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Airband whip antenna design part 1: theory and basic models

Introduction

A few years ago I bought Uniden UBC92XLT scanner, with Uniden AT218 0481001 Bearcat antenna. The default antenna is a wideband,12cm long helical one and on the base of my experiences it has very poor performance in range of air frequencies, so quite quickly I’ve bought a new one for the band closest to airband: 2m/70cm – it has much better performance, usually it works fine enough, but finally I’ve decided to bulid my own, airband dedicated antenna.

The requirements for antenna which shall be met:

  1. Good performance at airband – from 118 to 136MHz (the full airband range is 108 – 136MHz, but I’m not particularly interested in listening to VOR or ILS transmitters)
  2. Omnidirectional horizontal characteristic
  3. Antenna parameters independent of the conditions like: type of ground, haight over ground, connection type (directly to the scanner or to coaxial cable)
  4. Be handy and mobile, attached directly to the scanner – so no ¼ GPs, dipoles, loop antennas etc.
  5. Robust and flexible
  6. As simple as possible and relatively cheap

There are many sources about whip antennas theory, but in almost all cases they describe perfect case when whip is ¼ (sometimes ⅝) wavelength long, standing on perfect ground, so such antenna has very nice gain approx. 5dbi and quite decent 35 ohm impedance. In real world whip antenna is connected to the scanner held over real ground and its own ground plane is reduced to ground of PCB inside. I had thought, that there had to be some differences, due to my very small experience in antennas division, lack of time and equipment for testing I’ve decided to play with some antenna simulation software to understand better design of antennas. I’ve chosen Arie Voors’ 4nec2, because, first off all, it is totally free software, moreover quite easy to use, with good documentation and a lot of examples, so I should be far sufficient for my needs.
Last but not least notice in introduction to : as I mentioned above despite of being electronics engineer my knowledge and experience in antennas design and modeling in 4nec2 are small, so if I’ve made some mistake please don’t hesitate to correct me, any critical feedback is highly welcome.

1/4 lambda monopole model with perfect ground plane

First of all I’ve created a model of almost perfect ¼ wave antenna placed on perfect ground to check if theory and simulation results for my model are the same. Results show pretty decent correlation between theory and simulated model:

Radiation pattern

Perfect 1/4 monopole radiation pattern

Perfect 1/4 lambda monopole VSWR figure in range from 100 to 150MHz

SWR has smallest value for 127MHz (middle of airband) and as expected from theory I’ve read, for lower frequencies than resonant frequency antenna had resistive in capacitive component, while for higher frequencies resistive + inductive component.

Input impedance for perfect 1/4 lambda monopole antenna

1/4 lambda monopole model with real ground plane

In next step I have changed ground settings from “perfect” to “real-good” in 4nec2 and the results were different:

The radiation pattern – maximal antenna gain is -4dBi comparing to 5.19dBi in previous case and max gain angle is 66 degrees (was 90), input impedance also distinctly increased to value 89-j391 ohms

Radiation pattern and input impedance at 127MHz for 1/4 antenna placed on real groundon real ground

Frequency sweep from 50 to 200MHz revealed, that the saddle on VSWR figure has been shifted to higher frequencies and for 127MHz VSWR=36.6, and also that within range of airband antenna is seen by receiver as capacitance.

VSWR for antenna placed on real ground

Input impedance for antenna placed on real ground

1/4 lambda monopole model 10 cm above real ground plane

When antenna is placed even a little bit above ground (it is not connected) things are getting even worse:

The gain is a little bit better: -3dBi, but impedance is 24 – j3195 ohms at 127MHz, so VSWR for 50 ohm scanner input equals 8559! Capacitive component of impedance varies from -j3462 (118MHz) ohm to -j2936 (136MHz)

Of course in real world antenna is attached to scanner or feed line, so I’ve modeled scanner by adding simple 10×5 cm rectangle frame, it could be done much better, but at this point such simple model is enough.

4nec2 model of whip antenna attached to radio frequency scanner – the rectangle on bottom emulates ground on scanner PCB

In such configuration antenna works as asymmetrical dipole

Radiation pattern as well as input impedance varies with the height above the ground

Variation of radiation pattern for whip antenna attached to scanner vs. height of scanner above the ground

VSWR and input impedance of antenna vs. its height above ground

Yet another interesting dependency of input impedance for alone antenna and antenna attached to the scanner:

The biggest VSWR variation is visible, when antenna height is from 1 to 60 cm (quater length of 127MHz wave) , then the parameters are getting stable on levels approx.: VSWR = 20 000 Zin = 11 – j3000 ohms

Even relatively smal ground plane as modeled for Uniden scanner attached directly to antenna makes, that input impedance and hence VSWR is much smaller

The characteristics for this case are similar to previous ones – VSWR and Zin varies much for first 60 cm, then it is quite stable, the main difference are the much lower values: VSWR= 112 , Zin = 31 – j413 ohms

A few conclusions after analysis of simulation results:

  • a big mismatch between theoretical, perfect case vs. real world case
  • capacitive reactance of antenna is dominant on the whole with of airband
  • even small ground plane attached to antenna significantly reduces input impedance hence VSWR
  • antenna matching will be necessary

At the end of part 1, 4nec2 model for modeling antenna with scanner used during simulations:

CM 
CE
SY antH=0.1 'antenna height over the ground
SY antL=0.5647 'antenna lenght
SY antR=1e-3 'antenna radius
SY uniH=0.1 'scanner height
SY uniW=0.05 'sanner width
GW 1 41 0 0 antH+uniH 0 0 antH+antL+uniH antR
GW 2 1 0 0 antH+uniH uniW 0 antH+uniH antR
GW 2 1 0 0 antH uniW 0 antH antR
GW 2 3 uniW 0 antH+uniH uniW 0 antH antR
GW 2 3 0 0 antH+uniH 0 0 antH antR
GE 1
GN 2 0 0 0 17 0.015
EK
EX 0 0 1 0 1 0 0
FR 0 0 0 0 127 0
EN

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