…April 11, 2016 Based on the number of hits on this page (by far the most in the site) guys seem interested in variometers…feel free to contact me directly for addl. info or suggestions for more content … I’ve built more than a few, of various sizes. WA3ETD at g…l address pretty easy to figure out while beating spambots! …jm
So, what is a variometer and why would we need it?
In almost all cases, a vertical antenna for 630m is going to be far shorter than the optimum length of 1/4 wavelength, also expressed as 90 degrees of length. Such a short antenna appears to be capacitive and requires an inductance in series to resonate the antenna.
That being said, the vertical antenna is the subject of entire textbooks and its analysis is beyond the scope of these pages. The interested student (that’s why you want to get on 630m, right!) is strongly advised to buy, beg, or steal of the following ham radio reference…
LOW-BAND DXing by John Devoldere, ON4UN I own the 4th edition.
Chapter 9 of the above edition ‘Vertical Antennas’ is must reading and highly advised. (Available from ARRL bookstore or AMAZON)
So, understanding that a short vertical is highly capacitive in nature and some unknown amount of series inductance is required to achieve resonance, enter the variometer ( Vario). A vario is nothing more than a variable inductor, tuned by turning a shaft. A common vario consists of fixed outer coil and an inner coil free to rotate within the outer coil. The two inductors are wired in series, and due to magnetic coupling between the two, the total inductance of the system may be varied over a considerable range. I have posted several links to variometer theory on the HOT LINKS page.
As usual, a picture is best. Above is my current vario. As seen, the inner coil is free to rotate around the fixed outer coil. Maximum inductance occurs when the inner coil is parallel to the outer, and if rotated 1/2 turn, or 180 degrees, the magnetic fields buck and minimum inductance occurs. I try to design a vario, initially, such that the inductance needed to resonate the short vertical occurs when the inner coil is ‘halfway’, thus inductance may be added or subtracted. In practice, minimizing the amount of inductance buck improves the overall Q of the bucket. In my current station, now knowing how much the antenna changes depending on the weather, I have the vario set to minimize bucking, or subtracting total inductance…
BUT HOW MUCH INDUCTANCE DO I NEED, you correctly ask?
After reading ON4UN, you should have some idea on what is involved in designing a vertical, without or with top loading, etc. Three pages of formulae are presented, and with a scientific calculator you can determine the required base coil series inductance required to resonate YOUR vertical. To spare you the exercise, I created an EXCEL spreadsheet that does the math for you.
VCalc LEFT click this link and select “Open In A New Tab”
Problem? E-mail me and I’ll mail the app to you. WA3ETD (at) (firstname.lastname@example.org@ !
This spreadsheet will run on MS EXCEL or OpenOffice. Click on the above link, and open the project. In the upper left, enter the length of your vertical, the length of any top loading wires, and the wire guage of the vertical…common antenna wire diameters are listed in a small table on the middle left. Enter your middle frequency of proposed operation. The answer, as well as all intermediate calcs (from ON4UN) are presented. The answer in uH is accurate within 10%, close enough. Again, I encourage one to ‘read the book’ to get a feel for what’s going on!
Follows is a Cheat Sheet based on the above discussion and spreadsheet. Note the impact of top loading wires on the size of the base loading coil (in uH)! LESS COIL, LESS LOSS! NOTE that the frequency resulting in the table below is 475 kHz…
Dave, W1VR asks if the top wire length entered is measured from the center, or is it the length of ALL top loading wire. Answer: Enter the total length of all top wires as if they were one continuous segment. The combined capacitance is essentially the same…
BUILDING YOUR VARIO
OK, now we have an idea of how much base inductance is needed but how to translate that into a working device? The total inductance of two series inductors is additive. IN GENERAL, the total max inductance of the vario is the APPROXIMATE sum of the inner and outer coil inductances, approximate as there is magnet coupling between the coils. This coupling allows it to be tuned.
It is very desirable to build a large outer coil and a small inner coil, the goal is to not induce a huge inductance change with a very small angular movement as this makes tuning critical and painful.
For example, assume 200uH is required to resonate your vertical. I would design a vario with about 180 uH of outer inductance and 25 or 30 uH inner. Since the outer coil will be tapped, this should allow reasonably non-critical tuning.
All materials for vario construction are found at Home Depot or similiar outlets. In general you will need:
— a two or three gallon bucket, like a drywall cement container -avoid the 5 gallon size, too big. Must be plastic, do not get the ones with a metal bottom. This is the ‘outer’ coil form. See the PIX, below.
— a piece of 2-4″ od PVC – the ‘inner’ form. This piece must be able to rotate freely within the bucket, with ~2″ of clearance on each side.
— a length of 3/4-1″ PVC – the shaft to rotate the inner coil form.
— the ‘next size up’ piece of PVC based on the shaft OD. You want the shaft to tightly fit INSIDE this piece. Or almost fit, as will be seen. It will be used to form the collets that secure the shaft to the inner coil, and to lock the position of the shaft where it passes thru the bucket. Home Depot has a bin in the PVC section with short lengths of telescoping PVC.
— WIRE – single strand TFFH house wire, either #12 or #14 solid. Get the spool. Avoid anything less than 14 guage as losses will be too high.
— Can of cement appropriate to the selected PVC.
As usual, the pictures above should give you the idea. The holes in the bucket must be exactly opposite each other, and the same height from the bottom. To do this, I wind a non-stretch piece of fishing leader around the bucket, marking the exact point where the free end of the string touches the running string from the spool. Cut the string at that exact point, find the exact halfway point and blacken that point with a marker. Tape the string back on the bucket such that it is parallel to the base all around. The center mark and the touching ends on the opposite side are the drill points. Drill such that the selected PVC shaft is a snug fit.
Employing exactly the same technique, prep and drill the inner form at its exact center.
Having determined the desired inductance of your inner and outer coils, it is necessary to determine how many turns of wire are required. These are air inductors, and many calculators exist on the web…search for ‘air inductor calculator’. You will need to know the exact OD of the bucket (at its center as they tend to be slightly tapered) and the inner PVC. Since the actual wire is insulated, a good assumption is that the turns are spaced by a wire diameter for entry into the calculators. Here’s one:
The pictures should give you a good idea of the scheme. I secure the ‘bitter end’ (leading wire) at the bottom of the form by drilling 2, 1/8″ holes an inch apart and threading about 2′ of wire thru to form the bottom of the coil. John, WG2XIQ suggests placing several strips of double-sided, sticky tape from the bottom to top of the bucket, which makes winding the coil on a slightly tapered bucket MUCH easier. Winding the outer coil can be tedious if you possess a Type A personality!
Be patient. TAP the coil every three or four turns at the bottom. Four taps is a good idea. These taps are used to match the vario to the feed system and subtract inductance if desired. Form the tap by making a one or two inch twisted pigtail, then continue winding. Knowing the length of the final coil (you entered it into the calculator!), plan on centering the coil around the shaft holes. The pictures tell that story. I used small brass woodscrews to form guides for the wire directly below and above the shaft hole. Use no steel hardware when constructing a vario.
Wind the inner as above, centering the winds around the shaft hole. The coil on the inner should be exactly split by the hole. Centering the outer coil is unnecessary.
The shaft should extend past the bucket as desired, leaving enough to easily turn it or add a motor drive system. Trial assemble the entire vario – insure the inner rotating coil is exactly in the center of the bucket. Drill a 3/8″ hole in the shaft to one side of the inner form – this to pass the inner coil wires outside the bucket thru one end of the shaft. I used lengths of flexible, stranded #12 wire soldered close to the inner coil ends, passed thru the hole and out the shaft to the outside world. This flexible wire allows the shaft to turn freely without the lash that would occur had the stiff solid wire been passed out thru the shaft.
If you have in inductance meter, such as the very useful ‘Almost Digital’ L/C Meter IIB found here: http://www.aade.com/lcmeter.htm , check your winding work for ballpark planned inductance – NOT CRITICAL, as the vario is meant to be adjusted! IF SATISFIED, wire the inner and outer in series. Good, soldered connections, please! The AADE meter is currently unavailable, according to their site…hopefully it will return.
The shaft is locked to the inner form and the bucket using the telescoping PVC piece noted in the parts list. The inner shaft should tightly fit, or ‘almost’ fit the outer telescope piece. Cut four pieces of that outer PVC, about 1 1/2 inches long. If necessary, slice the piece lengthwise, soak in hot water, and force fit them right up against the inner form to lock the shaft. The other two pieces lock the shaft against the bucket. Picture ‘AA’ way above clearly shows how this works. If happy, a SMALL drop of PVC cement in the slit or around the collet secure everything – knowing that now only destructive techniques are required should you ever desire to disassemble the vario!! Having built three of these, I have found the ‘almost fits’, slit method collets provides enough friction to lock the shaft, without glue.
So there is your vario. This cookbook approach worked for me, clearly there are many ways to do this…be creative if necessary. This is not rocket science…
In general, at least here at WG2XKA, one of the flying leads from the inner coil directly connects to the vertical. The bottom of the vario, cold end, is grounded to the radial system. Power is applied to one of the taps, usually via whatever matching is required to allow the 50 ohm feedline to see a 50 ohm resistive load at resonance.
Another method for matching the base coil to a 50 ohm coax transmission line is the shunt feed. In this case the variometer or fixed inductance base coil (cold end) is connected directly to the center coax conductor. A second, small value, inductor is placed between the center coax conductor and ground (radial system, etc). Coax shield is also directly grounded. The value of the shunt inductor is typically quite small – a few uH, and is directly related to the antenna resistance. The variometer tunes the system to resonance, and the shunt inductor facilitates the 50 ohm match. Refer to “Small Antenna Tuners” in the references…
It turns out that when using #14 TFFH wire, the green HD stuff I used, 10 turns closely spaced, insulation touching, results in a coil almost exactly 1.0″ long – useful to know when planning the coils with an inductance calculator. This fact will assist one in determining where to start the coil at the bottom of the bucket to roughly center it around the shaft hole. A smaller outer coil can be wound directly below the shaft hole with no adverse issues.
Finally, the monster 1.2 mH, not uH vario on the orange bucket in the leading picture is going to be too big and difficult to tune at 630m. It is more suited to a LF setup. However, it serves as a practical example. The form is a Home Depot orange, 5 gallon tapered plastic bucket – avoid any that have a metal base. The outside diameter is 12″ at the top and about 10.25″ at the bottom. The following picture and scale provide of rough indication of the inductance when winding with solid #14 wire…
Have Fun, CU on ‘The Band’ !