Carriers or Interference on Guard Freqs

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Gator596

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Hi all,
Why is it that 243.000MHz and other mil air freqs sometimes have what sounds like an "open microphone" - no voice, just the sound of an "empty" transmission? Is this what is called a "carrier"? If so, what is the purrpose of it? Or is it just that the mil air band is easily affected by electrical noises from computers, flourecent lights, microwave ovens, etc?
It is frustrating to have the scanner lock up on to these. Should I be patient and keep listening? Will somebody eventually say something over the air?
Sorry for all the questions - I'm fairly new to mil air.
 

splash1

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I live in eastern Iowa and have that problem also. Scanner constantly locks up on 243.000 and 121.5, so i have to lock those out .I would also like to know whats going on with those two freqs.

rick
 

Tech792

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Same here in central NJ. Pretty odd its on both guard freqs. On days when its bad, I use the built-in attenuator on the scanner.
 

kny2xb

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I think that some interference/leakage is coming from our cable TV line. At times in my room, and outside our front door [where a junction box? is mounted] I've experienced the 'open mike' effect on 121.500. I'll take my hand-held outside with me while I'm washing my car in the front yard, and I've had to lock-out 121.5

Just for reference, 121.5 x 2 = 243.0
243.0 is the 2nd harmonic of 121.5
So if something is screwing up 121.5, it may also show up at 243.0

This may or may not mean anything to anyone having this problem, just my experiences is all.
I hope that it helps out.
 

Alliance01TX

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121.5 and 243 Guard Freqs..

Often we show the same issues in North Texas and we have heard several possible reasons, some of which coinside with interference we have on ham bands in both vhf and uhf spectrum.

First, if you are in a large city, several known issues are present with various cable companies having 'leaky' rf from distribution points. Normally this appears to impact the uhf bands more from what we can tell... If you get the same rf interference out of town we suspect the next likely canidate below...

Second, the rf test gear/test radios and CAP gear used to test and-or exercise for SAR missions is very close to the same two frequencies and unless you have a great filtering capability, this would be a suspected and most likely canidate for the 'open carrier' that we all are hearing from time to time. The test gear signals can be in low-power or high power mode, especially when running an exercise. So if folks are testing at 50 watts or higher, or from a plane, we suspect this will 'open up' reveivers for many miles in radius.

Finally, as the 121.5 and 234.0 will be phased out and in the digitial mode in our future, much like the ERB's now that no longer use the above freqs, and move to the ~408.mhz range and digital, this will be a mute point in years ahead perhaps?

You might get a couple folks to try an trianglate the signals and determine the general location the rf is coming from, like an airport, military base or plant...also check NOTAMS to see if any exercises are on tap in the area too...

Bill
 

datainmotion

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CATV leakage is a high probablity in many areas.
Check the frequencies between 108-113 for an odd warbling (or similar sound). Comcast uses 109.275 AM here in Denver for their "Sniffer" transmitter (warbling).

Since CATV occupies frequencies straddling most of the VHF and UHF frequencies (and in many areas, HF for reverse ops such as broadband internet services) and tries to achieve at least 0 dBmV at the back of the cable box, signal leaking from a loose connector or damaged subscriber or distribution line can cause significant interference.
 
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KE5CCA

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As I was leaving for work one day, I noticed a Cox DF SUV driving through our subdivision ... so I chased him down.

:D

I forget how many antennas were on top, but not enough to have really good resolution. He was a super nice guy to talk to, but either didn't really know anything about RF or was holding out on me. IIRC, the threshold for them to take a look at something noisier -20 db.

The basic impression I got from him was that as long as the Feds are not complaining about RFI and they mosey through some streets every once in a while, they aren't going to get in too much of a hurry to fix anything.

Which is a shame, because as I drive around my Kenwood is constantly unsquelching almost to the point of being annoying. And I don't need any DF gear installed to tell me something is wrong!!!
 
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datainmotion

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While more interference exists than just CATV (like gas pumps with my Spectra :lol:), CATV is always worth considering when trying to find RFI.

Having spent nearly 8 years chasing down leaks myself, I can tell you that the "hot" ones get the most attention. If you want to know more, you can always PM me.
 

datainmotion

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I think that some interference/leakage is coming from our cable TV line. At times in my room, and outside our front door [where a junction box? is mounted] I've experienced the 'open mike' effect on 121.500. I'll take my hand-held outside with me while I'm washing my car in the front yard, and I've had to lock-out 121.5

This may or may not mean anything to anyone having this problem, just my experiences is all.
I hope that it helps out.
I have written an RR wiki article on CATV RFI. You can read it here
 

eorange

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Finally, as the 121.5 and 234.0 will be phased out and in the digitial mode in our future, much like the ERB's now that no longer use the above freqs, and move to the ~408.mhz range and digital, this will be a mute point in years ahead perhaps?
Just to clarify:

Both freqs serve(d) a dual purpose - they carry distress beacons and voice guard traffic. Only the beacons are being transitioned to 406. 121.5 and 243.0 will still be around to carry civilian and military voice guard traffic.

---

This is an interesting thread, because I almost always hear an open carrier on 243.0. No other neighboring freq is affected.
 

Gator596

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Spacing

Thanks to all who have contributed to this thread!
Looking into this further, I see that 121.5 has a fair amount of "spacing" set asside on either side of the spectrum to protect it from spurious trans and bleed overs and I imagine 243 does too.
What I'm hearing sounds level and constant in power and tone. Like a purposeful transmission. It doesn't snap crackle or pop or wheeze in and out. I'll be driving down the road and it will suddenly happen to BOTH freqs at the same time, but never in the same geographical location like you would expect if it was from leaky connections.
I dunno - it is wierd! Any other theories?
Thanks again,
 
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