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Author Topic: Building a butterfly capacitor  (Read 7827 times)
AB1PA
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Posts: 3




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« Reply #15 on: October 05, 2012, 06:33:09 AM »

If I ever will solder the plates I will let you know about the results, it may take a while though.
I built the loop because I moved to a place with very limited space and I got the material for free. I didn't know at that time that I would live there only for a month. Now space is no concern anymore, but I like that little antenna, I don't really wish for anything else at the moment, maybe a dipole or a simple end fed wire for 160m, 80m and 40m, but that's it.
From what I have read, you are planning your antenna with a lot of attention to every detail, I am sure you will be pleased with the result of your effort.
I have learned in the past that it's not always that important to get the last dB out of everything, sure every little bit helps to be heard in a pileup and it does not make much sense to generate a lot of RF just to turn 80% of that into heat. However, I like simplicity and I like to utilize what I have at hand, I like to use RG6 75ohm coax because I have some of it, and it is much better than rg58 especially at higher frequencies, never mind the slight mismatch. I've used dipoles without balun, for some people unthinkable because of the somewhat unpredictable change in radiation pattern and possible RF on the shield of the coax but they worked very well for me. If the propagation is right then almost everything will bring you into the air.
By the way, the copper I used for the capacitor is 1mm thick.

Good luck with your antenna project, I hope you will share your experience.

73
Jens
« Last Edit: October 05, 2012, 06:38:02 AM by AB1PA » Logged
JAHAM2BE
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Posts: 280


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« Reply #16 on: October 05, 2012, 06:44:55 PM »

If I ever will solder the plates I will let you know about the results, it may take a while though.

Thanks - that would be very interesting information. I've searched long and hard but have never seen anyone quantify how much difference soldering (or welding) the butterfly cap plates makes. It varies from case to case of course depending on the other relative losses, but it would be interesting to see some numbers.

One thing I find interesting about loops is that their very low radiation resistance allows observation of and comparison of small differences in RF resistance. I hope one day to be able to compare various connection methods, metal types, metal thicknesses, metal topologies (flat strip, crumpled strip, round tube), etc. to get an idea of how much RF resistance is introduced by such changes.

Now space is no concern anymore, but I like that little antenna, I don't really wish for anything else at the moment, maybe a dipole or a simple end fed wire for 160m, 80m and 40m, but that's it.
From what I have read, you are planning your antenna with a lot of attention to every detail, I am sure you will be pleased with the result of your effort.

I have a loop built now with 30 micron copper tape and a split-stator capacitor that has pressure connections connecting the vanes to the shaft and frame. I haven't been on the air much, but I was able to make domestic contact on 17m, but none on 40m. This experience convinced me firstly that small loop antennas really work even in my confined, stealth-mode, balcony situation, and secondly, that the antenna losses need to be reduced to work on 40m.

Good luck with your antenna project, I hope you will share your experience.

Sure thing. I've received so much specific, in-depth advice here on eham on how properly to build a small transmitting loop, that I'm looking forward to sharing the final result. It will be a long journey though. First step is to build a prototype cardboard octagon with 1m diameter and 10 cm tubing thickness, using several flat cardboard sheets rolled into wide tubes and connected at 135-degree angles at the edges. That will pave the way for building the real version out of copper flashing (which is expensive...). Finally will be the butterfly capacitor (or other sliding/compressing plate arrangement if I can come up with one, but the good old butterfly capacitor seems like the most mechanically stable and easiest to motorize).

After going through all that trouble, I want the antenna to last, so I'm also interested in how to weatherproof and protect the copper capacitor plates from corrosion, as you mentioned in your previous post. Anyone have any advice?
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STAYVERTICAL
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Posts: 875




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« Reply #17 on: October 06, 2012, 12:17:11 AM »

I have just built a box around my motor driven capacitor with rubber grommets where the tubes enter at the top.
Copper will oxidize slowly but without constant water exposure it is quite minimal.
I remember reading years ago, that even in microwave work, the oxidation on copper did not have that much effect.

If you weather proof the capacitor, it should all be ok.
Your loop is going to weather anyway, and provided the oxide is not conductive, it should have little effect.

An interesting website by a guy who has done these experiments with inductors should give you some insight:
http://www.w0qe.com/Technical_Topics/inductor_Q_tests.html

I don't know if I would have the patience to bury inductors in the flower bed for five years to check how corrosion
affects the Q - but he has.
More power to him.

73 - Rob
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