explain:

Commercial microwave ovens as applied in restaurants have two magnetron tubes and compared to domestic kitchen counterparts they spread the higher RF power and radiated heating energy more evenly. The domestic kitchen or residential microwave ovens have only one magnetron tube. 

Please understand that the radiation from the commercial type of microwave ovens is more difficult to characterize than the radiation from the residential ones. The commercial type of microwave ovens radiate a CW-like interference that sweeps over tens of MHz during the two bursts per mains power cycle. The residential ones give a CW-like interference that has a more or less stable frequency near 2.45 GHz occurring once per mains power cycle. 

However, in both cases, and assuming you found a way to flash a microwave oven with the AREDN firmware (debatable?) and use the oven to establish a link with other ovens, this will still be done using a signal that is transmitted on a bandwidth considerably larger than the frequency content of the original information. This constitutes a frequency hopping situation. Frequency hopping is a basic modulation technique used in spread spectrum signal transmission.
Taking in consideration that most microwave ovens run in a power range of 600 – 1200 watts and the only allowed transmission power for spread spectrum is only 10 watts, we come to the conclusion that microwave ovens cannot LEGALLY run the AREDN software.  
However, it will be perfectly legal to establish a USB connection between your microwave oven and your computer to download a pizza pie right from dominos[dot]com to it, eliminating delivery charges.

K5DLQ's picture
haha!  nice!

haha!  nice!

BTW, 802.11n is NOT spread spectrum, it's OFDM.

wa2ise's picture
Even if I had a full kilowatt

Even if I had a full kilowatt and a half on my AREDN nodes, I probably still couldn't connect to any neighbor nodes, with all the trees around me... 

AE6XE's picture
Maybe after a day or so, you

Maybe after a day or so, you'd burn a hole through the trees :) .


"Always a tree!"

I suppose a 31dBm Mikrotik 5SHP at MCS0 into a 30dbi panel antenna would get you 1000W+ EIRP theoretical. Less the MIMO split, cable losses, and the fact it probably won't work well at balls out power. But still we're getting 200-300W on most of our Rockets and better dishes :)

K0PHP's picture
Unless your neighbor was

Unless your neighbor was running a gallon and a half as well!

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