A Periodic Switching Technique to Compare Receiving Antenna Performance in the Presence of Strong Fading
Chavdar Levkov, LZ1AQ
Rev.1.0 Dec 2016
There is a strong and fast fading on HF bands. When testing my phased array  with small active antennas, the measured front-to-back (F/B) ratio is usually between 20 to 40 dB with local stable signal sources with vertical polarization. The real signals coming from the ionosphere are with undetermined polarization. The signal levels are also unstable and at the RX output the combined effect of polarization and level changes are with a range between 5 to 30 dB. It is difficult to estimate the F/B ratio on sky wave signals since the fading effects mask the effect of F/B switching. Manual switching and S-meter readings are very unreliable methods to obtain accurate measurements.
A setup to measure accurately the signal difference between F/B directions of a phased array or between any two different antennas is shown on Fig. 1. The idea is to toggle periodically to the receiver input the two signals under test and display on the screen the signal strength graph as a function of time. It is much easier and accurate to obtain a reliable measurements of signal difference on such a display since we have a periodic process and also we can monitor the changes of this difference .
Fig.1 Periodic switching between 2 antennas (or F/B of a directional antenna) to RX antenna input.
Software: The main instrument is the Linrad SDR (SM5BSZ) software. It has an S-meter which gives graphically the signal level as function of time. It is very accurate and can be adjusted in very wide range to suit the needs of this experiment. How to use Linrad can be read from . A very good user guide for Linrad (from ON4KHG) can be obtained from .
SDR: Linrad supports numerous SDR receivers– from simple Softrock type, RTL dongle to DDS receivers such as Afedri and Perseus –any one can be used.
The switch: The array is switched periodically front-to-back (F/B) or two antennas are toggled periodically. The duration of the two positions of the switch must be different in order to distinguish on the S-meter graph Front from Back position or which antenna is active. The optimal period of switching depends from the fading speed. For slow fading almost any period can be used but when the fading is fast, shorter periods must be used. Practical experiments showed that the switch duration ratio must be from 3:1 to 10:1 and the shortest period is somewhere at 150 to 200 ms .
Fig.2 Switcher schematics. The relay is switching periodically 2 antennas to the RX input.
An example of a switcher is shown on Fig.2. CMOS 40106 Schmidt-trigger inverters are used. A 12V small communication relay is used which is driven by four 40106 invertors in parallel. The device can be set to logic state 0, 1 or periodic pulse mode. The duration of each state can be set independently by P1 and P2.
For users of VDL-1 phased array  the periodic switching mode is implemented in the software  which controls the array directions and modes. There you can choose the duration of each direction and the sequence of switching which permits even to switch between all 4 directions in circular sequence.
Fig. 3 Periodic switching window in VDL-1 control software. +Y/-Y directions are chosen to evaluate F/B ratio of the array.
Experiment: First of all we must choose a proper sky wave signal for this method. The best results are obtained with wideband (2 to 50 KHz) digital signals: commercial modems, DRM stations, OHR and RTTY stations. AM stations (Linrad must be in AM mode) can be used also but digital stations are much more stable. Set the receiving bandwidth equal to signal bandwidth to avoid selective fading effects as much as possible. AM carriers can be used (Linrad in CW mode, with very narrow filter) but very often the selective fading is very fast. CW can be used if the settings of the graphic s-meter and switch period are properly chosen. The SSB mode is very difficult since the speaking power is very changeable. To obtain a good results the user must also play with S-meter settings - speed, averaging etc.
Examples: The following pictures (Fig.2, 3) explain the method.
Fig. 2 Measuring F/B ratio with Linrad software and Perseus SDR. The receiving frequency is 5229 KHz. A digital signal with 6 to 12 dB fading is displayed on S-meter graph. The scale is in dB. The array is switched periodically between North and South directions where the North is with longer duration. The signal obviously is coming from North. The F/B is not stable but can be measured accurately as a difference between high and low levels in the switching fronts and is from 8 to 15 dB. The antenna under test is a 2-element wideband array as described in .
Fig.3 Periodic switching between two small identical active magnetic loops  mounted horizontally and vertically on the same mast . Digital signal at 13918 KHz is used as a source. The longer period is for vertical loop state. Note the change in polarization from horizontal to mostly vertical component.
 Linrad , http://www.sm5bsz.com/linuxdsp/linrad.htm
 Gaëtan Horlin, ON4KHG, Linrad Installation and Configuration User Guide, http://on4khg.be/linrad-by-leif-sm5bsz/
 VDL-1 phased array http://active-antenna.eu/variable-delay-line-kit-vdl-1a/
 VDL-1 control software http://active-antenna.eu/variable-delay-line-kit-vdl-1a/technical-documentation/
 Active antenna AAA-1, http://active-antenna.eu/amplifier-kit/