Schwarzbeck HFBA 9122 HF-VHF Broadband Balun

The 1:1 Balun/Holder HFBA 9122 was originally designed as passive field probe to measure very high fieldstrength at low frequencies from 100 kHz up. Depending on the element type in use the HFBA 9122 can be used up to 500 MHz. The charcteristics of HFBA 9122 in the VHF range are comparable to common biconical antennas. The HFBA 9122 balun accepts several biconical element sizes, e.g. BBVU 9135 or BBUK 9139, which allows an antenna factor shift to the desired application. In general the antenna factor rises (reduction of sensitivity) with smaller elements.

Application:

The HFBA 9122 can be assumed as an electrical short antenna (Hertzian Dipole with LE << l). The Antenna Factor decreases by 20 dB / decade of frequency below 100 MHz. The result is a 20 dB / decade of frequency decreasing fieldstrength measuring range as indicated in the diagram (a constant noise floor or a constant voltage reading was assumed). For measurements of high fieldstrength the following difficulties may arise: A measurement error might be induced by insufficient shielding effectiveness of coaxial cables and measuring receiver. The HFBA 9122 offers with by far more than 120 dB shielding effectiveness a value which is at least one order higher than the respective value of cables or receivers. Braid currents may affect the measurement results, therefore the used cables should be as short as possible. The presence of interfering braid currents can be checked by rotating the polarisation for 180 °. In cases where critical braid currents exist, the measured fieldstrength differs by far more than 1 dB.

A further method to determine the validity of the measured results is to remove the biconical elements and watch the reading (do not touch the element fixtures). If there is a considerable reading without elements, the reason might be either insufficient shielding effectiveness or braid currents. Braid currents can be supressed by using ferrite loaded cables. Typical commercially available ferrite loaded cables are nearly useless in the frequency range of interest (< 30 MHz) because of their insufficient outer conductor inductance. A workaround can be achieved by the following technique: A thin double braid coaxial cable is wound 4 to 6 times through a ferrite core of very high permeability. The spacing between the ferrite cores on the cable should be 0.5 m each.

In order to improve impedance matching the use of a fixed attenuator (e.g. 6 dB) is recommended since the short antenna elements have only capacitive characteristics and do not provide impedance matching therefore.

Calibration:

The Calibration of the HFBA 9122 was performed using two different methods: In the frequency range from 0.1 to approx. 20 MHz (depending on the element size) the electric field for calibration of the antenna factor is provided with a capacitor. Above 20 MHz the antenna calibration is made under quasi free space conditions using the 3 antenna method. The measurement uncertainty is +/- 1 dB for both methods (k=2).