Ground plane reflects RF or brings it to ground

Question

I'm still confused with the purpose of a ground plane purpose in an antenna.

My understanding is this. If an antenna is perfectly balanced, there will be no common-mode currents going to ground. If an antenna is unbalanced in any way, there will be common mode currents returning back towards the radio.

If the feeder is coaxial cable, does the outer shield carry this return current?

If so , we must try and get this return current into the ground and away from us and the equipment?

Why do I then read that ground planes or radials are used to reflect this RF away from the ground and not carry it to the ground?

Feel free to correct me, and / or criticize any statement I made:) I'm still learning here.

Answer 1

I think you're right that common-mode currents will flow if the antenna system isn't balanced. It's my understanding that common-mode currents will flow on the outside of the shield of coaxial cable. There are some excellent explanations in answers to the question "Can a common-mode current exist on the inside of a coax shield?". We often try to reduce common-mode currents because they make the coax radiate, since the RF caused by the common-mode current isn't opposed by equal-and-opposite RF from an equal-and-opposite current. Radiating coax often causes problems because it skews the antenna pattern and often leads to RF-in-the-shack problems. Also, if the feed line is radiating then it is also receiving, so the feed line will pick up more undesirable noise from the other electronics in the house.

Your term "return current" seems misleading and unhelpful to me. Power is delivered to the antenna, which requires current to flow in the feed line to deliver the power. The transmitter, feed line, and ground connections form a circuit in which all current that flows "from" the transmitter returns "to" the transmitter. I put quotes around "from" and "to" because the current is alternating, so it would be just as accurate to say that current flows from the antenna to the transmitter. In this case I think it's better to think in terms of common-mode currents and differential-mode currents. If you're wondering what those are, Jeff K6JCA wrote up a nice explanation. Both current modes describe currents flowing between the transmitter and the antenna via various paths, but differential-mode currents are equal and opposite through parallel conductors and therefore the emitted radio waves tend to cancel each other out, but common-mode currents aren't equal and opposite and therefore their emitted radio waves don't cancel.

You can try to ameliorate RF-in-the-shack problems by connecting the coax shield to a good RF ground so that some of the common-mode current flows to ground. That was a popular approach to the problem for hams in earlier years. But that really only works if the RF ground is quite good, such as a field of buried radials (the ground rod at the electrical service entrance is typically a poor RF ground), and the ground connection is a small fraction of a wavelength long. But connecting the coax shield to a good RF ground still doesn't solve the problem of the radiating feed line. In practice keeping the antenna system balanced, with the help of a balun as necessary, usually yields better results for the effort.

Vertically-polarized radio waves (from vertically-polarized antennas, naturally) interact differently from the ground plane than horizontally-polarized waves. Vertically-polarized waves are said to reflect off the ground, but horizontally-polarized waves are said not to. It turns out that this is a simplification; see the question "Why do horizontally-polarized RF waves reflect better off ground than vertically-polarized waves?". I think talk about RF current being absorbed by radials on the ground and returned to the antenna is also a simplification. These simplifications can be helpful, until two different simplifications seem to conflict, which causes confusion. Your question could be rephrased, "Does the RF from a vertical reflect off the ground plane, or does it induce currents in the ground that are returned to the antenna via the radials?" I'd say the answer is yes; they are both simplifications that help to describe what happens.

The questions you ask are about complicated topics. It took me years to better my understanding of them, since I learned as a ham in my spare time rather than as an Electrical Engineering student. Don't be surprised if it takes a while for you to get your head around these ideas also. This is a good place to learn.

Answer 2

The actual antenna is always balanced (as there are no magnetic monopoles in nature at any frequency, including RF). However, the actual antenna may include stuff that most people do not normally include in what they call "the antenna". And the balance may or may not have geometric symmetry.

This stuff is many, such as, with a handheld HT, ones hand, arm, and body. Or in a typical HF setup, the feedline, mast, guy wires, radio case, transmitter power feed, ground lines, computer network cables, household wiring, gutters, metal fences, trees, salt water/moisture, and etc. All of which can end up with accelerated current flow from the transmitted RF field or feed, and thus affect the RF radiation pattern, even if completely disconnected, but still reasonably close to the RF near-field (such as with electrically isolated Yagi-Uda director and reflector elements).

In the case of a VHF HT, the operator's body being part of the antenna system may actually be an advantage. Same with the feedline when operating an end-fed antenna SOTA QRP. But in the case of HF at higher power levels, touching the case of a transmitter (etc.) connected to an unbalanced feedline located at an SWR peak may cause unpleasant surprises.

For safety, an operator might want to minimize these unpleasant surprises in those parts of the antenna that are not called "the antenna". For communication efficiency, one might not want those parts of the antenna other than what one calls the antenna to distort the expected radiation pattern and aim it in useless directions, such as directly into the dirt, or sideways from the desired target.

Radials, or a solid ground plane, are one mechanism to aid in reflecting the antenna's RF pattern back up out of the dirt, and more into more useful directions. But even directly into the dirt might be more pleasant (and safe!) than though your body into the dirt. Thus safety grounds.

But sometimes, grounding a portion of the (named) antenna just makes the antenna modeling (completely correct math with mostly bogus assumptions) easier.