FlightAware Discussions

19.5cm vs 66cm 1090 Antenna

Actually the lobes are symmetrical now when they weren’t before.

Can you do the original design again and make sure you select “Far Field” ?
There is an entry in the top bar for it in the “Pattern” window.

I ALWAYS select far field when generating simulation.
All the simulations I have posted are with “far field” setting.


Pattern generated by selecting “Far Field”


Pattern Generated by selecting setting “Near Field”




(1) The unsymetry is not severe.

(2) The unsymetry is caused by the stub as there is an unbalanced 6mm bottom part of U.

(3) Please see my last post. I have done simulation with Horizontal U, and unsymytry is still there.

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@abcd567 , looking at the 20.2 SWR, is it possible that would improve by using -45 angle radials in the model ? I have wire on the workbench today, I just may try one of these designs for grins … :man_mechanic:

I just need to push aside a few dozen previous bits and pieces of “fine craftsmanship” to make room to work… :rofl:


  • When radials, instead of being horizontal, are bent down 45°, the SWR improves from 20 to 18. Not enough improvement :sob:

  • Don’t get too excited by simulation results. When model is built, the performance may be much less than what is shown in simulation. The reason is accuracy. For 2 meters (144 Mhz) or 4 meters (70 Mhz) antenna, few mm +/- wont matter. For 0.3 meters (1090 Mhz) antenna, few mm +/- means disaster.

Click on image to see larger size

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Any one interested to run simulation himself/herself, can download my model files from DropBox and run in software 4NEC2.

Monopole with Coil and Ground Plane.nec

Super J-Pole with Coil.nec

Download 4NEC2 Software (free of cost):

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Another variant Lower Radiator reduced from 137 mm to 60 mm

SWEEP- lower radiator length vs SWR & Gain

Monopole with Coil - Sweep Lower vs SWR

Monopole with Coil - Sweep Lower vs Gain

SIMULATION: Lower Radiator = 60 mm

SWR = 1.69
Gain = 4.98

Click on image to see larger size

You removed the feed match but appear to have replaced it with a ground plane. Could you explain the design change please.

Also, as it is the end of a half wave radiator in free space (ignoring the phase coil and second half wave) you would expect the impedance to be quite high.

The purpose of the impedance matching network is to transform the actual impedance to match the feed line impedance.



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As the RF feed is on 2 wires, not on 1 wire, the other wire has to be connected somewhere. Ground plane is where 2nd wire is connected.

If the 2nd wire is connected to natural ground, then antenna being in free space, the single wire running from receiver up to antenna will itself become a radiator.

Taking RF feed on ladder line/twin wire/coax upto antenna requires a point where to terminate 2nd wire or shield, and that point is provided by the ground plane.

The purpose of removal of stub was to highlight its effect on radiation pattern/lobes. I knew very well that the impedance will be very high, but it was the lobes, and not the impedance, which was the object of this excersise.

True, and this is a well known and well utilised fact.

The 1/4 wavelength stubs are mainly used for two purposes:

  • Impedance matching.
  • phasing shifting, so that currents/voltages in two adjuscent 1/2 wavelength sections (connected through stub, and not directly) are in phase and do not cancel each other.
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I’m afraid this doesn’t clarify why you are including the feed and the impedance match in the antenna model.

If the feed and the match are affecting the performance of the antenna then you should reconsider your design.

If the feed and the match do not affect the antenna performance then I suspect that your modelling is seeing them as radiating elements and is skewing the output.


This is the requirement of simulation software. Without including the feed, the simulation fails.

Simulation of Super J-Pole with stub removed

Because in reality you always will have those two connected to a functional antenna.

Looking to fix one of the FA 66cm ones to a round tubular mast - the supplied U bolts don’t do a great job as there is only 1 saddle for each bolt. (So the figure 8 of the antenna/mast is clamped with circular/oval clamps.)

Anyone have some good suggestions how to fix the antenna to the mast?

Can you post a picture - I found the supplied clamp worked perfectly well with a round mast.

Agreed- my FA antenna is on a stand-off @
45 feet AGL on my tower and hasn’t moved in the 1.5 years it’s been there; despite strong winds, ice, snow, rain and temperatures -30c to +30c. The supplied mount hardware works fine