Is this effect fairly localized?
If so, I can suggest a reason on "general principles."
High fequency radio signals are pretty sensitive to being bounced around by the wierdest things... which is how radar works.
These echoes can re-combine to form dead spots.
My FM radio signals (88-108) out here in the boonies are sensitive to whether or not my neighbor's metal front door is open or closed, and to where I am in the room. And these are rather long waves, around 3 meters.
My higher-frequency digital off-the-air TV signals are sensitive to whether the line of pine trees outside are wet or covered with snow and blowing around a little in the wind, leading to pixel dropouts beyond the buffering capability of my television set.
The trouble with digital signals is when they're good, they're excellent, but with the least attenuation, every single byte of information has to be correct to reconstruct the original information.* This is what leads to pixel dropouts and in severe cases, a freeze up of the images.
With analog signals, rather than getting a signal drop-out, the signal just gets weaker and weaker, but is still "readable" by the receiver.
This image of wave patterns interfering with each other shows what I mean. Assume the signal coming from the left source is the original outgoing set of waves, and the signal coming from the right side source is a reflection off some object. It can be seen that there are many spots where the signals reinforce each other, and just as many spots where the signals
cancel each other.
With high-frequency shorter waves, the reinforcing and cancellation spots are pretty close together, which is why slight movement of reflecting surfaces (or slight differences in the location of the receiver) can drastically affect the signal quality.
With longer waves, the cancellation and reinforcement spots are still there, but they are much further apart, and therefore less noticable and the dead spot may be half a mile away. (It also takes larger reflecting surfaces to reflect a strong reflection.)
The difference between your two units is probably just due to a difference in the receivers' inherent "front end" sensitivities.
Just a suggestion regarding your problem based on general principles. It might even be due to someone parking a panel truck near you or the transmitting tower.
Terry
* Search for "cliff effect."
