Toroids for crystal receivers

LC combinations can be done with different type of coils. Most used are cilinder, honeycomb and spiderweb coils. But you can also make your coil using a toroidial coil. But there are lots of toroids available made out of different kind of materials. Some are Iron, Ferrite Mumetal, and lots of compositions of different materials.

For crystal radio use the most used are ferrites, like the FT240-61. Meaning: Ferrite Toroid 2,4inch outer diameter, No61 material. The No61 material is for the frequency range of 0,2-10Mhz.
There are a lot of materials available, with all kinds of final outcome when made a coil useable for the 531-1600Khz AM broadcast band.

A toroid mostly used is the 4C65 or the FT140-61, I tested these toroids, wound using 300/44 litz wire. Combined with my best variable C to get a LC network and measured for Q on different frequencies across the BCB band.

As seen in the excel sheet, you can see the Q difference using a contra wound toroid coil. Connected in parallel you get a Q almost doubled at the high end of the MW band. The same applies for a contra wound spiderweb coil, or cilinder coil.

Here i`ve wound toroid type 4C65 using 300/44 litz wire and type R40C1 with the same litz, but the R40C1 has a second contra wound coil.

You can see the differences in Q when the R40C1 coils are in series and parallel. And how bad the Q is using the ordinairy 4C65.

Next measurements are taken using my spiderweb coil, made out of a PE coil form, and wound with 420/44 litz wire. I compared this coil against a 4C65 toroid, and to a combination of R40C1 toroids with the size of 37x23x7mm and 45x26x8mm. Combinations are just one or more toroids stacked on top of eachother. Each R40C1 toroid is wrapped with PTFE tape. Not the cheapo one used in water pipe systems or drainage, I used MIL-Spec PTFE tape as used at the QRL for high-end LOX equipent for aircraft oxygene.

Capacitors used are: One old modified spacers using PTFE, and completely ultrasonic cleaned, rotorshaft now connected using litz wire. And a high-end capacitor also cleaned and added litz wire to the shaft, I got this one from Ming, a fellow radio enthiousiast from the USA.

Best results in Q where measured using the 4x stacked R40C1, size 37x23x7mm (small one). A Q of around 1300 at 600Khz and a Q of around 800 at 1600Khz. But litz wire used in that combination was 660/46. But
the value of the inductor is to low to use for the MW-BCB
As for the contra-wound coil, the 45x26x8 with 2x 30turns has a good Q within the MW-BCB band. But needs a extra C for the lower range. But there`s still space for extra windings.

Winding a contra wound toroid coil is a bit more complicated as just winding one complete toroid with for example 60 windings to get 225µH. You need to wind the toroid with one coil with a value of 32 windings to get 70µH and another one CONTRA wound with the same turns to get also 70µH. Putting these in series will get you 225µH.
But putting these two in parallel you get 58µH.

The series to parallel ratio is 1:3.2. The ratio of 2 windings in parallel to measured value is 1:0.832. So 2x70µH makes 58µH in parallel. Knowing these ratios you can calculate how big those two windings on a toroid need to be in conjunction with the value of you capacitor to spread the BCB acros the dail.

In my example of 2x 70H I need a variable capacitor of a value of 100-440pF. As I only have one of a value of 19-361pF, a smaller trimmer needs to put in parallel of the bigger capacitor. I choose a trimmer 100pF, trimmed to 80pF.
Now I can tune the lower BCB with 2x70µH in parallel from 501-1052Khz, and the higher BCB with the 2x70µH in series from 996-2090Khz. Don`t bother the range towards 2090Khz.

Another amateur from china made an impressive coil with a Q value of exceding 2000. This coil is made using 25 pieces of R40C1 toroids.
Here a translated PDF file of the building of this coil.

I also made this coil, here a picture of this coil using 25 R40C1 (37x23x7) toroids separated by PTFE foil 660/48 Ltz wire on PP stands. Serial/parallel switch using two reed contacts and neodymium magnets. The white stuff is all PTFE and brass srews/nuts.