[TowerTalk] Radial length question

[RLVZ at aol.com]

Hi Guys,
 
I'm at a portable location setting up for Field Day.  One of our  antennas 
will be a 40-meter 1/4 wave ground mounted vertical... with  a good number 
of radials under it.
 
The soil is very poor: 6" of sand with gravel under the  sand.  About as 
bad as possible for a Vertical.
 
Common 1/4 wave Vertical theory often states things like: "radial  length 
does not effect a Verticals  resonance when the vertical  is ground mounted". 
 However, I am finding in with this super  poor ground conductivity QTH 
that the 1/4 wave Vertical wants 1/4 wave  radials in order for the Vertical to 
resonate anywhere close to where it  should.  (with odd length radials, the 
Vertical's length must be changed by  several feet)     
 
With the above in mind, with this super poor ground  conductivity, the 
ground mounted vertical behaves more like a Ground  Plane antenna.  I must 
assume this is normal when you have about zero  ground conductivity.  Is that how 
you see it?
 
73,
Dick- K9OM