On Fri, 20 Nov 2009 13:18:15 -0600, "amdx" wrote:
"Jeff Liebermann" wrote in message
.. .
On Thu, 19 Nov 2009 18:46:59 -0600, "amdx" wrote:
Sure, as soon as I try to make you King you find work for me :-)
I hate to tell you this, but that's what kings are paid to do. They
give orders. Do you require a public proclamation, executive order,
or pontifical bull (in Latin), in order to be properly inspired? Now,
pleae do some testing. After all this is your antenna, your question
and your problem.
Ya, I have quad panel on the boat, when I get some time I'll try the
comparision.
Doing it on a boat might be a problem. You're too close to the water
which will probably be inside the Fresnel Zone. The water also acts
as a great reflector. Thinks also move on a vessel, making stable
readings difficult. I do my testing across a valley, where there's
little chance of ground reflections (and there's a convenient 2.4GHz
RF source from the local mountain top WISP). The path also has a wall
of 40 meter high trees on both side to attenuate any interference.
Jeff Liebermann
All that may be true, sorry King, is true.
What a king says is by definition true, even if it's wrong.
My antennas are 14ft above the water and the signal only goes over water for
about 30ft
before a concrete parking lot fills the rest of the distance to my targets.
Total target distance
is about 600ft. The antennas are tipped upward about 10 degrees.
Sounds good. You're out of the Fresnel Zone and the parking lot isn't
going to contribute any reflections.
Here's the test I performed, I aligned the panel and the yagi on a pole
with about the 2ft between the yagi and the center of the panel.
Not so good. You can't leave a bunch of potentially resonant metal
hanging around that close to the antenna. The unsused antenna could
easily be re-radiating anything it pickups up from the other antenna.
One antenna at a time please.
I aimed at
the target as
accurately a I could see. (Center of condo building) During testing I did
not terminate the unused
antenna.
Ugh. If you're shooting into what's essentially and indoor wireless
router, you're going to get reflections from the building and in
particular, from the room where the AP is located. Also, not so good.
I used net stumbler for signal strength numbers, I received 42 signals with
the Panel,
and 44 signals with the yagi.
How steady were the readings? With my experience with Netstumbler, if
I wait a few minutes, the readings will move around perhaps 2-5dB. Are
you doing this on Channel 6 (middle of the band) or near the band
edges?
The one signal I usually use went from -48db (panel) to -50db (yagi),
That's +2db for the Panel.
Then I did a Netstumbler screen print of the first 35 signals using the yagi
and then the panel.
I added all 35 signal strengths and divided by the 35, to get an average
signal.
(35 is all that fit the screen without scrolling)
The Panel AVE = -58.11db, the Yagi AVE = 59.9db that is + 1.79db for the
Panel.
Good idea but not really valid as you're probably measauring the side
lobes of the antennas. An antenna with many side lobes is going to
hear more stations than one with just a single major lobe.
So if you have any faith in my method, it looks like the yagi is down about
2db from the Panel.
I'm impressed, the MFJ-1800 yagi is advertised as a 15dbi antenna, the
panel
as 19dbi antenna.
I have less faith in the RSSI linearity of thatever you're using for a
wireless device. -50dBm isn't particularly saturated, so presumably,
the comparison is valid. There might be some gain compression at high
signal levels, which explains why you're seeing 2dB difference instead
of 4dB. I would have predicted a larger difference as my NEC model of
the MFJ-1800 predicts a gain of about 13dBi. I would have expected an
even larger difference.
Here's the Panel seller for the 19dbi panel antenna.
http://www.fab-corp.com/product.php?...cat=255&page=1
At one time I found antenna plots for it, but can't locate them now.
Here's the yagi seller.
http://www.mfjenterprises.com/Produc...uctid=MFJ-1800
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558