Questions about Plane reflector
 

I am "playing" with a plane reflector at 920 MHz.

The physical construction is a copper sheet as reflector (290mm x 240mm).

A single dipole is mounted in the centre vertically.
The dipole consists of 2 aluminium rods that are 6mm diameter and
the length between the tips is 123mm.

I seem to get a double dip of lowest SWR at 908MHz and again at 945 MHz
(I am not sure of exact SWR but it is closer to 1:1 than 2:1 - I set up
a SWR brodge and noted 1:1 level and 2:1 level using calibrated loads).

The design was scaled from Cebik's 432 MHz version. The calculated
length of the dipole should have been 134.8mm.

My questions are...
1. Why do I see a double dip in SWR instead of single dip
2. Why is the element length so short compared to calculated lenth (I
want antenna to operate at 920MHz)
3. Should I have a choke (Say 5 turns of the coax on a 10mm diamtere at
the feedpoint ?

Thanks in advance for any help.

(Just in case you are curious, I wanting to use the finished unit as a
reference antenna to compare antenna gains)