Tweaking of Groundplane (Spider) Antenna

ab cd

Senior Member
It is well known that in a Groundplane antenna, the slope of radials affects its impedance, and hence SWR.

1) If slope = 0 degrees (i.e. radials are horizontal), Antenna Impedance is around 30 ohms
2) If slope = 45 degrees down (i.e. radials are slanting), Antenna Impedance is around 50 ohms
3) If slope = 90 degrees down (i.e. radials are vertical), Antenna Impedance is around 75 ohms

Before the advent of ADS-B, the Groundplane/Spider was mainly used by Hams. Since the defacto standard of ham transmitters & receivers is 50 ohms, the 45 degrees slanting Groundplane/Spider, which has an impedance of 50 ohms, well suited ham systems. Generally other antennas were also designed to have 50 ohm impedance, either inherently, or by use of impedance matching components. The coaxial cables used were also of 50 ohms rating.

When ADS-B receiving started, its equipment (receivers antennas and coax) followed the already established ham standard of 50 ohms impedance. The sites like Flightaware, Flightradar24, Radarbox24, Planefinder also followed the suite and offered 50 ohm equipment.

With introduction of DVB-T for ADS-B, the scenario changed. The DVB-T is designed for TV reception. The defacto standard for TV & Satellite is 75 ohms, hence the DVB-T and RG6 Coax both have impedance of 75 ohms.

Unfortunately, the 50 ohms Groundplane/Spider with 45 degree slanting radials was BLINDLY used for 75 ohms DVB-T & RG6. Logically a 75 ohms Groundplane/Spider (i.e. the one with vertical radials) should suite the DVB-T+RG6 system.

I have done some simulations, which support above arguments. However Antenna making without proper test equipment & technical know how is a "Dark Art". Hence the only way left to know if the simulation results are true, is to make prototypes, and put these on trial run. I intend to do this when I find time, as there are 4 models, and trial runs should be lengthy to be meaningful.

Tweaking Groundplane.PNG

Last edited:
Very useful post ab cd,
Such a popular antenna this one, a good performer and very easy to make.
Interesting as I constructed 2 antennas recently for another Planefinder member Phillip Robinson.
The thread is here

During testing/final commissioning I found that bending the radials to a point close to the vertical gave the very best results.
This seemed to defy what I thought was best for an antenna of this type which is normally around 50- 60 degrees from the horizontal.

"I have done some simulations, which support above arguments. However Antenna making without proper test equipment & technical know how is a "Dark Art""

Indeed, A very Dark Art, Almost completely Black on occasions !
Even if you are armed with the theory .
There are so many variables such as spacing in free air and the properties of that air on a given day on a given site.
A we in adsb are working in the Ghz range, with a 1/4 wavelength at just 69mm, antennas are not very forgiving.
On occasions, micro adjustments can make a good station out of a bad one.

Photo of Phillips antennas after I completed testing attached



  • IMG_20160227_1603276371.jpg
    581.7 KB · Views: 249
  • IMG_20160227_1605058391.jpg
    488.8 KB · Views: 228

Hi Mike!

Thanks for your supportive reply. Although I have done these simulations sometimes back, but did not post these results because I prefered to post my finding after I test these experimentally. I did not find time (or was just lazy) to do this lengthy exercise.

Philips' recent post, followed by your yesterday's Conversation (PM), confirming that the Philips' groundplane/spider with 90 degree bent radials was made by you, and has performed better than 45 degree radials, triggered today's posting of my previous findings.

A conclusion cannot be drawn on only one prototype. When at least 3 prototypes, made by 3 different persons at 3 different locations are tested, then a conclusion can be drawn based on these 3 results.

Thanks for contributing to the never ending quest for the best.

Last edited:
Hi ab cd,
Strangely my actual spider antenna builds for Phillip did not in part agree with the theory ( nothing new there !)
I started in the conventional way by first cutting the radials and the centre driven element to 1/4 lambda (69mm) 69.5mm actual then put the unit in test.
Then I started trimming back until I got to a point where performance started to drop off .
Normally this is a situation akin to an over enthusiastic use of hair clippers, you have gone too far with the clippers and cannot put any hair back !
In this case though fine tuning is quite easy, I removed sleeving to wire & added a blob of solder to the cut wire element/radial and whilst still hot stretched it out sp that an extra 4mm or so could be put back to the electrical length.
Fine tuning adjustments can then be made with a discarded fingernail emery board taking off a little at a time until optimum performance has returned.
Slide over the pvc sheath to the bare wire, adjustments hidden.

I found the best results with radials @ length 69mm measured from the outer ring of the so-239 delrin insulator/ aka the start of the flange giving an electrical length of 69mm the same as the centre element. The flange being part of the groundplane (an integral part of the radial arrangement)
Radials are in fact actual length of 65mm.

I have a picture attached to show what Phillips antennas looked like cut to correct length before bending so show what I mean.

I think perhaps this could be further improved by using a 75 ohm flanged connector to achieve better matching for a 75 ohm unbalanced dongle input.
In theory, the swr would be lower and the antenna would be slightly more efficient especially on weaker adsb signals.
Again , in theory, as we are dealing with microwave signals very small changes and tweaks could mean a station seeing a distant aircraft or not seeing it.

I am glad I got nice performance from this pair built for Phillip, tweaking took a while , but he is very happy with the results


  • IMG_20160227_133007468_HDR1.jpg
    702.9 KB · Views: 263
  • IMG_20160227_1318066751.jpg
    517.3 KB · Views: 260
  • IMG_20160227_1322393021.jpg
    637.7 KB · Views: 241
Last edited:
Hi Mike!
Here is simulation of your Spider.
Simulation shows it has an impedance of 50 ohms. If it gives good results to you, it is likely your system is 50 ohms. Do you use 50 ohms coax?

Groundplane - Radials 10mm Horizontal + 49 mm Vertical.PNG