Gollum Coil

[라디오]

Hints and Wrinkles

Coils. Resonance circuits. Wave traps.

Tips und Tricks

Spulen. Schwingkreise. Sperrkreise.

Cylinder coils with lower internal capacity

Normal cylinder coils have a relatively large distributed capacity (compared to a honey comb coil or a spider web coil). That is why the whole MW (B.C.) band (520-1620 KHz) sometimes cannot be covered in a single sweep .

With the variable capacitor in the low capacity position, the frequency of the tuned circuit will not coincide with the upper band edge because frequency variation of the tuned circuit is not wide enough.

If such a coil is divided into several sections (coil wound with several (3-5) sections spaced 4 mm apart), the internal coil capacity is reduced quite a bit. Example: b.c. band. Coil diameter 2 cm, 4 pie windings of 30 turns each.

Even better is a small space between each turn.

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Low loss coils

When you want a receiver with both high selectivity and high sensitivity , low loss coils are a must. If available, use r.f. litz wire (stranded wire).

The question about the best coil is already a religious question almost. Ask 10 crystal set specialists. You will hear 10 variously opinions. I prefer at coils without ferrite cores spider-web coils (simple and certainly to construct) and at ferrite core coils pot coils.

Best coil forms are

• Honey comb coils (basket weave coils)
  Using solid copper wire lets you build air-wound coils. After winding remove the coil former and diptinto a stabilizing compound and let dry, or use polysterene strips to strengthen the coil. The use of r.f. litz wire is problematic because of low mechanical stability. Here or here you will find a construction plan for a honey comb coil. Low internal capacity and high Q.

• Spider web coils
  Spider web coils are also good (and easier to wind as honey comb coils). Take a look at my construction plan for a crystal receiver with spider web coil. You can use r.f. litz wire! Low internal capacity and high Q. Better build up as an equal cylinder coil. When using this type of coil former a higher Q is the result.
  Photo of spider web colil with a skeletized acryl coil former and a close up. The Q is 202! More information about the Q of different coil (circuit) types here.

• Cylinder coils and ylinder basket coils
  Almost an "air coil". You can use r.f. litz wire. It is more difficultly to fix the necessary space from turn to turn (One diameter of wire space between each turn).
  
  As large as possible coil diameter (for b.c. band approx. 10 cm or approx. 4 inches) and as short as possible coil length. Low internal capacity (when space between each turn) and high Q. Coils with full material as a coil former tube are not to be recommended. For low loss use polystyrene or teflon as coil former.

  
  

• Frame coils
  Use only thin coil former of low loss material like Pertinax. Remove as much as possible material of coil former. Useful Q factor. Necessary space from turn to turn for reducing the internal capacity. You can use r.f. litz wire.

• Ferrite core coils.
  Thes coils have also a high Q (short coil wire lenght, when using crisscrossed winded coils also low internal capacitivity). The do-it-yourself of a ferrite core crisscrossed winded coils is quite difficult. The core material must be appropriate for the desired frequency range.
  
  Thin RF litz wire is to be procured already heavily. Ready-made coils often have no tapping for the optimal adaptation of the diode. When using two loose coupled coils in double tuned xtal receiver, the construction is quite difficult too.
  
  
• Ferrite pot core coils with high Q.
  Pot core coils are also well suited using M33 (Siferrit) material at the sizes P18*11 or RM6 (less) in relation to r.f. litz wire. Coil Q´s up to 600 are possible. (Many thanks for information to Berthold Bosch, DK6YY. See his DX receiver here.)
  Both coil core types including teflon cord and r.f. litz wires are in my radio store available. Size comparison of coil cores P18*11 and RM6.
  
  Construction hints for a ferrite pot coil using RM6 core, M33 material for b.c. band (MW). Important is the use of teflon cord in the middle of the coil former space (for details see photo 2, photo 3, photo 4, photo 5). This cord should guarantee a free space the gap of the two ferrite closure halves. As a result, the Q of coil is even better. The room should include no coil (According to manufacturer's specifications for medium wave use is some spcace near to the gap of core necessary). At first you wind the turns only left from the teflon conic and then on right side. Now you wind normal further on. Construction details here.Construction plan for a high performance triple tuned dual circuit receiver using this coils here.
  
  Example 1:
  For a 160 uH coil you need approx. 125cm (49.21 inches) 40*26 AWG r.f. litz wire. This coil in combination with a 330pF tuning capacitor produce a frequency range of about 600-1650 kHz (upper medium-wave band) .
  
  Example 2:
  For a 230 uH coil you need approx. 150 cm (59.05 inches) 40*26 AWG r.f. litz wire. This coil in combination with a 330pF tuning capacitor produce a frequency range of about 520-1200 kHz (upper medium-wave band).
  
  Why the subdivision of the range? Because so a optimal adaptation of the Q to the diefferent frequencies is guaranteed.
  
  Construction hints for a ferrite pot coil using P18*11 core, M33 material for b.c. band (MW). Important is the use of teflon cord in the middle of the coil former space (More details see RM6 cores). This cord should guarantee a free space the gap of the two ferrite closure halves. As a result, the Q of coil is better. At first you wind the turns only left from the teflon conic and then on right side. Now you wind normal further on. Construction details here.
  
  Example 1:
  For a 120 uH coil you need approx. 140cm (55.11 inches) 45 strands and total diameter 22 AWG r.f. litz wire. This coil in combination with a 330pF tuning capacitor produce a frequency range of about 720-1800 kHz (upper medium-wave band) .
  
  Example 2:
  For a 275 uH coil you need approx. 185cm (72.83 inches) 40 strands (48AWG) and total diameter 26 AWG r.f. litz wire. This coil in combination with a 330pF tuning capacitor produce a frequency range of about 450-1200 kHz (upper medium-wave band).
  
  Why the subdivision of the range? Because so a optimal adaptation of the Q to the diefferent frequencies is guaranteed.
  
  

You will find here more information about low loss coils.
More information about the Q of different coil/resonance circuit types here.

The coil is decisive for the quality of your receiver!

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Litz wire coils better than solid wire coils?

Litz wire coils have a better "Q" than solid wire coils of the same wire gauge. Litz wire is available with different numbers of strands. For medium wave b.c. - short wave and long wave (LF) band different sizes of litz wires with different numbers of strands are optimal.

Usable litz wires for B.C band coils
Number of strands	strand diameter (AWG)
25	44
45	44
160	44
420	44

Solid copper wire also offers good results when the wire diameter not too small. Wire diameter for b.c. band coils should be number 20 to 24 AWG.

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How to solder rf litz wire

Ensure that EVERY single strand is tin-plated. Heat up well and for a long time enough with the soldering iron, tin-plate well.

Your type of litz wire can directly be tinned with the soldering? Then skip the following lines.

Among other things there is litz wire which connnot be tinned because the strand isolation resist soldering:

Here is some advise for those who are not familiar with the handling of this type of litz wire. It is stranded wire but each wire is enameled to isolate it electrically from all the other wires in the bundle with temperature resistant lacquer.

Since each wire is so thin (about numbers 40 to 44 AWG) it is out of question to scrape off the enamel of each wire with a knife in preparation to soldering. Just remove carefully about half of an inch of the silk covering of the wire bundle. Then hold the fine strand bundle into the flame of ethyl alcohol (spirit).

Watch the wire become red hot and then imediately immerse into the spirit liquid below the flame to cool off. The litz must be bright and shiny after this. This procedure cleans the wire and it is ready for tinning. Attention! Every fine wire (strand) of the bundle must be tinned. Better try a couple of times with some spare wire to get the knack of it.

Be safety - conscious ! Beware of the open flame ! Don´t start a fire!

Hint by Peter Stepponat

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Which coil former is for b.c. band high "Q" best?

The coil formers reduce the coil "Q" factor of the coil. A coil is at best without coil formers ( air coil). But such coils are hardly realizable. Dry card board tubes are suitable. I have made experiments with air- and balsa- and PVC- and card board- and Pertinax coil formers, ranking from good to lesser good. (Click here for a low loss example of spider web former.)

It should be attempted generally to use only few material. A compromise between mechanical stability and "Q"-factor is the destination. You will find many different opinions on this subject.

Thin balsa is appropriate for spider's web coil former if it is dry. Such coil is to be seen here. However, these coil formers are mechanically quite sensible.

I made good experiences with acrylic glass (perspex), I used a CD-ROM disk with removed silver film (You can remove this film from some disk types). Here a coil former made by this material. More information about the verification of coil (resonance circuit) Q´ s here. A construction plan using this coil former type here.

Read also the notes about ferrite pot core coils above on this page.

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Basket weave coils? Spiderweb-coils? Cylinder coils?

Some solutions to a tough question.

Having the same inductance, it is principally more difficult to obtain the same Q with cylinder coils as compared to spiderweb-coils or basket weave coils (honey combs). I personally give precedence to spider-web-coils over basketweave coils and cylinder coils.

• Spiderweb-coils are easier to construct than basket weave coils. High Q because of less coil former volume and you are able to use r.f litz wire. The cardboard coil former nessessary to wind the coil and to keep it in shape only means minimal loss.
  
  These effects altogether may weigh more than the adventages of the basket weave coil. I use always 13 slots for the coil former. Why? The turns are so located more rigidly than at only 9 or 11 slots. The distance between the turns is to be adjusted more simply also.

• One advantage of the basket weave coil is that the coil former can be dispensed with after winding (provided you have used enameled wire of some stifness). This type of coil is an "air-coil" with low distributed capacitance. Basket weave coils wound with litz wire are mechanical unstable after removing the coil former. Nevertheless the use of litz wire offers higher quality than enameled wire (magnet wire) in b.c. band range.

• Cylinder coils should have a relatively large diameter and should be winded with turn distance. R.f. litz is well suitable for medium wave (B.C.). The coil former should have low loss material.
  
  A coil basket coil is well suitable. A low capacity and low loss high performance coil can be built. Notes on the principle: 1 = wodden coil former lateral part. 2 = wooden slabs. 3 = coil wire. 4 = grooves in wooden slabs.
  
  So that the wire (winded to distance) do not slip. Do not coat or waterproof or isolate the wood. Employ as thin as possible wood. The distance from turn to turn should have 1 wire diameter size.

You will find differing opinions in Internet discussion groups, web-pages and publications, but also confirmation of my view.

I have measured the q-factor of the above mentioned coil types. These measurements support my opinion

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Rejector circuits (wave traps) for B.C. and SW band crystal receivers and single circuit radio sets.

In the past, rejector circuits (wave traps) were used to weaken the reception of strong transmitters. The normal crystal receiver has only a poor selection. Was a local station too strong, this transmitter could disturb other far away stations.

Today we have additionally another problem with detector reception. Crystal receivers have secondary resonances next to the main frequency range.

These resonances arise from antenna - circuit resonances. After darkness, short wave stations become particularly audible in the upper frequency range. Example: SW broadcast stations become audible in Berlin around 9 and 7 and 6 MHz. It is known that these problems occur also in the U.S.A. area.

Within the years 1920-1930 there weren't any stations in this frequency range. Therefore this problem didn´t exist either. In other countries it will be other stations on othter frequencies. A rejector circuit can help here. You can use these wave traps with your B.C. and SW crystal receiver!

SW wave trap.
Construction plan for a rejector circuit 5-8 MHz. When you use only 30 coil turns, the frequency range is approx. 6-11 MHz. (Remark: 4 mm = 0.157 inch. 0.2 mm = 0.0078 inch)

MW (B.C.) wave trap.
Construction plan for a wave trap 500-1650 KHz. Use a spider web coil. 48 turns rf litz wire (45/41 AWG) or isolated solid copper wire (magnet wire) 24 AWG. Thickness of card board coil former 0.06 inch.When you use r.f. litz wire and an air spaced tuning capacitor (2*320 pF), a high "Q" wave trap is the result.

Insert the rejector circuit in the antenna line.

Please, consider that the wave trap reduces the sensitivity somewhat. However, this is still better as to hear the interferences. Double tuned xtal receivers may also need a wave trap in addition .

Special hints:

• You can add more rejector circuits in series, also in different frequency ranges.

• You can use a second crystal receiver as "rejector circuit", if you have a strong local transmitter in the original frequency range. Connect the antenna with the antenna socket of the "rejector" crystal receiver, the ground socket of this receiver connects with the antenna socket of the main crystal receiver. By carefully tuning, you can attenuate the unwanted station.

When you are interested in"crystal receiver DX- reception" you need some rejector circuits or use a double tuned receiver! (DX = dark x-rax, long distance.

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Loose coupling of tuned circuits.

An excellent means to improve the selectitity of the crystal receiver is using two tuned circuits. Inductive coupling of these circuits should be as loose as possible. For a quick check you will need two single-tuned crystal receivers.

Preparation: Tune crystal receiver # 1 to a radio station. Repeat this with crystal receiver # 2.

Now reconnect the antenna and the ground to receiver # 1 (without earphone). Place crystal set # 2 (without either antenna nor ground connection but with an earphone connected) near receiver # 1. Try to place the coils of both receivers as close as possible to each other.

Now you may listen to the previously tuned radio station in receiver # 2. Carefully readjust the tuning. You have a real double-tuned crystal receiver in operation! You may now slowly increase the distance of the coils (and receivers).

The signal level in the earphone will decrease but selectivity incrases. Here a schematic of a simple double tuned crystal receiver. Photo 1 and Photo 2 of this quick test.

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One circuit, two circuits, double tuned, triple tuned?
Definition of terms.

In America crystal receivers witht two circuits are called as "double tuned sets" or "dual tuned sets". When an antenna matching variable capactitor is also in use these sets are sometimes called "triple tuned set".

In Germany the antenna matching variable capacitor is not counted. A set with one circuit (with or without antenna tuning cap.) is a single tuned xtal set. A set with two circuits (with or without antenna tuning cap.) is a two-circuit set.

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Do not use bad insulation material.

Using ceramic solder tag strips, the resonant circuit Q can be increased up to 5 percent. In particular the "hot" connections of tuning caps. and reactance coils and diodes are critical. The lines must not come with other materials in contact. See the dconstruction plan of my triple tuned ferrite pot core coil receiver. These ceramic tag strips are available here .