Hearing trans on two freqs!

Was wondering what could explain the following;
A local cab company is in the DB at 162.510 but I sometimes also pick them up on 924.0875 when I am driving around.
What is up with that? Don't these two freqs seem to too widely seperated for them to be tx/rx on a commercial repeater set up?

What radio? Does it have a 380MHz 1st IF frequency? If so, then the 924.xxx frequency is just an image. This is particularly likely if the 162.51 MHz signal is very strong.

Thanks cMallen
My radio is a bcd396xt
Did some research on frequency image. Even though that radio is supposed to have features to help stop it, that is probably what is happening.

There are 2 other possibilities here - at least here in my neck of the woods, taxi radio systems are notoriously dirty; overmodulating all over the place. A ham club I belonged to once had to take a taxi company to the FCC to get them to fix one of their transmitters that was interfering on our 70cm freqs - and they were on VHF hi.The other possibility here (possible, however unlikely) is a link of some kind.

73 Mike

Gator596 said:

Thanks cMallen
My radio is a bcd396xt
Did some research on frequency image. Even though that radio is supposed to have features to help stop it, that is probably what is happening.

Click to expand...

If you are referring to the feature of being a "triple conversion receiver" this does make it less
prone to receiving images but does not make it totally immune and it can still happen. It all
depends on the radios IF freqs and the offending transmitters freq and how strong the transmittions
are being received.

The bcd39txt is also supposed to (from the wiki) provide; " Intermediate Frequency Exchange - changes the IF used for a selected frequency to help avoid image and other mixer-product interference on a frequency. "
I did some looking on the map - I think I was pretty close to their transmitter each time I was getting them on the image freq.
Thanks all!

Here's the calculations of image frequency. 924.0875 MHz - 162.510 MHz = 761.5775.
761.5775/2 = 380.78875 MHz.

You would get an image as noted, if the local oscillator is +380.xxx MHz for the 162 MHz band, and is -380MHz for the 924 MHz band. That is a fairly common way of designing scanners.

The spec sheet for the 396XT says that the normal 1st IF frequency is 380.7 to 380.8 MHz ---- the observed offset of 2 x 380.7885 is consistent with that.

As you noted, there is a IF freq shift function. That changes the 1st IF to the 265.5 to 265.6 range. In this case, the image frequency from the real 162.51 MHz signal will appear around 2x265.55 + 162.51 = 693.61 MHz

Thanks for the explanation. The math for this is really interesting!