Wednesday, March 2, 2011
As we saw in a previous article, An Unassuming Antenna, The Ferrite Loopstick, the groundwave signal decreases in strength when tilting the radio (and thus the ferrite antenna) towards vertical. This effects not just the station tuned to, but all signals across the mediumwave band because the pickup of the magnetic component is lessened when the ferrite rod or bar is rotated away from horizontal. During reception of nighttime, skywave signals, the effect is often less pronounced depending on atmospheric bending of the wave. So, how much is a signal reduced?
I decided to perform a static test. At mid-day I tuned my Sony 2010 to station KBLU-560 (1KW, omnidirectional) out of Yuma, AZ, 69.6 miles distant on a bearing of 200 degrees. KBLU is not a powerhouse here at this distance, but does present a lower-medium strength signal to a sensitive receiver. KBLU was arriving with a respectable 6-7 leds lit (out of 10) on the Sony. Rotating the 2010 to vertical reduced the signal to barely 2 leds. It was weak, but still copyable.
Next I fired up the Tecsun PL-380. Now, the PL-380 is an interesting receiver because it has a direct signal strength readout. KBLU, not as strong on this ULR, peaked at a strength of 25,07 (25 dBµV, 7 dB signal to noise ratio) during normal reception. Rotating the PL-380 to vertical reduced the signal to below the noise level at 15,00. There was no trace of KBLU in the headphones.
Another local, somewhat stronger station is KLPZ-1380 (2.5KW, omnidirectional) out of Parker, AZ, 33.5 miles distant on a bearing of 354 degrees. It registers 31,25 (31 dBµV, 25 dB signal to noise ratio) on the PL-380 during normal, peaked reception. Rotating the PL-380 to vertical reduced KLPZ's signal to a level of 16,06. It was still copyable, though very weak and just above the noise level.
On the Sony 2010, KLPZ registered a solid 7 leds when peaked in the normal horizontal position. However, when tilted vertically, the signal reduced to only a strong 2-3 leds. It was easily copyable. Overall, the Sony 2010 does not seem to do as good a nulling job as the PL-380 in the vertical position.
Similar results were had for the Sony SRF-M37V and Tecsun PL-600.
It was interesting to note that while in a vertical attitude, each radio could be spun about the vertical axis to a certain position causing a marginal improvement in the station's signal strength. This, proving some possible interaction with the radio's circuitry and the ferrite loopstick, or imbalance in the ferrite loopstick itself, causing some pattern skewing.
That the radio receives any signal at all while in the vertical position is interesting. At 90 degrees to the magnetic component there should be no magnetic component to receive on a local, groundwave signal! Again, skewing of the ferrite's receiving pattern might cause this, or perhaps a little magnetic component sneaking through due to path shift of the signal. Or perhaps the cause may be a curious effect usually attributed to open loop antennas known as "antenna effect", where the loop, if unbalanced, does in fact respond to and receive a small amount of the transmitted signal's electrical, or vertical component.
Nighttime reception using this technique is a mixed bag. Although mediumwave DXers routinely use tilting of a radio from horizontal towards vertical to help null distant pest stations at night, using the vertical technique to generally reduce the strength of all signals works erratically at best. Example: powerhouse KOA-850, Denver, CO was actually equal strength at times the other night in both the horizontal and vertical positions, at the same time! With skywave signals booming in from all directions and at many different angles due to atmospheric distortion of the wave polarization, you have your answer as to why.
Could this odd way of nulling signals be used to the benefit of the mediumwave DXer? Maybe at least during the daytime for those of us who are daytime DXers, when conditions are stable?
I set to wondering if it would be possible to nullify the radio loopstick's direct reception of signals while at the same time coupling a passive, external antenna device to the radio - all without tearing into the radio. This sounds counter-intuitive. Why would we want to reduce signal pickup of a radio's ferrite loopstick?
Passive antenna devices like loops and QStick-like devices are routinely used by mediumwave DXers to enhance signal pickup. The problem, and what bothers me the most about using these devices, is that the radio's own ferrite loopstick is still receiving contrary signals of its own at the same time, degrading the effect of the passive device. Further degradation may come from the radio's loopstick having poor nulling ability or an overly wide broadside peak, also compromising the purity of the passive antenna. The radio also might be positioned somewhat differently than the passive device, causing unwanted signals to interfere with the natural peak and null of the passive device.
So, the question remains: What if it were possible for the radio to only see the passive device as its antenna, and largely ignore directly-received signals from its own ferrite loop antenna? The vertically-held radio with its corresponding vertically-oriented ferrite loopstick seems to fill this bill. But how do we couple to it in this position?
This creates coupling problems in several respects. In the case of the external tunable ferrite loopstick or QStick device, it must be placed parallel to or off the end of the radio's own ferrite loopstick in order for it to work. That means it would be vertical too, and it wouldn't receive much signal itself, since when vertical it is 90 degrees to the magnetic component of the received wave just like the radio.
The usual passive loop, with its coil wound in a solenoid fashion on a square frame (sometimes called "depth-wound"), won't couple much signal at all to a radio held in the vertical position due to its loop being 90 degrees in relation to the radio's ferrite coil winding. If we rotate the passive loop to a horizontal position we have good inductive coupling to the radio, but then the passive loop is 90 degrees to the wave's magnetic component and signal pickup is reduced to nil. Perhaps a spiral-wound loop?
And another possibility becomes evident. Why not put the radio inside a Faraday cage, eliminating all outside signals? But then, how do we couple the passive device to it, and still not tear into the radio? All food for thought. Perhaps more in a future post.
Hope you have enjoyed this series on a most interesting device, the ferrite loopstick.