Tuning capacitors

Might give this site a look:

http://www.surplussales.com/Variables/AirVariables/AirVar1.html

And this site has some larger ones:

http://www.midnightscience.com/catalog5.html

RFB

Other than resistance is there a reason it has to be large?

I was considering a small capacitance range because it's just a final trim for a loading coil antenna which is already tuned to the broadcast frequency via a combo of no. of turns and length adjustment.

It resonates well, but, as you've indicated in another thread, AM is affected by atmospheric and soil conditions which can make a difference at our wee power levels, so I want to do what might be called "very fine tuning" ... i.e., no more than necessary to help compensate for those changes.

So am I right, or should I go for a full range to 365 pf?

Here is one of those I'm considering:
Capacitor, 365 air variable with 8:1 reduction drive shafCapacitor, 365 air variable with 8:1 reduction drive shaft
http://www.midnightscience.com/catalog5.html
'Bout 1/4 way down the page, red background

Pursue your idea using the variable cap but here's another thought which worked for me. My loading coil is wound on a 3 1/2 inch diameter Plexiglas tube and as a means of tweaking the tuning I added a 5 turn movable winding in series with the coil and mounted on the coil form with the loading coil. The 5 turns are lashed together with wire ties and slides easily on the core which is how I make the fine adjustment. When adjusted for resonance this coil is about 1 inch from the main coil. Moving it back and forth about 1 inch pulls the resonant frequency +/- 10 kHz. There is nothing magic about 5 turns since more or less will work. It can be wired series aiding or opposing as required.

It took a bit of experimenting to get it right but the precision it gives is worth the effort. I started with the intent of building a varometer but this was easier.

Neil

I found the larger caps were a really good choice for the course tuning. On the first site I listed, the smaller caps will be perfect for the fine adjustments.

The alternative of using the variable inductor is also a good choice. It really just depends on which method you want to use.

If your box outside has enough room within it, the variable inductor approach may be worthwhile. Just remember that the more material making up the loading system for the antenna, the better chances of those systems being sensitive to the temperature changes.

Indeed with the power levels involved, even a 2 percent change from center resonance of the system will drastically reduce your station's signal coverage.

Let us know how it all works out!

RFB

Apparently the post had double posted.

RFB

I attached a picture of the variable loading coil to another thread. Here's the link:

This view is of my "test antenna" which is mounted to the rafters in my basement. The fine tuning coil is on the left and slides left to right. The main coil is secured by having the ends of the coil pass through holes drilled in the coil form.

Neil

Say, I like you guy's thinking. Falls right inline with what I've been considering:

Use a pair of basket weave coils, 1/2 inductors if you will, for course tuning, and another few winds for fine tuning.

Power windows in autos have a flexible plastic rack & pinion set. Getting a few such parts from Amazon Automotive Section, I can probably engineer a little movable system with a motor drive ... or if not ...

A mechanical cable system. The steering gear on the fly bridge works like that. One end is a flexible rack, and the wheel. of course, has the pinion.

Hmmm ...

An issue of QST showed a remote tuned loading coil for 160 meters (1.8 - 2.0 mHz.)

The coil was huge, made with 3/8" copper tubing. He used a motor driven "jack screw" to stretch or compress the coil for tuning.

Given the maximum power hams can operate at 160, and the many times I have seen under rated designs of both radio output tuning tanks and antenna loading methods that end up with arc damage and burnt up ham shacks..I would make it as big as possible as well.

RFB

As I recall, his stated purpose for such a large coil was to obtain a very high Q as he was loading a short monopole. The high Q was needed as he was running QRP (low power.)

He used tubing as he felt that with "skin effect" solid conductor was not needed.

In one of my earlier Part 15 AM transmitter
setups, I used a homemade variometer.

The outside coil was wound on a bucket from the
hardware store - I don't remember the
dimensions, but it wasn't that big.

The inside rotatable coil was wound on a round
plastic margarine container.

It worked. I had some kind of plastic rod that
went through holes on the outside coil form,
which was attached to the inside coil form.
It was a very long rod, so you could turn it
and be pretty far away from the variometer.
You could hold one of those cheap CB/ham radio
field strength meters in your hand and peak it
up just by looking at the meter.

The set-up tuned over a small range. It took
a long time to set it up and get it running, but
it was worth it.

At that time, the AM transmitter was a fixed
up Ramsey AM-1, which was also modified for
crystal control. That set-up didn't go as far as
my SS-Tran set-up, but it did work well. I think
it got out 1/2 to 3/4 of a mile. When I say the
AM-1 was fixed up, I just mean that a friend of
mine put some better caps into the transmitter,
because it had some stability problems.

It was a lot of fun putting that set-up together.
It was my first set-up that actually went some
significant distance.

Bruce, Dog Radio Studio 2

P.S. The 1700 kHz transmission from the
AM-1/variometer set up also came in real
well on my Heathkit CR-1 crystal radio.
Actually, it blew it away!