A concept of a good voltmeter is routinely applied to

interpretation of electromagnetic wave spectra measurements

(noise spectra) in space plasma. According to this concept,

plasma density may be inferred from specific banded

structure assigned to upper hybride band. Good voltmeter

concept is quatified by proximity of a transfer function of

equivalent circuit to a constant value. Good voltmeter concept

was inaplicable to interpretation of noise spectra registered

on the ionospheric satellite Intercosmos-19 (IK-19). With

plasma density derived from topside sounder data and

equivalent circuit numerical simulation, it was shown that

parameters of specific banded structure correspond to a

parallel resonance. The parallel resonance follows from a

preamplifier input capacitance and antenna inductance

between electron cyclotron and upper hybride frequencies, in

cold magnetoplasma approximation. In subsequent satelite

projects (IK-24, IK-25, IK-27), noise and antenna impedance

measurements were integrated in one instrument in order to

have plasma density from two independent techniques. The

presentation is aimed to elucidate relations among the most

typical spectral structures in noise and impedance data,

plasma density and equivalent circuit parameters, possibly

influenced by spacecraft plasma interactions. Global

consistency between plasma density signatures in noise and

impedance data and those in simulations support statement

that basic missinterpretations in broad set of data were

avoided. Our data set is characterized by orbit inclination of

~80deg, altitude range of ~500km-3000km, frequency range

of ~0.1MHz-30MHz and antena lenght of 15m. Despite

limited applicability of the good voltmeter concept to

interpretation of wave measurements in ionospheric electron

plasma, it is show that plasma density can be deduced from

paralell resonance and that resonance components can be

separated from components of electromagnetic field in the

vicinity of the antenna. To reach these goals, the

preamplifiers with variable electrical parameters should be

implemented and noise measurements should be integrated

with independent technique capable to provide sufficiently

accurate values of plasma density. Inherent relation to wave

measurements, simplicity and absolute accuracy point

antenna impedance measurements as complementary source

of plasma density, required for interpretation, calibration and

verification. Numerical simulations of radiospectrometer and

impedance equivalent circuits are essential at instrument

design and data analysis stages. A sample of simulations

pertinent to long antennas in a magnetoshere is presented.