Follow up- I tried a couple of different indoor "flat" antennas, one is an RCA with an in-line amp, and the other is something called a Leaf antenna. They are both about 8 x 10 inches, but the Leaf is super thin, almost like a sheet of paper encased in a plastic laminate. I also spent more time with my original Philips, trying new positions, closer to outside walls, etc.

Both the RCA and the LEAF are able to get all the local stations easily, even the one that is 46 miles away. The Leaf basically matches the RCA in signal strength, but does it without the in-line amp. I was also able to get my Philips to get all the stations when I used the in-line amp that came with the RCA.

So, I think the tuner in the Panasonic is not as sensitive as the set top converter feeding my old analog TV, but both the new antennas work well enough, and I was getting higher signal strength readings from both of them than from the Philips. I'm going to keep the Leaf and take back the RCA.

Thanks a lot for all the comments and advice. Now, I've discovered another problem with the TV (not reception related), a well documented issue called "floating blacks". When high contrast scenes change, the TV makes abrupt, but delayed, changes in black level to adjust for different contrasts in the scenes. It's pretty distracting, but there is supposed to be a fix from Panasonic. So, one more issue to hopefully solve before my return period expires.