WEAK SIGNAL RECEPTION WITH POLARIZATION
Ralph Wallio, WØRPK WØRPK at netINS.net
Diversity reception has been used in military and
commercial skywave HF radio networks for many decades to reduce the impact of
circuit fading. This practice has included the use of Frequency Diversity,
simultaneously transmitting and receiving on two or more frequencies, Space
Diversity, simultaneously receiving on two or more antennas separated by
several wavelengths, and Polarization Diversity, simultaneously receiving on
horizontally and vertically polarized antennas. This discussion covers
experiments with Polarization Diversity to enhance reception of weak signals in
the Amateur Radio Service.
An ARRL QST and QEZ search with the keyword, "diversity", finds a few interesting article titles but most of them are in QEX. I do not have a QEX library and would appreciate comments on the relevance of these more recent articles before I order reprints from ARRL. I have looked at cited QST articles going back to 1966 and the current ARRL Antenna Book but find very little discussion of the value of diversity reception. Experiments here at the Middle River Valley Black Hole have been limited to polarization diversity because it does not require participation at the transmit end ( as does frequency diversity ) and has equal potential in all directions ( as doesn't space diversity ).
Without polarization diversity, a station has one receiving antenna, typically either horizontal or vertical. Over the long term, ionospheric fading mechanisms impact both polarizations equally so the choice of polarization of this single antenna can be made for other considerations. However, over the short term measured in fractions of a second to seconds, ionospheric fading can be substantially different between the two polarizations. These brief but important differences in polarized fading can be seen in this graph of a constant amplitude 10m signal from California (each horizontal division represents 1 second). Note the radical differences in horizontal vs. vertical signal strength as ionospheric fading mechanisms impacted one polarization and then the other. ( More on the system that acquired this graph follows. )
ONE WAY TO USE DIVERSITY RECEPTION
This graph gives enough reason to find a way to effectively use a polarization diversity receiving system. We could feed both antennas through a switch that is controlled by receive signal strength. This is one of the ways the big guys do it but they don't have to worry much about QRM. Unfortunately this system can be easily fooled by signal strength from other stations contributing to co-channel or adjacent channel interference. We appear to need a receiving system comprised of two antennas, vertical and horizontal, feeding two separate receivers and a super computer to select only the station of interest. Fortunately we all have an available super computer, what some folks call "gray matter DSP" (or for very weak signals, "gray matter ESP").
Here in the Black Hole I am using two antennas, a commercial vertically polarized multi-band 40-10m product and a homebrew horizontally polarized "Log Periodic of Another Sort" ( see http://showcase.netins.net/web/wallio/LPDA.html ). These antennas individually feed two identical Kenwood TS-430S transceivers. The interface to "gray matter DSP/ESP" is a set of stereo headphones yielding one polarization in my left ear and the other polarization in my right ear. This block diagram includes a simple interface box that includes a selection switch and audio level control.
The primary result of this arrangement is improved weak signal reception through ionospheric fading, e.g., when signal strength is fading in one ear it is often increasing in the other ear. But there is more, with a little operator training slight differences in audio phase give the weak desired signal characteristics different from all other signals, even much stronger ones, in the receiver passband. A quickly trained ear concentrating on these differences appears to put the weak signal out in front of all QRM.
I have found that a simple interface box is helpful to organize and use this polarization diversity scheme. The box provides individual cables to plug into headphone jacks on both receivers, a jack for the stereo headphones, a stereo volume control and a three position selection switch arranged for vertical only, horizontal only and both. This schematic shows interface box wiring.
Graphs included in this discussion were produced by amplifying voltage drops across the two TS-430S S-meters approximately 25x with a homebrew dual-channel instrumentation amplifier and then sampling these voltages with a DATAQ WinDaq Starter Kit ( see http://www.dataq.com/ ). Instrumentation amplifier and photo that also shows the DATAQ DI-194 board:
Two Receivers, One Tuning Knob
Any two receivers can be used but they both need to be independently tuned to the desired signal. This tuning chore can be eased by using the same "VFO" to tune both receivers in lock step. Toward this end I use a Kenwood TS-430S transceiver that has been my shack since it was a pup circa 1983. Mike McQuiston, WA0MM, provided the second TS-430S of the same litter from his bone bin that I have now had so long that I am forced to charge storage fees.
The TS-430S "VFO" starts with the front panel tuning encoder and, after wandering over the river and through the woods, ends up as a tuning signal traveling in a coaxial cable between the PLL and RF boards. It has been a reasonably easy task to sample this signal in one transceiver and use it in the slave transceiver. With this arrangement only one transceiver can be used for tuning but both transceivers and attached antennas can be used to receive and transmit on the selected frequency.
WØRPK Home Page