"dansawyeror" wrote in message
...
The programs vertload.exe and loadcoil.exe produce similar results
for radiation
efficiency and radiation resistance. In both cases the antenna is
configured as
a 6 meter vertical with a 10 cm long, 75 mm diameter coil in the
middle.

However the coil dimensions or the input impedance do not match
between the
programs. Vertload predicts a 50 turn coil with an ATU capacitor of
1289 pf.
While loadcoil predicts a 31 turn coil and 0 feedpoint reactance.

What causes the difference? Which one is the better model?

Thanks - Dan kb0qil

========================================
Hello Dan,

Both programs will accept input parameters for a short coil-loaded
resonant vertical above ground. But they are designed to output
different computed parameters and do not use exactly the same models
or calculating methods.

The input parameters are stated in different forms and when comparing
one program with the other it is important to ensure the correct
antenna dimensions are entered. In particular, the overall heights
should be the same and the centre of the loading coil must be at the
same height above ground on both models.

I am unable to check your figures. You have given me insufficient
information. The resonant frequency is essential.

But to satisfy myself there is not a bug I have modelled the following
antenna using both programs -

Using program LOADCOIL
-----------------------------------
Overall height = 6 metres.
Mast diameter = 10 mm.
Rod diameter = 5 mm.
Coil Length = 100 mm.
Coil Diameter = 75 mm.
Location of coil center = 59.5% of overall height.
Ground loss resistance = 5 ohms.
Resonant frequency = 3.8 MHz.

Using program VERTLOAD
------------------------------------
Mast height = 3.52 metres.
Mast diameter = 10 mm.
Coil length = 0.1 metres.
Coil diameter = 75 mm.
Rod length = 2.38 metres.
Rod diameter = 5 mm.
Ground loss resistance = 5 ohms.
Resonant frequency = 3.8 MHz.

The height of the coil above ground is that which maximises radiating
efficiency for a given physical size of coil.

As a check on agreement between the two programs, compare the few
computed output parameters which are common to both -

Programs VERTLOAD and LOADCOIL
----------------------------------------------------
Radiation resistances = 5.6 and 5.1 ohms.
Feedpoint resistances = 12 and 14 ohms.
Efficiencies = 47% and 35.1%.
Turns on coil = 43 and 40 turns.
Coil wire diameters, 1.63 and 1.64 mm.

The difference between efficiencies is 1.3 dB.

I think the some of the differences can be due to program Vertload
neglecting the increase in effective coil diameter due to being wound
on a former of nominal diameter.

For different frequencies and antenna heights there will be different
sets of differences but of similar magnitudes.

The more accurate program is probably Loadcoil because it was
specially designed to find maximum efficiency by sliding the coil up
and down the antenna. The effect of the length of the coil on
radiation resistance is probably more accurately taken into account.
It is also a more recent program.

In practice there's nothing to choose between them. The uncertainty
in results will be controlled by the uncertainty in input data,
particularly the uncertainty in ground loss resistance.
----
Reg, G4FGQ.