reading a field pattern graph

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The attached photo is that of a field pattern for a yagi antenna.Result of a graph when running the code from balanis on Yagi_Uda.m. My problem is that I cannot understand the graph that I am having.I understand that it is a plot of E-field agains theta and H-field against phi. But I cannot interprete the result obtained.??

I would also like to know if there is a way to know the directivity, E-plane and H-plane at half-power beamwidth from just those graphs??
The directivity obtained was 8.68 dB and the E-Plane and H-plane was 48.3 and 55.1.:?:
 

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The radiation pattern plot you show only indicates the main lobe width to -3 dB and side lobes reduced according to the scale. Antenna directivity or gain is not included in your plot. When the plot is taken on an antenna range, a "standard-gain" antenna should be tested first or last, to calibrate the complete system from transmitter output to receiver input. Knowing the standard-antenna gain you can then determine the maximum (on-axis) gain of the antenna under test. The plot is then taken as a function of power level vers. azimuth or elevation angle.
In addition to the above, similar tests are made in the cross polarization if needed.
 
Thank you for replying!!from what i can gather, you are saying these for a practical antenna!!here it is a simulated antenna pattern from a matlab code by balanis. I would like to understand the field pattern plot shown for interpretation!! I understand that the field plot uses the poynting vector and the theta and the phi.
 


I do not simulate but rather try experiments. If you make the calculated antenna, you will see the difference. In practice we use the RF power in Watts instead of Poynting vector, and all values in plotted radiation patterns are related in decibels to the peak antenna gain. The gain or directivity value itself is not a part of the plot, it is obtained by comparing the antenna with a standard one having a known gain, like a dipole, etc.
In practice, theta and phi angles correspond to azimuth and elevation angles, and linear polarization is related to the ground plane.
For a good information, check Agilent white papers on antenna testing.
 

Actually i have made three antennas and the boom element is wood!! I am waiting a lab appointment date to test the antennas!!Most probably i will test the antenna by this week.
I have not implemented the impedance matching. I have just connected the driven element to the coax cable of 75 ohm. Do you think it will work??
 

If I understand correct, you writes that you want to measure a impedance unchecked system, feeding a balanced antenna via unbalanced cable, and asks if it will work?
Suppose that you is aware that a lot of radiation maybe radiates from your coaxial cable under these circumstances. How much is depending on amount of mismatch but guess main radiation will be emitted from coaxial cable.
If that is what you want to measure, sure it will work.
 
thanks you have convinced me that the balanced antenna requires a balanced cable. So, what is the easiest way to make the balun using a coax of 75 ohm for a center fed half wave dipole??
 

As I not know your antenna impedance, and what impedance your measurement lab can handle, can I not recommend what you should have in between.
Most measurement instruments are designed for 50 Ohm, and most RF radios with coaxial input expect cable impedance to be 50 Ohm, so that is what I had designed a Yagi-Uda antenna for, or a multiple of that.
A popular and easy to do balun is 1:4 coaxial balun. If you wind your own transformer, can that be done in somewhat tricky ways for improved balance. If winding on a core is it easiest to design for a discrete number of turns, impedance ratio can then be selected 1:1, 1:2, 1:3 ..
It is rather common that commercial Yaga-Uda antennas have a balun integrated with dipole feeding point, which then also is a part of total impedance correction network.
For homemade TX dipoles are gamma-matching a common technique as it is easy to adjust and fine-tune afterwards that the antenna have been built and mounted on an high antenna tower.
 
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Isn't the impedance to be 73 ohm for 1/2 wave dipole??if not this impedance that is to be used then can someone guide me through the procedure to calculate the input impedance??

---------- Post added at 18:16 ---------- Previous post was at 18:11 ----------

BTW after inputting the values for one of the antenna that i built in PCAAD, the impedance i got is 15.5 +j16.1 for a gain fo 12.08dB!! I would really require the method to calculate the impedance as i will have to put this in the dissertation!!
 


for hfss, take r and Xl square add them & substract its square root from ur desierd value.
is it right for PCCAD ?/

thanking you in advance
 

As per my understanding both figures are same. One is polar plot and the other is linear plot. But the figure shows the directivity is poor, because it has a strong back lobe. To improve the directivity of the antenna, the back lobe needs to be minimized.
 

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