Wednesday, March 2, 2011

The Vertically-Challenged Ferrite Loopstick

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.


Stephen said...

Hi, Bill...

I've found that I can even reduce the signals of some fairly strong locals to near zero, or at least very weak with severe phase distortion / SSB-without-decoder sound.

For example, about 9.3 miles north of me, there's a local 50kW Salem conservative talker, 1170 KCBQ, which I believe I have mentioned in the past. With their signal peaked with the built-in ferrite antenna, the PL-380's signal strength meter pegs at 63,25 and the Tecsun PL-606 hits 81,25. Add the Select-A-Tenna and a utility pole grounding wire, as pictured...

...and it overloads both radios, the PL-606 more severely because of its few dB higher sensitivity. (The sound clip is from the PL-380 - first barefoot, then placed in position coupling with the big guns. (The PL-606's meter pegs at 98,25.)

Back to barefoot reception, by tilting near vertical, if I get the azimuth just right, I can severely reduce the signal strength. An audio clip follows...

Signal indicator sometimes drops to 15,00 - listen for the severely distorted audio portions. I don't think I was quite able to COMPLETELY null the signal, though. Also, getting the azimuth right to get this severe nulling requires the steady hands of someone like a brain surgeon, jeweler, watch maker, piano repair technician, etc. Bump it off by maybe a fraction of a degree or millimeter, and you'll boost the signal by several dB, if not 10-20dB or more.

So far, I haven't been able to null out the two powerhouses near my grandma's house when I've been there, though - 23kW 1300 KAZN and 50kW 1430 KMRB, both 0.54km distant at a heading of 318° (reverse 138°). (BTW is there a way to calculate what the approximate field strength of those two stations would be at that location? Also I'm wondering if she's too close for the directional pattern to have much of an effect - KMRB is typically stronger in spite of having a deep null, while KAZN is a few dB weaker in spite of having a minor lobe toward her in the daytime. I haven't had the PL-606 there, yet (missed an opportunity a week or two ago cause I was sick), but I'm fairly sure both would be indicating in the upper 80s dBu, possibly even hitting the low 90s on KMRB, barefoot.)

What about weaker stations, though? For example, on Feb 17 at 1pm PST, I recorded this station, complete with TOH ID. You may recognize the station. ;) (Headphones strongly recommended.)

Splatter is from 690 XEWW, 32 miles SSW of me.
I suppose those should be easier to null, right? Or would it be possible to tilt-null the splattering local, while still bringing in the DX? Also in a comment in another post you mentioned doing the tilt test to check for the presense of skywave. Would that test work for weak signals like that one (and weaker such that maybe all you detect is a carrier (by way of rotate-nulling or tuning off then back on), but no audio), or is there another preferred way?


Hi Stephen,

Thanks for your notes and audio clips.

I have found the same - it is hard to get the radio perfectly perpendicular and azimuth right to afford the greatest null. It does require the hands of a brain surgeon.

I would concentrate on tilt-nulling the splattering local, then see if the weak station nearby in frequency improves any.

I have found that tilt-nulling (I like your new term) seems to work well even on the weak, distant stations during daylight hours. KFI-640, about 240 miles distant, nulls quite nicely here. So do other stations farther away, indicating that daytime, groundwave polarization is maintained out to much greater distances than I thought.


Stephen said...

Hi Bill,

I was reading this earlier today, and realized I posted the wrong link for the audio from tilt-nulling 1170. Here is the correct link:

Also, I'm in Burlingame, CA (near San Francisco) right now, but I remember that I can usually null KFI with just "standard" rotate-nulling on my PL-606, usually, in spite of them being only 99 miles away from where I live. Same goes, IIRC, for 1070-KNX, even though they're a few dB stronger, in spite of being 12 miles farther away (111 miles distant from my home QTH). KBLU-560, btw, is very faint outdoors where I am, and is impossible to receive inside the house. Signal readings may be as low as 15,00 or 15,01, or as high as 24,00 or so, depending on how effectively I am able to null strong signals on 600 (IBOC), 690, 760, 910, 1130, 1170 and 1360.

As I tune closer to 1130 and 1170, the desense gets worse, peaking at around 45,00 to 47,00 within 15-20 kHz of 1170. BTW I don't consider the Tecsun radio to be free from desense until it's possible to have a reading of 15,25 as pictured below:

I just wonder how well tilt-nulling will work on some of my strong locals vs distant targets, though. In some places I might be listening, the local might be upwards of 94,25 to 98,25 or so using only the built-in antenna, whereas the target only 10 kHz away, assuming the "noise level" was low enough to allow a 15,25 reading (which in reality wouldn't be the case, obviously), might itself only be 15,00 to 15,01, WITH a tuned loop AND coupled to a utility pole ground wire. :|


Thanks for the update, Stephen.

Tilt-nulling may not be very effective for extremely strong local stations due to the desense problem. When I get home to NY the end of next month I'm going to do some more tests with local WYSL-1040. Again, you may have to remove yourself by distancing yourself from the problem.

As far as calculating signal strengths (a previous question you had), no calculation I know will be accurate or even close to what you would read on the PL-380, etc. There are too many variables to contend with. I have a program which does calculations, but I use it to compare projected signal strengths between different stations, not as actual expected values.


Stephen said...

I did a few tilt null tests (as well as a few desense tests) with my Tecsun PL-606 and recorded some video clips. (Unfortunately there's no sound, as the microphone on the camera isn't working, but you should be able to see the signal strength indicator in the display.)

On the tilt nulls, I show the null, then rotate the radio for maximum reception.
As for the blocking/desense, I start off tuned off frequency, then tune to the desired station. The signal indicator briefly shows a fairly high number, then refreshes down several dB.

A few links follow. More are available in the same Picasa folder.

Tilt-nulling 760 KFMB - 20,00 to 87,25:

Tilt-nulling 1070 KNX - 15,00 to 49,25:

Tilt-nulling 1170 KCBQ - 22,00 to 81,25:

Desense on 1130 KSDO - 71,25 to 54,25 (SAT tuned to peak 1170 KCBQ which is ~81,25 barefoot):

Desense on 760 KFMB - 94,25 to 85,25 (with Select-A-Tenna - normally it's about 88,25 barefoot at this location):

And, last but not least, one time the PL-606 couldn't decide whether to display 84,25 or 88,25 on 760 KFMB (after dropping from 91,25 to 83,25):

Have you ever noticed behavior like that on your PL-380?

Also, yesterday I was experimenting with a (admittedly drift-prone) signal generator to see how desense affected the PL-606. I have no video & audio clips at this time, but have noted a few interesting results so far...
With a test signal on 1630 (which peaks at 98,25 on the PL-606), a station on 1700 went from 71,25 (clear signal aided with the SAT and with the signal generator turned off - was about 22,00 on a nearby blank channel) to 50,00 with the pest signal turned on, and unreadable. Also, with the test pest on 840kHz at 98,25, it took 760 KFMB from a clear 80,25 barefoot to a faint 50,00.
I'm hoping I can maybe experiment with 1170's 2nd harmonic tomorrow. With proper antenna positioning, it's possible to have a reading of 15,25 on 2340 kHz, and a fairly clear signal at around 36,25 to 40,25 or so. I'm wondering how the test pest, if I set it to 2300 or 2400 kHz, would affect the PL-606...