Re: yagi's, yogis and other methods of twisting things (was: Re: How to calculate increase of home wireless router range?)
- From: Jeff Liebermann <jeffl@xxxxxxxxxxxxxxxxxxxxxx>
- Date: Tue, 04 Jul 2006 19:30:49 -0700
"Wolfgang S. Rupprecht"
<wolfgang+gnus20060704T143125@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> hath
wroth:
Jeff Liebermann <jeffl@xxxxxxxxxxxxxxxxxxxxxx> writes:
Type Gain Cost Cost/dB
dBi
dish 15 $35 $2.3
yagi 15 $65 $4.3
panel 19 $68 $3.8
omni 12 $70 $5.8
A while ago I posted about an open (non-radome enclosed) yagi that
claimed 15dbi for ~$30
(http://www.mfjenterprises.com/products.php?prodid=MFJ-1800).
In ham radio circles, MFJ is not known for quality products. That
antenna might be suitable for temporary installation, but the exposed
copper driven element and unprotected connector is just asking for
problems outdoors. Also, I made no attempt to price shop or optimize
the cost/dB ratio. Interestingly, at one time, the yagi was the most
expensive, while the omni was the cheapest. I can do it again using
approximately the same gain antennas, which should produce more useful
results, but I was just hoping that the OP would do her own
calculations based on prospective antennas.
Well as
you mentioned elsewhere "most manufacturers lie, but thats ok, nobody
calculates", I'm coming to the conclusion that they related the gain
of this yagi to a reference bag of male bovine excrement. Comparing
it to a 15dbi bbq-type dish, it is 5dbi lower in signal strength.
Call it a 10dbi IFF the bbq is rated well.
Well, the maximum gain of a dish is very easy to calculate.
http://en.wikipedia.org/wiki/Microwave_antenna
Gain(dBi) = Pi^2 * Dia^2 / wavelength^2 * feed-efficiency
where Dia and wavelength are in the same units and efficiency is a
function of feed illumination accuracy and is no more than 50% at best
for typical small dish antennas.
Climbing onto my roof and trying the numbers on a PacWireless 24dBi
dish:
Gain = 9.87 * 68cm^2 / 12.5cm^2 * 0.50
Gain = 9.87 * 4624 / 156.25 * 0.5 = 146 = 21.6dBi
Close enough to 24dBi.
The gain of a yagi is approximately the gain of a dipole, 1.66 scalar
(or 2.2 dBi) times the number of elements (including the driven
element). So, the MFJ yagi should have a gain of:
1.66 * 16 = 14.2dBi
Close enough to 15dBi.
Basically, if the antennas were built perfectly, they would be
sufficiently close to theory to make the advertising correct. However,
this is rarely the case. Manufacturing tolerances, surface coating
inefficiency, connector losses, sloppy construction, VSWR losses,
boresight errors, effects of the mounting hardware, local reflections,
and sloppy installations all conspire to reduce the gain below the
maximum. Your mileage and gain may vary.
(Is there a department of the government thats the slightest bit
interested in chasing fraud like this?
It's not fraud. You might try the better biz burro or Federal Trade
Commish. I doubt that they care.
If the gas pumps were
calibrated 5dbi short you can bet that the establishment would be shut
down pretty damn fast.)
It's not 5dB short. There are plenty of other abominations in the
antenna business. I've often considered going into the commercial
antenna business because few people can understand how they work, make
valid comparisons, and have the necessary test equipment. Perfect for
an evil exploiter like me. Anyway, I can do without hand drawn
antenna patterns, unrealistic free space antenna patterns,
non-standard scales on antenna patterns, claims of range improvement
(compared to what?), forgetting about coax cable and pigtail losses,
ignoring bandwidth issues, failing to measure up-tilt on omnis, crappy
mounts, use of easily corroded materials for unsealed outdoor
antennas, etc. It's not really distorting the numbers as much as
conveniently forgetting to mention some important items.
--
Jeff Liebermann jeffl@xxxxxxxxxxxxxxxxxxxxxx
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
.
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