I’m working with two RTK Facets (not L-band) and a pair of RFDesign RFD900x-US Telemetry Modems for corrections. The prototype setup works great. Credit to @toeknee for inspiration. I’m refining it right now and I have a question about the serial connection between the base Facet and the RFD900x. In an effort to reduce signal loss between the radio and the antenna, while still mounting the antenna high enough, I’d like to lengthen the serial cable between the radio port on the facet and the RX/TX/GND pins on the RFD900x. There is no need for the power pin to be involved in this cable run since I’m providing +5V from a 2nd battery situated inside a box with the radio unit. The minimum length I would need is 15 feet (antenna mast height), but the longer the cable can safely be, the more flexibility I would have when trying to find a location with good view of the sky for the GPS base and line of sight from the top of the antenna mast toward the rover’s area of interest. These are not always the same exact location. My questions are: 1) what is a best practice maximum cable length between these two devices and 2) what specifications should I consider when choosing bulk cable for this purpose? A quick search turned up 50 feet, but I’m wondering about this particular scenario since “serial” is such a broad category. Thanks in advance!
I’ve had decent luck with using twisted shielded cable to reduce the impact of noise. Ground the shield Only on one end.
I like to keep the radio down at the Facet where I can see the link LED. I use a quality N extension cable (N male to N female ) with the below antenna and adaptor cable when I put the antenna up high.
Quality RF cables are not inexpensive and they can be hard to source. Getting the correct RF connectors can be confusing as there is a huge variety of connectors and cable available.
Using twisted pair as @rftop suggested for a longer serial cable is much easier and much less expensive and should work fine.
I have lots of CAT5 cable laying around, I
might first try using a pair in that because free.
Thanks for the reply! I’m using the 5.8dBi antenna already and it is working really well. The longer RF cable is, as you mentioned, a bit of a weak link, which is why I’m attempting a shorter run RF and longer run serial. One needs to be long if the radio antenna is raised. Regular old CAT5 is an interesting idea! I’m curious about your mention of the link LED. I monitor the link LED on the rover side of the telemetry pair, but since I’m mostly solo, I haven’t spent much time looking at the base station radio. In your experience, has base radio’s LED been really helpful to you in troubleshooting?
Thanks for the suggestions of twisted shielded cable and maybe trying shielded CAT5! – any ideas about a smart max cable length? I could just try out a long run, but it’d be nice to have a high target. I’ve seen mentions of a max of 50ft when working at 57,600 baud, but cable capacitance seems to be a factor too. I’ve also read that you can get much further in certain cases. 100ft would be amazing since I’ve run into situations where I want the radio antenna on a ridge and the facet in a clearing. Thanks!
I’ve had good success with extremely long analog signals in situations where I didn’t have a choice, in the neighborhood of 80’ I think, with shielded cable grounded at one end. BUT, I’ve also had situations where the noise environment caused havoc at 10’, because we are basically building an antenna to pick up noise, in addition to carrying your signal.
I suspect the answer will generally be dependent on location.
I really didn’t mean to say the RF cable was a weak link. Frankly it’s the sturdiest cable in the whole setup.
In my opinion the weakest link in the Facet is the JST connector for the radio port and the resultant very thin wires. A twig will yank the little pins right out of the JST cable end, or yank the cable out of the socket.
Don’t forget the serial data is TTL level, not RS232. RS232 was designed for longer cables. So if you’re reading up about cable lengths be sure you’re reading about TTL serial.
So when I use my RFD900x radios, I keep the radio right on the Facet to protect that weak JST TTL serial cable.
But I digress.
To answer your question, at the base I look at the LED on the RFD when setting it up to make sure the link is connected and data is flowing, so basically at the start of the job. The LEDs are not bright so I keep them where I can see them. At the rover I look at the LED more frequently, usually when my solution latency starts increasing.
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For my longest range radio work, I use a pair of Satel UHF radios. I have an FCC license for them as it’s required in the US. These radios, like
most surveying equipment use RS-232 with DB9 (aka DE9) or better modern connectors. With this, I can use a sturdy serial cable.
I built SparkFun TTL to RS232 converter into a very small package with JST and DB9 connectors. I put one of these on each Facet and use old-school RS-232 cables. My converters are mounted on the Facets to keep the JST cable short.
Thanks @toeknee and @rftop for these very helpful replies. I think I’m almost to a solution here. A UHF system that requires a license(!) sounds like the holy grail, but I’m committed to the RFD900x for now. Knowing that RF environment will be a big factor in how well this works, I would like to go for a resilient solution. I was already planning on using an adapter box attached to the base facet for cable management due to the JST connector, and your TTL to RS-232 package sent me in the right direction. So I’m thinking: If I want a longer cable length, why not go with a balanced solution using RS-422 or RS-485? This is my first rodeo with long distance serial, so maybe I’m off base. Amazingly, I have a free supply of super long cables that fit the bill. I’m wondering if two of these breakouts from SparkFun would work on either end of the Facet - RFD900x connection to get me where I need to be?
The voltage spec looks like both the Facet radio port and my 5V supply on the radio side would be fine to provide power, but this is a half-duplex arrangement. Would that be a problem for the RTCM correction data? Or does it need full-duplex communication over that line?
Just for Giggles :
RFD900x once had a FW version for Mesh support.
By adding a 3’rd radio (to act as a repeater), you open up many possibilities for local Base/Rover combos, not just this one specific use case.
I’d keep it simple to start and try extending the TTL serial and see how that works.