John Langridge KB5NJD / WG2XIQ (Originally written in June 2012)
Last month I detailed a modification for the MFJ 259B antenna analyzer to extend the range to the 470kc region. The next step in my pursuit of 630 meter Shangri-la, also known as Nirvana, was to build a loading coil and variometer to match my existing low band antenna structure to the new band. Prior to designing this coil, I considered the option of erecting a new antenna to keep things simple. The simple fact was that I could not replicate the awesome radial system my current low band system employs (130 count, 100 foot radials – killer on 80 and 160m) anywhere on my property, nor would I want to as copper prices have made the purchase of bulk wire by the common man virtually impossible. I was fortunate to construct my radial system by way of a few “lucky” deals with wire suppliers, which no longer exist. A quick glance at the stations in the 600m-research group indicated that I had a better radial system than 85% of the participants. The answer was clear – use the existing antenna and switch in the new coil and variometer for 630 meters. Using the calculations that I detailed some months ago, I determined that I would need about 700 µH of inductance to resonate my existing structure on the band. After some research to see how others were “doing it”, I chose to wind my coil, which I was going to make about twice as big as I needed for “safety factor”, on a $5 Ace hardware bucket. The variometer would be wound on PVC and a PVC pipe shaft would allow rotation for tuning. The wire used is #16 silver plated copper with Teflon insulation. Ring lugs and tie wraps from Harbor Freight round out the assembly to keep things nice and clean looking.
Before I talk about construction, lets talk about the function of the variometer. A variometer is basically a moveable coil within a coil. This moving coil allows for modification of the inductance by way or capacitance and inductance and functions much like a tuner would. As indicated in previous articles, the narrow band between 472kc and 479kc still represents a 7-foot difference in quarter wave radiator length between the band edges. Even with the fact that feed line loses are generally low at these frequencies, that represents an appreciable mismatch if one were to use a tuner in the shack. The variometer is like a tuner at the feed point of the antenna. My intent is to make the variometer motorized so I can literally tune the antenna from the shack in real time. That’s a simple task with limit switches and a gear-reduced motor. I should point out that the variometer is a simple solution to apply to other bands if remote tuning is necessary but generally the losses are higher at higher frequencies so I would not recommend using this technique above 80 meters.
So lets build this beast. Since I know I have to drill some holes and work on the mechanical aspects of the variometer, let’s start there. The first thing I do is pick the point where I think the variometer should go. I have no real concept of how many turns I should use (some references indicate 20% of your turns should go on the variometer) so since I was planning big on the coil with many taps, I simple choose a point about half way down the bucket and drill a hole large enough to accommodate the PVC shaft through both sides of the bucket.
Next, we have to determine the rotational characteristics of a cylinder inside of a non-parallel bucket. Yes, the bucket, like pretty much all of them on the market today, has a slight pitch to it, sloping slightly from top to bottom. I can’t really give you a good way to accomplish this except through cut and try. I had decided to use a 4-inch piece of PVC pipe for the variometer and about 50 total turns (25 per side of the shaft) and ended up with a variometer form of about 9.5 inches in length.
Selecting the center point of the variometer form PVC, I drilled a hole all the way through and ultimately expanded the hole to handle a ½-inch piece of PVC pipe for a shaft. I also found that I had to cut the ends down slightly for free rotation of the tube. This is a normal part of the cut and try process.
Note the two screws used on the shaft near the bucket to prevent motion of the shaft.
Next we will start winding the coil on the bucket. Calculations indicate that I need about 50 turns to achieve my required inductance. This is a big bucket and I have surplus wire so I am just going to find a starting point at what is traditionally considered the bottom of the bucket and start winding. I used brass hardware since it is lower resistance at these frequencies than typical zinc nuts and bolts.
From this point, put a ring lug on the wire, mount it firmly on the brass screw and start winding. I had decided that for the coil portion below the shaft, I install taps about every 15 turns. Duct tape made holding turns in place easy while working on the taps.
The liberal use of duct tape makes holding turns in place easy when taking a break or making a tap point.
Taps are easy to construct if you use duct tape to hold turns in place while working on the tap. Simply tape the wire in place, select the tap point and strip off the insulation. Form a loop with the newly stripped wire and twist together, soldering them to form a stump to later attach to. Repeat every 10 or 15 turns. Above the shaft, I opted for taps every 5 turns. It’s in this region that I expected to be the most likely location for a tap at 472 kc.
Pick the location, strip the insulation, twist and solder.
Arriving at the shaft, simply work your way around the shaft and keep winding. You want to leave a gap for the shaft to rotate freely. This is not a critical step so be creative in how you make the transition.
At the top of the coil, terminate like you started, with a lug and a brass nut and bolt. This will not only hold the wire in the place but will give a connection point for the coil on the variometer.
Next wire the variometer in a similar manner. I put 25 turns on each side of the shaft and rather than using hardware to hold the wire in place, I used 3-holes in the PVC form at each point where the wire is sewed in and out with both wires terminating inside of the PVC form.
Since the variometer represents a moving coil, I did not want any loose wires floating around inside of the bucket. In order to get the feeds back outside of the bucket to connect to the coil and antenna, I drilled a hole in the shaft at the center of the PVC form and simply pushed both leads to the outside.
Note the hole in the center of the shaft with one wire from one end of the variometer passing through. The second wire is floating around and has not yet been pushed through and outside.
Outside of the main coil, one wire from the variometer connects to the top of the main loading coil and the other side connects to the feed point of the antenna.
A little hot glue was used to secure the PVC form of the variometer to the shaft as well as secure the wires exiting the shaft. I would have used PVC glue but didn’t want anything that permanent at this point since no testing was completed. I also soldered ring lugs to each of the tap stumps on the main coil. While I have not figured out how I will interface to the tap at the antenna, this seemed like a logical means of connection at the time. One additional item that is very important: I pointed out early on that there is a pitch to these buckets, that is, they are larger at one end than the other. In order to hold the turns in place, I used a little spray varnish on the coils and buckets to act as a glue to hold things in place. There are other ways to accomplish this but if you notice your coil slipping and sliding on the bucket, pick a solution and go with it. I did not experience this problem with the variometer – the PVC pipe is a true cylinder with no pitch.
This is where things get interesting. At the time of construction, I was not prepared to test this coil with the antenna. In fact, at the time of writing this article, I have not acquired the vacuum relays to switch high voltage RF between my main station running 80 and 160m and the new home brew transmitter I have been working on. This is one of the complications of sharing antennas. My switching arrangement is vastly complicated and will be described in a later article. In the short term, I needed a way to test this coil to make sure it was going to play nice with my antenna. Referring back to my calculations, I had determined that the surge impedance of my antenna system was around 480 ohms at 472 kc’s. This translated to almost 2200 ohms of capacitive reactance. From this information, I was able to work backwards and determine the equivalent capacitance that my vertical exhibits with respect to ground. Doing so would allow me to come up with capacitors of equivalent capacitance of the vertical and test the coil using my newly modified antenna analyzer.
Calculations indicated that I needed about 250 pf of capacitance to mimic my vertical. My inventory of capacitors in the pF range had been decimated while building loops for 80m-direction finding but I did find some value to connect and series and get “close enough”
Using the analyzer, RF is fed to a tap (middle picture). The antenna wire from the variometer is attached to the capacitor at one end and the other end of the capacitor is connected to the ground of the analyzer (first picture). By adjusting the variometer position (90 degree rotation of the shaft), and testing several taps, it is possible to find the resonance point (last picture), or at least near zero reactance. This test gives me some generalized data and assurances that I should be able to get the antenna to resonate between 472 kc and 479 kc. In fact, I have full band coverage by adjusting the variometer using this test setup. That is encouraging considering that there is a huge difference in impedance from one end of the band to the other. That’s also the reason I will implement a motor to tune the antenna remotely.
A word of caution: I would NOT consider doing this type of testing in the shack with a transmitter. Even five watts at these frequencies with a coil this large can result in a Tesla coil effect, generating dangerous voltage and current. The analyzer operates at the microwatt level and loses are huge but resist the temptation to setup the coil in the shack and drive RF to it. Save that for the antenna feed point where you hopefully implement safety barriers and solid grounding.
Next time I will either discuss my home brew VFO/transmitter/amplifier for 472 kc or I will actually implement this loading coil at the antenna. It all depends on what goodies I find at Hamcom to facilitate the remote switching of extremely high voltage. Vacuum relays can be expensive!
If you are interested in undertaking this project, I am available to answer questions. There is a lot of engineering type stuff that goes into this and sometimes you just have to figure it out while doing it.
73 and see you in the pile up!