Hi Larry, Thank you for your review on our vcap, and I am hoping to also have the vcaps available on my website aswell as Etsy. I am currently trying to create an online shop for this. If all goes well I wil be able to offer this to radioboard members only with a discount (probably around 12%) as sales expenses will be less.

Hi Larry, Just to let you know your 400pf vcap has been sent to-day by registered air mail - thank you for your order and I have discounted you on this and issued a partial refund, for all your help. Etsy has proved to be excellent for selling the Vcaps. When you have had a chance to test and use this, any feed back would be most welcome.

The whole range of Vcaps has now been finalised and is now available together with details and prices on the etsy link below, for anyone interested. fhttps://www.etsy.com/shop/armstrongradio?ref=shop-header-name&listing_id=1533702752&from_page=listing

I had a quick look at the elliot 3 gang cap, and I would not be able to produce this. A 3 gang version of the butterfly cap would be possible i think, but this would be larger than the elliot type, and I am not sure if the performance would be the same as the elliot version. Also the cap would have a linear rotation to capacitance characteristic, which I imagine may not suitable for your purpose.

Below is a dimension diagrams and detalils on the range of Vcaps currently produced Also a couple of pics on the 250pf and 500 pf versions. All caps use the same bases and differ in the number of blade used, and the front to back depth of the Vcap.

Maybe in the future, but to start with I will only be doing single gang caps on this range. Should be able to put details of these on my next post. What purpose would you require the 3 gang capcitor for, will this be for low power or high power application? I am asking this as I am looking into producing variable butterfly type variable capacitors, which would be available for low and high power applications. These would be easier to produce for 2 and 3 gang versions.

Yes your right, for high Q variable air spaced capacitors are hard to beat. As I have a reasonably well equiped workshop, I decided to put it to good use to try and make these, and offer this to anyone intereseted.

Back again, after a long break, due to family health problems, now resolved . The design of the variable capacitors has been finalised and I have now started producing this in small quanities, and should be able to supply these to anyone interested. 16 values will be available from 75pf to 500pf In my next post I will have details including photos, dimensions. Also I will include DXF files of the final blade designs for anyone interested in experimenting further.

Larry, Very interesting on the AB amplifier, I remember in the radio constructor magazines ages ago an article for a class ab amplifier using 2 transistors. I believe the bias to the transistors varied according to the input signal and the article was by sir Douglas hall . I also sold a kit for an FM pulse counting receiver, and maybe I will do a new post for this, as I am thinking of producing a few more of these to sell.

A long time since my last post but I have been quietly busy. Firstly a simple test jig was needed to enable to quickly test various test caps to see the characteristics of rotation to capacitance. Basically it consists of the tuning capacitor under test with a graduated scale marked in degrees, with a capacitance meter connected to this. By measuring the capacitance at various points it should be possible to test the rotation versus capacitance of various capacitors easily. The more serious problem I have is to be able to construct and test variable capacitors to give a consistent performance, without long construction, setting up and alignment times. It is quite different to produce capacitors reasonably quickly in quantities, as opposed to making just a prototype. After a lot of work I believe I am now slowly winning so I will battle on.

I have at last managed to do the first tests on measuring the rotation to frequency/wavelength characteristics of the prototype 3 vcap. A parallel LC circuit consisting of the prototype 3 vcap together with a MW ferrite rod coil was tested at various frequency/wavelength resonance points. The results show that the cap has a virtually linear rotation to wavelength characteristic as shown in the photo below. The units are the wavelength in meters. This is not what I expected as the blade profile I used which was shown in two different articles I read was supposed to give a linear rotation to frequency characteristic. After checking online I noticed that virtually all older vcaps have a linear rotation to wavelength characteristics, whereas the newer ones have an approximation of linear rotation to frequency characteristic. The smallest commercial vcap I have is of the newer type (made by ALP ) and tests to see how this compares would be useful. Also the larger commercial vcap I have is the older type (from an old valve radio) and again tests to see how this compares would be useful.

Test results on prototype3 and the two commercial V.caps has now been completed and is very encouraging. I have now used a 2 pf coupling capacitor from the RF generator in order to minimize loading on the tuned circuit. Results have been double checked and are as follows: TEST FREQUENCY 800KHZ Prototype v.capacitor Bandwidth=6.0 khz Q=133 Smalll v capacitor Bandwidth=6. 1 khz Q=131 Standard v.capacitor Bandwidth=6.0 khz Q=133 TEST FREQUENCY 1200KHZ Prototype v.capacitor Bandwidth=18 khz Q=66.7 Small v.capacitor Bandwidth=19 khz Q=63.2 Standard v.capacitor Bandwidth=18 khz Q=66.7 The tests indicate that all three V.Ccaps now have almost identical results and there has been a large improvement on prototype 3. The only reason I can see for this, is that since the blades of the prototype 3 capacitor have been reduced in size (approximately half the area of prototype 2), the conductance has improved substantially. Also the small commercial V.cap shows that the Q is slightly lower in comparison and I noticed that the blades on this V.cap were thinner than the other 2 V.caps, so again this could be due to conductance of the plates. Although tests have not been done at higher frequencies, I believe for at least the MW band the performance should be equal to commercial V.caps. The next tests will be to see if there is a linear rotation to frequency characteristic.

Just finished the final prototype3 as per photos. In order to make alignment of the plates easier the size of the plates has been down sized. The new cap has a width of just 4 cms and a depth of 2.5 cms with a maximum capacitance of 260 pf. For other values the depth will vary accordingly. Also as to-day MHZ are used for most tuning dials, the shape of the rotary blades has been altered to give a linear frequency to rotation angle. Thus minimum frequency is now fully anti-clockwise and maximum full clockwise, Further benefits of the final prototype now are: 1) More compact footprint / smaller volume 2) The plates are now more rigid 3) Alignment is substantially quicker 4) Precision miniature roller bearing 5) Values up to 500 pf possible To finish, just need to replace the front screws with countersink screws and put solder tags for connections. Also to speed up alignment further, the space washers can be replaced with more accurate and flatter shims. And then more tests on bandwidth, Q tests and rotation/frequency tests

Larry, I have just finished the final prototype, and I will be posting photos, info Etc shortly. I haven't used Etsy before, but I assume it is similar to Ebay, so I will need to register with them. The final prototype is now quite compact and initially can offer various values up to 300 pf to start with. Also the MK484.s have come, so I should be able to do real time tests.

Thank you for your feedback and I believe your interpretation of the results is correct. This means however, that in order to keep the Q high the gap spacing cannot be increased, so I am going to have another go with prototype 3, to see if it is possible to change the design, in such a way to make alignment of the plates easier and thus more practical to build. If successful I will post DXF files for the blades and the base parts, so that anyone interested in building their own V.cap or experimenting further, may find these useful.

Good and bad news. The brass spacer washers (0.5mm an 1mm thick) finally arrived and the original washers have now been replaced with I mm spacers. Also the vanes spacers have been installed as in the photo below. Tests were re-done and surprisingly the Q now at both 800khz and 1200khz is almost identical to the commercial larger V.cap, I am not sure if this is due to the closer spacing between vanes, which now is 0.25 mm (the same as the larger commercial V.cap) or the improvement is due to the replacement of the steel washers with brass washers. Re- testing, now the capacitor when fully meshed measures around 500 pf Now the bad news, to align the vanes correctly to the 0.25 mm without any shorting between the vanes, took ages to align, and I believe, at the moment it would be very difficult to construct these on a consistent basis and in a reasonable time. So to start with I will use a larger gap of 0.4mm or 0.55mm between vanes and limit the maximum capacitance value to 250pf, so the V. cap would not be excessively large. Once enough experience is gained the gap size can be reduced soV.caps with larger capacitance values can be constructed precisely.

Mechanically it is smooth and the same as the commercial samples. Unfortunately I am still waiting for the MK484's to arrive to do some real time tests.

Finally constructed the second smaller V.vap. The original design was used and scaled down so that the vanes are half the area of the original. It was possible to reduced the gaps between the stationary and moving vanes from 0.55 mm down to 0.35 mm. The spacing washers used were 2 x m4 washers (one 0.8 mm + 0.4 mm = 1.2mm thick This now increases the capacitance between vanes by 0.55/0.35 = 1.57 times but as the area of the vanes is now half, this results per vane set = 1.57/2 = 0.78 times the original approx The new prototype uses 11 moving vanes and 10 moving vanes so there are 20 gaps between moving and stationary vanes. The total capacitance measured with the V cap fully meshed was 300 pf (i.e. conviniently 30 pf for each moving vane) The photo below shows the difference in size between the two prototypes The new V. capias now is far more compact and uses 5 mm thick acrylic for the front and back bases, which reduces the depth by a further 1 cm. To finish this I still need to cut the vanes spacers, so the spacing is more accurate and rigid. Now for the test results TEST FREQUENCY 800KHZ Standard v.capacitor Bandwidth=6.2 khz Q=129 Prototype v.capacitor Bandwidth=6.3 khz Q=127 TEST FREQUENCY 1200KHZ Standard v.capacitor Bandwidth=19 khz Q=63 Prototype v.capacitor Bandwidth=21 khz Q=57 I did the measurements twice to make sure and the measurements for the commercial V.cap were different from the previous results. I think this maybe because this time the ferrite rod/ coil was resting on a 5 cm polystyrene block, whereas the first test were done with the ferrite rod/coil resting on the bench. Also tests were done with the bases spaced close to the end vanes and then I cm away, but there was no difference in the results. The results between the second prototype and the commercial V.cap are almost identical now at 800 kHz but at the higher 1200 frequency there is still a significant 2 kHz bandwidth difference, although better than the first prototype.. As soon as the MK484 ics arrive real time tests can be done which should be interesting. Also I am waiting for 1 mm thick brass washers to arrive and this will give an even smaller gap spacing of 0.25 mm between vanes, but I'm not sure if it will be possible to align the vanes accurately enough. Maybe with brass washers the results may improve.

After some thinking about the lower Q of the prototype I am wandering if this is due to the much larger vanes I am using to the two commercial V.caps, The thickness of the vanes on the prototype is 0.5 mm, the same as the standard commercial V.cap I have, but the size of the vanes must be around double or more, and maybe this results in greater conductance losses. To test this, a second test prototype will be needed with the vanes around half the area of the original, but with closer spacing. Hopefully the gap between vanes can be reduced to about 0.3 mm from 0.55 mm, so this will also result in a smaller V.cap for the same value. Time to start prototype 2 !!

Thanks for the info, what a big range of litz wire. I would definitely be interested in getting litz wire/ferrite rods from Mike, once the V.cap is successful as I would like to do a new post on coils to use with this. At the moment I am using the coil andferrite as below, as I have a few of these, which for Q comparison tests are OK