My result looks like circles with no terrain effect
If you use the Free Space Path Loss model with Signal-Server, the terrain is ignored so the result will always appear as perfect concentric circles. If you want to see the impact of terrain but also with the Free Space Path Loss model then the new Sleipnir engine will show you this.
My result is too optimistic / pessimistic
Most of the models are empirical which means they were produced from real-world measurements with fixed heights. Under the same conditions they would deliver good results but as you deviate from these conditions the accuracy worsens.
Models perform differently at different frequencies but in general they can be ranked in order of optimism as follows, assuming 2.4GHz:
Free Space Path Loss - Most optimistic
Irregular Terrain Model - average
SUI WiMax - Most pessimistic
Why can't I select ITWOM in the new engine
ITWOM was not included in the new engine because it's license is questionable as it was first incorporated into SPLAT! under a restrictive copyright license with a contributor claiming copyright of the model and proposing it to the FCC for formalisation (FCC docket 10-152). ITWOM was derived directly from the ITM model which was US Government (NTIA) property until it was public domain licensed.
This file is copyright(c) 2011 by Sid Shumate and Givens & Bell, Inc. All rights reserved. Commercial use, and resale, including when compiled with
wrap-around software, is prohibited except under Givens & Bell, Inc. license.
In 2014, SPLAT!'s maintainer rightly removed the copyright claim according to the change-log which was encouraging but if it was swapped out for GPLv2 then it still could not be used in a closed source application due to how GPL works. This is why it only features in the open source Signal-Server engine.
The copyright restriction on the ITWOM 3.0 propagation model has
been removed. ITWOM 3.0 code may now be used and modified under
the GPLv2 license.
Despite the complex license, the model itself has not satisfied academics according to published research.
...SPLAT! v1.4.0 with ITWOM gives worse simulation results than SPLAT! for Windows and Radio Mobile. Additionally, in distances bigger than 40Km the simulation results are much worse. Most probably, SPLAT! with ITWOM overestimates the attenuation by obstacles and this is currently under investigation.