W242AA

[Mike Fitzpatrick]

>Image overload, or intermodulation, from strong
>but off-frequency HD stations does not seem to 
>activate HD at all. 

I have seen HD intermod activate the HD indicator off frequency, and here is how I usually tell if it is that or not.
When the HD light flashes, you should have the call identifier right away. (i.e. "WCRB-FM" or "WGBH-HD2"). 
If the IBOC units aren't setup correctly,  or have the call sign portion left blank, it will show up as "HD -FM"
Even really weak HD signals I almost 98% of the time get the call sign to show up. At 50 milliwatts, the HD signal 
would come in solid in parts of Cambridge. 50 milliwatts from the roof of the MIT building , outside, directional is
a lot more digital power than you may think.

If you do not get a identifier after several seconds of the HD light flashing, every time, you are probably getting a mix factor.
It is very possible that what you are seeing is the mix factor from the PRU that is combined with the W242AA analog signal.
Thus you only see it when you're within a certain spot of the transmitter.