Antenna Puzzle #1
The more I learn about antennas, the “less I know”… or more precisely: the more I realize what I don’t know. Here’s one of the puzzles I ran into recently…
Right before ARRL Field Day this year, I hastily strung up a long wire antenna to try to snag some QSOs. I used a spool of insulated wire that I had in the basement. Being an odd length, it required a tuner to get working. But, my first contact was 59 from Hawaii on 20M, and the second was Guam, so I felt reassured.
A few days later, while working field day, John, W6FGX, made a comment that the insulation would have an effect on the signal. It would not necessarily be a good or bad effect, because there were quite a few variables involved, for instance, the random length of the wire.
I sat there for a minute under the big oak where our Mendocino group was stationed… contemplating my knowledgeable lack of knowledge on EM theory, and emphatically replied, “no way.” The plastic insulation was a dielectric, similar to air, so there wouldn’t be much effect.
John went on to tell me a story about some 2M VHF wire antennas he made and how they changed resonate frequencies when inserted into ordinary PVC pipe. The effect was quite noticeable… it electrically shortened the antennas by a few percent.
So, it seemed to me an odd puzzle and remained in the back of my mind for more than a month.
Well, the answer turns out to be fairly simple. When an EM wave enters the surface of any material (from some other material), two waves are created. One passes into the dielectric, the other is reflected. Think about light on glass as an example. Some of the light passes through; some of it reflects back. In addition, the angle at which the light enters the glass makes a big difference. The steeper the angle, the more the reflection.
You might think of it a bit like an impedance mismatch… where energy gets both transmitted and reflected, depending on the difference in the values involved. And, in the case of my insulated wire antenna, the dielectric properties of the plastic will cause part of the wave to be reflected back into the radiating wire, all at a slightly different phase, and thus changing the flow of the current in the wire. The net effect is to change the velocity factor of the wire, and hence, its electrical length.
Well, good to know. One antenna puzzle solved. Only 99,999 more to go.
If you have more to add.. feel free to post your comments.