handheld transciever reading 2 frequencies... Help

pigpenracing

En-Route
Joined
Feb 8, 2012
Messages
2,757
Location
Brenham Tx
Display Name

Display name:
pigpen
Hey guys,
I just put a Yaesu VXA-220 handheld in my Cub along with a remote antenna. Everything works and radio is nice and clear. I have one issue I can't figure out. The CTAF at my airport is 123.07 and the AWOS is 121.12 for this airport. I am keeping my radio on the CTAF 123.07. Whenever I am close to the airport the 121.12 tries to bleed in. It only last for a second or two but I can hear that AWOS trying to come in and it goes away. Why would I be picking up that other frequency??? The radio has the dual watch for 2 frequencies but that is disabled. This is driving me crazy.
Would it help to turn the squelch up and make it less sensitive? This does not make sense to me.
 
How close are you to the transmitter? It could be bleed over from a cheap front end circuit.
 
It's not a sensitivity problem, it's a selectivity one. Welcome to cheap handheld receivers with front end filtering as broad as a barn door.

I'd have to go do the math to see if it's an IF mix, but nevertheless totally normal on modern handhelds. Could even be that the AWOS transmitter is out of spec on harmonics, too.

Distance from the AWOS transmitter is the solution... The signal strength will drop off rapidly as you move away, per the distance-squared rule.

Squelch does not do anything to selectivity, nor sensitivity even. If the radio has an attenuator function, that might help.

For the record, it doesn't have one...

http://www.yaesu.com/downloadFile.c..._20111031.pdf&FileContentType=application/pdf

Thank Yaesu for making the receiver broad enough to receive NOAA broadcasts... And making the radio "submersible" ... before putting decent front end filtering it it.

Heh. Just kinda normal for handhelds these days.
 
The other thing could be mixture product. 1950KHz is the old LORAN frequency (though there might be other users of this).
 
The other thing could be mixture product. 1950KHz is the old LORAN frequency (though there might be other users of this).

This is true. I didn't go check multiples. The filtering usually is so bad the receiver front end is overloaded but it can also be strong local sources that create a mix. I discounted the easy one since it was 1950 instead of a little smaller, since I was thinking "is it AM broadcast" when I quickly checked the obvious mix via subtraction.

But... I didn't check 3x mixes. 1950/3 is 650. Is there a strong AM station nearby on 650? Usually you can make out some of the "content" of the AM product in the audio, even on an FM receiver.

(Around here, a mix with KHOW would be likely with those two frequencies.)

But usually even in crappy handhelds the AM broadcast band would be filtered well enough to wipe out a direct mix. What I've usually seen is those are IF mixes, and I'm not sure of the IF frequency in the typical aviation handheld. (11.3? I forget...)

I've done the "lets go figure out what's really strong" game with a spectrum analyzer before to see what's likely to be mixing. It's a pain. Especially when the mix is happening somewhere on a tower at a rusty bolt acting like a diode in the near field of a transmitter. Grrrrr.

The guys with high power 600 KHz broadcast in their towns really love whoever came up with 600 KHz as the standard offset for VHF Ham repeaters long ago... I'm sure. :)
 
The mixing may not be occurring inside your handheld. Have you tried other radios nearby?

Having been active in the maintaining the repeaters in Baltimore where we have a WCAO at 600 KHz (which is the offset between the transmit and receive frequency, mixing was always a nightmare).
 
The mixing may not be occurring inside your handheld. Have you tried other radios nearby?

Having been active in the maintaining the repeaters in Baltimore where we have a WCAO at 600 KHz (which is the offset between the transmit and receive frequency, mixing was always a nightmare).

I feel for you on that one! Ouch.
 
Back
Top