rtl-sdr and similar rf devices (1 Viewer)

Tekamthi

Pilgrim
Joined
Sep 18, 2018
Messages
24
Location
Great Lakes
I've recently swapped out my cheapo baofeng 2-way (i bought for $30 off CL) for an rtl-sdr dongle (i also bought for $30 off CL) that connects to my phone while travelling... i find having an RF device useful primarily for listening to the rail channels and figuring out wtf the train i'm on is stopping, alleviating my anxiety somewhat when this occurs. Sometimes it helps me decide which one to get on, too. As a bonus, its something interesting to listen to in strange towns i've never been to, without hitting wifi or data charges etc.

Its all still a little fresh, and I'm not sure I love the change, as this new setup is a little more fragile and less instant than just grabbing the old drop-proof/water-proof radio i used, but on the upside, it cuts weight and bulk of adapters and charging cables etc, and seems to open up some additional capabilities, albeit non-essential.

On my wishlist, i'd love to grab something with a wider frequency range (hackrf one?), but imagine this would be a little silly to travel with, and is beyond my budget anyway ($30 vs $300). Have any of my fellow tech nerd travellers tried anything similar? Have any pros/cons to share? Or can recommend specific RF devices that are better suited than others for travel?
 
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ali

Wanderer
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Aug 9, 2020
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132
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BC
What antenna are you using, or can you use the dongle without an antenna?
 

Tekamthi

Pilgrim
Joined
Sep 18, 2018
Messages
24
Location
Great Lakes
I've been using the chrome-finished telescopic antenna mast that came with the bundle... its about 20cm when collapsed and close to a meter fully extended... like a single ear off the old tv rabbit-ears from a few decades ago. Its very light and easily packed.. maybe 0.5cm in diameter. The antenna base is a bit of a nuisance, but altogether still easier to carry around the full radio handset i was using for these things previously, and I've threaded the cable strategically through my pack, just connecting the dongle and extending the antenna when i need it... setup this way its reminiscent of a WW2 backpack radio operator, though my pack is mostly filled with other non-radio stuff.

I didn't order it from this vendor directly, but it is very similar to the package listed here: Nooelec - Nooelec NESDR SMArt v4 Bundle - Premium RTL-SDR w/ Aluminum Enclosure, 0.5PPM TCXO, SMA Input & 3 Antennas. RTL2832U & R820T2-Based - https://www.nooelec.com/store/nesdr-smart.html

I haven't had a lot of time to look into what's best, and though I had become reasonably proficient with my baofeng, I am still a bit of an RF noob. This antenna was just the most convenient to get started, and as I understand things, the variable length is better-suited to multiple frequencies, whereas the fixed-length options are best for specific frequencies. The signal on some channels is less than stellar, but works okay for getting an idea of what's going on around me anyway. The baofeng i had with its default antenna seemed to work a little better tbh, but just barely.

For anyone else considering doing the same, note that my phone specifically required a usb 3.0 otg adapter (another CL purchase, $10) in order to recognize the dongle -- the more common usb 2.0 otg adapters just didn't work at all, but that may just be my phone, ymmv. Also should note there is no transmit, compared to a 2-way radio, but i never transmit when travelling anyway.
 
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ali

Wanderer
Joined
Aug 9, 2020
Messages
132
Location
BC
Thanks for posting all this info. I don't have much useful to add, but i have been pondering picking up a scanner so it's cool to find there is a more "open source" solution than going with a Baofeng or Uniden.

The size of the dongle is really great, as is being able to multi-purpose your existing smartphone. Having used some other USB dongles on my phone before, i think the challenge might be using it when your battery is low. Is there some kind of splitter cable you can use to plug your phone into a power pack and also leave the OTG adapter for the radio?

Putting the antenna on your pack seems like a good idea, since those things can get huge. I don't know much about radio either. From what I've gathered online the whip antenna formula is something like 300/x where x is the MHz. So picking up 160MHz means 300/160 =~ 1.8m antenna. Apparently you can quarter it to still get decent reception, so 45cm might work. Still pretty long, eh.

On waterproofing: i've noticed a lot of bike/moped couriers using a special waterproof bag for their phone. Most of them could fit a dongle as well (although you would still have the antenna cable coming out). One of those is on my list to buy before the next time i'm out traveling.
 

Tekamthi

Pilgrim
Joined
Sep 18, 2018
Messages
24
Location
Great Lakes
yeah your knowledge is roughly where I'm at, from the sounds of it. I can't really learn about these things just by sitting and reading though, so now that I have one to experiment with I'll probably increase my skills via trial and error.

When I next settle in an apartment or room of some kind for an extended period with a laptop, i'll experiment a bunch, and hopefully by next summer have this setup well refined. From what i've heard even just a flexible cable of the right type and length could serve as an antenna when extended out straight... possibly a small array of these improvised together just right might be able to pick up a lot of signals. I'm thinking i could keep a coil of whatever antenna cable and then mount my antenna(s) in a tree or on a handheld branch for good effect.. maybe I'm dreaming though

As for waterproof, I'm gonna try a 3 ft usb extender cable that runs into my pack, and keeping the dongle tucked in there permanently (it does get a little warm after being on for a while, mind you). That just leaves the phone with the cable connected to worry about.

Battery consumption certainly is higher with this thing running, but it hasn't had drastic effect as I mostly use it in brief stints. I've heard that some otg adapters have an extra port built-in for power, but I could not find any of the usb 3.0 variety that i needed. There were a few usb 2.0 options with this feature built-in. I imagine a powered usb hub could do the job too, but now we're talking about adding a lot of extras...
 
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ali

Wanderer
Joined
Aug 9, 2020
Messages
132
Location
BC
So, i recently picked up one of these to use on my journeys. I just added a USB OTG adapter and rubber ducky marine radio antenna. The marine radio antenna is tuned exactly around the frequencies that i care about (136-174MHz) and only cost a couple of bucks. The whole kit is way smaller than i anticipated, check it out:

rtlsdr.jpg


The dongles either come with R820T2 or E4000 tuner chips. From what i understand, the R820T2 is the most common and it has a cleaner and more consistent reception, but it may use a bit more power. The E4000 has a bit more patchy reception, and a gap around a particular set of frequencies used by pilots (which i don't care about), but it might use less power, so i opted for that one.

For Android apps, RF Analyzer is a free/open source app that gets the radio working (you also need to install Rtl-sdr driver app). I haven't played around enough yet to see if it's worth upgrading to one of the paid apps, but so far this one does the job of finding a frequency and listening in.
 
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ali

Wanderer
Joined
Aug 9, 2020
Messages
132
Location
BC
Over the weekend i messed around a bit more with this RTL-SDR and the various apps, and it's definitely a bit more DIY than i imagine a dedicated scanner to be. Here are some more notes for anyone who is curious.

It helps to program in a bunch of the frequencies you want ahead of time, because scanning around trying to find them by hand is a pain in the ass. When you're listening, use the "squelch" control to set a threshold just above the static so that the radio stays silent until a louder signal comes along.

Apps...

I liked using CubicSDR on my PC (Mac/Windows/Linux) to test the dongle and find the main frequencies i was interested in.

In CubicSDR you can set up a bunch of "active modems", which means listening to multiple frequencies at the same time. If you hit "S" you can switch it into solo mode, then you can tab between the active frequencies to do manual scanning. The shitty thing is that changing the active modem you are listening to does not retune the radio itself, but if your radio center frequency is somewhere in the middle of the range of modems you are listening to, you should be okay. If you are getting a weak signal, you can tune the radio center frequency (left and right arrow) closer to the current active modem to get better reception.

I'm about 1km from the CP yard using a marine rubberduck antenna and pretty much get crystal clean reception on all the channels in use. Here is an action shot of me picking up a new frequency i didn't expect, turns out it's CN channel 3 (CN yard is 8km away).

picking-up-cn.png


CubicSDR stores its bookmarks in an XML file that you can save and load. I'm thinking of writing a little script that will generate a list of bookmarks in that format so it'll be easy to set up a bunch of interesting frequencies without having to tune them all in using the UI. I'll post it here if/when i get around to it.

On the phone (Android) RF Analyzer is great because it is fully-featured and free, but you need to manually enter the bookmarks one by one, and if you touch the touchscreen anywhere then it will retune the radio and fuck up your listening.

I also tried SDR Touch (i won't link because it's not open source), which you need to pay for if you want to use the waterfall view, but the demo version still lets you load bookmarks from XML (different schema to CubicSDR) and tune in to certain frequencies. It also has the problem of if you touch the touchscreen it will fuck up your listening, but at least it's just one click on the bookmark button to get back where you were (RF Analyzer needs a bit more fiddling).

I have no idea why these apps don't have a "lock" button, but i guess they are not designed for people who want to monitor a single channel, or switch between a couple of pre-programmed channels. This definitely feels like gap in the market. It doesn't seem like it should be too hard to write an Android app that could simply scan through several known channels without all the visualizations and pinch to zoom gimmicks. If anyone knows of one, let me know.

Other tips for radio noobs...
  • for most of the frequencies you can pick up in the VHF range, they are encoded as FM - you can make out some kind of speech if you set your software to AM, but it's much clearer if you set it to FM and put your bandwidth around 20kHz
  • there is also "FM narrowband" which seems to just be FM but set the bandwidth thinner than 11kHz, here in BC that seems to mostly be used on the logging roads, so i will check that if/when i head into the back country
  • you might notice two peaks happening close to one another at the same time when people talk, it turns out this is because you are seeing both the Tx/transmit which is the signal coming direct from the sender's radio and also the Rx/receive which is the frequency it got rebroadcast on after getting boosted by a repeater - the receive frequency will be stronger and it's the one you want
All in all i am happy with the purchase. Even if it's a bit fiddly, it's fun to play around with, and it's pleasingly small. I love tiny gadgets that do a bunch of cool stuff. Picks up local FM radio stations too, which is a good way to test if it works. Or you can check the weather report on 162.4MHz.
 
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ali

Wanderer
Joined
Aug 9, 2020
Messages
132
Location
BC
Update on RTL-SDR... I now think using FM narrowband is better than FM for railfanning, at least in this neck of the woods. There is a bit less noise picked up when your bandwidth is around 12kHz instead of 20kHz.

I also think it is worth the money to pay the $10 for an SDR Touch key, mainly because you can't listen to multiple channels at the same time, so it's useful to keep the waterfall view open so you visually get an idea of which channels are active and then quickly switch over. I also highly recommend putting your device in landscape mode, because that makes it much easier to select from the bookmarks and adjust the squelch without accidentally tuning to a different channel.

For Canadian radio nerds, here are some bookmarks for CubicSDR and SDR Touch that have the most frequently-used CN and CP channels as far as i can tell. This doesn't include utility channels, which are far more entertaining to listen to but not very useful for finding out which trains are coming and going. You will have to rename from .txt to .xml to load the into the apps.

I think i might write a script to load in all the "AAR" channels like on the Baofeng so you can easily jump to a "Baofeng address" like when someone says "AAR 87" you know that's 161.415MHz = CN channel 1. But i'll do that when i'm bored someplace with electricity. If anyone has a BNSF hi-line frequency list, let me know.
 

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