Welcome to W2BRI's Magnetic Loops
Magnetic Loop Antennas for HFAll About Variable Capacitors
for Magnetic LoopsAfter recieving emails from many of you asking me where you can find the right parts for loops, and more specifically variable capacitors, I decided to put forth my sources. I hope this makes it easier for those of you out there that want to build and design your own loops. You'll find in this section web addresses and phone numbers where appropriate. I'll also explain my opinion on each device and talk about my experiences. I hope you find what you seek.
I make no guarantees for any of these vendors. Use them at your own risk! I do not accept any responsibility, period!
1. Vacuum Variables Capacitors:
If you are looking for the ultimate in frequency flexability and power handling then a vaccum or ceramic capacitors are the path to success for your magnetic loop. Pay attention though, they are small, expensive and easy to break -- so watch out. Be careful when buying vacuum variables. The plate spacing can be askew if
they are knocked around, and therefore the capacitor will not have the voltage rating you expect. It is better to buy the vacuum vairiables from a good vendor that has a hipot tester. I would stay away from a flea market or online auction site in regards to these capacitors. I'm a fan of Allen Bond, the president of Max-Gain Systems. He has a personal collection of items. You can check them out on his site.
Look at the "Presidents Page" or just call him. His number is on the site: (770) 973-6251
Let him know you got his number from W2BRI's site.He is a reputable source who tests the capacitors before he sends them. I like and use a Jennings 5-750pf vacuum variable rated well over 10KV. The cap itself is officially a 5KV cap, however, Allen can find one with a high rating way above the official number. You see, there is a little secret to all this capacitor business. Jennings makes a lot of capacitors and then sells them for the rating they test at on the hipot machine. So you may have a cap that says 5KV, however, it will rate far beyond that official designator. If you call Allen and ask him, he'll pick one out for you and test it and find one with a high rating well over 10KV. They will cost you around $204 US. They are well worth it, I own two of them. I've had an excellent experience with these capacitors and regularly run between 100-800 watts on my magnetic loop on 40 and 80 meters. If you want a more durable cap, go with a ceramic variable over a vacuum, but keep in mind, they are heavier and larger.
2. High Voltage Air Variables
More contraversial
and less common are the use of large and well spaced air variable capacitors. These high voltage air based capacitors are outlined on this site in the "Images and Notes from Other Loop Builders" section. The reason I say contraversial is because most of literature claims all connections must be soldered. These capacitors are not of the soldered variety, nor are they butterfly or stator capacitors which are OK according to the literature to use for loop antennas. However, these high voltage transmitting capacitors work very well from real world testing and can also be disassembled and remade into stators. The wipers sometimes according to some people can cause losses in the capacitors that are just too great. So you may want to go with turning your cap into a stator. I would try it both ways and see what you think. There are many strategies for creating high voltage air variable capacaitors. Try them. However, for those wanting to purchase and try these solutions, these are large spaced capacitors that can be bought cheap. If you would like to start sooner rather than later, get yourself a 4KV rated Johnson capacitors, they can be ordered from Ocean State Electronics in 100pf sections for nine dollars a piece. They work very well for 100 watt SSB applications, and have not arced over in tests. I myself own many very large 10KV and higher air variables with one inch spacing in the 40-600pf range. I bought them used for about 40-50 dollars. You can probably find them for about that price or cheaper from used equipment dealers. They work well in my tests. Ocean State Electronics has many types of high voltage rated air variables, I suggest calling them and asking for info on what capacitors they have in stock. They claim to have a lot more than what the catalog lists. They have stators as well. The site is listed below.
Order a: two gang 50pF/secttion large air vairiable for 9.95.
Here is picture of my larger spaced capacitors in case you wondered how big they can get. Yes, that's a quarter next to the device. The cap on the top is about 2 feet long. No wonder some call the caps breadslicers.
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