Dipoles have the 2 wires going to each leg of the antenna completing the circuit.
Big AM towers use the second wire as a ground plane
But how do handheld rigs transmit and receive with just one wire sticking in the air?
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1$\begingroup$ Just the right topic. 38 years later, I 'm back in the shack. Or better yet, I'm walking home. KABH 8895, from 1976. Wow. Today's project: a flat-side dipole or other antenna for these 2-watt CB hand helds. Realistic? Hope so. TRC-218. EBay win because they...work! A 102" wire direct out of the Motorola plug roughly triples my ears. But both WT's work better when only the hot , center, wire is connected. Soon as the outer ring touches ground, well, friend, no benefit. Yes, I got the periscope down. These WT's have the brushed metal side panels. When I hold them, yup, I become the dipole. When $\endgroup$– user1363Jul 23, 2014 at 21:13
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2 Answers
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Your body acts as the ground when using a handheld radio. By holding the radio, you are capacitively coupled to the radio and make the other leg of the antenna. This is one of the reasons to hold the radio in your hand away from your body while using it, rather than using it while attached to a belt clip. (The other reasons pertain to RF safety.)
Of course, you aren't a very good ground system, so you can improve the efficiency of a handheld's antenna by attaching a counterpoise to the radio to be a better ground. There is a product called a "rat tail" designed for this purpose, but you can easily homebrew your own by attaching any handy piece of wire about a quarter wavelength long to the radio's case.
As noted in a comment: further details about adding your own ground/counterpoise to an HT.
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1$\begingroup$ I just wanted to add a comment to this article which explains in detail how to do this and some of the theory behind it. $\endgroup$– DanNov 1, 2013 at 3:08
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$\begingroup$ But my radio can still receive when it is across from the room when I switch on VOX and talk into it when I am not even touching it, the radio still works fine $\endgroup$ Nov 1, 2013 at 12:08
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1$\begingroup$ Most HTs have an aluminum chassis inside the plastic that works as a (poor) ground plane when it's sitting on the shelf and one plate of the capacitor when you are holding it. Given that you are usually using this on 2m or 70cm locally, often with a repeater, you don't need much of an antenna. $\endgroup$– WPrechtNov 1, 2013 at 13:39
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$\begingroup$ The lack of an RF ground/counterpoise isn't going to prevent the antenna from working altogether. It will improve the functionality fairly dramatically, though, and it will affect the resonant point of the antenna, which in turn can affect the SWR on your frequencies of interest. I used to use a vertical antenna on HF with no ground radials, and it wasn't great, but it worked well enough for the situation. $\endgroup$ Nov 1, 2013 at 14:01
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1$\begingroup$ @Skyler440 Place a pizza pan or some al foil under your radio while listening. You may be surprised by the improvement with distant repeaters. $\endgroup$– PaulNov 2, 2013 at 2:30
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If you are using a monopole antenna, when you hold the radio, your body acts as the image of the antenna to make a full dipole. This works for both transmit and receive.
If you are receiving a strong signal, it doesn't make a lot of difference, but if you have a marginal signal, you will notice that it gets a lot worse if you set the radio down or put it on your backpack. However, if you set it down on a piece of metal, like a metal table, it'll be about as good as when you are holding it, because the radio can capacitively couple to the metal. (It doesn't have to make conductive contact to do this.)
However, if you put an antenna on the radio that is a full dipole, like an end fed half wave whip, a loop, or coax going to a J-pole, you'll find that this matters a lot less.
Similarly, you can add a counterpoise to the monopole and it should help.
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$\begingroup$ It is a truism in radio that all monopoles are just antennas trying to behave like a dipole and they'll use any image antenna they can find; regardless if this image antenna is provided explicitly or not. $\endgroup$– user21417May 10, 2022 at 14:16