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Messages posted by: stevech
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Yes, that has been my experience.
I recall that hams have a band around 1.2GHz.

Though I've been a ham licensee continuously since high school (never mind what year that was)...

propagation losses at 1.2GHz would be a bit onerous, for a low cost/small system. Just a bit more than at 868 (EU) and 902-928MHz (US) and 918-928MHz (AUS), unlicensed.

"CP" antenna is?

1.2GHz - I don't think that band is unlicensed in No. America and much of the world.
Is it unlicensed in your region?
This USB/Serial board with FTDI pinout has a switch to go between Vcc = 5V and 3.3V.
The mega328p seems to be 5V tolerant on the UART.


Spansion is ID 1 in all I've worked with.
These would be plug and play. No messing with soldering up radios or level shifters or the BIG Uno boards.


Good price, too.

The sub-GHz radios have better range than the 2.4GHz radios - by laws of physics.
It's all about frequency and choice of coax cable.

The very best solution is for radio and antenna to be collocated. A weatherproof box on top of the mast. Low voltage cable with DC power up the mast (or tree or rooftop). Coax is 1 ft long. Or 0 ft. if you can get away with a "rubber ducky" antenna's gain.

At 900MHz RG8 coax is normally used to connect to a small omni antenna or yagii. Short coax. Pre-made cable w/connectors.
At 2.4GHz, 6 inches or so of LMR400 or some such. But most often, in a point to point bridge link, one uses a 1 ft. square weatherproof box that is on one side, a patch antenna and the radio is inside, with cat5 or some such cable down.

The first thing to do is a link budget. Simple math.
Need site survey...
Path length to 10%. Can use Google Earth or other means.
Verify clear line of sight visually - binoculars. If not clear, photo and description- terrain vs. trees vs. man-made structures.
We then choose the required antenna heights at each end.

Now the math and prediction
Then we do path loss calculation with a spreadsheet that has embedded formulas. I have such. Do for two freq. bands.
Then we choose some radio products for one or both bands. Plug into the spreadsheet their transmitter power and receiver signal strength required for your goal data speed.
We then see how much extra antenna gain is needed, if any.
We include at least 10dB margin for fading and link reliability.

At 700 ft. it'll be pretty easy.

Glad to help with this if you can provide the site survey info as above.
can you get line of sight or nearly so? How obstructed might the path be for practical antenna heights (say, 15 ft)?
Strength means .... ?
(best to not leave it floating?)
The RFM69 reset is not connected to the board's reset, as I recall, looking at the schematic.
I have encountered no issues with this.

I think you'll find that the Km and more ranges in cluttered urban - refers to the 100bps or so bit rates. Lots of marketing liberties taken.
Low bit rates and spread spectrum and forward error correction bits permit it to run at a -110dBm, -120dBm or so. But if there's interference from other radio systems, that'll not let the -110 or so work.
But a carefully chosen frequency in the 900MHz band might find little interference, or infrequent interference. Avoid the top of the band due to paging systems just above 928MHz that slop over.
The SCADA systems kind of dominate this band. Supervisory Control And Data. The little antennas you see on big power transformers, some traffic signal controller boxes, and others, are SCADA. The good news with SCADA is that they tansmit infrequently and briefly.

In the US 433MHz band, there are lots of odd radios. And there are stringent FCC regulations on how often the transmitter can be on and for how long. Not that the FCC will take you to jail - they respond sometimes only after repeated complaints of interference by licensed users of the same band that also permits unlicensed use.

Beware making brief measurements and basing a conclusion on that. Unlicensed bands are famous for having frequently changing conditions.
we can do an educated guess on line of sight (LOS) range at modulation for 1200bps. It'll be better at 433MHz than 900MHz. But US 900MHz regulations are more accommodating of transmitter on-time/duty-cycle.
For non-line-of-sight (NLOS), you'll have to determine this experimentally as the blocking material-types vary widely.

I have some LoRa radios but no time now to make measurements.

Here's one quick SWAG
Antenna gain at both ends: -3dBi (lossy, assuming crude antennas)
TX Power 20dBm (0.1Watt)
Path length: 1 mile LOS
Fade margin: +24dB (good)
Receiver sensitivity: -100dBm for a reasonable packet error rate at 1200bps, spread spectrum. You can go to -105 or -110 if you dare. Receiver sensitivity spec. has to be in context of packet SIZE and packet error rate. More bits in packet, more chance of bit error. Some protocols (like RadioHead's Reliable Datagram) have error correction via ACKs and retransmission. This effectively improves the required received signal strength OR allows larger packets, if the margin is low.

Required antenna height BAD NEWS: to clear 1st Fresnel zone at 1 mile path length: 50 ft.
Using lower antenna height still LOS, yields less fade margin. At 0.1 mile LOS, required height is 10 ft.
The Fresnel zone losses are because RF power "spreads" in space as distances increases. Visualize two antennas, your view is broadside. The Fresnel zone is a football shaped "cloud" of RF power. The Fresnel zone at the half-way point of the path, on flat terrain, touches the earth. The longer the path, leaving antenna height the same, the more occluded the "football" shape is - and that RF power is lost to absorption by the earth. The common design for point to point links is to keep the loss from the Fresnel zone to less than 30% of the power.

Also, the fatness of the shape changes with frequency; smaller (better) at, say, 2.4GHz. But at the higher freq., the path loss increases. And then.. it's much easier to get, say, 10dBi of antenna gain at each end (total 20dB benefit) at 2.4GHz than at 433MHz - due to the size of the antennas (smaller at the higher freq). A 10dBi gain at 433MHz is a large antenna, say, a 6 ft. long yagii boom. At 2.4GHz, 10dBi is like 8 inches of yagii, or a square patch antenna.

This sum of gains and losses is called a link budget. One uses a link budget with a certain fade margin according to how LOS the link is, path length, and frequency, and tolerable packet error rate. A good receiver product will spec the error rate vs. each modulation mode (bit rate) in the form of packet error rate (PER) for a 1% rate, and a certain number of bits in the packet, and for each coding (FEC) rate option. Cheap radios normally have no FEC.

Last point: The LoRa's I've seen use spread spectrum and "long" spreading codes and get their best range at speeds much lower than 1200. Spread spectrum adds a bout 6dB of benefit to the link budget. This is called post-correlation gain and it comes from the "correlation gain" of a spread spectrum receiver.

To all this, we have to add allowances for interference from other radio systems, or from other stations of our own system.

So, it's quite a balancing act.
The key issues are the packet size, use of (or not) of forward error correction bits (FEC, aka. coding), or error correction from ACKs and retransmissions.

"Wireless isn't a hundred times harder than wired, it's a million times harder". Professor Paulraj, Stanford.

very much dependent on your application's required data rate (bps)
serial UART line may go to indeterminate voltages in sleep, causing garbled data to be perceived at the receiving end.
Best to use some sort of protocol on the serial line, such as a few special characters after wakeup.

No-antenna - usually OK, no damage to TX. Range limited to 10's of feet. Benchtop OK.
I'm in the habit of using a marking pen on my boards <grin>
What does Dr. Ohm say to Dr. Ampere about 3.3 / 10,000?
maybe the wrong commands to the RFM69 are being used for deep sleep?
The Moteino uses a different part for the SPI flash so the commands may differ from the Spansion chip used on the Anarduino boards.

Is it possible that the software you're using to sleep the AVR is not using the right methods?
w1ll14m wrote:Hi Rick,

Great update! I was wondering, when will the production start of the MiniWireless Mega?


I beta tested the mega1284-based boards usjng my bootloader with over the air reprogramming. I suspect a production run is in process now, but Rick is the authority.

I recall reading here or somewhere that the "C" model is functionally the same but is pin-compatible with the RFM12B. There are a couple of software control differences - and these would be reflected in software such as RadioHead.
I looked at several bootloaders that handle RAMP for AVRs up to 128KB of flash (mega1284p and mega32p). Not interested in mega2560.
I used the write-flash macros (I think they came from Atmel, not sure), and some of the notions for STK500 subset commands.
I wrote the SPI flash I/O from scratch to keep it small.
And the technique for detecting what's in flash and where is my own scheme.

Optiboot has evolved and been tweaked by so many individuals that I chose not to just modify it.

I didn't elect to download directly from the wireless radio link and write to flash... though I did that on a prior bootloader. It was hard to make sure there could be no deadlock with only a partial download, a power failure mid-download, and all sorts of errors. This did work, and it had to... as there are 100+ of these all around the globe, remotely reprogrammed via cellular and IP. The flash size on that was 120KB and there was no mass storage like big RAM or SPI flash. So each time the MCU reset, the bootloader ran again to check if the last remote reprogram was successful (CRC OK, etc.).

This could be done for the little AVR with a 2KB bootloader, I think. Might be best done with a mega1284 as the larger bootloader would have less impact on the 128KB of flash, and the 16KB of RAM will help in buffering for larger sized flash pages. Alas, it's a TODO item. Meanwhile the intermediate buffer is the SPI flash chip which serves other purposes... the bootloader use of the SPI flash is a small percentage of the flash chip's capacity. I'm using the top-most sector in that 16MB flash.
I don't recall that HopeRF has a non-volatile memory update that's meaningful. User must do all settings (which RadioHead does do, with many alternatives).

The radio per se is the same for 433 and 915MHz; the key difference is in the filter components on the PCB external to the radio chip.
I've noted that if you tune a radio for one band to the other band, and adjust the antenna length accordingly, the filter diminishes the signal strength about 10-15dB. That's fine if you have lots of excess signal strength due to using high power and/or have a short or clear path, or have 6dB or better gain antenna(s)
Did you remove the blink-the-LED code? It's 40 or so bytes.

I've gotten my bootloader with support for flashing from UART as well as recall from SPI flash - at about 950 bytes of C code.

I heard that Rick's been on travel recently. And the shipping fellow is swamped with work.

license for non-commercial use would have to be on a case-by-case basis, after review.

Apologies, but there is just too much theft going on this this world we find ourselves in.
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