Antennas

After my successful experiment with winding a 4:1 balun, I started to think about how to package the balun. One of the problems with the Ruthroff design is that the input and output are close to one another. In the Sevick book, all of his Ruthroff baluns are shown packaged in square or rectangular boxes, with an SO-239 on one side and a pair of binding posts on an adjoining face, at a 90 degree angle from the coax connector.

So, I began reading about the other types of baluns. The one that seemed most promising is the Guanella current balun. Unlike the Ruthroff, the input and output are on opposite sides of the core, making it easier to package in a PVC pipe. Sevick also claims that the response is flatter than the Ruthroff balun as well. One disadvantage is that you need almost twice as much wire, as there are four windings, instead of only two. Here’s the schematic:

It took me about a half hour to unwind the first balun and rewind it with two bifilar windings instead of just two. I then connected it up to my antenna analyzer, and found that the response did seem to be flatter, the impedance measurement varying between 58 and 60Ω from 1.8 MHz to 18.1 MHz, then dipping to 56&OMega; at 28.0 MHz.

Although I wound my balun on a 2.4-in. core, the Sevick book shows a design wound on a 1.25-in. core. This core should be substantially cheaper than the 2.4-in. core, and the design uses 20-ga. hookup wire, which is certainly easier to get than magnet wire. Sevick notes:

They are conservatively rated at 150W of continuous power and 300W of peak power. They have been exposed to 500W of continuous power (in a matched condition) for a considerable length of time with virtually no rise in temperature.

That’s plenty good for most ham radio applications.

Assuming that these baluns actually play nice with some real RF, I was thinking about possibly even building a bunch and selling them. RadioWorks charges $60 for a 1.5 kW 4:1 current balun. I should be able to sell a lower-power version for half that or less. And not only would it be cheaper, but it’s smaller and lighter, too. Why pay for something you’ll never need?

Links
Here are a couple of links that look useful:

• Step-By-Step Construction of a 4:1 Current-Type (Guanella) Balun
• Low Power Balun Manual. This is a design by Charles Greene, W1CG.

I’ve written about building baluns before. I’m still quite proud of myself that I was able to build and deploy a couple of nice 1:1 baluns.

Of course, the next challenge is the 4:1 balun. I even bought a couple of AB240-125 kits to build one. Since my 40m dipole came down in the ice storm we had here over the weekend, I couldn’t really get on the air, so I decided to try it tonight.

The book Building and Using Baluns and Ununs by Jerry Sevick, W2FMI, has a whole chapter on building 4:1 baluns. After reading through it, I decided to build the Ruthroff design. Here’s the schematic:

The book shows several different designs, but the one that caught my eye was the low-power version on page 24. What I liked about it was that it was built with hookup wire instead of magnet wire. Using some cheap hookup wire, I could play around without possibly ruining the magnet wire.

One problem with using hookup wire is that it’s not as ductile as the magnet wire. This means it has a tendency to want to unravel. I did finally manage to get ten turns around the core, though. Here’s what it looks like:

Next, I clipped a 220-ohm resistor across the input and connected the output to my antenna analyzer. Here’s what I measured:

MHZ	Z
1.9	57
3.7	58
7.1	59
10.1	58
14.2	56
21.2	53
28.2	53

Amazingly, the thing actually seems to work! Not only that, it’s relatively flat over the HF frequency range. The next step is to package it into a nice enclosure, then put it to work.

I think my next antenna project is going to be the Cobra Antenna. They use a 4:1 balun to get the impedance of the antenna into a range tuneable by a common antenna tuner. K1JEK sells the antenna for $90-100, but you can build the thing for about a quarter of the price. More on that this weekend.