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This is a very simple question and I'm sure if I had a bit more background knowledge, I could figure it out myself.

Let's say I have a 1/4 wave vertical antenna that is slightly too long, but it is impractical to cut shorter or fold back on itself. What do I need to add to raise the resonant frequency slightly higher (10 - 100kHz) without using a full-fledged tuner? To put it simply, I can't use a tuner. Can I do it with one or two components?

With electrically short antennas, I know I can add an inductor in series with the antenna to lower the resonant frequency, but how do I do the opposite?

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    $\begingroup$ Um, one or two components constitute a tuner if you use them to tune your antenna :) $\endgroup$
    – Marcus Müller
    Oct 20, 2019 at 10:18
  • $\begingroup$ Also, how much is "slightly too long"? What's the target frequency? What will you use that antenna for? The match-over-length-relative-to-wavelength plot of a simple antenna really isn't all that steep around its actual frequency, so how much are you actually losing through this $\endgroup$
    – Marcus Müller
    Oct 20, 2019 at 10:21
  • $\begingroup$ It is a wire based 1/4 wave vertical (inverted L to be exact). The bandwidth is very narrow. It's only about 20 kHz before the SWR goes above 1.5 and I know that's not too bad, but I like to be easy on my radio because it's a solid state. $\endgroup$
    – Synaps3
    Oct 21, 2019 at 18:33

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An antenna which is electrically long at the desired frequency usually presents inductive series reactance at the feedpoint. This inductive reactance can be compensated by adding series capacitive reactance - a single capacitor - between the feedline and the antenna at the feedpoint. This will probably be a relatively narrow-band solution.

Be sure to use a capacitor which can withstand the RF current flowing through it and the RF voltage that will appear across it. You must use a capacitor that presents high Q (low loss) at the operating frequency. I know from sad experience that using a "transmitting" or "doorknob" capacitor is no guarantee of success. To avoid converting your capacitor into smoke, start by applying very low power and, at the very least, measuring the capacitor's temperature before increasing the power. Duty factor will also affect heating; extended application of a steady carrier - for example, tuning, AM, FT8, etc. - will produce more heating than CW or SSB.

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  • $\begingroup$ Thanks for the help. What is the type of capacitor you would most recommend? $\endgroup$
    – Synaps3
    Oct 21, 2019 at 18:35
  • $\begingroup$ It appears that ceramic disc capacitors will probably work for you at HF. Place capacitors in series to increase their voltage handling ability, place them in parallel to increase their current handling ability. Check the manufacturer's datasheet for the candidate cap to calculate its resistance at the frequency you want to operate so you can calculate how much power the cap will have to dissipate. $\endgroup$
    – Brian K1LI
    Oct 21, 2019 at 20:07
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If adding an inductive load half way up each leg of a dipole “lengthens” it, I might try the opposite to “shorten” it. Add a capacitive non-hat part way (half?) up each leg. This might partially shield the tips of the dipole, and make them seem shorter. Vary the size of the non-hat to vary the effect.

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  • $\begingroup$ No, this still makes it longer. It might be possible to add a trap to shorten it, but any sort of hat, extension or thickening will make it work at a lower frequency. $\endgroup$
    – tomnexus
    Oct 20, 2019 at 17:45
  • $\begingroup$ Have you modeled the addition of a "capacitive hat" to evaluate its effect? $\endgroup$
    – Brian K1LI
    Oct 20, 2019 at 22:04
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Cut down your ground plane radials or counterpoise.

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    $\begingroup$ Hello and welcome to ham.stackexchange.com! This isn't a forum-style site, this is a question-and-answer site. Please expand your answer and explain why one should do what you recommend. $\endgroup$
    – rclocher3
    Nov 12, 2019 at 14:15

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