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Photo I. 90-degree elbow with turn loop.

Photo J. j'circui? phone plug mounted in pipe cop.

long, l/2-in.-diameter ferrite rod. These rod> are av ailable in two formulations: "mix 33" and "mix 61." At the risk of' significant oversimplification, use mix 33 for frequencies below 500 kHz and mix 61 for everything else,

I spent two months building and analyzing more than 50 different ferrite loop antennas. Rather than attempt to explain how to design and optimize a ferrite loop. I'll instead concentrate on explaining how to duplicate the ones I built. If you wish to depart from these precooked designs, the hesi readily available reference is Chapter 5 Of The ARRL Antenna Hook (1994). However, in building test antennas i found that the information published in the amateur literature greatly oversimplifies many factors and that differences of 50^ between theoretical and measured parameters were common — so be prepared to do some experimenting!

1. Secure the material and cut the PVC pipe sections to length.

2. Drill a clearance hole in the pipe cap for your 3-circuit phone plug, I used Radio Shack part number ~74-139A piugs that required a 7/16-inch drill, There seems to be some variation in shell si/e, however, so check the size required before drilling, Epoxy the 3-circuit plug in place and allow the epoxy to cure* Since the plug mechanically supports ihe loop, make sure the plug is centered in the pipe cap and is straight.

3. Wind the coil following a few pointers:

a. Don't wind directly onto the ferrite rod. Instead make a paper core. Wind two layers of typing paper around the rod and glue the paper into a cylinder using glue suitable for paper. The rod should be a snug lit into the paper cylinder, hut it should be possible to siide the rod in and out. The paper cylinder should be the same length as the rod.

ht Calculate the winding length; for example. 60 turns at 10 turns/inch means the winding length will be 6 inches, Mark the paper eorc with the start and finish as well as making a w mark evcr\ half-inch.

w c. Wind the coil, striving for as much uniformity as possible. Leave a foot or so of wire at the start and end of the windings. To avoid collapsing the paper core, insert ihe ferrite rod. Please treat the ferrite rod with care, as it is brittle and will fracture if stressed, (1 used a mandrel made from a sieel rod and wound my coils on a lathe, inserting the ferrite only when the coil was ready for testing.)

d. It is necessary to apply tension to the wire while winding, and while finished, it is necessary to restrain the

Photo A". Coil for ferrih rod showing ¡Hiper core and ends anchored with adhesive tape.

m tJ

m tJ

1 - —n. 1 ^ i i'hoto L. Ferrite coil antenna w ith ends removed.

i'hoto L. Ferrite coil antenna w ith ends removed.

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wvvw.rbelectronics.com winding. I taped the start and finish windings to the core with 1/2-inch adhesive tape.

4. Press-fit the 2-3/4-inch stub into the center of the TEE. Insert the coil into the TEE section, routing both free wire ends through 2-3/4-inch stub. Cut off any excess wire, strip and tin the ends, and solder to the tip and ring pins of the 3-cireuit plug. (Polarity isn't important.) Leave the sleeve (ground) pin unconnected. Temporarily assemble (without glue) the 2-3/4-inch stub and the cap. Install the loop on the tuner and check the tuning range,

5. If the tuning range doesn't match expectations, adjust the loop;

a. Squeeze the turns together or stretch the turns farther apart. Squeezing the turns together increases inductance (lowers frequency) while strctching reduces inductance (increases frequency), b* Slide the ferrite rod away from center position. This will reduce the inductance.

If you can't achieve your desired tuning range, then rewind the loop,

6. When you have achieved your targel tuning range, unsolder the loop and remove it from the TEE. Cement the winding to the paper core with Q-Dope® gtue. If you donnt have Q-Dope, household cement may work. When the glue is dry, reassemble the loop, including the PVC legs, and resolder to the 3-circuit plug. To prevent the loop from rattling inside the pipe, fill the TEE with packing. I've used plastic "peanuts'1 and scraps of foam rubber successfully. Fill the legs and then slip the caps over the end. I've found that friction fit holds the assembly together without the need for cement. If you wish additional mechanical strength, cement the assembly together, but

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Step One: Pick up microphone. Step Two: Transmit.

(Please note: H F Tuning doesn't get much easier than this J

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Hams of all ages have long had a love affair with vacuum tubes. Arguably\ the 1950s were the golden years for amateur use of vacuum tubes. However, hams who remember those years probably have only a vague understanding of the vacuum tube's origin and history. AlsoT the demanding nature of vacuum tubes has long been forgotten, while the memory of their beautiful glow lingers on,

The early 1900s marked the first "solid state7' era for radio communications, with the emphasis on the word "solid," It was an age of spark transmitters, crystal detectors, and other physically solid devices. It was an unlikely setting for the birth of the fragile, glass bottle vacuum tube.

In those early years, radio communications utilized sparks. To understand a spark transmitter, think about the small spark you create when you walk across a carpet on a dry. cold, winter day and touch a doorknob. If you were carrying a portable AM radio, you would hear the static generated by that Spark. That's how spark transmitters worked, only they used much larger sparks, requiring a significant amount of electricity.

An important need In the 1900s was for a better detector (receiver) of spark signals. To try to meet this need, John Ambrose Fleming, an English scientist, began experimenting with the "Edison effect" that had been noted in elcctric, glass light bulbs.

The Edison effect was named for Thomas A, Edison, the American inventor of the electric light bulb. In the late 1800s, Edison was troubled by the fact that the glass in his light bulbs 16 73 Amateur Radio Today * August 2002

would eventually become darkened. This was caused by particles of the carbon from the light bulb filament being transferred to the glass. In investigating this problem, Edison placed another element (a plate) inside the bulb and discovered that a very small electrical current flowed from the filament through the vacuum to the plate. This action came to be called the Edison effect" and seemed to be of no practical value.

To investigate the Edison effect, Fleming built his own two-element vacuum tube. Sn the course of his research work, he discovered that his tube could be used as a detector (receiver) of radio waves. Fleming called his two-element vacuum tube a valve.

Lee De Forest

At about the same time, Lee De Forest, an American inventor, was also trying to develop a heller detector of spark signals. He invented a flame detector that had two platinum electrodes held close together in the flame of a Bunsen burner As strange as it sounds, it actually worked.

De Forest knew that, for practical reasons, the flame needed to be inside a glass mantel. He also realized that the flame could be replaced with a burning filament. He ultimately created a vacuum tube that he called an audion. It was similar to Fleming's valve, biu De Forest got there via a different intellectual path, or so he said. Actually. De Forest was quite familiar with Fleming's work with vacuum tubes. However, it was De Forest's decision to add a third element — called a grid — to his audion that produced the start of the vacuum tube revolution,

Nevertheless, for the first few years after its creation, the three-element audion vacuum tube languished. Even its creator was unclear about how it worked and could be best utilized. Then, in the 1910s, engineers like Howard Armstrong and corporations like AT&T began to turn the vacuum tube into a truly useful device.

Through their efforts, the vacuum tube became more than a mere detector of radio waves. It also became an amplifier, and even more importantly, it became an oscillator that could generate continuous waves, which would eventually replace spark transmissions. The development of new electrical circuits had turned the vacuum tube into an all-purpose electronic device.

During the 1910s, there was some limited use of vacuum tubes by hams. It was limited because at that point in time vacuum tubes were very expensive, their quality was poor, and their life was short. However, hams could clearly see their potential.

During the 1920s, vacuum tubes continued to be refined and developed. More grids were added to increase their performance capabilities. The power handling of transmitting tubes continued to increase. It was clear that vacuum lubes were here to stay.

The creation of radio broadcasting* in the 1920s, greatly expanded ihe demand for vacuum tubes. This meant thai ihe volume production of tubes could create economies of scale and reduce the cost of tubes.

During the 1920s, ham use of vacuum tubes greatly expanded because of their lower cost and greater availability. Vacuum lube receivers and continuous wave tube transmitters were increasingly common: spark was no longer king.

During the 1930s, vacuum tube development continued. However the great depression of the 1930s impacted ham use of vacuum tubes. In that era. it was not unheard of for a ham to have a one-tube receiver and a one-tube trans* mitter but have only one vacuum lube.. This required physically moving ihe lube between ihe receiver and trans-milter. Hams learned to make the best of what they could afford.

During World War II, hams went off the air. but vacuum tube development continued at an accelerated pace. New tubes were invented to work on the very high frequencies (VHFh miniature tuhes for military applications were created, and tubes with a much lower current drain were developed. All of these vacuum lubes would greatly benefit the civilian market after the war had ended.

The golden years

For most hams, the 1950s brought the return of relative economic prosperity, They could now afford more complex, multiple-tube transmitters and receivers, and they wanted them.

In that era, many hams were able to

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