TheRadioBoard

I'm considering building an "army loop" style tuner like this one:

source: http://homepage.ntlworld.com/g4kki.will ... _tuner.htm

I need a high-quality butterfly capacitor (welded vanes and solderable shafts with no lossy pressure connections) for the loop resonating capacitance. The problem is, the only butterfly caps available locally (in Japan) are only 20pF max. I'd want maybe a 20-200pF capacitance swing and a small form factor (as this is for QRP and portable use) - something hand-held size or smaller would be ideal. I know MFJ sells a large butterfly cap (10 inches) that is used in their own loops, but I'm looking for something smaller, like the butterfly capacitor used in the smaller MFJ-932 mini loop tuner, which is a hand-sized unit.

I am also considering buying the ready-made MFJ-932, but something in me resists the idea of paying $140 for a unit that consists of little more than two variable capacitors. Also, I guess MFJ shipping to Japan will probably run about $40 dollars, making a total of $180 dollars.

I'm thinking for $180 dollars surely I can find some source of good, wide-range, small-form-factor, and cheaper butterfly variable capacitors that will ship to Japan. Thus I ask TheRadioBoard: any ideas?

Thanks in advance.

uhm, I have regular two gang cap in approximately correct value range, it is from transmitter ( 100W), should be OK for this.
i will check the exact values and post back in a few min

Quick check, it was 11-240Pf according to multimeter (experience tell me to give or take up to 20Pf ).
it it would occupy approximately 80X80X75mm and has ~1mm spacing between plates.
should hold up well inder QRP levels:-),

also have a few smaller from recievers, this means spacing is smaller and thus tolerance to high voltages is smaller, id say QRP is still OK but no guarantees.

oh, also why butterfly cap?

Thanks for checking your stock of dual-gang caps, but I'd really like to use a butterfly cap for this application. The reason is that the tiny radiation resistance of a small transmitting loop (far less than 1 ohm) mandates minimizing all sources of loss resistance or equivalent series resistance.

Most of my second-hand knowledge about this comes from reading the postings of one source, W8JI, who worked on the original design of the acclaimed MFJ magnetic loop antenna. Basically, my understanding is that capacitor loss must be minimized through all of the following factors:

- minimum resistance in current path (no pressure or screw connections; welded vanes; large-diameter and tapering connections from loop conductor to capacitor)
- symmetrical construction
- box shape
- small size with respect to loop diameter
- minimum path length for current
- minimum inductance

Split-stator caps force the RF current to go from one stator, through one vane, into the shaft (possibly a lossy connection if not welded), through the shaft (possibly lossy since it was maybe not designed to carry high currents and is small-diameter compared to the loop conductor), into another vane (another lossy connection), and finally into the other stator.

In contrast a butterfly cap allows current to pass only through the vanes from one stator to another, avoiding any lossy or long path through the shaft.

W8JI also found that the often-proposed trombone capacitor (concentric cylinder capacitor) designs were lossy, due (I think) to path length. He also stated that it is easy to delude oneself into believing one has built a high efficiency loop, because it is very easy to lose several dB in the capacitor with a poor design.

Since I don't have the capability to directly measure loop efficiency or losses, I'd like to stick with a proven capacitor design, namely, the butterfly cap with welded vanes.

OK, just say so and I will send it your way.

OErjan, please check your private messages on the board for my reply.

I'm a lucky man today. After a lot of web searching, I could only find one shop that said they had butterfly variable caps, but they only listed 20pF units. I went there today and asked the man if he had any larger units. He lamented that they don't sell well at all, and the only available ones are the leftover stock he has in the drawer. He opened up the drawer, and, lo and behold, a 50pF and a 30pF were to be found! Both with welded vanes and small form factors. Total cost: $25. A heck of a lot cheaper than $180!

The 50pF unit should get me on 20m through 10m with a 1m-diameter loop.

Finally, I'm starting to think I have a real chance at building a low-loss, mechanically stable, magnetic loop antenna.

Looks like they are silver plated as well.
Use silver-content solder if you solder to them.

If you organised "stops" on your dial somehow, you could solder a flexible rotor connection to the back of the rear rotor vane.

Does 50pF each side (fixed to rotor) = 25pF max when in series in the resonant loop circuit?

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Thanks for the advice. I've never used silver-content solder. Why is it important here?


Good idea about soldering to the back of the rear rotor. I'm still thinking about how I will feed the loop (inductive coupling opposite of the capacitor, or capacitive coupling at the resonating capacitor). With inductive feed, no rotor connection is needed at all. With capacitive feed (army-loop style) the rotor needs to be grounded, but that ground connection doesn't carry so much current (it's not part of the resonant tank) so I understand that a wiper connection here might be acceptable.

About the meaning of "50pF" capacitance, I was hoping that the meaning was "stator-to-stator capacitance is 50pF". I'll find out when I hook it up to the loop. It's going to be several days before I get around to testing it though.

The first loop conductor I will try will probably be a 50mm-wide copper tape strip, affixed to an RF-neutral backing of polyethylene packing foam (making for a collapsible, portable loop). The big unknown here is the effect of the adhesive substance that totally covers one face of the copper strip, which may kill the loop Q. At least with a good butterfly cap, though, I have optimized the capacitive half of the resonant tank. If the antenna doesn't perform, the it's simple to try other loop conductors to optimize the inductive half of the tank.

I think you're going to find each side is 30 or 50 pF, so 15 or 25 pF when in series.

Silver solder has a lower resistance and will help preserve the silver plating.

73,

_________________

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

As KR1S said, normal solder supposedly leaches the silver plating from the "substrate" so you lose your low-R, silver-surface contact between the soldered joint and vane.

For it to be really a problem though, you would also need to be using a silver plated, flexible lead . As you say, it won't be in the Hi-Q resonant circuit so it most likely would not matter.

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Yikes! I definitely don't want to be causing a permanent, irreversible increase in loss resistance in this loss-sensitive application (loop antenna). I'll research some silver solders - I hope they're available locally.


Well, I will probably not solder the rotor yet, but I am planning to solder the stator posts - which are in the high-current path - to the loop conductor, so any loss here must be minimized.