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TECHNOLOGY |
![Text Box: Antenna IQ Brainteaser #3 by Glenn DJ0IQ and W9IQ
Introduction Antenna theory can sometimes be a tad dry. For the typical amateur radio operator who is not a practicing engineer in the field of antenna engineering, the association of practice to theory can become a bit strained leading to incorrect assumptions and generalizations that creep into the articles, vernacular, and assertions of our hobby.
To this end, I offer this Antenna IQ series in a fun, brainteaser format. The questions are constructed so that you can easily re-use the brainteaser during your next QSO or club meeting. They are guaranteed to spark plenty of disbelief, debate, and ultimately enlightenment. The number of Antenna IQ Brainteasers produced will be determined by your feedback. I hope you enjoy them.
Consider the Following Know it all Neal was in rare form at the local establishment after a recent monthly club meeting. The beer was flowing and the conversation drifted around to how to pick the best coax for use between the shack and the tower. Neal listened to a few people espouse decibel loss per one hundred feet at the frequency in question and the effect of power loss due to skin effects. Finally, he couldn’t contain himself any longer.
“People, the answer is very simple,” preaches Neal with an intonation that makes most reach for their beer and role their eyes. “You buy the coax with the thickest center conductor and then the highest velocity factor you can find.” At this revelation a few club members involuntarily sprayed their mouthful of beer as a prelude to spontaneous laughter. The club station trustee, Tom couldn’t believe what he just heard. “Are you serious – that is your entire criteria?” challenged Tom.
“Well, it works well for me,” slurs Neal. “It will give you the lowest losses. You shouldn’t knock it until you have given it a try.”
After the laughter dies down, Neal continues to defend his position but the club members are relentless and Neal’s retorts are growing fuzzier. “Look Neal, I have been drinking club soda all night long. Why don’t I drive you home and we will come back in the morning to get your car,” offers Tom. “We can talk later about how you get your credibility back with these guys.”
Now Answer the Following Question
Did “know it all Neal” have it right or should he have remained quiet until his head cleared? [ Stop reading here if you don’t want to see the answer just yet. ]
Working Out Your Answer The equations dealing with the nuances of coaxial transmission lines look like the dreaded final exam question in Professor Thompson’s 304 Advanced Transmission Lines Theory class. Even with these advanced mathematical models, the real world results can have perplexing minor variations. Fortunately, the amateur radio community has a wide variety of commercial sources for well specified coaxial cable so we are spared from hacking our way through the theoretical morass. With a trip to our local store, we return with the number crunched results of others in hand.
A quick glance at a typical coax spec sheet shows several parameters that may or may not be of interest to the buyer. More often than not, we probably narrow our selection based on loss, flexibility verses rigidity, and price. Sometimes size and weight can enter into the equation. If we have done our homework, we might even add UV or water resistance to our criteria list. Velocity factor is of interest if we are making transmission line transformers so we can compute the right cutting length. And of course, we are always looking at 52 ohm cable except for some esoteric applications.
So it seems that know it all Neal is off in the weeds - or perhaps we should say, suds. On the other hand, one man’s weeds is another man’s salad. It turns out that for the most part, despite his questionable state of mind, know it all Neal is right! Read on…
Continued on TECHNOLOGY page 2](image1228.png)
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