Re: Q: why are impedances strange with type 5 sources?

Charlie,

I can give you some unscientific experience:

Never use source type 5 if you require accurate input impedance info.

The advice above does seem to be fairly arbitrary but is based on the
following: We have been using NEC2 for about 10 years and very often
to obtain input impedance information. The following studies were
perhaps the most rigorous:

- skeletal wire antennas using rotational symmetry (very careful
measured and computed impedances - source 5 always give larger errors)

- Loaded dipole studies (as above)

- About 5 months ago we performed perhaps the most "careful"
measurements yet where we measured half wave dipole input impedance
when approaching a metalic plane. (meticulous HP NW
analyzer calibration to actual feed point using surface mount
resistors, quarter wave "bazooka" balun, thin wire dipole (all
measurement wires moving perpendicularly away from dipole), in an
unechoic chamber at 350MHz. We obtained input impedance versus
spacing (to metal plane results) with the maximum error being 3 Ohms
(typical error less than 0.5 Ohms) in real and imaginary values when
moving from 0.2 to 1.8 wavelength in very small steps. Also measured
dipole only input impedance versus frequency in the range 340-360MHz
with real part errors smaller than 1 Ohm and imaginary part about 2
Ohm maximum errors. Resonant frequencies and mutual impedance related
cross over frequencies were within 1 MHz from measured values. MOST
IMPORTANTLY all this was acieved using ACTUAL lengths (no "length
correction" and other fiddling) as well as actual wire radii.

Some additional experience which helps with input impedance
computation is to use exact segments adjacent to feed segment and
when connecting wires at the feed (such as in wire conical antennas)
rather used multiple feeds then have a connection close to the feed
segment. (None of this obviously applicable to your simple dipole
study).

I hope this is useful and will like to hear from people with similar
experiences.

Regards
Andre Fourie

e-mail: fourie_at_odie.ee.wits.ac.za
Tel: intl + 27 11 716 5386
Fax: intl + 27 11 339 4610
Address: Dr. APC Fourie, Dept Elec Eng, PO WITS, 2050, South Africa
Received on Thu Sep 14 1995 - 13:11:00 EDT