NEC-LIST: Re: Antenna factors of bicone antennas

Sara Fletcher <sara.fletcher_at_npl.co.uk> wrote:

> 2. Putting a 1V voltage source on the central segment of the feed
> wire and finding the E-field at a distance of 100 m from the antenna
> 3. Illuminating the bicone with a plane wave of 1 V/m and placing a 50
> Ohm load on the central section of the feed wire, and then finding the
> AF from the current across this load.
>
> ... The problem is that the
> antenna factors are different depending on the configuration, with the
> figure derived from the SA being exactly between the values from
> methods 2 and 3. ...

It's just inaccuracy of NEC. I tried your job and then changed it,
giving each wire 31 segments (403 in total) and feeding at the 16th of
the 1st tag. All results together (with Nec2d):

                              AF(100MHz) AF(200MHz)
 39 segm, receiving: 31.77377075 m^-1 8.281573499 m^-1
 39 segm, transmitting: 26.63664522 m^-1 6.931275027 m^-1
403 segm, receiving: 33.69953495 m^-1 8.855042947 m^-1
403 segm, transmitting: 33.58607806 m^-1 8.825061315 m^-1

NOTE: AF here is in inverse meters, NOT in dB. In dB we'd have:

                              AF(100MHz) AF(200MHz)
403 segm, receiving: 30.55247816 dB(m^-1) 18.94381343 dB(m^-1)
403 segm, transmitting: 30.52318586 dB(m^-1) 18.91435463 dB(m^-1)

So with more segments the problem is reduced. If you use a very
large number of segments, however, loading only the central segment
would be wrong (load a/o excitation shoud be spread over a number of
segments covering a finite length in the center.)

Greetings,
Jos

----
    Jozef R. Bergervoet                         Electromagnetism and EMC
    Philips Research Laboratories,            Eindhoven, The Netherlands
    E-mail: bergervo_at_natlab.research.philips.com   Phone: +31-40-2742403