EFHW Experiments continued

Further to my EFHW experiments. 

I have now performed some measurements on my system as follows:

Fres = 7020Khz

Toroid = T-200-2

Capacitor = RG58 cable ~ 115pf

1) Primary link turns = 3.5

1.5:1 Counterpoise length = 31ft (I could not get the SWR down below 1.5:1)

Fmax at 2:1 = 7.24MHz

Bandwidth = 220*2=440Khz

2) Primary link turns = 2.5

(Turns ratio = 18/2.5 = 7.2:1. Impedance = 50*7.2*7.2=2592ohms)

1:1 SWR Counterpoise length = 31ft 

Fmax at 2:1 = 7.15Mhz

Bandwidth = 130*2 = 260Khz

3) Primary link turns = 1.5   

(Turns ratio = 18/1.5 = 12:1. Impedance = 50*144 = 7200ohms.)

1:1 SWR Counterpoise length = 5ft

Fmax at 2:1 = 7.090Khz

Bandwidth = 70*2 = 140Khz

If you have a short counterpoise the bandwidth drops, and the input impedance rises.

Why?