chicken

[POTM] dAISy - A Simple AIS Receiver

286 posts in this topic

No Chicken,

It only worked once in the first trial. Is there the problem about transponder VHF antenna positioning or excessive power transmit by transponder?

Thanks.

 

-Rian 

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Excessive power would be my guess if the receiver doesn't work anymore. The radio IC is only rated to an absolute max of 10 dBm, equivalent to 10mW.

You can somewhat protect the radio with two parallel PIN diodes from RF to ground. One with cathode to GND and the other the opposite way. This clamps the signal to a maximum of less than +/- 0.7V, the equivalent of 10dBm. Though not sure how long that would help when directly connecting the receiver to an AIS transponder.

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Dear Adrian,

I tested the configuration file that you sent me for Si4467. But result is not better. However, in interrupt service routine for Rx data clock, I don't check CCA condition (clear channel assessment, it belongs to RSSI threshold in MODEM_RSSI_THRESH register. I always read Rx Data pin and decode data (check preamble, flag 0x7e, ...)). Dose it makes lost message?

Thanks Adrian!

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I usually have the check for the CCA condition turned off as well. It only did make sense for single-channel receivers where RSSI level can be used for more intelligent channel hopping (didn't make much of a difference).

I still suspect noise to be your main issue.

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Initial test results

dAISy Hat with an FT232 RS232 convert - Half wave vertical dipole at approx 30 ft fed with 80 ft 75 ohm RG-6 quad shield with balun at feed point (The mismatch is largely irrelevant in modern active front end **receivers**, transmitters are a different ballgame altogether)

Noise, no Antenna = -126 dbm

Noise with antenna = -123 dbm 

Results from 800+ messages:

Minimum received signals = -117 dbm

Maximum received signal = --67 dbm

Average received signal = -91 dbm

 

NOTES:

(1) The more than acceptable 3db increase in noise from the antenna is mainly due to using quality coax and a vertical dipole with a proper balun which is a balanced antenna and tends to work more with the magnetic portion of the electromagnetic signal. A 1/4 wave vertical ground plane antenna would likely add another 3 - 6 db to the noise levels because it is an unbalanced antenna that works mainly with the electro (voltage) part of the electromagnetic signal and thus is more sensitive to RFI noise. Another engineering term for a vertical ground plane antenna is a Voltage Probe Antenna. However on most vessels a 1/4 to 5/8 wave vertical will be used since they are shorter (1/4) and/or are end fed rather than in the middle making them much easier to mount and feed on most smaller vessels.

(2) While this could benefit from a 10 db preamp with input and output filters and scaling one of the 2 meter amatuer band designs would be relatively simple, trying to get there with only 5 volts is tough as most are designed for 12-14V. Plus you are going to need to use fairly large air coil or a slug tuned ferrite coils (like in IF circuits) which have a higher Q and thus lower loss. filters using SMT inductors while convenient and small are inherently low Q and lossy and that is why everyone is getting poor results putting a preamp with SMT filters in front of these. No preamp will increase the SNR of a receiver only increase it as they will amplify both the noise and the signal present on the input in equal amounts. The only way to lower noise is by decreasing the input bandwidth and filtering out the noise before it reaches the input to the preamp. 

(3) My receive range roughly tripled over a RTL dongle and doubled compared to a HackRF SDR. This is most likely due to the poor dynamic range and SNR of the 8 bit ADCs even in the otherwise well designed HackRF

Now it's off to mate this with my boat's GPS and the output to a RS-422 converter so I can add it and the GPS to my Lowrance Elite 7 Ti NMEA inputs and I can give it on on the water test this weekend .......

chicken likes this

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20 hours ago, chicken said:

Excessive power would be my guess if the receiver doesn't work anymore. The radio IC is only rated to an absolute max of 10 dBm, equivalent to 10mW.

You can somewhat protect the radio with two parallel PIN diodes from RF to ground. One with cathode to GND and the other the opposite way. This clamps the signal to a maximum of less than +/- 0.7V, the equivalent of 10dBm. Though not sure how long that would help when directly connecting the receiver to an AIS transponder.

Ok thanks for your suggestion. I think i will use attenuator for the antenna port. I will try it first.

Regards,

Rian

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@nazmibojan your radio IC may be toast by now, so you may have to replace it.

@longjohn119 thank you for the detailed report. My own knowledge about antennas is very basic.

It's definitely much noisier around here in suburban Seattle.

Also always interested how the receivers compare to SDRs, as basically it's a very specialized SDR. For a fairer comparison, you could try to add a bandpass filter in front of the SDRs. My 1-channel dAISy has a discrete LC filter built from SMT components, and it holds up surprisingly well to the SAW filter used on other models. In real world tests the advantage of the SAW was maybe 10% more messages.

An external filtered preamp can help. I observed better range when adding this one in front of the HAT:
https://store.uputronics.com/index.php?route=product/product&product_id=93

Adding a preamp on-board is still on my to-do list. I did early experiments which resulted in worse reception. However by now I know that the small dAISy is limited by noise from the USB side. I may give it a try with the HAT, which despite the integrated splitter performs as good as the single-channel dAISy, i.e. the design likely has a lower noise floor.

 

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The Mississippi River finally dropped to near normal levels so I got a chance to try this out 'on the water' fed to my Lowrance Elite 7 Ti  through a RS-422 converter and then the  AIS, GPS, as well as depth and water temp data are sent out the Elite TI via the built in WiFi (NMEA 0183 over Ethernet) to my Windows tablet running OpenCPN and other mapping software. I was getting approx. 7 miles range upstream and 7 miles range downstream which is on par, maybe slightly better than voice communications with a Marine Band radio when it was hooked up to the same antenna. Since it's mounted on the rear deck of a modified Jon boat the base of the antenna (1/4 wave vertical) is only a couple of feet above the water level which is the main limiter for range. Still 'seeing' a barge 7 miles out that is only moving 4-7 Mph gives me plenty of warning (At least an hour) and suits my main purpose perfectly. Also as I suspected the noise levels went up about 4 db with the 1/4 vertical over vertical 1/2 wave dipole I have at home although in practice I didn't notice any adverse affects. 

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Happy to hear that, and thank you for the review on Tindie!

I had to lookup how a Jon boat looks like :smile: With that low height, I'm surprised of the 7 miles range.

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2 hours ago, nazmibojan said:

does dAISy support for receiving AIS Message 24 (Class B Static Data)?

Yes, message type 24 is supported. dAISy itself is not really aware of the AIS message contents. It will forward all valid AIS messages it receives.

You may see fewer Class B messages than expected because:

  • Class B transponders are much weaker than Class A, and therefore more prone to noise interference. Especially for long messages like static data.
  • Class B messages are typically sent less frequently than Class A

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