Ground plane questions


I just observed a phenom that I cannot explain, so I thought I’d ask here as my reading hasn’t come up with a clear answer.

I understand that for the coke-tennas and other cantennas of differing wavelengths that the can becomes ( emulates ) the ground plane in much the same way as the 4 radials of a spider antenna. I believe we are saying that it forms a surface that reflects RF signals, rather than an electrical ground, per se.

Once could have a flat plate, a car roof or whatever also serve as a ground plane. Where it gets fuzzy for me is the antennas that are wire or collinear, etc.- I have seen comments that they do not require a ground plane. I imagine that people mean that the real earth ground plane is employed. OK, so absent a local metal reflective surface, does the chassis ground and real earth become the ground plane ? And if you are on a laptop on battery ?

I don’t see how they would perform very well with a flat radiation pattern without a ground plane effect. In a sense, its always there.

I suppose my other questions are whether the size and direction of the ground plane affect RF, distance below antenna, whether the antenna models have a ground plane assumption ( eg perfect )


Holy smokes…I suspect we would need a book to cover all that.:grin:

To make it absurdly simple, if the radiator is a quarter wavelength, you need the other quarter to make the antenna efficient. In the quoted case, a plane is formed by either a sheet of metal or radials. To be efficient to the max, the plane radius should be as close as possible to the quarter wavelength or multiples. If using radials, I think you need a minimum of 4 per band/frequency, and there are improvements up to 16 radials. After that the ‘law of diminishing returns’ kicks in.

This is why, when it comes to antennas, one explanation does not fit all. A Quick Spider is one type of antenna, the Co-co, J-Pole, Yagi, Cubical Quad, loop, e-probe, random wire, etc. require different analyses.

Receive only antennas are more forgiving, if one does not follow all the ‘rules’. I’m ‘speaking’ in the context of antennas as radiators, that is, transmit antennas.

Bottom line is, the antenna must not only be impedance matched but efficient. A dummy load is a perfect match, but not efficient, unless one wants to produce heat.

Suggest you get a copy of the ARRL Antenna Book. It’ll be money very well spent. It does not need to be the latest edition. Buy a used copy, antenna theory has not changed recently.:grinning:


The are normally called a sleeve or flower pot antenna.


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So I’ll share that my observed results were that a 1/4 wave spider ( previously featured here, using a 3/4 inch copper pipe cap as a base), Flight Aware antenna and my Frankenstein 8 element Coco at the same location all achieved more or less the same results at the same location, plus or minus say 5%.( My Coco was towards the higher end. The FA and Spider were closely matched)

Not what I expected. At. All. My primary test site reveals very different reception results for each of these antennas… I bought a new FA antenna intending to replace the Coco with a known performance antenna and continue testing the Coco elsewhere.

Talk about a snickers bar in the punch-bowl. :open_mouth:

I’ve seen some other odd things with ground proximity in some testing, it seems to make a difference.

At this site I am mounting antennas 3 feet above a raised wood deck ( perhaps 10ft agl) , which has an 8ft x 12ft tin sheet roof right underneath to keep rain off of the area below. My assumption is that this tin roof plane is somehow improving/ modifying the reception of all of them.

My wife thoughtfully suggests I stop paying attention to RF…


@Dxista I will read the book, I promise.


That can be a wonderful journey. I’m not that ‘academic’. Once it’s working, I leave the electrons alone.:joy:


Here is result of my effort to calculate the precise & correct length of Coco element:


classic :sunglasses:


In Brazilian Portuguese, coco can mean something else. On second thought, it’s a perfect description for the antenna as well. If the length is not precise & correct, it causes constipation.:rofl::rofl::rofl:


Having the dimensions correct is one thing (ie quarter-wave section) and having the impedance correct is another thing (ie 50ohms or 75 ohms). The nice thing about the spider antenna is that the impedance is adjustable by changing the angle of the legs. A large part of the antenna efficiency is from matching the impedance. There is a post by abcd567 with the impedance vs angle of the legs.

Some antenna don’t need a ground plane because they can handle the return current better than others. There is a lot of information about antenna return currents and ground planes. This is where I would start if you want more answers.

As with everything there is a lot of things to know if you want to know more how it is actually works. Getting something to work is much easier with trial and error. This is that how evolution works :stuck_out_tongue: … right?


The dimensions have to do with the signal capturing ability of the antenna.
The impedance has to do with how much of the energy is transferred from the antenna to the coax.


I found one of the discussion about angle of the legs and impedance. There are probably more if you want to look.


My following post is from thread QUICK SPIDER - No Soldering, No Connector. During conversion of forum format to Discourse, the images in this post got knocked out (converted to links to images, instead of displaying images). I cannot fix it as editing of post older than a month is blocked in this new Concurse .

I am now re-posting it here with images restored


I thought of building a few experiments over the course of the next two weeks. One is building a ground plane tuned for the 131.55 range for ACARS.