Variometer Construction Notes
or - Divide & Conquer!
March 13, 2002
After building the 10-gallon variometer depicted in my previous article, I discovered that it was far too large to fit into the tuning cabinet I had managed to obtain for it. (I seem to remember something about measuring twice and cutting once...) Well, it was too late to locate a bigger cabinet, so I decided to do the next best thing, and divide the variometer into two parts, the variometer assembly itself, and a separate loading coil. Each one would be constructed on a 5-gallon Home Depot paint mixing bucket. I reasoned that it would be fairly easy to simply unbolt the buckets and cut the wire between them.
I first applied several complete wraps of packing tape around the middle of the original variometer assembly, directly over the joint between the two buckets. This tape, I reasoned would keep the wire from unwinding when I took the buckets apart. I was almost right.
I carefully make a cut through tape all around the diameter of the bucket assembly directly over the joint between them. I then removed the bolts holding the buckets together, and carefully separated the buckets. As soon as the buckets were separated a bit, I cut the coil wire to free the two buckets. That turned loose the tiger. The wires promptly decided that even though the tape was holding the first few turns together, the now separated ends of the buckets were nicely tapered, so they could leap off the end of the drum and create a nice tangled mess! Obligingly, the wire did just that. (Picture omitted here to protect the guilty - or the dumb.)
Fast forward about 45 minutes - The wires had been wrestled back onto their proper place on the now separated buckets and fastened securely back into place. I also decided to add another 14 turns to the Variometer drum since I had room for the added turns. I did not put more turns on the other coil, but may do so if it turns out that I need a bit more inductance.
LOADING COIL MODIFICATIONS
This is a picture of the 5-gallon loading coil after the separation process was successfully completed. Note that the wire turns at the bottom of the bucket (in the top of this picture) is prevented from sliding off the bucket by using a series of wire retaining "clips". The next few pictures show the process of installing these retaining "clips". Note that about 25 more turns could be added on the two circularly ribbed areas shown at the bottom of the picture.
Here's a view of the bucket bottom showing the wire retaining "clips" in place. Note that they do not actually fasten over the coil wire, but instead simply present a "bump" on the outside of the drum to prevent the wire turns from slipping off the bucket.
Start by drilling a hole through the bucket wall just outside the bucket bottom. The hole is just large enough to allow the section of #14 THHN wire to slip through the hole with a tight fit. Leave the insulation on the wire.
Looking at the "back side" of the retaining "clip" hole.
Using a pair of pliers (or your fingers, if you're Superman) fold over the outer end of the wire down against the outside bottom of the bucket.
Clip the ends of the wire of short and bend the trimmed end down tightly to prevent it from moving.
You can attach the end of the coil winding to the end of the drum by slipping the end through a hole drilled in the bucket just as you did for the "clips". Since this end of the winding has an attachment loop, the wire end is cut off short after passing it through the bucket wall.
This shows the coil winding end threaded directly through the drum wall. In this case, there is no attachment loop used, so the free end of the winding is left long enough to attach to something else in the tuning system.
Remember those Homemade Standoff Insulators? Well, this is half of one of them. Here, I show one of four identical standoff legs I used to hold the Variometer assembly in the tuning cabinet. Each leg uses two 1/4" x 20 x 2 1/2" long full threaded hex head bolts. They are run through 1/4" holes in the bucket wall and then self-threaded through both walls of the PVC pipe. No nuts are required to hold them in place against the inside of the drum. Use flat washers between the bolt heads and the outer wall of the drum to spread the bolt-to-wall stress load and help prevent bucket wall cracking.
This shows the position of all four legs inside the variometer drum. This is looking up from the position where the bottom mounting panel would be located. The variometer coil wires are visible exiting through the right side of the variometer coil shaft. The left end of the shaft is threaded and will extend through the wall of the tuning cabinet.
The variometer assembly mounted inside the aluminum tuning enclosure. The tuning cabinet door has been removed for this picture. The 50 Ohm coaxial cable from the transmitter enters the cabinet through the SO-239 connector which passes through the bottom left of the cabinet. The connector is visible just in front of the brass-nutted ground bolt. A home made standoff insulator is shown to the left of the connector. (This particular standoff is almost 10 years old.) Also visible is one of two additional home made standoff insulators which support an E.F. Johnson 10 uHy inductor which will be used on MF frequencies. Note the bolted wire connection to the lower right of the Variometer coil and directly above the rear of the Johnson coil. Also, note that I have added about 14 additional turns of wire to the bottom of the Variometer main coil.
Overall view of the tuning unit under construction - The Variometer adjustment shaft extends from the left side of the coil through the tuning cabinet wall. The variometer adjustment coil leads are shown on the right side of the unit. One wire connects to the top of the main loading coil, and the other wire connects to one of two standoffs on the right side of the tuning cabinet. The two standoffs are designed to support an in-line RF ammeter at a later date. In this photo, they are jumpered since the meter is not in place. The heavy copper strap leading from the upper standoff goes to a large porcelain bowl insulator mounted in the right side wall of the tuning cabinet. The black crud on the rear wall of the cabinet is what happens when you let the Magic Smoke out of a large transmitting Mica Capacitor. It's nearly impossible to remove the mess without a sandblaster. I used lots of pieces of adhesive backed aluminum tape to cover the 30 years worth of holes in the cabinet. At long last, it's fairly waterproof again!
I'll post more on my project as I get it done! (Where ARE those 'round tuits??)
73, Ralph W5JGV
The entire contents of this web site are Copyright © 2002 by Ralph M. Hartwell II, all rights reserved.