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In the near future, I'm planning to build an indoor inverted V dipole. However, I won't be able to raise the ends significantly above the floor. I understand that this will adversely affect performance, but I'm okay with that. I'm more concerned about the length. Will I need to adjust the length due to the antenna's proximity to the ground?

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    $\begingroup$ If you intend to operate an antenna inside your home, you are in for a world of hurt. However, when you key up the mic, you can look forward to electronic gizmos doing strange things. The garage door is opening/closing, and Gramdma is being ejected from her power lift chair. Tingling sensations of RF shock and burns, wiping out your CATV and Internet modems. SWR and the length will be the least of your worries. The antenna will be death and mute. $\endgroup$
    – Dereck
    Commented Mar 25 at 20:11
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    $\begingroup$ @Dereck you might be assuming enormous signal powers. I've never built an antenna for any more power than 50 mW. $\endgroup$ Commented Mar 25 at 21:29

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Feed-point impedance of a capacitive antenna, like an Inverted V, is affected by four physical factors:

  1. Length of radiating elements. Adjusting the overall length so that the antenna is resonant on the frequency, achieves the lowest impedance in a current-fed antenna, like an Inverted V.

  2. Angle between the radiating elements: the narrower the V, the lower the impedance, or get the elements to 180 degrees apart to reach the maximum possible impedance.

  3. Feed-point offset; this is where you have an antenna that is 1/2 wave in total length, but you choose to have the feed-point somewhere off-center, you typically do this when you are too close to ground and have a very low impedance because of that fact. Like trying to get an 80M horizontal dipole, that is 40 feet above the ground, to be 50 Ohms at the feed-point, you might find the 50 Ohm feed-point is off-center, somewhere around 70%/30%.

  4. Reflections from the environment; this is where the ground under the antenna, or conductive objects around the antenna, are reflecting energy back to the radiating elements of the antenna, causing a change in the feed-point impedance.

The first thing you need to determine is resonance, and the best way to do that is by using the Smith Chart feature of a Nano-VNA (tuning for the lowest SWR might get you close, but it won't tell you if your impedance is too high, or too low), you can first adjust the antenna length to be resonant at whatever frequency you want it to be at, then, depending on whether the Nano-VNA tells you that your impedance is too high, or too low, you can adjust the angle of the V, or the offset of the feed-point to get it where you need it to be. However, if you have objects that are reflecting too much energy back to the antenna, you may not be successful in reaching your desired impedance.

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Yes, somewhat. We could model the effect of height above ground on resonant length for an outdoor antenna. The effect I would expect to see is that lower height means more capacitance to ground, which means more end-effect, which means the antenna will resonate at a shorter length than it would if it was up higher.

But an indoor antenna is surrounded by all sorts of conductive objects less than a wavelength away, which we couldn't possibly model without X-raying your house. That means that, even leaving aside the low height above ground, the performance, pattern, input impedance, and ideal length are all "who even knows?". You're going to have to guess and test. An antenna analyzer will make it easier, a radio with a good SWR meter will do in a pinch.

If it works, great. If it doesn't work... don't be too surprised. I once tried using a telescoping whip dipole on 20 meters in a big open room of a 20th-story penthouse apartment. I couldn't get an SWR below 2.5:1 with any adjustment of the lengths, and I couldn't get a contact even on FT8. The building was essentially a Faraday cage, and the wonky feedpoint impedance was nature's way of telling me that none of my RF was getting out, I was just creating one big standing wave in the space inside the apartment.

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