FlightAware Discussions

Why Antennas Have Ground Plane?


A Simple Experiment to Demonstrate How Ground Plane Radials Affect the Performance of An Antenna

A 1/4 Wavelength Monopole Whip was tested under following conditions:
(1) Only Whip, no ground plane radials.
(2) Whip with horizontal ground plane radials.
(3) Whip with 45 degrees slanting ground plane radials.

The attached performance graphs show the importance & affect of ground plane radials.

Image 1 of 5
Device Under Test: 1/4 Wavelength Monopole, without & with radials

Image 2 of 5
Three Test Setups

Images 3, 4, & 5 of 5
Performance Graphs

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Wouldn’t it be better to make parallel tests? Just because, for example, at my receiver there is usually high traffic time after 20:00… Like around 12:00… I would be interested in the difference at the same time, if possible.

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You are right, but this will bring in differences of 3 hardware systems used. Here the hardware, software, location every thing is constant except the radials, the only variable.

The other variable is of cource the traffic. The entire test was within an hour, and a period where message rate was fairly constant.

During the test, I kept an eye on graphs of another system, whose Antenna is just 1.5m away horizontally from antenna under test, and at same height. Its graphs showed fairly constant traffic during the period of test.


Here is the Message Rate graph for the bench mark/reference antenna which I used for monitoring traffic during the test period.
Unfortunately I did not save the hourly graph at that time, and now hourly graph shows current hour. Other graphs posted were hourly, and an hourly graph of reference antenna would have given a better visual comparison.

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Very nice demonstration! Applause!

Bonus points –

Performance difference between odd and even multiples of a quarter wavelength? Performance v. ground plane angle?

You’ve already done multi-legged versions, from the traditional 4 to 8.

Is there a significant difference in performance between 1/4 wavelength ground plane and 3/4 wavelength ground plane? At what point does increasing the ground plane size result in a less than significant performance increase?

A very good (and understandable) writeup on ground plane antennas is in the Radio Society of Great Britain’s (RSGB) VHF/UHF Manual, by Jessop – it’s a classic, and has oodles of good stuff for folks doing ADS-B work.

bob k6rtm

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Thanks Bob for good suggestions for further experimenting.
Thanks also for referring to the classic hand book.

Based on your reference to the book, I searched and found download link for the classic “VHF/UHF Manual, by Jessop”.
Here are the links if someone wants to read it.

Pdf version:

Kindle version:



Are your radials physically connected (contacting) the outer braid of the coax or simply zip tied to the outer skin of the PVC jacket?

I can’t really decide from the pics, but it ‘looks like’ they are simply strapped to the outer jacket, in which case the radials are only capacitively coupled to the braid and the impedance will not be the same as if they were physically connected to the braid.
I’m not sure how much of a detrimental effect this may have on receive performance, but it’s not ‘conventional’ !
I wonder if you would get an improved reception if the whip length were increase to 1/2 or 5/8 wavelength in such a configuration?
Might be interesting to solder an extra length to the whip and observe…


The radials are pushed in to contact the shield, direct electrical contact.

Yes, different lengths and different angles, and different numbers of radials matter, up to a point.

Quarter wave and bent down around 45 degrees is a good starting point.

That’s one of the great things about simple antennas – easy to build and experiment with!

(n.b. - the so-called co-co, particularly 6 or 8 sections, is not a simple antenna; at these frequencies, lengths in terms of mm are important.)

bob k6rtm


All the 4 radials were inserted between the outer PVC jacket and the braid of the coax. The zip ties were used to secure the radials in position, as these have a tendency to move and change 90 degree angle with each other.

After completion, I checked the continuity between the body of F-connector at other end of coax, and each of 4 radials by a multitester. All 4 were found electrically continuous, as the multitester beeped on each radial.

The radials were inserted in the same way as a center wires of a Coco are inserted between the PVC jacket and braid of adjacent elements.

Here is a larger size photo of the monopole with radials inserted and zip-tied.


OK, the larger image shows your construction much better as in the smaller original it seemed as though the radials were on the outside because of the one that punctured the skin.

I have read George Jessops VHF-UHF manual way back in the day and we still have a copy of it at our local radio club.
Interesting to see the original price of 21 shillings (1.1 GBP (1.5 USD) in today’s money).

I’m not a fan of the co-co either as it is difficult to get right, whereas the 1/4 wave ground plane is entirely predictable and so simple to build.

Nigel G4ZAL


I have optimized the dimensions of several types of collinears using simulation softwares 4nec2.

Using the optimized dimensions, I fabricated these collinears, and put to trial run. ALL BUILDS (MORE THAN A DOZEN) PROVED HOPELESS, much inferior to a 1/4 quarterwave groundplane antenna (Cantenna/Spider).

These designs included coaxial collinear (coco), wire collinears with coils between sections, wire collinears with stubs between sections, super Jpole etc. For ALL these designs, the simulation software gave optimized dimensions which should give a gain between 3.5dBi and 5.5 dBi and SWR between 1.5 and 1.1, but trial runs proved these to be completely wrong. Why? There is an error of about 5% to 10% in simulations output. This error is too much for a collinear, which is very intolerant to dimensional errors, and knocks it off far from tuning point if errors are as high as 5% to 10%.

Although the simulation software are not suitable to design a collinear for constrution purposes, these do give a very good picture of the performance of a design for understanding the general behaviour. I have therefore run a simulation, in which the 4nec2 software has varied the length of radials in steps of 1mm from 30mm to 285mm, and plotted results in the form of a graph.

The conclusion is that:
SWR: 1/4 & 3/4 wavelength radials give best SWR (1.91 & 1.65), 1/2 wavelength radials give worst SWR (13).
GAIN: 1/4 wavelength radials give better Gain (1.2 dBi) than 3/4 wavelength radials Gain (0.52 dBi). The 1/2 wavelength radial gives very poor gain (-12 dBi).

**These simulation results needs to be verified by trial runs. **

I plan to put on trial run a 1/4 monopole like the one in first post, but with 3/4 wavelength (203 mm) radials, run for 20 minutes, then trim the radials to 1/2 wavelength (138 mm), run again for 20 minutes, then again trim the radials to 1/4 wavelength (69mm) and run for another 20 minutes, and compare performance on performance graphs.

Will repeat above trial run, but this time with slanting (45 degrees) radials.

… Will keep me busy next weekend.


I suspect the polarity of the incoming signal makes a difference too. Also, just thinking about the incoming wave trains, I wouldn’t be surprised of horizontal radials work better when longer than 1/4 wave, and angled ones work better at 1/4 wave, for signals coming in horizontally, anyway. I could be wrong, but that seems intuitively to make sense to me.


Measurement of SWR of a 1/4 wavelength ground-plane antenna, with Horizontal, Slanting, and Vertical Radials

Photo 1 of 3 :
Horizontal Radials, SWR=1.93

Photo 2 of 3 :
Slanting (45 deg) Radials, SWR=1.343

Photo 3 of 3 :
Vertical Radials, SWR=2.174

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abcd567, when you get a chance try that with a cantenna.


I have done SWR measurement for Cantenna as well when I did for Spider (January 2017). Here is result for Cantenna.

Measurement of SWR of a Cantenna

Measured SWR = 1.463

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I used a Bushes beans can with a stainless steel whip and works great. I’ve seen your antenna projects on the web and that is what I based my project on.


Compared to Collinear antennas, both the Spider and the Cantenna are dimensionally very tolerant, and have a good SWR for 50 ohm & 75 ohm systems without need for tuning/trimming. This makes them ideal for a DIY novice.

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It will be nice if you post a photo.

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I know it is not pretty but they function.
CanTenna is on the left its hooked up to my other ADSB set up running on windows. The middle antenna is FlightAware’s ADSB antenna. The one on the right is a generic amateur radio 6 meter, 2 meter and 70 cm antenna I had. I use it for ACARS. I haven’t been active in ham radio for a long time so I use it instead for my ACARS.


Great! You have mounted antennas high, and clear of obstructions. This brings the biggest improvement in reception. Pretty or not does not matter.

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