Weather freq

[737mech]

I was doing a "Ham Radio" search on my bct15x and it stopped on 924.05 NOAA weather loud and clear? I thought weather was on 162.55? Is it also supposed to be on 924.05?

 

[jaymatt1978]

It's an uplink of some kind. They have a few of them across the country. Mostly common in the mountains

 

[N2KMB]

it may also be a Ham Radio Rpt With a weather radio tied in with it

 

[WA1ATA]

If your scanner is a triple conversion scanner with 380MHz 1st IF (or more precisely, 380.75MHz 1st IF), then you may be hearing an image frequency.

This is particularly likely to be true if you are near the 162.55MHz transmitter.

Although I couldn't find the IF frequencies in the BC15 manual, 380MHz is a common 1st IF frequency for Uniden scanners, so my bet is that you are just hearing an image frequency.

Charlie

 

[737mech]

Weather Freq

If your scanner is a triple conversion scanner with 380MHz 1st IF (or more precisely, 380.75MHz 1st IF), then you may be hearing an image frequency.

This is particularly likely to be true if you are near the 162.55MHz transmitter.

Although I couldn't find the IF frequencies in the BC15 manual, 380MHz is a common 1st IF frequency for Uniden scanners, so my bet is that you are just hearing an image frequency.

Charlie

That makes sense. I'll have to check the specs or info on the scanners, You are probably correct, I can duplicate this on bct-15 and bct-15x. Tried the PSR-500 and Pro-2051 and nothing. Is there a quick reference somewhere for the math on this? I'm off to read the wiki on "Image Freqs"

 

[WA1ATA]

That makes sense. I'll have to check the specs or info on the scanners, You are probably correct, I can duplicate this on bct-15 and bct-15x. Tried the PSR-500 and Pro-2051 and nothing. Is there a quick reference somewhere for the math on this? I'm off to read the wiki on "Image Freqs"

The math is pretty easy. The desired signal that you have tuned in and the image frequency will be separated by 2 times the IF frequency.

2 x 380.75 = 761.5MHz.
761.5 + 162.55 = 924.05 MHz -- the spurious signal you picked up.

===============================

The longer explanation ......

The antenna is connected to a preamplifier which then feeds into the mixer. The other signal into the mixer is the local oscillator.

The mixer has many frequencies in its output, but the main ones are + or - (input frequency) +/- (local oscillator).

A radio with a 380.75MHz 1st IF uses an oscillator 380.75MHz either above or below the desired frequency. That means that the desired frequency will be converted to 380.75MHz. The reason for this is that it is easier to design amplifiers and filters for just one fixed frequency.

To make the range the local oscillator needs to tune down to a minimum, it is common for the highest frequency bands to have the local oscillator below the desired frequency and the VHF frequencies to use a local oscillator that is at the desired freq + 380 MHz. So for the weather band, the oscillator will be around 540 MHz (543.3 for this particular case). This is also the same local oscillator freq used fto receive a 924.05MHz signal.

To keep from receiving both the 160MHz and 920 MHz signals at the same time, the radio uses a filter between the antenna and the mixer. It is setup to reject 920MHz signals when trying to receive around 160MHz and vice versa. Using a high (380MHz) IF frequency keeps the desired and undesired frequencies far apart and make the design of the filter easier. But a 160MHz signal will get through slightly even if you are trying to just receive 920MHz. How much gets through is shown in a spec call "image rejection". A specification of 75dB image rejection means that means that the gain of the undesired 160MHz signal is 75dB less than the desired 920MHz signal.

In not so fancy words, this means that you normally must have a strong frequency to generate an image. That's why I assume that the Wx transmitter is fairly close to you.

New receivers generally have better image rejection than old ones, particularly if they have 800MHz capability. This is requirement of the FCC, so that you can't receive cellphone signals simply by tuning to the image freqs. (Of course cellphone freq blocking is only relevant for analog cellphones, but that's another discussion).

Charlie

p.s. If you tune the PSR500 to 924.15MHz (0.1MHz higher than with the other scanner) you might find a weak image of the NOAA weather transmitter. The 1st IF freq for the 500 is 380.8MHz. It doesn't give a spec for image rejection, but it will be much better than the older BCD 15, so the image will be weaker.

 

[ecps92]

The NWS would use a 406-420 Mhz uplink from the Gov 'A' band

It's an uplink of some kind. They have a few of them across the country. Mostly common in the mountains

 

[737mech]

Image freq

The math is pretty easy. The desired signal that you have tuned in and the image frequency will be separated by 2 times the IF frequency.

2 x 380.75 = 761.5MHz.
761.5 + 162.55 = 924.05 MHz -- the spurious signal you picked up.

===============================

The longer explanation ......

The antenna is connected to a preamplifier which then feeds into the mixer. The other signal into the mixer is the local oscillator.

The mixer has many frequencies in its output, but the main ones are + or - (input frequency) +/- (local oscillator).

A radio with a 380.75MHz 1st IF uses an oscillator 380.75MHz either above or below the desired frequency. That means that the desired frequency will be converted to 380.75MHz. The reason for this is that it is easier to design amplifiers and filters for just one fixed frequency.

To make the range the local oscillator needs to tune down to a minimum, it is common for the highest frequency bands to have the local oscillator below the desired frequency and the VHF frequencies to use a local oscillator that is at the desired freq + 380 MHz. So for the weather band, the oscillator will be around 540 MHz (543.3 for this particular case). This is also the same local oscillator freq used fto receive a 924.05MHz signal.

To keep from receiving both the 160MHz and 920 MHz signals at the same time, the radio uses a filter between the antenna and the mixer. It is setup to reject 920MHz signals when trying to receive around 160MHz and vice versa. Using a high (380MHz) IF frequency keeps the desired and undesired frequencies far apart and make the design of the filter easier. But a 160MHz signal will get through slightly even if you are trying to just receive 920MHz. How much gets through is shown in a spec call "image rejection". A specification of 75dB image rejection means that means that the gain of the undesired 160MHz signal is 75dB less than the desired 920MHz signal.

In not so fancy words, this means that you normally must have a strong frequency to generate an image. That's why I assume that the Wx transmitter is fairly close to you.

New receivers generally have better image rejection than old ones, particularly if they have 800MHz capability. This is requirement of the FCC, so that you can't receive cellphone signals simply by tuning to the image freqs. (Of course cellphone freq blocking is only relevant for analog cellphones, but that's another discussion).

Charlie

p.s. If you tune the PSR500 to 924.15MHz (0.1MHz higher than with the other scanner) you might find a weak image of the NOAA weather transmitter. The 1st IF freq for the 500 is 380.8MHz. It doesn't give a spec for image rejection, but it will be much better than the older BCD 15, so the image will be weaker.

Thank you! This is great information! The 500 didn't respond as suggested but I'm not concerned. Right now I'm just happy I learned something new today. Wish everyday was like that!
Thanks again!

 

[Myles]

The newer BCDx36HP scanners pick this up too. Both by 436 and 536 hear this but I can use the IFX function and make it go away like magic.