RF question - Plane radio aliasing?

In summary: I see you're in the USA. That's about the top of the band for AM broadcast radio here in the US, so it's not likely that your radio is out of range. If it's still an issue, you could try calling the station and asking them to send you a sound file of their broadcast so you can test your radio.
  • #1
cctxbmx
5
0
RF question -- Plane radio aliasing?

I hope this is the right place to post it.

My buddy and I were working on a jet earlier and turned on the HF communication system to perform an operational check out of the system. We tuned the radio to 2.5 MHz and picked up a local AM radio station. The station is 1250 KHz.

Why are we able to receive this frequency when tuned to another exactly twice the frequency? We tried it on another jet and got the same thing.

I searched google and couldn't find anyting.
Any help would be awesome, thanks.
 
Last edited:
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  • #2


Interesting! It is not twice the frequency, one is 2.5GHz, one is 1.25MHz, it is 2000 times different!
 
  • #3


cctxbmx said:
I hope this is the right place to post it.

My buddy and I were working on a jet earlier and turned on the HF communication system to perform an operational check out of the system. We tuned the radio to 2.5 GHz and picked up a local AM radio station. The station is 1250 KHz.

Why are we able to receive this frequency when tuned to another exactly twice the frequency? We tried it on another jet and got the same thing.

I searched google and couldn't find anyting.
Any help would be awesome, thanks.

The HF band is nowhere near 2.5GHz. You're misreading something.
 
  • #4


yungman said:
Interesting! It is not twice the frequency, one is 2.5GHz, one is 1.25MHz, it is 2000 times different!

lol

berkeman said:
The HF band is nowhere near 2.5GHz. You're misreading something.

My mistake, I meant to put 2.5MHz.
The radio only let's me tune from 2MHz to 29.999MHz.
 
  • #5


So when tuned to 2.5MHz, you picked up a radio broadcast on 1.25MHz? How far is that local broadcast antenna from the plane, and do you know what power level they transmit? You can get more info on the station by googling their call letters.
 
  • #6


berkeman said:
So when tuned to 2.5MHz, you picked up a radio broadcast on 1.25MHz? How far is that local broadcast antenna from the plane, and do you know what power level they transmit? You can get more info on the station by googling their call letters.

Yes.
wiki said 1kw day and night. It is roughly 17 miles from the jet.

This site seems more credible though
http://radio-locator.com/info/KZDC-AM
 
  • #7


cctxbmx said:
Yes.
wiki said 1kw day and night. It is roughly 17 miles from the jet.

This site seems more credible though
http://radio-locator.com/info/KZDC-AM

Hmm, even 25kW is not that big for being 17 miles away. If it were a mile away, I could see it maybe overloading the input stage of the HF radio and bleeding into the 2x frequency. Was there anything at 3x the 1.25MHz frequency? Are you able to try it at night when the transmit power appears to be lower?

It could be a problem with the radio. Are there any other planes available to you with the same HF rig?
 
  • #8


BTW, I noticed at the website that they have different locations for the daytime and nighttime transmitters. Are you sure that the daytime transmitter is still 17 miles away? There was something mentioned about new construction...
 
  • #9


berkeman said:
Hmm, even 25kW is not that big for being 17 miles away. If it were a mile away, I could see it maybe overloading the input stage of the HF radio and bleeding into the 2x frequency. Was there anything at 3x the 1.25MHz frequency? Are you able to try it at night when the transmit power appears to be lower?

It could be a problem with the radio. Are there any other planes available to you with the same HF rig?

I work from 7am to 4pm but I might be able to get another guy from grave shift to try it on a few planes. We had tried it on two different planes today about 100 meters from each other and got the same result. But yes all our planes are equipped with them. Tomorrow I will try multiple haha
 
  • #10


berkeman said:
BTW, I noticed at the website that they have different locations for the daytime and nighttime transmitters. Are you sure that the daytime transmitter is still 17 miles away? There was something mentioned about new construction...

All three transmitters are roughly 16-18 miles following roads, maybe 12-15 line of sight.
 
  • #11


Hmmm

Give a listen to that station on a good radio (like a car stereo) and listen for distortion. If it sounds scratchy and 'S' sounds are exaggerated and "SH"-y, it might be worth a phone call to that station's engineer. If he's clipping and making a harmonic he'll want to know pronto. tell him you're receiving him on twice his carrier.

It could be your radio, though.
A common IF frequency is 455khz so your local oscillator is probably 2955khz (2.5mhz + 455khz),
which heterodynes with 1250khz to make 1705khz,
which heterodynes again with 1250khz to make 455..
in other words there's an "image" at half the frequency of interest and your radio doesn't have real good "image rejection"..

i think they call these "Mixer Spurs"

and they're discussed in simple terms around page 23 here.

http://www.google.com/url?sa=t&rct=...xtnTDA&usg=AFQjCNFL4-aaqrjyCbg6dLpMxP3yIaGrqwIs the "HF communication system" a high quality one? If so it should have decent image rejection.

Question to any genuine radio guy - is above plausible?

old jim
 
  • #12


good thoughts Jim.

this also could be just plain ol' intermod mixing

Dave
+
 

Related to RF question - Plane radio aliasing?

1. What is RF aliasing in plane radio?

RF aliasing in plane radio refers to the phenomenon where two or more radio signals overlap and interfere with each other, resulting in distorted or corrupted data. This can occur when the sampling rate of the radio is insufficient to accurately capture the signal, leading to errors in the reconstruction of the original signal.

2. How does RF aliasing affect the performance of plane radio equipment?

RF aliasing can significantly impact the performance of plane radio equipment by introducing errors and distortions in the received signal. This can result in poor quality audio or video transmissions, loss of data, and even complete signal failure. In extreme cases, it can also damage the equipment itself.

3. What are the common causes of RF aliasing in plane radio?

The most common causes of RF aliasing in plane radio include inadequate sampling rates, improper filtering of the signal, and overlapping frequency bands. Other factors such as environmental conditions, equipment malfunction, and interference from other devices can also contribute to the occurrence of RF aliasing.

4. How can RF aliasing in plane radio be prevented?

RF aliasing can be prevented by ensuring that the sampling rate of the radio is sufficient to accurately capture the signal without any overlap. Proper filtering of the signal and avoiding frequency band overlaps can also help prevent RF aliasing. Regular maintenance and calibration of equipment can also help identify and address any potential issues that may lead to RF aliasing.

5. Can RF aliasing in plane radio be fixed?

In most cases, RF aliasing can be fixed by adjusting the sampling rate and filtering of the signal. However, if the equipment is damaged or malfunctioning, it may need to be repaired or replaced. It is important to address any issues with RF aliasing promptly to prevent further damage and ensure the proper functioning of the plane radio equipment.

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