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Play Time With Antennas
By Marlon Schafer - Owner, Odessa Office Equipment - www.odessaoffice.com
Back in the old days (two or three years ago), when I could actually get all of the day's email and phone calls dealt with in one day, I took a look around my office and noticed that I had quite a collection of antennas laying around. Now I'd always been told that there was a lot of difference in the performance of different types and brands. Front to back ratios, side lobes and such.
Being the curious type that I am I decided to hook up a couple of radios and do some testing to see what all this theoretical stuff meant in the "real" world. I decided to conduct an informal experiment to learn something about how different types of antennas (sector vs. omni, panel vs. grid etc.) performed and gathered up all of the different antenna types (9 in all) and went to work in my shop.
Keep in mind, these results are just seat of the pants stuff. I do NOT have a nice clean RF free room. I DO have file cabinets, metal work benches, desks, and a lot more crap laying than you'd want in a real RF lab. And if it's not apparent by now, please remember that I do NOT have a PhD in RF physics - but I did stay in a Holiday Inn last night.
The following measurements were obtained from my home-brew test set up which I *think* I used a 2 meg Teletronics ap with a 3dB rubber ducky antenna at the "Access Point" (AP) side. I simulated the customer premises equipment (CPE) by placing a Lucent Silver card with an 18" pigtail on the other side of the room.
I first took a look at the panel antennas you'd typically find at a customer site. One of the things that surprised me most was how much difference there was between different models - take a look at the data below to see what I mean:
* * Comtelco 14dB panel antenna
o o -31dB Vertical
o o -37dB cross polarized
o o -46dB 180*
* * Teletronics 18dB panel antenna
o o -26dB Vertical (notice that there's more than the expected 4dB difference) o o -36dB cross polarized (less than the 4dB
expected) o o -48dB 180*
* * Andrew 16dB panel antenna
o o -27dB Vertical (within 1 dB of the 18dB panel, hmmmm) o
o -37dB cross polarized o o -49dB 180*
* * Unknown brand 12dB corner reflector
o o -31dB Vertical
o o -36dB cross polarized
o o -48dB 180*
* * Teletronics 18dB grid
o o -32dB Vertical
o o -41dB cross polarized
o o -48dB 180*
* * Andrew 24dB grid
o o -22dB Vertical
o o -29dB cross polarized
o o -47dB 180*
So what do we learn from that (besides the fact that this kind of testing should be done out in the middle of nowhere)? We learned that not all antennas are created equal.
I'd expected the 18dB panel to have had much better cross polarization isolation than the 14dB unit. Instead they were almost identical. I also expected more than 20dB of front to back out of both of them.
The similarities between the 18dB Teletroncis Panel and the Andrew 16dB panel are hard to explain. They are almost identical in every respect except the physical construction. The Andrew is smaller, has a better mount but is awful to weather seal at the connector (when will the manufacturers ever learn?).
Normally, the lower the gain on an antenna, the less cross polarization and such you'll get. The little 12dB corner reflector was a shock to me on that front. I expected gain in relation to the two panel antennas was right in there, but the back lobe (how much signal leaked right out the back of the antenna) was just as good as the higher gain antennas. And at a lower price too. Something to remember, for sure...
It's amazing to me how much difference there was between the $100 Andrew 24dB grid and the $50 Teletronics 18dB grid. We'd expect an 8dB difference. Not 10dB. I know that's only 2dB different from what we'd expect, but that 2dB is 66% more power! That's twice now, once with a panel and once with a grid. The more expensive ones have higher gain. So what should a person do? Go with the next lower gain antenna and save a few bucks? Or go with the good ones right up front? I've always tried to use the good ones in my system because sometimes even a few dB of margin can be priceless.
We also leaned that things like cross polarization are dependant to one degree or another upon the antenna on the other end. I think that if I'd had a good panel antenna or something like that for both ends we'd have seen more cross polarization isolation. Most of these antennas spec out near 30dB as I recall. I never saw that much, maybe some day I'll have the time to redo part of this test with a better antenna at the AP side.
I also took some measurements on a few antennas you'd typically use on an AP or tower:
* * Comet 10dB with 3* electrical downtilt
o o -37dB Vertical (surprisingly it was -34dB when physically tilted 5* in the direction of the AP!!!) (My notes also say that it had MORE downtilt with the ground guides removed than with them on.) o o -47dB cross polarized
* * Mobile Mark 9
o o -37dB Vertical
o o -34dB cross polarized (BIG difference between the two omni antennas on this one.) o o -34dB signal when tilted 5* toward the AP
* * Teletronics 9.5dB 180* sector
o o -30dB Vertical
o o -34dB cross polarized
o o -44dB 180*
It was interesting to see how the different antennas performed in relation to each other. I was especially surprised to see how much uptilt omni antennas have in their beam patterns. I'd always been told that omni antennas had a natural uptilt to them, but it was fascinating to see that played out right in my shop. This tendency to spray RF upward away from subscribers might explain why some operators not familiar with this phenomenon are running such large amplifiers on many of their towers.
Some day, I'm going to have to do this test again with some of the new antennas I'm using. Maybe - when I have more time - I'll even do a similar experiment but with identically spec'd antennas from different manufacturers. It's a shame that most of the original hardware I used has found its way to various field installations, or otherwise evaporated in the 2+ years since I ran these tests, but even the crude data I collected here should provide some useful comparisons with the new designs that have come on the market.
Whether or not my measurements were accurate to the 10th decimal place, they taught me a whole bunch about what antennas really do. I found these tests to be most helpful and I encourage you to try this at home for your own amusement and enlightenment - with the following advisory:
CAUTION! - Experimentation of this nature can be hazardous to your ignorance.
Until next time,
Marlon K. Schafer - Owner, Odessa Office Equipment email@example.com
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