xantegh
Member
Seems like AMC is transmitting over both AM and what sounds like P25 on 349.400 Mhz .. is this correct?
Are military using digital for comms with aircrafts?
Are military using digital for comms with aircrafts?
- Status
No, aircraft comms are still AM for the most part, some FM and digital on SATCOM systems.
349.4 is definitely still AM as I monitor the inbound aircraft on it daily.
You might be close enough that you are getting an image from the JBLM trunked system...
- Chris
349.4 is definitely still AM as I monitor the inbound aircraft on it daily.
You might be close enough that you are getting an image from the JBLM trunked system...
- Chris
xantegh
Member
Thank you Chris for the prompt response. my understanding is that McChord Field Airport (KTCM) is within the JBLM vicinity, so, i'm wondering how they would use the same frequency and wouldn't they interfere with each others?
devicelab
Whacker Extraordinaire
What scanner and antenna are you using? Also what was your location when you heard the above traffic on 349.4?
xantegh
Member
I'm located in Snoqualmie City. my setup is basically a diamond discone on the roof feeding a MCA204 multi-coupler from Stridsberg Engineering to a couple of BCT15X. I believe it's pretty good quality setup.
devicelab
Whacker Extraordinaire
I would speculate that the multicoupler is giving you too much gain into the scanner and that's where your intermod is coming from. With your discone you don't really need the preamp on the multicoupler.
As a test, you could just feed one scanner directly and see if you still get the intermod. I'm guess you won't. Stridsberg does make their passive model which may work better for you. However, a simple fix would be to put an attenuator between the antenna coax and MCA204. There are a few on Amazon which would work fine. They're not very expensive.
If you have a USB SDR dongle then you can probably replicate the conditions and it'll be easier to 'see' the intermod thru the SDR.
Obviously if this isn't a major problem then it may not need fixing. Just be aware of it.
JBLM = Joint Base Lewis McChord
Fort Lewis and McChord AFB used to be separate facilities, but DoD combined facilities. (For example: Naval Station Pearl Harbor and Hickam AFB merged, as did Fort Dix, McGuire AFB, and NAS Lakehurst.)
xantegh
Member
Thank you @devicelab .. while I test and revert back , I thought of posting a link to a recording, hopefully it helps identifying the root cause.
IMG_3444.mov
update: I have just received a normal reception from a C17 heading to JBLM at 11:10 PM
IMG_3444.mov
update: I have just received a normal reception from a C17 heading to JBLM at 11:10 PM
Last edited:
devicelab
Whacker Extraordinaire
Hmm from your movie it sounds like you have some local RFI. You shouldn't hear any tones like that. AM mode is unique in that it'll pick up a lot of electronic hash -- so it's a good indicator of other RF nearby. I just checked my SDR and 349 Mhz is nice and quiet here -- just perfect white noise.
Do you have a RTL-SDR by chance? If not then you should pick one up and play around with it. There are a ton of videos online to walk you through setting it up -- which is much easier these days than when they first came out.
It would let you 'see' the spectrum though and possibly track down any problems.
Here are a couple examples:
(Upgraded model -- slightly higher cost)
(A good starter kit)
And yes, you'd plug your discone into the dongle directly. (You'd need a BNC to SMA adapter of course.) Using SDR# (a free application) the software does all the work for you. You have gain adjustments and a multitude of features. You can pipe out digital signals to DSD+ for decode.
Do you have a RTL-SDR by chance? If not then you should pick one up and play around with it. There are a ton of videos online to walk you through setting it up -- which is much easier these days than when they first came out.
It would let you 'see' the spectrum though and possibly track down any problems.
Here are a couple examples:
Amazon.com: NooElec NESDR Smart XTR SDR - Premium RTL-SDR w/Extended Tuning Range, Aluminum Enclosure, 0.5PPM TCXO, SMA Input. RTL2832U & E4000-Based Software Defined Radio: Computers & Accessories
Amazon.com: NooElec NESDR Smart XTR SDR - Premium RTL-SDR w/Extended Tuning Range, Aluminum Enclosure, 0.5PPM TCXO, SMA Input. RTL2832U & E4000-Based Software Defined Radio: Computers & Accessories
www.amazon.com
(Upgraded model -- slightly higher cost)
Amazon.com: RTL-SDR Blog V3 R820T2 RTL2832U 1PPM TCXO HF Bias Tee SMA Software Defined Radio with Dipole Antenna Kit: Electronics
Buy RTL-SDR Blog V3 R820T2 RTL2832U 1PPM TCXO HF Bias Tee SMA Software Defined Radio with Dipole Antenna Kit: Radio Antennas - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
www.amazon.com
(A good starter kit)
And yes, you'd plug your discone into the dongle directly. (You'd need a BNC to SMA adapter of course.) Using SDR# (a free application) the software does all the work for you. You have gain adjustments and a multitude of features. You can pipe out digital signals to DSD+ for decode.
xantegh
Member
xantegh
Member
@devicelab ,
Just tested it with an SDR and Cubic SDR software, it's clean on the waterfall while hearing the same noise on the uniden.
both SDR and the scanner are connected to the multi-coupler.
video: https://www.dropbox.com/s/z1gta6cv9epvq08/IMG_3452.MOV?dl=0
Thanks again for the help.
xantegh
Member
@devicelab , surprisingly, i found out that the issue is 1 scanner.
have a look at the video below.
IMG_3454.MOV
any idea?
have a look at the video below.
IMG_3454.MOV
any idea?
devicelab
Whacker Extraordinaire
I have seen weirdness with scanners that close to each other. I wouldn't think it'd affect the 15x since they're enclosed but you never know. Try taking the 2nd one out of its bracket and see what happens.
I presume they both were plugged into the coupler? If you unplug the left scanner, do you still get the noise? If so, then try a whip antenna or even a generic rubber duck and see if you still get the noise.
BTW, do you have unused ports terminated?
The only thing I could think of is some kind of ground loop or a power issue. Do you run these off AC mains using the switching adapters or DC power?
PS. Lastly you could check coax cables It doesn't take much to create a very tiny break in coax.
I presume they both were plugged into the coupler? If you unplug the left scanner, do you still get the noise? If so, then try a whip antenna or even a generic rubber duck and see if you still get the noise.
BTW, do you have unused ports terminated?
The only thing I could think of is some kind of ground loop or a power issue. Do you run these off AC mains using the switching adapters or DC power?
PS. Lastly you could check coax cables It doesn't take much to create a very tiny break in coax.
ecps92
Member
Does the SDR show anything in the room, if hooked to an indoor whip?
Have you tried swapping power cords for the scanners?
xantegh
Member
@devicelab ,
Yes both scanners are connected to the same multi-coupler with 3 used ports out of 4. actually 2 ports to BCT15X and another going to an SDR. the forth port is not terminated unfortunately. i have asked about some time ago and i understood that it is not necessary. so, i ignored it
Scanners Power source is DC supplied from a Switching Power Supply while the multi-coupler is powered thru the provided AC Wall Adaptor.
i did swap as well the coax cable between the scanner; the noise stayed on the same scanner and by the way, the noise lasted for 30-40 minutes and then disappeared .
devicelab
Whacker Extraordinaire
IIRC, I received a switching AC adapter for the MCA204 and I replaced mine with a Jameco linear. You might want to consider powering the coupler thru your power supply -- since it doesn't use much power. You shouldn't have any issues but you never know.
Still seems odd that you get the noise on one scanner. If the noise comes back then try plugging the antenna directly into that particular scanner.
PS. Yeah I used (2) couplers on my MCA204 and I never really had any issues either way. At least from the SDR perspective I couldn't see any change when I had them on or off.
devicelab
Whacker Extraordinaire
From today: https://www.dropbox.com/s/0tak1v2l5d6xo3f/AMC28.mp3?raw=1
I rarely hear the tower since I'm 15+ miles away.
I rarely hear the tower since I'm 15+ miles away.
Did you swap the whole cable, using the same port from the multicoupler? Swap cables between scanners, leaving them on the same coupler ports? Did you just swap the cables at the coupler ports? Some other arrangement? All of the above? (Trying to isolate if you have a cable, port, or scanner problem.)
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