8' Loop Crystal Radio
by Josh Young

    About eighteen years ago my Grandfather introduced me to crystal radio fun.  He told me some really neat stories of receiving long distance stations with no batteries or power.  Ever since he bought me my first Radio Shack Crystal set I have been hooked.  I have been experimenting and tinkering with them off and on for about 16 years, and I am continually amazed at their performance.  I can’t say exactly when it happened, but along the way somewhere I got interested in Loop X-tal sets.  My first Loop made was a “X” frame made from some cheap wood and a scavenged Variable Capacitor from an old Heterodyne AM radio.  It was about 1 foot per side, and had about 15 turns to resonate the BC band.  It was a very poor performer, but gave me interest to keep on advancing.

    The current set I have worked up to is the 8’ Octagonal  double tuned X-tal set.   I have included a Led light, a Digital spotter radio, Ceramic insulated Silver Plated VC’s, and some nice calibrated reduction tuning knobs.   These additions have made for some pleasurable Dx-ing.  It seems to be a real performer, as I have logged over 85 stations at night, and DX at 1400 miles.   There are certain aspects that have drawn me to loop sets, some include: precise station direction finding, portable, deep signal nulls, low noise signal source, neat looking, straight forward construction, no dependence on external ground/antenna, very good signal gathering capability, and very high Q set potential.

    I choose the Octagonal shape because it was easily constructed, and gives near Round-Loop results.   This particular set is made out of 1x3x8 cheap lumber, has been stained and sealed to keep it looking fairly nice and to preserve longevity.  Wood was used because it was very easy to work with, and was light weight.

    I used a dado blade to make the “Slots” in the wood frame material that allows it to be assembled.  I also used the dado blade to produce the slots on the ends of the frame that allows Styrene insulators to be inserted.   This would also be a good time for me to mention a problem I have encountered with my current Loop.  One of the Horizontal spreaders now sags due to the weight of the Variable Capacitor on it.  It would be wise in the future to support the V.C. on the Tuned Loop from the interior of the Set as opposed to the exterior.

     Since the function of the main Tuned Loop is to gather/reject signals it is necessary to build it in a manner to preserve all the “Q” possible.  Keeping in mind the afore mentioned, another aspect I tried to keep in mind on this Loop Radio is to minimize metals near the Loop.  Metals in proximity to coils have been shown to distort reception patterns, so where ever fasteners were needed I tried to use less lossy materials.  Fasteners considered should include:  brass, stainless steel, or nylon.

    In past articles on crystal set design, I had seen some information about insulation materials and their benefits when used properly on components that are exposed to RF energy.  One of the materials mentioned was Styrene, which usually is a white colored plastic and is very easy to work with.  Styrene is used in many ways, one of which is for drain type plumbing.  Genova products is one such company that makes Styrene fittings, and should be easy to locate at local plumbing supply houses.  Since Styrene has been said to work well for RF insulators I thought I would try my hand in using it.  As you can see in the photos, the 18 gage wire never touches the wood directly, to help minimize RF losses.  The wire is suspended above the wood by the Styrene insulators, and has yielded very noticeable results.   Usually the Styrene purchased from local plumbing supply companies comes in fitting which are sometimes curved.  One way I found to straighten the rounded pieces to make them into a more useful shape is to cut a certain amount out of the fitting, dunk it into boiling water and straighten it to the desired shape.   The Styrene needs to be heated by the boiling water a few minutes, and then can be easily formed.  I made a flat Stainless Steel jig to flatten the Styrene to make the flat pieces seen in the photos behind the Tuned-Loop knob.  My jig is two pieces of Stainless Steel about 5”x 5” w/ small holes in the corners to facilitate screws for tightening the two halves together.   I place the Styrene to be flattened in the jig, insert screws w/ nuts into the four holes, place it in boiling water, and every couple minutes pull it out of the water to tighten the screws to flatten the Styrene.  This may not be the best or easiest way of making flattened pieces, but it has worked well for me.

    The wire I have chosen for this particular tuned Loop was 18 Gage solid enamel coated wire.  I choose this particular wire because it was the largest diameter solid wire I had available.  I actually ended up reclaiming it from a large power transformer!  The next time I create a new monster Loop, I will seriously be considering high count Litz.  There has been much speculation that high count Litz wire may yield better results on a loop-type set as opposed to a standard Antenna/Ground type set.  I just haven’t saved up enough money yet to give it a spin, but with any luck I will eventually.  After perusing Ben Tongue’s web pages I decided to attempt a Contra-wound winding scheme on the main tuned Loop, and have noticed good results.  Information on the Contra-Wound winding scheme can be found at www.BenTongue.com.  I also tried to maintain approx.  ¼ inch spacing between adjacent turns, in order to minimize stray capacitance.  The wire ends also terminate into the Styrene insulators mentioned above, but other insulating materials may be used with good results.   It is also necessary to get the turns as tight as possible to restrict movement, as slightest movement of the wires can cause frequency variation or other undesirable results. 

     I choose to use a double tuned configuration to reduce direct loading on the main loop, which can increase selectivity and sensitivity.  When using a double tuned circuit one can distance the two coils to reduce coupling, although other electrical configurations could yield similar results.  The “Hobbydyne” type circuit is one I have in mind that could be interesting to try when directly connected to the main tuned loop.   I haven’t tried it yet, but probably will soon.   I have often wondered if it is better to reduce the coupling by physical coil separation, that way the receiving pattern is not distorted by the extra components it would take to electrically reduce coupling.   Only time, and experimentation will tell.

 
     I currently use RCA Sound Powered phones with a UTC-015/Bogen  STM Impedance matching circuit w/ a “Benny”.  The two diodes I have had excellent luck with are (1)1N770P10, and (2) HP 5082-2835’s in parallel.  Those have given me the best weak level sensitivity, but I haven’t tried all that there is to try.  I am still learning, and it seems as my set progresses with better VC’s and wire etc. I also find that the best diode changes from time to time.

 
    I have attempted to give a general synopsis of my Loop set.  It is an ever-changing design, and gets better all of the time.  Much of this information was learned on the Rap & Tap, and spending time on the many great websites doing research.  I need to give much credit to all the helpful guys on the Rap, they are great!  Hopefully this will inspire some to reevaluate Loop performance, and possibly build models with their own hints and wrinkles.  I know that I am always amazed at how well it performs.    Enjoy, and have fun!

Josh Young