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OK.  I walked around the house with the HT.  The house is clearly full of RF signals of the sort that are not helpful to receiving clear signals.  In most parts of the house, these unwanted signals (noise) are stronger than the squelch, even when it is turned all the way up.  Some corners of the house are relatively quiet, noise wise, such as the garage.

Now with both radios in the same room, with the antenna for each being in the same room at the same level, the 710 mobile has much cleaner reception.

My new hypothesis is that the 710 is much better at rejecting unwanted frequencies.  I believe this characteristic is called "image rejection."  It is a specification that is NOT provided for either radio, but is an important spec.

Here is more on image rejection from Radio Electronics.com :

 

Image rejection is distinct from either Sensitivity or Selectivity.  I suspect that the HT has significantly inferior image rejection compared to the 710. 

What are your thoughts.  And does anyone know why manufacturers don't bother publishing this specification?

Possible lessons learned subject to results from larger samples:

• HT's generally have poorer image rejection than larger units [probably] because of their compact size
• If an antenna needs to be installed indoors, find the quietest part of the house.
• Avoidance of RF interference from electrical circuits and transmissions in the house is right up there with "higher is better" as a reason to get the antenna OUTSIDE of the house
• Attics are generally a quieter part of the house and achieve both height and separation from RF interference.