plz help or my teacher will not let me pass

why i have different answers ? half wave dipole antenna
i have used three programs in order to get gain and impedance (MANAMA,PCAAD,EXNEC) i have give the same parameters but each program give me different answers why i cant understand as i know these programs uses the same method (MoM) and i want to ask how does MoM operates (how does it work ..i need information about MoM... i have done this work and MY TEACHER likeD it but he asked me why u have differnt answers and u have same parameters ....he said the answer is relatd to mom think and write a report about that so he will not let me pass so please i need a help
these are the parameters


plz see the file or



f cm g r x program
500 MHz 30 2.2 77.5 42.2 PCAAD
500 MHz 30 2.16 85.3 48.2 EZNEC
500 MHz 30 2.15 76.88 44.02 MMANA
500 MHz 30 2.15 75 50 Theoretically

please dear friends i need answers and help

well, im not really into this very well. The calculations are based on aproximations, which each company gives at their choice, which makes the result different.

Edit: i also know that when you solve integrals in the calculations the computer uses approximate values. so thats why each software has its "own" values.

tkowd thanks alot mmmm,my teachers give me a hint he told how does mom work ? from there u can find your answer so friends any more answers thanks

the difference is in the implementation of the MOM> For speed reasons, one uses delta functions as expantion functions. Delta functions represent very bad the current.
Other functions are entire domain functions (sin/cosine) or PWS (Picewise Defined functions), which can represent the currents better. THe MOM has many integrals, so (again for speed reasons) some of the calculations for these integrals is not very accurate.
THese differences are very visible for impedance calculation, and not very important ofr radiation patterns

Hello,

there is a number of issues which can influence the solution of a dipole antenna:

1) integration must be precise enough to give a stable system of linear equations (which basically means that if you increase the accuracy of integration it no longer influences the results significantly). Each MoM software should have a parameter that would allow you to control how accurately it calculates integrals. If it doesn't, it's not a serious code.

2) Current approximation over the wire should be accurate enough. If your codes don't take of this automatically than you must check it instead. If they use low-order basis, wire must be segmented into lambda/10 segments or smaller. Decrease this value until you get a stable result. If the code is using higher-order basis, than you are able to increase the order of approximation on the wire. I guess your codes are using low-order, although I didn't work with any of them.

3) What is the wire model? There are several wire models on the market and in the academia, but very popular one is thin-wire approximation. This means that the wire is treated as one linear current along its axis. The wire thickness is then separately taken into account... Anyway, make sure that all of your codes "see" the same thickness of that wire. This is particularly the case for the NEC family of the software, if you decrease the wire thickness, the result for the impedance will approach the theoretical 75 ohms...

4) Excitation model is significant for impedance calculation, so you should be careful to check if the excitation models in all of the programs is equivalent.

At the end, my advice to you is to download a free demo of WIPL-D, which is a really serious commercial code ... www.wipl-d.com -> register and download the demo.

Regards,
drasko

thanks im very happy to read the answers