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Author Topic: Some Thoughts on Resonant Loop Antennas  (Read 18297 times)

Online jFarley

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Some Thoughts on Resonant Loop Antennas
« on: July 24, 2013, 2149 UTC »
Back in the day I built one of the NRC A-A Medium Wave loops which, at the time, was the de facto "Gold Standard" for MW receiving loops.  This was a Resonant Loop which tuned the MW band using a common mechanical variable capacitor.  I was floored by the performance which such a small antenna exhibited in such a relatively small package.  I was very interested in seeing if this performance could be duplicated in a Resonant Loop modified to tune the Tropical SW Bands.  After many prototypes and many errors, I finally came up with a loop which is similar to the one I currently use.  Not too many DXers use Resonant Loops, and I thought I would share some of my experiences here in the form of a list of Resonant Loop advantages and disadvantages. Some of these pros and cons are shared with Shielded Magnetic Loops such as the Pixel and Wellbrook loops, and some are unique to the resonant design.

What this loop is...

The loop I use is a remotely tuned Resonant Loop which has been designed for performance at the lower end of the HF spectrum.  It is a basic no frills loop whose band coverage can be pushed up or down a bit by re-sizing the loop windings.  It consists of four main parts.  The loop winding itself is a 20" square four turn winding which has a 3" gap between the center two turns.  This winding is tuned by a pair of NTE618 tuning diodes which are housed in the same aluminum box which holds the loop amplifier.  This is a balanced JFET cascode amp of minimal gain.  The third component in the loop is the transmission line, which is a continuous 180' run of direct burial rated double shielded 75 ohm coax.  This single coax provides both the amplifier power and tuning voltage, and carries RF back to the shack.  Finally, in the shack there is a variable DC supply which is used to tune the loop, and to strip off RF for the receiver.  This single coax system makes siting the loop a very simple matter.

Some Metrics for this loop...

This loop has a tuning range which covers 2400 to 7800 kHz approximately.  Over this range this loop has an Effective Height which varies from about 10 meters at the low end to 40 meters at the high end.  The loop exhibits a Loaded Q which varies from about 150 at the low end to about 250 at the high end of the tuning range.  This high value of Q sets the minimum discernible signal to a very low value which will generally be below ambient atmospheric noise for most listeners' locations.  All loop design is an exercise in compromise, in which one metric can be enhanced at the expense of other metrics.  Since it is generally difficult to DF the skywave signals common to HF propagation, the FOMP (Figure of Merit with respect to Pattern - the ability of the loop to exhibit deep nulls) has been traded away somewhat in favor of loop sensitivity and Effective Height.

Some Advantages of using a small Resonant Loop at HF...

1)  In the typical residential lot, it is getting increasingly difficult to be able to site a larger wire antenna structure away from all potential noise sources such as power lines, neighbors' houses, and even one's own house.  A small loop samples RF in a very small volume, and because of this, it is far easier (at least for me) to site a small loop in an area which is relatively far away from all noise sources.

2)  Small loop does not mean small performance.  With a high Loaded Q, a Resonant Loop will have an output which rivals or exceeds many large wire structures.  A Loaded Q of 250 means that the winding-tuning capacitor tank circuit will show a voltage rise at resonance of about 250 over an equivalent non-resonant winding.  This adds a free passive gain of about 48 dB to the loop, and only a small amount of gain is required in the loop amplifier.  Unlike an SML (such as the Wellbrook or Pixel loops which require a large amount of gain following the winding) the loop tank circuit does virtually all of the heavy lifting. 

3)  With a relatively high Loaded Q, a Resonant Loop will exhibit a low minimum discernible signal which allows it to be an effective DX antenna in a small real estate footprint.  I have had the opportunity to test this antenna against an Wellbrook ALA 1530, and I can say the following:

In a relatively quiet environment, the Resonant Loop will produce JBA to useable audio from very weak 43M pirate signals which are only rendered as weak carriers sans audio on the Wellbrook.  In the 60M tropical band (which is not even the region of maximum sensitivity for this loop) the Resonant Loop will show fade-up carriers on very weak African DX often 5 to 10 minutes before they are seen with the Wellbrook.  This is a big claim, but it has been proven by hundreds of receptions.  YMMV?

Please don't get me wrong here; the Wellbrook is a fine loop (as I imagine Pixel loops to be also) and I am very satisfied with it.  Using the Wellbrook, I can program many benchmark and target stations in the SDR, and bang through them without having to re-tune a loop.  This makes for very efficient DXing, and from this viewpoint SMLs live up to their typical marketing claims as being "the perfect antenna for SDRs".   Based on comparisons, however, I do wonder if SMLs are the perfect antenna for DXing.  When it becomes time to dig in and dig out weak DX, the advantage almost always goes to the Resonant Loop.

In a relatively quiet environment, of course.

4)  While the FOMP directional properties do not generally allow DFing at HF, the "nullpression" can be very effective in combating local QRN via loop rotation.  I have been able to get stable reductions of 18 - 24dB against the noise from power poles gone wild.

5)  One of the real strengths of the small Resonant Loop (and also the SML) is that it looks at the RF differently than a wire antenna.  A small loop can be a great complement to another antenna because of this.  Depending on propagation, either a loop or a wire antenna may be the favored receptor at ID time; being able to switch between two antennas which receive differently can be a great advantage.

6)  I have found no real advantage in mounting the loop at an elevated height; it hears just fine when mounted on a low mast (2" PVC pipe) 8 to 10 feet off the ground.  This makes the loop a safe antenna during thunderstorm season, and it is generally left on 24/7.

7)  Resonant Loops are flexible and can be designed to solve particular DX scenarios.  Loops can be built specifically for LF NDB DXing, MW DXing, and for other portions of the HF spectrum.

Some Disadvantages of using a small Resonant Loop at HF...

1)  The Resonant Loop I use is a unshielded loop, and because of this it is far more prone to QRN from local sources than the Wellbrook Loop I use in conjunction with it.  Serious consideration has to be given to the location where the loop is ultimately sited, although it is rather easy to move it around to find the best site.

2)  Resonant Loops are not commonly available, and to get what you want you will most likely have to build it yourself.

3)  Resonant Loops have a finite frequency range over which they may be tuned, and this is typically a 3:1 frequency range.  Additionally, the loop must be retuned for significant frequency changes.  With a Q of 240, this generally means that my loop must be retuned for peak reception for frequency shifts of around 30 to 40 kHz at 6925.

4)  Not only does a Resonant Loop have finite tuning range, but the receiving properties may vary significantly from the bottom of the tuning range to the top.  In general (with this loop which uses Tuning Diodes):
    - Loaded Q increases with frequency
    - The Effective Height (antenna output) increases with frequency
    - Sensitivity (minimum discernible signal) gets better with frequency
    - FOMP (the ability of the loop to generate a pattern null or depression) decreases with frequency

A Resonant Loop like this should generally be designed to have the most important frequency band located near the top of the tuning range.

5)  The Loaded Q I state is typical for dry environmental conditions.  The actual Q may and will be depressed somewhat by weather conditions such as rain, snow, fog, or ice loading.  Perching birds, flying squirrels, and wet foliage nearby will also affect the Q. 

6)  If you are designing a Resonant Loop, remote tuning via tuning diodes may not always be feasible if you have a target Q in mind.  For some Resonant Loops, it may be necessary to use tuning via mechanical capacitors, and in some scenarios only a vacuum capacitor may offer the necessary Q.  This is a major complication to any Resonant Loop which must be tuned remotely.

7)  Like the Shielded Magnetic Loops, the Resonant Loop may be subject to very peculiar reception properties from time to time.  For instance, I could barely hear a pirate (which generally puts a great signal into my area) on the loop for almost the entire duration of one of his shows, yet the pirate could be heard S8 on a balanced dipole at the same time.  The rest of the evening and overnight, the loop heard normally.  One might guess that some unique combination of wave polarization and/or ground reflection was causing this, and it tended to be a stable situation for an hour.  Luckily this is the exception to the rule as the loop tends to be a dependable performer.

A Reference Design...

Here is a link to a PDF of an article written for Fine Tuning's Proceedings a while back.  This article describes a loop very similar to the one I am currently using.  Several improvements have been made to the loop since then, and I would not build one this way right now.  But this article does describe the methodology of the single-coax interface to a remotely tuned Resonant Loop.

http://www.mediafire.com/download/373043kw6d4oasf/RLA_Article.pdf

Towards the future...

The concept of the Resonant Loop at HF has not IMHO been pushed to the limit, and there are some areas in which serious research and work needs to be done.  Two areas I am exploring are:

1)  It has been mentioned that the High Q Resonant Loop is generally unshielded, and is prone to noise from local sources unless sited carefully; the Shielded Magnetic Loop is far less susceptible to noise.  What if both concepts could be combined to create a Shielded Resonant Loop?  I have had some partial success with this, and I will post some more info shortly in a picture gallery of some of the loops I have built.

2)  The loaded Q of my loop is around 240 at 43 meters, which results in a 3dB bandwidth of about 29 kHz, and a 10dB of about 80kHz.  These bandwidths are far too broad to permit successful frequency discrimination in the antenna, that is, they do not allow one to "slice off" an offending station from a desired station.  Crunching a lot of numbers, running some models, and building some prototype coils suggests that it may be possible to create a Resonant Loop which would exhibit a loaded Q of approximately 1200 at 43 meters (my design goal) and possibly as high as 2000.  At a Loaded Q of 1200, the 3dB bandwidth would be around 5.8kHz, and the 10dB bandwidth would be about 16.2kHz at 6925kHz.  This level of selectivity would really allow one to do some neat tricks when it comes to weak signal reception!

Joe Farley, Near Chicago
SDR-IQ / R8 / R7
Remote Resonant Loops for HF and LF / ALA 1530
Active 60" Whip / PA0RDT
QSLS appreciated to:    jfarley44@att.net

Offline Pigmeat

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Re: Some Thoughts on Resonant Loop Antennas
« Reply #1 on: July 25, 2013, 0249 UTC »
I remember building the Radio Netherlands MW loop years ago. I had to make it a bit smaller than the one square meter they recommended.

That thing wouldn't tune up for MW below about 800 kHz. due to the compromised size,but it had good coverage through the 90 meter broadcast bands. Heck, it worked on 40 meters if I decoupled a few turns.

Loops are easy to build and are stellar performers during the summer static season. I love the things.

Offline BoomboxDX

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Re: Some Thoughts on Resonant Loop Antennas
« Reply #2 on: July 25, 2013, 1555 UTC »
I can see how a loop for lower SW frequencies would be useful, especially to null out QRM.

The Realistic DX-375 receives the Low SW bands off the internal loop as well as its whip antenna, and can be directional on the low shortwaves using the internal loop.

If a loop can work on a digital portable, there's no reason a larger air core loop can't work well on the low shortwaves.
An AM radio Boombox DXer.
+ GE SRIII, PR-D5 & TRF on MW.
The usual Realistic culprits on SW (and a Panasonic).

Offline Sealord

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Re: Some Thoughts on Resonant Loop Antennas
« Reply #3 on: July 25, 2013, 1558 UTC »
My god JF - you are truly the mad scientist!

This is a heck of a lot work you've put into this, cudos :)
North East Florida
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Online jFarley

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Re: Some Thoughts on Resonant Loop Antennas
« Reply #4 on: July 25, 2013, 2143 UTC »
Rather than trying to muck up a bunch of quotes in a single post...

@Pigmeat

You bring up an excellent point I forgot to mention re summer static.  A common scenario for me in Chicago would be storms to the south in the Texas-Gulf area, and listening targets presumed to the east.  By loop rotation, the noise from the south can be depressed at times, while the pirates are aligned with the lobe of max reception.  Additionally (and I cannot prove this, but it often does seem to be the case) the lightning strikes which are heard on a resonant loop seem to be "softer" than those heard on a wire antenna; the audio is less harsh and fatiguing.  No explanation I would hang my hat on, but just sayin'...

@BoomboxDX

My loop does not really have the ability to null out either QRM or QRN; it can depress things a bit, possibly 3 or 4 S-Units, but making nasties go away entirely is a pretty rare occurrence.  In my loop I have traded away some of the ability to possibly generate deeper nulls in favor of other metrics.

There is no reason that one could not use a loop with a portable; an attenuator might be required if the rx does not have adequate dynamic range.  Having some physical separation between the receiver and the loop may also be a consideration so that the loop does not hear receiver junk.

@SL

Please reserve all judgments on my sanity until my next post on the subject, and that will be coming shortly.  There is definitely some real whack there!
Joe Farley, Near Chicago
SDR-IQ / R8 / R7
Remote Resonant Loops for HF and LF / ALA 1530
Active 60" Whip / PA0RDT
QSLS appreciated to:    jfarley44@att.net

Offline Lex

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Re: Some Thoughts on Resonant Loop Antennas
« Reply #5 on: July 28, 2013, 0543 UTC »
Great info, looking forward to more!  I've dabbled in various homebrewed passive loops with some success.  They're pretty crude but effective in my apartment complex for minimizing household RFI.

Reminds me, I've been intending to do a video demo of the Villard aluminum foil loop I made last year.  It worked amazingly well with a small portable.  But my old digital camera had terrible video so I never posted it.  My new digicam has really good video and audio so I'll redo the demo.

The Villard loop is darned impressive for a cheap, crude thing.  It's just aluminum foil on a foamcore board, using a flap of foil as the tuning capacitor.  It's passive and coupled through the telescoping whip on a small portable through proximity.  I was mostly astonished to see how directional it was from around 8-15 MHz, with sharp nulls and peaks.  I need to make a larger version for the usual pirate funny bands between 6-7 MHz.
That li'l ol' DXer from Texas
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