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Take for example, the Hustler 4BTV. It has traps formed by an inductor, covered by a sleeve that's connected at the lower end, and the upper end is covered by a non-conductive cap:

sleeve trap diagram

The tuning instructions for this antenna advise the installer to move the sleeve up or down relative to the coil it covers, rather than adjusting the length of the intermediate elements. What's the electrical explanation for why this effects a change in resonant frequency?

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The trap is an LC parallel circuit:

capacitor and inductor in parallel

At some frequency, the inductive and capacitive reactances are equal, and the trap looks like a very high impedance. No significant current can flow through this impedance, and the rest of the antenna is effectively disconnected.

When the sleeve is adjusted up, more of it overlaps the top section of the trap, increasing the capacitance. Now the trap isn't precisely resonant, and starts to look more like a series capacitor. This electrically lengthens the antenna, effectively making the rest of the antenna a capacitive hat, making it resonate at a lower frequency.

Adjusting the sleeve down has the opposite effect: it reduces capacitance and now the trap looks more like a series inductor. This is not like a loading coil: adding series inductance at the top electrically shortens the antenna, making it resonate at a higher frequency. This is not something you ordinarily see since people usually want to make a small antenna work as a bigger one, not the other way around.

schematic

simulate this circuit – Schematic created using CircuitLab

With these kinds of adjustments, you can get a very large change in resonant frequency without moving the trap very far, but it's not without disadvantages. Since current is no longer flowing in a quarter-wave section, it won't behave like a quarter-wave antenna. The feedpoint impedance and radiation pattern will be different. There are most likely higher losses as well, since currents are flowing through more metal, increasing resistive losses. The magnitude of these problems is proportional to the distance from resonance the trap has been adjusted.

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  • $\begingroup$ Are you quite sure that it isn't acting on the inductance instead? A non-ferrous "slug" inside a coil will reduce the inductance, causing the resonant frequency to increase instead of decrease. $\endgroup$ – HarveyB Jun 9 '14 at 15:17
  • $\begingroup$ @HarveyB how would moving the sleeve significantly alter the permittivity of the coil's magnetic circuit? $\endgroup$ – Phil Frost - W8II Jun 9 '14 at 15:52
  • $\begingroup$ If in close proximity to the coil, current induced in the sleeve produces a field opposite that of the coil. This effectively "shorts" the turns that are covered. This technique is used extensively in VHF/UHF tuning coil "slugs" instead of the traditional magnetic materials. $\endgroup$ – HarveyB Jun 9 '14 at 16:43
  • $\begingroup$ @HarveyB I guess I'm not sure -- I'm not familiar with that technique. In these traps, all the turns are completely covered for normal ranges of adjustment, if that makes a difference. If you have a better explanation, please do write an answer. I mostly figured this stuff out by primitive experiment and could easily be wrong. $\endgroup$ – Phil Frost - W8II Jun 9 '14 at 16:58
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    $\begingroup$ OK, I went to the source and (forgive me!) READ THE DIRECTIONS. Moving the Trap Housing down 1/4"-1/2" moves the frequency UP by 500 KHz. That means that the capacitance is indeed the dominant factor and you are correct sir! My Bad. $\endgroup$ – HarveyB Jun 9 '14 at 17:53

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