Antennas and Microwaves

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Current & Voltage Distributions

There  are plenty of good text books around these days that have diagrams showing the current and voltage distributions on antennas. However there is nothing quite like making some measurements for yourself !

The distributions are in the form of colour-maps with the antenna outline overlaid in black. For each antenna there is amplitude and phase information, resolved into two components, X and Y. The measured antennas were all essentially planar structures so the scans correspond to X and Y components in that plane. It should be noted that a  Z component, although small, is also present but is not measured.

The scans were produced using a modified  X-Y plotter hooked up to a Vector Network Analyser (VNA) and a PC with a data acquisition card. The voltage probe consisted of an electrically small dipole, approx 1/30th wavelength. The current probe consisted of a balanced loop approx 1/30th wavelength in diameter.

Planar Scanning

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Links to the plots

Dipole currents

Dipole voltages

Patch currents

Patch voltages

Slot currents

Slot voltages

Some points of interest in the plots

Looking at the phase of the X-component currents on the dipole’s pawsey  stub balun the 0/180deg phase difference can be clearly seen.

If you compare the Y-component of the currents on the dipole itself with the X-component of the voltages on the slot, the complementary nature of the two antennas is evident.

Looking at the patch Y-component currents it can be seen that those on the patch itself are balanced against those in the groundplane to the sides.  These can be considered ‘non-contributory ‘ to  the radiation pattern.

However , the Y-component voltages on the patch are in phase, looking like a pair of slot antennas, which is actually not a bad model for calculating radiation patterns.

Equipment

The principal items of equipment used are as follows :

HP 8754A  Vector Network Analyser

HP 8502A  Transmission/Reflection Test Set

Gould Series 6000  X-Y Plotter

Advantech PCL711B PC Multilabcard

The PC L711B has only one D/A output (need 2 for X-Y drive) so another external DAC chip was included in the interface box and driven from the lower 8-bits of the digital output channels.

The software was written in Turbo Pascal running under DOS.  I prefer this approach for real time applications, as the PC isn’t tempted to nip off and tidy up its hard drive when it should be making my measurements!

Post processing of data was carried out using a combination of MATLAB and FASTCAD.

Measurement  Setup

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EMC

Using a Spectrum Analyser in place of the VNA , the sources of spurious unwanted emissions can be located. The image on the right is of a mobile phone board. The ‘hotspot’ at 900Mhz was expected as this is the fundamental frequency in the GSM900 band and located physically at the antenna feed point. The 2nd harmonic emissions were not expected and are clearly originating from elsewhere on the PCB.

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