Electronics > RF, Microwave, Ham Radio

Caps for RF transmitting loops, that handle power well?

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Wallace Gasiewicz:
 2 inch Copper and aluminum "pipe" is 2 inches internal dia

2 inch copper or aluminum "tube" is 2 inches outside dia.
It's hard to "flair anything without a good tool.
You can however slit the opening of the 2 inch "pipe" to make it easier to insert the VVC. Yhen you can use big hose clamps on the outside to make a tight connections. Some folks use silver paste to make a better connection.You need another hole in the pipe for the tuning rod, this does not have to be isolated since the tuning rod is connected to the end of the cap anyway, however a piece of new fangled PEX pipe can be used to connect the tuning rod to a knob or a tuning motor, to insulate it from the user.

You need to consult the charts to determine the value of your cap. The surplus Russian ones have had good reviews by some folks.
Count on $100 or so for a transmitting VVC. For Rx any good, not corroded air variable cap will do nicely.
Some VVCs come with mounting brackets for connecting. You might get lucky and find one like this. But again, the more connections the worse the performance.

Silver solder is mostly silver, many solders only contain some silver (1-5%)

Anyway. these things are hard to tune. They have sharp tuning and the better they are made, the sharper the tuning.
The more the surface area the more the radiation resistance and better performance. Hence the helically wound loops.

If you have a Vector Network Analyzer, you can look at you antenna's characteristics with just regular solder joints and then with good connections.

The more the turns of this type of antenna the lower the radiation resistance,  I think someone did get an award for making a two turn 80 Meter loop.


I think the flaring tool would need to be custom made on a lathe

Wallace Gasiewicz:
Harbor Freight sells flaring tools that work somewhat
A muffler shop may be able to flare your ends.

it also might be an option to use thick copper pipe for the loop then make a braze to a thin copper pipe of same dimensions to make the holder

Most vacuum variable caps are adjusted simply by turning a rotating part on one of the two ends.

Only a few companies seem to be selling them new, and they are very expensive it seems for what they are.


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