Greetings,
I'm a bit stuck on a problem involving the interpretation of various radiation patterns that are depicted in antenna spec sheets. I get the general orientations (XY, YZ, XZ, or, alternately, Azimuth, Elevation (phi=0), Elevation (phi=90)), but the polarities are throwing me.
A typical spec sheet lists two curves on each of the three orientations, one for horizontal polarity and the other for vertical. I believe that the main determining factor for the polarity is the orientation of the longest element in the antenna (assuming linear polarity to start with).
So, if my assumption above is true, why does the XY-V cut not look like the XZ-H cut? Is it due to the position of and reflections from earth ground?
Thanks for any help in getting my head straight on this,
Mike
Thanks for the reply.
I guess my main problem is that all of the spec sheets I've been reading don't state the initial assumptions of horizontal vs. vertical polarization. They show the cuts with regards to the antenna, so I assume that the "default" position is horizontal and that rotating the antenna 90° with respect to the ground is the vertical one (like if default were in XY plane, parallel to ground, the "vertical" measurement would be with the antenna rotated into the XZ plane).
Anyway, I've pretty much convinced myself that the big rocky ball underfoot is the culprit in the variations I was worried about. Again, thanks for the help.
I've got an unlisted (i.e. non-Internet) spec sheet from Fractus with no orientation.
There's this one from Johanson Technology with no real indication of H vs V.
There's **broken link removed** from Furukawa. It's not a full spec sheet, but the reference for H/V isn't obvious (to me, at least).
Some others: 1 , 2 , 3 , 4.
Finally! Here's **broken link removed** with the HV orientations listed, but I'm still having trouble interpreting it.
I just had a thought though. Is it possible the H&V listings are for the polarization of the TX antenna (assuming, for the sake of argument, we're taking measurements on the RX side)? Thinking about it, that seems fairly likely since I hadn't considered the "other side" of the link at all. And the TX polarity would have a significant effect on the reception.
Anyway, again, thanks for the help,
Mike
Heres one more question/double check:
antennas are usually referenced to a dipole antenna or isotropic radiator.
I have computed the gain of a half-wave dipole to be 1.61-1.64 dB.
If I build an antenna, and it has a gain of 4dB :
1: if I reference it to dipole which is 1.61 dB. is the gain 4 dBd or 5.61 dBd or 2.39 dBd? some normalize the gain of a dipole to 0 dB and then
2: if i want it in dBi, i have to add 2.14 dB to it. so my antenna would have a gain of 6.14 dBi? if its gain =4dBd?
Its d@mn confusing and i already talked to a fellow engineer, and he gave me some bull$hit. i read books, and white papers and they bull$hit.
I want a straight concise answer with a few examples. also a why and how about why we would want to reference to a dipole and iso would be great but not required.
Thanks.
Now i have a question. i already know the answer, but this engineer i talked to seems like he doesnt know wtf he's talking about. or at least i haven't heard of it in school and the industry.
We know radiation patterns can tell us the relative field strength or power in some direction, xy, yz, zx or theta or phi. the pattern can be normalized, which it usually is, and it can be linear or it can be log based.
what am i getting at?
This one engineer told me, this one chip antenna is referenced to an isotropic antenna. fine, so its gain is below 0 dBm. so its lossy. so the company gives a picture of the orientation and then diagrams of the pattern ploats at different cuts.
However, he said the pattern on this one plot is a plot of constant gain around the antenna. I don't understand, i thought radiation patterns were plots of field strength around the antenna. Depending on the antenna type, its response will be stronger or weaker in certain directions.
I think it has to do with the surface charge density since, an area that has more surface charge induced on it, will have greater electric field strength. Also, the polarity(+/-) of the lobes dictate how and where lobes might form-theory. The impedance along some point on the surface also would have an affect. i have to read some more.
but anyways, he started blabbing on that to get the circle or so of constant gain, you have to move non-AUT antenna farther or closer in. i believe that, but thats not how tests are done right? I have done some work in anechoic chambers, and what they do is have the scan antenna, move in geometric patterns-planar, cylindrical, spherical or starburst.
We do this at a fixed distance, usually some where in the far field at some predicted field strength. and scan the antenna at that fixed distance. we don't move it in, becoz that'll throw off the readings anyways.
for example, let look here
on page 5, it gives the oreitnation and plane cuts of this particular antenna. so lets look at the aziumth, theta =90. although in my EM text books the azimuth is given in phi, but alas. anyways, the plot looks fairly uniform at 0 dBi. at 0 degress, the gain is 0 dBi but at 180 degress, it is a bit greater than -2.5 dBi. and by greater, i mean that a field strength at -1.5 dBi is greater than one at -2.5 dBi, not -3.5 dBi is greater than -2.5 dBi, which some folks get confused about the number line and log values.
so thats how i interpret it. the circle is not at constant gain. so am i reading it right? i just want some clarificatin. yeah thats alot.
Thanks though.
i'll go read JPUG.
Hi Q
I just read this thread and wanted to say something about your post. I too am puzzled about the reference to a constant gain plot. The term constant gain is often used in work with the Smith Chart, in, for example, the plotting of an amplifier's constant gain circles. In all my years of antenna work, I have not seen a "constant gain" plot of an antenna. In fact, it would seem that plotting constant gain would require 3-dimensional visualisation. Can you provide a copy of that controversial plot for us to interpret?
I also wanted to point out a little error in that you used the term " so its gain is below 0 dBm". This term doesn't make any sense, since gain is never described in units of dBm.
regards
Ron
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