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- Central Mass. Amateur Radio Club
- Greg Algieri
- WA1JXR
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- What is antenna modeling?
- Antenna Modeling is a computer simulation of a antenna.
- Computer simulation allows the antenna performance to be analyzed and
parameters changed to see how they effect antenna performance
- What Parameters do we look at?
- Antenna Gain, input impedance, VSWR, Far Field Plots, 3-Dim. Plots,
Azimuth Plots, Elevation Plots
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- How Does it work?
- Antenna modeling software uses the NEC formula
- NEC stands for “Numerical Electromagnetic Code”
- NEC was originally developed by Lawrence Livermore National Laboratory
at University of California
- It was designed to operate on Large Main-Frame Computers
- MiniNEC was then developed to operate on PC’s
- There are a few different versions out there.
- EZNEC - www.eznec.com
- NEC-Win Plus+ - www.nittany-scientific.com
- Antenna Model - www.antennamodel.com
- This Presentation will use EZNEC
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- What parameters can we control in the model?
- Physical Layout of the Antenna - Dipole, Yagi, Quad etc.
- Frequency
- Height above ground
- Conductor (Wire) type and loss
- Ground type and loss
- Sources (transmitter)
- Loads
- Transmission lines
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- What is a isotropic radiator?
Light bulb - no shade
- Point Source Radiator - Mathematical Source
- Conservation of Energy Applies
- Your Antenna is a passive device
- It does not generate any power to create gain flashlight
- Antenna Gain is just directing the power available from the isotropic
radiator to a particular direction.
- Antenna gain
- Referenced to isotropic radiator - Gain ref. to dBi
- Referenced to Dipole radiator - Gain ref. to dBd
- Gain of dipole referenced to isotropic is 2.15 dB
- Show in Dipole1.ez plots and how point source looks
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- Ground effects
- Ground is like a mirror to the RF energy
- Ground losses absorb some of the energy
- Antenna height above ground effects antenna radiation
- This is where the term radiation resistance comes from
- Add in Ground and talk about Gain /Losses
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- Antenna Patterns
- 3 - Dimensional
- Plot in the X-Y-Z plane of overall radiation
- Elevation plots
- This is a plot looking from the side of the antenna
- This is how you determine antenna take off angle
- Azimuth plots
- This is a plot looking from above the antenna
- This is how you determine antenna directional gain
- This is where you determine front-back gain
- Open G5RV no feedline.ez and
show plots
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- Analysis of antenna impedance and SWR as a function of frequency
- EZNEC does allow you to analyze the impedance of the antenna and
therefore the SWR
- Remember SWR is not an indication on how well your Antenna will work.
- You can put your dummy load high in the air at the end of your
transmission line and you will get a great SWR but it will not be a
very good antenna.
- EZNEC models transmission lines but they are “ideal” (lossless)
transmission lines.
- Open G5RV no feedline.ez and show Impedance/SWR plots
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- Therefore, in order to evaluate the antenna we must look at the overall
“System” gain and losses.
- One way to do this is to use EZNEC to model the antenna calculate the
Gain and Impedance at the feed point.
- Then use TLW (Transmission Line for Windows) program (comes with ARRL
Antenna Handbook) to calculate the transmission line loss.
- Total up the Gain and Loss figures and you have overall “System”
performance.
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- Let us compare a couple Antennas using EZNEC and TLW and evaluate
“System” performance.
- The infamous “G5RV” and “CF Zepp” Antennas
- The G5RV Antenna
- 3/2 Wavelength Antenna (102ft.) for 14.2 Mhz.
- 1/2 Wavelength Transmission Line (32 ft. 0.92 Vf)
- Center Feed Zepp. Antenna
- 1/2 wavelength of wire at the lowest frequency in use
- Put up as much wire as your lot will allow
- Feed it in the center with balanced transmission line
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- Loss in Transmission Lines
- Matched Line Loss
- Transmission Line terminated in it’s Characteristic Impedance Zo
- Matched Line Loss - Loss due to material used (heat)
- Added Loss due to Reflections (SWR) on the line
- Transmission Line terminated in a Impedance that is other than the
Characteristic Impedance
- Added Loss due to multiple reflections.
- Total loss in a Transmission Line is the Sum of these two losses
- TLW calculates this for you.
- Open TLW and show some of the features an loss calculations
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- What you should take away from this talk
- Antenna modeling is a easy way to experiment with antenna performance
- It is a approximation made with certain assumptions and does not
replace actual antenna pattern measurements, but gives a good idea of
performance
- Low SWR does not mean you have a good antenna
- The G5RV is a good 20m antenna, and a compromise on the other bands
- Keep the coax feed line length as short as possible
- Use the lowest loss coax you can afford
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- JXR’s advise on HF Multiband antennas:
- Put up as much wire as your property will allow
- Feed it in the center with balanced transmission line
- If getting balanced line into the shack is a problem, then you can use
a balun and a very short run of coax.
- Or two coax lines taped together with the shield open at the balanced
line end, and the two shields tied together and to the tuner ground at
the other can give you a shielded balanced line.
- This is the most efficient multi-band antenna you can find
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- Question and Answers
- Do you have Questions on Antenna Modeling?
- Questions on your Antenna System at home?
- Questions on “What type of Antenna should I put up”
- Open Forum for Questions.
- Ask now or during the Coffee Break
- THANK YOU For your Attention
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Notes
