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I've read the post here:

https://electronics.stackexchange.com/questions/77256/why-do-some-radio-antennas-require-a-path-to-ground-and-some-do-not

which says

The antenna system requires a "ground plane" for the radio wave to form. With dipoles, one half of the antenna is your ground. In verticals, something below must provide ground to complete the RF wave. This is usually achieved with radials, either attached to the base of the vertical, or buried below a ground mounted antenna. The presence of a metal mast/tower (which is lightning grounded) may contribute/interfere with the ground plane but it is not considered the antenna ground.

But, for example, 2m verticals on boats don't have ground planes. Two questions.

  1. Am I right in saying that a ground plane results in optimal radiation, but that radiation will still take place without one e.g. boats antennas still reach the horizon, though likely height and more power are required.

  2. Is a ground plane necessary for receiving?

Thanks for reading

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    $\begingroup$ Water is a very good ground plane. $\endgroup$ – Pete NU9W Jan 17 '17 at 22:35
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    $\begingroup$ At least some of the verticals I've seen on boats are vertically mounted dipoles. $\endgroup$ – Phil Frost - W8II Jan 17 '17 at 22:51
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    $\begingroup$ A ground plane contributes to a more stable SWR. Water does make a more stable ground. That's why some commercial AM antennas are built in wet sometimes swampy areas. $\endgroup$ – Optionparty Jan 18 '17 at 2:58
  • $\begingroup$ Note the post you link is talking about ground planes only within the context of monopoles. $\endgroup$ – Phil Frost - W8II Jan 18 '17 at 13:27
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    $\begingroup$ What do you mean precisely by "vertical antenna"? Do you mean any antenna that is vertical, or a monopole, which is colloquially called a "vertical"? $\endgroup$ – Phil Frost - W8II Jan 22 '17 at 3:24
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Every antenna has two halves, meeting at the feed point (where the coax or other transmission line is attached). Both halves affect the properties of the antenna (radiation pattern, efficiency, etc.).

A vertical element and a ground plane is just one possibility for what those two halves can be. If you omit the ground plane from a ground plane antenna, then there are only two possibilities for what happens:

  • You have an ineffective antenna — it does not radiate/receive well.
  • Something else is serving as the ‘second half’.

In most designs of coax-fed antennas, what ends up being that second half is the shield of the coax cable and (if applicable) the conductive metal structure the antenna is mounted to. I don't know how antennas on boats usually are, but I bet there's a metal structure there.

This can work fine, but there are two potential problems:

  • This conductive shape wasn't particularly designed to be an antenna, so it may not have the best characteristics for that (irregular radiation pattern, lossy, wrong impedance, etc.).
  • It may be closer to other electronic equipment, the operator, etc. and thus (for receiving) pick up extra noise or (for transmitting) deliver significant RF energy where it is not wanted.

If you're interested only in receiving for now, put up whatever antenna you can and don't worry too much. If you find you need better performance from your antenna, install a proper ground plane or other design of antenna.

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  • $\begingroup$ On HF wouldn't it still be necessary to make some attempt to make a relatively low impedance connection to the Earth, sea, or whatnot? For example, letting the coax lie on the ground to feed a vertical does not work especially well: with the impedance to the Earth being much higher than to the common-mode on the coax, it's approximately a vertical with one radial. $\endgroup$ – Phil Frost - W8II Jan 17 '17 at 22:49
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    $\begingroup$ @PhilFrost-W8II Rather than figuring out how to explain it better, removed the HF bit as it's not really necessary to the answer. $\endgroup$ – Kevin Reid AG6YO Jan 17 '17 at 22:56
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Any vertical that you may have seen mounted on a nonmetallic surface should have something like a metallic sheet or λ/4 copper tape radials (of proper dimensions) underneath it (and the coax shield connected to that). Otherwise, the outside of the shield will try and act as the return and have common-mode currents on it. How well that works depends on many factors, but that's not the proper way to do it to maximize communication range.

Of all the physical laws that there are, none are more well-established than the fact that a bottom-fed vertical (vertical monopole) or an inverted-L needs something to "push against" to be most effective. Call that 'something' a counterpoise, radials, or whatever you want to; but to be effective, it's got to be there. Period.

K5UJ put it nicely when he said "The reason for radials is to collect RF currents and return them to the feedpoint since you don't have the other half of the antenna to do that, the part that would make it a dipole."

If we make the statement that adding a counterpoise to a λ/4 vertical (for example) is not really necessary, then we might as well say that opening the window blinds won't make the room any brighter. Sure, maybe we can see our way around, but when we let the sun in, life is so much better. Or, someone could say that we don't really need tires on a car. Sure, with enough ground clearance we can drive around on the rims, but isn't the car much more fun to drive with the rubber attached? Likewise, a proper RF ground makes the radiated signal stronger while preventing unwanted common-mode currents on the outside of the coax shield.

From this web page.

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    $\begingroup$ Nitpick: It is only called a counterpoise if it is not mounted on the ground - like on a building roftop. $\endgroup$ – SDsolar May 13 '17 at 0:35
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If a vertical antenna is on a boat then the water will reflect the other half. Even better is its salt water. As for road vehicles then the vehicle creates a capacity shield away from the earth.

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Note that some writers believe that a primary function/purpose of elevated radials is to shield an elevated vertical monopole such as a vertical ground plane antenna from the earth.

This does seem intuitive, but in reality the free space radiation patterns of a λ/4 ground plane driven against pairs of elevated, λ/4, horizontal radials at its base and a center-fed, λ/2 vertical dipole are very similar (see the NEC4.2 analysis below). Other things equal, any effects/changes due to reflections from the earth or another surface will be very nearly the same for the radiation patterns of both of these configurations.

When pairs of horizontal elevated radials are separated by horizontal spans of 180° then the net far-field radiation from them essentially is zero. This is the result of the radiation from one side of the pair arriving 180° out of phase with the other side of the pair, as r-f currents are flowing in opposite physical directions on each side of that pair. All of the useful radiation from a vertical ground plane antenna originates from the r-f current flowing along the vertical conductor located physically above the horizontal radials.

As shown in the graphic below, the elevated, co-linear pairs of horizontal radials of a ground plane antenna do not shield a likewise-elevated vertical conductor above them from interaction with, or radiation toward the earth. They serve as part of the current path connecting the antenna system to the r-f "ground" return of the source (the transmitter). Without such a path, no antenna system will produce useful radiation.

  • The source for the r-f current flowing on the buried radials used with a monopole antenna is the r-f current flowing in the earth within λ/2 of the base of the monopole, as a result of radiation from that monopole. The sum of that current at the common point of the buried radials exactly equals the r-f current that can/will flow on the monopole, itself.

  • The source for the r-f current flowing on the elevated radials used with an elevated monopole antenna is a direct, wire connection to the r-f source (transmitter), using an all-physical conducting path such as the outer conductor of a coaxial transmission line between the antenna system input terminals and the transmitter.

Elevated radials function much differently than buried radials.

NEC4.2 Analysis:

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Certain types of vertical antennas require a 'ground plane' and some do not. Most verticals are 1/4-wave (not a dipole) and require a 'ground plane' to complete the 'virtual dipole'. On the other hand a 5/8-wave vertical does not require a 'ground plane' and is usually used on boats and cars, such as Corvettes, which have fiberglass bodies.

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  • $\begingroup$ "a 5/8-wave vertical does not require a 'ground plane' ". Why do you say that? $\endgroup$ – Mike Waters Feb 1 '17 at 3:27
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A vertical on good ground in the open will do well with a small number of 1/4 wave radials. Unfortunately that's not always possible, so a large amount of radials become the norm. How many depends on the environment your pushing against. Trial and error plus London gin could be the perfect answer.

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Also a good air choke 1/4 wave from the antenna will improve things...plus a good bottle of London gin.

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    $\begingroup$ Ah, the good old days, where alcohol was a prerequisite for doing any kind of radio work, amateur or professional. Surprisingly few of us got killed by it. Meanwhile, @2e0sir, welcome, and please take the tour and earn your first badge here: ham.stackexchange.com/Tour - you will find that you'll fit in better with more elaborate answers. $\endgroup$ – SDsolar May 8 '17 at 23:17

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