TheRadioBoard

I posted earlier about my copper-strip-based magnetic loop antenna. I originally thought the loop was too long to be able to resonate it at 21 MHz. But I found out that it is just barely possible to resonate the loop - a store-bought 5m-long and 50mm-wide continuous strip of copper tape - at 21 MHz with a minimal amount of capacitance. The capacitance is formed by laying the two loop ends flat and overlapping them both with a small dielectric layer and another copper strap. This forms a low-capacitance "split stator" capacitor where each loop end and the common overlaid strap forms one capacitor, and the two capacitors thus formed are joined in series by the common strap. The common strap can be repositioned slightly (e.g. curling up one of its edges) for fine tuning.

I mention this technique because it was difficult to get a low-enough capacitance to resonate the long loop at 21 MHz. I was trying various ways of laying the straps on top of one another but was always getting too much capacitance - just bringing the strap ends into close proximity (with the strap ends facing each other) introduced too much stray capacitance. The above technique (with thle strap ends laid flat and not facing one another) finally let me peak the loop at 21 MHz.

Just for fun I also tried hooking up an air variable capacitor, 450pF max, to the loop ends with alligator clips. The variable capacitor can be continuously rotated (no stops) and at its minimum capacitance has the rotating vanes completely separated from and oppositely-positioned from the stator vanes. With this arrangement it was not possible to resonate the loop above about 11 MHz. So the above capacitor arrangement is both lower loss and lower capacitance.

Don't know if I completely understand your capacitor, but, how about making your loop and then at the bottom have two tangs that hang straight down, these could be an inch long or more. and adjust the spacing between the tangs for proper resonant frequency.
Maybe thread a nylon screw through the tangs to adjust the distance.
Threaded polystyrene rod may be better than nylon.

Ok, just went and looked at the link you gave, I was thinking your
foil was thicker and would hold a bend and a screw. but you still might be able to work something out.
If your copper is 2" wide and your tangs are 3" long you have 6 sq inches of plate. If you space them 1/4" apart that is 5.4pf.
(air dialectric) http://www.daycounter.com/Calculators/P ... ator.phtml
What capacitance do you think you need?
Mikek

Hi,
I went back and read some more of the old thread where you described the problem of a small physical change to the overlap caused a large frequency shift.
A solution to that is more distance between the plates.
Having 6 sq in overlap with 1/4 inch spacing equals 5.4pf
Having 5 sq in overlap with 1/4 inch spacing equals 4.5pf
So that means sliding one plate 1/2" changed the capacitance by
only 0.9pf. Again I don't know the capacitance needed.
Alas, you still need to figure out a mechanical device to hold the plates firm and make the adjustment.
Mikek

Mike - I think he has tried separation.


.........................

In theory I think what you propose would work. But in practice, the mechanical precision needed to finely adjust the plate separation is daunting. For a 5m-long, 50mm-wide loop (equivalent conductor diameter assumed to be 20mm) resonant at 21 MHz, the AA5TB calculator says a capacitance of 19.883pF is needed, i.e. 2.892uH loop inductance. Changing the capacitance just by 0.9pF as in your example yields a whopping shift in resonant frequency down to 20.540270 MHz - about 460 kHz or 5 times wider than the predicted loop bandwidth of 91 kHz. (EDIT: whoops - the above calculations are for a 0.25-wavelength circumference loop at 21 MHz. My loop is 0.33-wavelengths long, meaning even more inductance is present and even less resonating capacitance is required.)

So even finer mechanical precision would be needed to adjust the spacing, and that is complicated by the fact that the copper foil I am using is so thin. Also the foil is not perfectly flat (with small twists and wrinkles) so the capacitance change with distance separation might not be linearly controllable, possibly leading to jumping over the resonant peak.

The method I used now (which, actually, is the same as the proposed PCB-based homebrew differential variable capacitor in the other thread) has the advantage that the loop ends are (a) fixed in place relative to one another and (b) have little stray capacitance (since they are flat beside one another). Then the overlaid common strap can be mostly fixed in place with just one tail end left loose and acting as a book capacitor. This allows easy mechanical stability of most of the capacitor bulk (everything is affixed to the common plane of the dielectric) while leaving only a small dangling flap for the fine tuning.

Also the effective capacitance is divided in 2 by the series capacitor arrangement.

Frankly, I'm still a little (pleasantly) surprised that the latter method (flat tails with common overlapping strap) works when the former method (spacing between mutually-facing straps) didn't. Since the latter method works, then it should also have been possible to lay the strap ends flat (eliminating mutually facing surfaces) with a tiny bit of common overlap and eliminate the third common strap . But that didn't work for me - maybe the loop ends in such close proximity already introduced too much capacitance. Probably, with enough precision in construction, any method could be made to work.

Right now the dielectric is a flimsy polyethylene freezer bag. Next, I need to visit the dollar store to find candidate polyethylene-based products that are stiffer (1mm thick) and could be repurposed as the dielectric.

Any chance of a capacitor setup diagram qrp?

......................................

Here is the high-capacitance configuration. Note the loop is suspended from the top.

A is a plastic barrier tape, sticky-side in, and wider than the copper foil. B is the copper foil tape, narrower than A.C is the polyethylene dielectric. Since A is wider than B some margin of A's inner adhesive surface can be used to affix A (and B) to the dielectric C. At the bottom of the capacitor, A is not affixed to the dielectric and is allowed to hang loose to act as an adjustable book capacitor (the stiffness of B allows some degree of bending and retaining shape of the loose ends). This whole arrangement thus forms a fixed capacitor and a variable capacitance section.

The above configuration can resonate the loop up to about 14 MHz.

Here is the low-capacitance configuration:


Again A is the barrier tape and B is the copper foil. A is affixed to dielectric C. Above the dielectric a common copper strip D is laid slightly overlapping the two flat ends of the bottom B strap ends. I haven't decided how to affix D to C yet (in my experiment I just laid it on top of C and poked it with a stick to adjust capacitance). I suspect most of D can be fixed in place with tape and one end of D can be left loose to act again as a book capacitor.

The above configuration can resonate the loop up to about 21 MHz.

Currently the polyethylene dielectric C is thin and flexible, but I plan to make it solid, then place the capacitor at the top of the loop (away from the ground to prevent ground losses and accidental human contact).

Armed only with the knowledge that polyethylene can be and has been used as a dielectric in high-voltage capacitors, and that some common household materials are made of polyethylene, I set out to systematically inspect every plastic item at the local dollar store for potential use as a dielectric.

Most solid plastic materials I found were polystyrene or polypropylene. Polyethylene was only used in freezer bags, thin packing foam strips, an ice tray, a squeezable sauce bottle, and the lids of some food storage containers. I was ready to give up and try polypropylene, but then I found this:


It's a 1mm-thick cutting board made of solld polyethylene. Perfectly sized and shaped for my capacitor dielectric, and also much cheaper and easier-to-source than an equivalently-sized block of teflon, which was my other choice for a dielectric (which has also been explicitly recommended as a high-voltage capacitor dielectric).

Does anyone have any idea if polystyrene or polypropylene would make good high-voltage capacitor dielectrics?

Some say the loss tangent is of more interest WRT capacitor Q.
See list of insulators with associated dielectric constants and loss tangent values.
http://www.rfcafe.com/references/electr ... engths.htm

Chart - note polyethylene and teflon are in the lowest loss tangent column.
http://www.eccosorb.eu/sites/default/fi ... -chart.pdf

Dielectric comparison chart
http://www.avx.com/docs/techinfo/dielectr.pdf

...............................

No doubt it would be job to build, but note that was with 1/2" of movement.
If you used a rod with 28 threads per inch that .064 pf per rotation.
I'm sure my plate size and spacing isn't optimal, and maybe you need
a second trim cap.
Just throwing out ideas, that's easy, putting them into a mechanically stable piece is harder, but rewarding.
Btw, as I was reading the thread I was thinking, he needs some cutting boards, then I paged down and there was a picture of a cutting board
You might clamp the ends of your copper strap between to pieces of cutting
board to hold them firm.
I got it, use a fixed plate for the main capacitance then small adjustable plate to fine tune.
Here are a couple more ideas.


Mikek

Gee guys, didn't mean to put a stop to discussion
I had a better thought about thefine tune capacitor.
The sliding plate should be wider and shorter.

Mikek

I've done some more experiments. The difficulty with tuning the loop from 7 MHz to 21 MHz is the vast range of capacitance required, which requires vast changes in either plate spacing or overlap.

I'm starting to think adjusting plate spacing may be better than plate overlap. Adjusting plate overlap over a wide capacitance range, as I have been doing, changes the dimensions of the loop, which complicates a rigid mounting structure.

Currently I have about 60cm of strip on each side of the loop that is used as the capacitor plates. I'm now using the solid polyethylene which allowed me to do some spacing experiments. Very, very roughly: overlapping the 60cm ends with a separation of 1mm resonates around 7 MHz, while the same amount of overlap requires a separation of around 50mm to resonate at 21 MHz.

Designing a mechanically stable assembly to controllably adjust the spacing between two 60cm-long, 6cm-wide, and 1mm-thick polyethylene plates seems pretty challenging. More thought required.

http://www.mfjenterprises.com/Product.php?productid=MFJ-19

http://www.mfjenterprises.com/Product.php?productid=MFJ-23

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

Use of those caps would however require a solder connection to connect to the loop conductor, which for this design I'm trying to avoid. It seems to be a toss-up whether the losses incurred by a homebrew capacitor with no solder connections (formed by the loop conductor itself) will be greater or less than the losses incurred by a solder connection to a low-loss capacitor.

A high Q aircap should give better results than any homebrewed cap with a non-air dielectric and dodgy tuning mechanisms one would think?

An expensive option might be to clamp (avoids soldering) both ends of the foil loop around the poles of a vacuum variable.

http://qro-parts.com/images/KP_1-4_1000/1271.gif

..............................