Hunting a 450mhz signal? - Page 1
Forum section: electronics / rf-microwave

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: July 04, 2016, 06:25:21 pm
content: Last night I went out and got some wok's to make dishes for a long range wifi link. There is a STRONG signal I can recive all over my town for about 20 miles at 450.xxx Im not sure what the heck it is, very narrow ear piercing droney sound. Anyways I would like to investigate this signal more I figured maybe I could use a wok to build a dish to get HI directional and find the source. I know I wont get much gain on a 14 inch dish at UHF but it is a strong signal. What would you guys suggest I feed the wok with a biquad?

author: donmr
author_rank: Regular Contributor
timestamp: July 05, 2016, 11:10:16 pm
content: This is why radios need band-pass filters!That can start with an antenna that is less sensitive to other frequencies.Here in the US, there is a shared allocation for "Radiolocation" (Radars) and "amateur" from 420.0 to 450.0.Are you near the coast or anything like a radar installation?Above 450.0 is allocated to "Landmobile" which can include linking systems between sites that might always be on.If you can narrow down the frequency you can lookup licenses (civilian only) at <http://wireless.fcc.gov/uls>But its probably legitimate so you have to live with it.

author: retrolefty
author_rank: Super Contributor
timestamp: July 06, 2016, 12:16:53 am
content: AT 450Mhz it's not hard to build and tune out a specific frequency using a high Q cavity filter to suck out the signal with little attenuation to other even near by frequencies. Hams have been doing so for decades on their longer range repeater sites.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: July 06, 2016, 01:30:48 am
content: You huys have this all wrong, the signal is strong but narrow, it doesn't bother me or cause bad aliasing etc... I just want to know what in the heck it is on the commercial band. I am in southern california but im at least 150 miles from the coast so I dont think it would be raido location. Im just trying to figure out the easiest way of figuring out the orgin of the signal and then maybe figuring out what it is so I can run it through some DSP in GNU radio.I have a commercial 450-470 9dbi 5 element yagi but the signal is so strong it seems to pierce right through the back reflector, so im trying to figure out how to build a very higly directional antenna for signal hunting. So a WOK and a bi quad tuned to 450? Maybe a quarter wave ground plane, Im not sure the best way to feed the reflector. Like I said gain is not terribly important here but I dont want to make a feed so bad I cant recive the signal or that wont have a tight beam widthI do live near 29 palms marine base which is the biggest base in the US but im about 20 miles away, looking up the license is a good idea I dont know why I didnt think of that, but if this were a military signal how would I determine that? Although I doubt it is it may be important for a future signal hunt. I know the FCC shows bands allocated for military use but im sure not all signals from the base are in those bands, they can use commercial HTs and things right?

author: donmr
author_rank: Regular Contributor
timestamp: July 06, 2016, 09:15:08 pm
content: Try moving away from your house in different directions. If you can find the edge of the "strong signal" region you will have an idea where it is coming from. (You might find its very local too)

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: July 07, 2016, 06:20:00 am
content: Well dogging through fcc search i cound the acual station, even drove by its antenna it has something to do with fm broadcast... An auxillary pick up or something. I will most post more info including signal tommorow maybe someone can help me figure out exactly what the signal does and if i can decode it.

author: David Hess
author_rank: Super Contributor
timestamp: July 07, 2016, 08:04:55 am
content: Quote from: rwgast_lowlevellogicdesin on July 06, 2016, 01:30:48 amA 2 or 3 element quad or similar antenna optimized for minimum sidelobes would be idea however your 5 element yagi should work fine. If the signal is that strong, then there is a good chance it is leaking through your feedline or enclosure directly into your radio.One solution to the signal leakage problem is an active or offset attenuator. I built the adjustable LC oscillator version from QST long ago but most people like the crystal controlled variation:http://www.homingin.com/joek0ov/offatten.htmlhttp://www.theleggios.net/wb2hol/projects/rdf/a_atten.htm

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: July 08, 2016, 09:02:36 pm
content: Ok here is the deal I found the signal buy going through the FCC database and drove by the facility where its coming from. In order to get any directonal signal loss using this antennahttps://www.tessco.com/products/displayProductInfo.do?sku=42395I had to get my LNA out of the chain and set my dongles gain down to 2.7! This is strange considering the broadcast is at least 5 miles away and is only 9 watts ERP. Here is the stationsFCC listing 450.98mhzit says it is used for Broadcast Auxiliary Remote Pickup, I couldn't find a definition for this exact term online, but after doing some research I think the signal is a digital route of 93.7FM which is about 30 miles away and is 95.5FM in my town. Here is the signal, these were recorded my yagi pointed at station and AGC turned on, even 3.7db of gain was overloading and causing aliasing.This is a recording of just the IF or selected bandwidth, it works fine in audio programs and SDR# but im not sure if it work in other SDR software. It is also not as clear as the the full IQ file as far as the waterfall, it takes a lot of playing with contrast, offset, and range in SDR# to get it reasonable. I uploaded this because it is a 10mb file with 40 seconds worth of data.https://dl.dropboxusercontent.com/u/27653605/IFonly.wavThis is a minute and a half file a 80mb it is recorded regular and is very clear in SDR#, should work in all SDR programs. I recorded it using .25msps i figured the less bandwidth recorded the smaller the file.https://dl.dropboxusercontent.com/u/27653605/FullIQdot25msps.wavSo this is my thing, its why I love SDR I want to find signals and demodulate/unencrypt them if possible. My problem is if is not a cw or analog voice signal, and isn't in the SDR signal wiki I dont know how to analyze the signal of figure out its modulation. Sometimes using Audacity works well but thats really only for low speed binary stuff FSK, OOK, etc. So knowing the info I posed above about the station would you all agree its an FM station being transverted and encoded to a digital stream to route from town to town. If thats it where would I start trying to figure out its modulation scheme etc...?? Also why does the standard 3 inch rtl sdr antenna pick this signal up so strong all over a 20 mile radius, and it breaks through my yagis back reflectors when the gain is to high. Admittedly I am higher than the antenna and my yagi is direct line of site, but I receive this thing on the junk antenna all over and im not in line of site!!! How??!! They have an antenna design I have NEVER seen but its still only a 1 watt signal?