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I'd like to add a few details to Jeff's excellent info in this and
his previous post.<br>
The penalty for cross-polarization (i.e. transmit and receive
antennas being "at right angles" wrt polarization) may be greater
than 3db because of the relative advantage (or absence of advantage)
of ground return. That is, with horizontal polarization the ground
acts as a reflector and often reinforces the field taking a direct
path to/from the antenna. <br>
The lack of a decent RF ground is surely at the root of some
disappointment with vertical antenna arrangements, just as Jeff
said. Because the RF ground is effectively the "other leg" of a
vertical (i.e. vertically polarized) antenna system, if it's wimpy
the efficiency of the antenna system can be pitiful. Arranging a
decent ground plane for the "piece of wire" (ostensibly 1/4wl)
vertical antenna should make a difference. Running a feedline to an
"outboard" monopole/ground plane setup would also be totally valid
and possibly very effective (except for the lack of a "DC ground".
See next bit).<br>
A well designed dipole on the end of a decent piece of coax cable
that is properly connected to the radio could make a huge difference
for a number of reasons. This is going to sound weird, but in my
opinion the really huge difference is that a DC ground connection to
the antenna could be arranged such that the coax shield can act as a
"lightning rod", with a decent connection to earth ground. This
might enable to antenna system to up several feet and still have a
rat's chance of surviving a season of southern thunderstorms. <br>
But you can't just stick coax on one end and solder the other two
pieces of wire and get a dipole antenna that performs properly. The
RC plane example and example where the guy uses the mouth-watering
good Agilent test instrument are surely improvements, but without a
balun or equivalent matching network on the antenna side what you
really have with those two examples is a radiating feedline that's
part of the antenna system. Worse, the length of the coax feedline
gets involved in the tuning of the antenna. This can drive a person
to distraction in no time flat. So forget about cutting those wires
to the super-precise lengths and expecting to be able to duplicate
the match with resonant frequency shown on those instruments: it
would only happen by luck. Also, *the gauge of the antenna wires is
important*. So it's fine to say "I cut these wires to 77.4mm", but
if it's 22 gauge wire vs 14 gauge wire that will make a big
difference with respect to tuning the antenna to the desired
resonant frequency. Finally, he said "The length of each side is
77.4mm." BZZZZZT! WRONG! That defines a "full wave dipole", not a
half wave dipole (the universal meaning if somebody just says
"dipole" is "half wave dipole"). A full wave dipole has a very high
feedpoint impedance. This tells me that by fantastic luck (or maybe
design that the forum poster didn't share for some reason), the
feedline constitutes a transmission line transformer that translates
the 50ohm feed of the RFM69 to the multi-thousand ohm impedance of
the full wave dipole. Or else he just wasn't thinking and quoted the
length of both wires and so each is really 77.4/2mm.<br>
But back to the DC ground. It's possible to have the dipole be a
solid wire/rod that's connected directly to the coax braid and the
coax center conductor feeding a gamma match or something similar.
This is dead common with amateur radio yagi antennas and has the
huge advantage that the entire antenna is very well connected to the
feedline coax braid and the transceiver and of that feedline can
have its braid well connected to a copper ground rod. (The ground
rod does not have to be pounded vertically into the ground. Burying
it horizontally a few inches under the soil surface is effective.)
Finally, a gas discharge tube/capacitor arrangement could be used to
isolate the radio from the feedline in the case of a storm-induced
high potential, shunting the energy back into the ground path before
it can get in an destroy the radio chip. <br>
Yep, this sounds totally off the wall to a casual hobby person. But
have you ever noticed that folks are not climbing all over cell
towers after every area thunderstorms? Likewise, have you noticed TV
stations rarely go off the air during storms despite the fact that
their towers are taking direct lightning strikes frequently during
the storm? That's 'cause the equipment is designed to handle storm
situations. So, as I imagine Chip having 100 or 500 trail counters
spread all over this part of the state I want to imagine them all
humming along vs 5-10 of them being blown to bits every week or two.<br>
The catch-22 is that if the trail counter antenna can be kept on the
ground, preferably right next to a low lying creek bed, the
equipment might be eternal, but communication may be a real problem.<br>
But another approach might be to focus the time and money into the
relatively small number of data collection systems. With those you
could use a (DC-ground), high gain yagi (cocked 90 degrees to
maintain vertical polarization) that could be aimed at the various
trail counters. Or if you want to go nuts, use a parabolic dish.
This would be big at 915mhz, but could provide 20-35db gain that
might compensate for the terrible performance of the logger radios.
On the other hand, aiming a dish would probably require some
specific equipment 'cause the beamwidth at those gains is
razor-thin.<br>
As for induction of RF back into the sensor wires, one could use
relatively cheap shielded cable, possibly in combination with
ferrite filters, and I'm confident that would eliminate that issue.
But it's very useful for Jeff to point that out, as this is the kind
of problem can be very subtle and initially mystifying. The noise
margin for I2C has got to be close to pitiful as it is.<br>
Gotta go do family stuff, but I thought I'd share some notes.<br>
<br>
<br>
<div class="moz-cite-prefix">On 05/28/2014 09:51 PM, Jeff Highsmith
wrote:<br>
</div>
<blockquote
cite="mid:35A89FB9-5BB5-44DB-A3BB-0857A1CC8ECA@jeffhighsmith.com"
type="cite">
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charset=ISO-8859-1">
<div><span></span></div>
<div>
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<div>Chip,</div>
<div><br>
</div>
<div>Four to six feet could be enough, but the forum post where
the user mentions 1.5 mile range at similar antenna height
states that that was line-of-sight, and he only got 900 feet
through "thick woods" (though at 900MHz). Given the choice,
I'd accept a little transmission line loss in exchange for
increased height, especially in rough terrain. You don't want
a big chunk of ground (dirt-wise and electrically) or wet
foliage between your antennas. Luckily, height is an easy
variable to experiment with. </div>
<div><br>
</div>
<div>Another popular antenna option is the twin-lead J-Pole. It
does not require a reflecting ground plane, so you can hang it
high without worry. For your application, it'd be about 17"
long:</div>
<div><a moz-do-not-send="true"
href="http://www.qsl.net/wa3yxk/jpole.html">http://www.qsl.net/wa3yxk/jpole.html</a><br>
<br>
If your making your own antennas, you might want to seek out
the use of an SWR analyzer (ask at local ham club) to help you
quickly dual in the length. </div>
<div><br>
</div>
<div>Devil's Advocate bit here: I see what you mean about
directional antennas being more work to setup, as you'll be
aiming them with a map and compass. The gains from yagis could
be significant, though (around 12dB, depending on the number
of elements). Are you trying to hide the sensor nodes from
view? Is it a mesh network topology or is there one home
station receiving multiple sensor nodes. Directionality would
be ruled out if you need to tx/rx different directions. </div>
<div><br>
</div>
<div>One more thing to keep in mind will be RF interference.
This is getting beyond my experience, but my understanding is
that if the wire between the sensor and the node is an
effective antenna at your frequency, you might get strange
issues while transmitting. </div>
<div><br>
<div><span class="Apple-style-span"
style="-webkit-tap-highlight-color: rgba(26, 26, 26,
0.294118); -webkit-composition-fill-color: rgba(175, 192,
227, 0.231373);">Jeff :)</span></div>
<div><span class="Apple-style-span"
style="-webkit-tap-highlight-color: rgba(26, 26, 26,
0.296875); -webkit-composition-fill-color: rgba(175, 192,
227, 0.230469); -webkit-composition-frame-color: rgba(77,
128, 180, 0.230469); "><br>
</span></div>
</div>
<div><br>
On May 28, 2014, at 11:22, Charles McClelland <<a
moz-do-not-send="true" href="mailto:chip@mcclellands.org">chip@mcclellands.org</a>>
wrote:<br>
<br>
</div>
<blockquote type="cite">
<div>
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charset=ISO-8859-1">
Jeff,
<div><br>
</div>
<div>Thanks for the response, very helpful. I had not
considered your point about the rubber ducky - I doubt my
small sensor node will provide a very good or very large
ground plane. </div>
<div><br>
</div>
<div>As for height, I guess that getting the antenna off the
ground would be helpful. My sensor is i2C based so I
don’t want to have too long a wire between the Moteino and
the sensor. At the same time, my understanding is that
the longer the length of the cable that connects the board
to the antenna the more losses there. My first attempt
will put the antenna close to the board (3-6”) and the
sensor 6’ from the Moteino, If I place this well, that
would place the antenna 4’-6’ off the ground. Does that
sound reasonable?</div>
<div><br>
</div>
<div>Based on your answer, it seems like the first option
below may be preferable as it will include a full quarter
wavelength ground and emitter arm and it could be “tuned”
by trimming the wires. Agree?</div>
<div><br>
</div>
<div>Thanks,</div>
<div><br>
</div>
<div>Chip</div>
<div><br>
</div>
<div><br>
<div>
<div>On May 28, 2014, at 10:17 AM, Jeff Highsmith <<a
moz-do-not-send="true"
href="mailto:jeff@jeffhighsmith.com">jeff@jeffhighsmith.com</a>>
wrote:</div>
<br class="Apple-interchange-newline">
<blockquote type="cite">
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charset=ISO-8859-1">
<div dir="auto">
<div>Chip,</div>
<div><br>
</div>
<div>Forgive me for my rusty RF theory, but my
understanding is:</div>
<div>• Dipoles are still somewhat directional, just
not as much as a "Directional" antenna like a
Yagi. Your best radiation and reception is at 90
degrees to the wire. In other words, the
performance drops in the directions that the tips
of the wires point. If you are vertically
polarized, this isn't as much of an issue, since
you are unlikely to be above the tip of the
antenna. </div>
<div>• Polarization matters. You lose 3dB or so when
switching polarizations, so make all your antennas
horizontal or make all your antennas vertical.</div>
<div>• So-called "Rubber Ducky" dipoles rely on the
radio's case or, more frequently, the human body
holding the case to be the other side of the
dipole. If you don't have a good ground like that
under your rubber ducky, it won't do as well. </div>
<div><br>
</div>
<div>At what height relative to the ground are you
placing your antennas?</div>
<div><br>
</div>
<div>I hope that helps. Others, feel free to correct
me-it's been awhile since my ham days.</div>
<div><br>
<div><span class="Apple-style-span"
style="-webkit-tap-highlight-color: rgba(26,
26, 26, 0.294118);
-webkit-composition-fill-color: rgba(175, 192,
227, 0.231373);">Jeff :)</span></div>
<div><span class="Apple-style-span"
style="-webkit-tap-highlight-color: rgba(26,
26, 26, 0.294118);
-webkit-composition-fill-color: rgba(175, 192,
227, 0.231373);">WJ3FF</span></div>
<div><span class="Apple-style-span"
style="-webkit-tap-highlight-color: rgba(26,
26, 26, 0.296875);
-webkit-composition-fill-color: rgba(175, 192,
227, 0.230469);
-webkit-composition-frame-color: rgba(77, 128,
180, 0.230469); "><br>
</span></div>
</div>
<div><br>
On May 28, 2014, at 9:42, Charles McClelland <<a
moz-do-not-send="true"
href="mailto:chip@mcclellands.org">chip@mcclellands.org</a>>
wrote:<br>
<br>
</div>
<blockquote type="cite">
<meta http-equiv="Content-Type"
content="text/html; charset=ISO-8859-1">
<div>To all,</div>
<div><br>
</div>
<div>I am playing with the Moteino and recently
tested the range at a local park. I was a bit
disappointed and discussed this at the last
meeting. As with all things embedded, there
seems to be a number of things that can be done
to improve performance both hardware and
software. For this note, I wanted to ask for
any advice on the hardware front. Once I have
that set, I plan to tweak the software settings
(primarily data rate but some others as well) to
see how much range I can get - ideally 1.5 miles
outdoors through wooded terrain.</div>
<div><br>
</div>
<div>The Environment:</div>
<div><span class="Apple-tab-span"
style="white-space:pre"> </span>- <a
moz-do-not-send="true"
href="http://lowpowerlab.com/moteino/">Moteino</a> base
and sensor nodes </div>
<div><span class="Apple-tab-span"
style="white-space:pre"> </span>- I went with
the <a moz-do-not-send="true"
href="http://lowpowerlab.com/blog/2013/06/20/rfm69-library/">lowest
frequency transmitter</a> - 433MHz - and the
highest power</div>
<div><span class="Apple-tab-span"
style="white-space:pre"> </span>- The Moteino
site states that there is not much improvement
from <a moz-do-not-send="true"
href="http://lowpowerlab.com/moteino/#antennas">monopole
antennas</a> </div>
<div><span class="Apple-tab-span"
style="white-space:pre"> </span>- In the
forums however, there is a posting promising
greater range from <a moz-do-not-send="true"
href="https://lowpowerlab.com/forum/index.php/topic,112.msg288.html">dipole
antennas</a></div>
<div><br>
</div>
<div>I understand that a directional antenna would
be better but my application would not easily
support this as the sensors nodes may need to be
moved frequently.</div>
<div><br>
</div>
<div>My limited understanding of a dipole antenna
theory is that there are two elements - one with
signal and one with ground with each sized to
match the specific frequency. I plan to add an
SMA connector to the board so I can install a
commercial antenna and I found two options that
both claim to be sized to the 433MHz frequency:</div>
<div><span class="Apple-tab-span"
style="white-space:pre"> </span>- The <a
moz-do-not-send="true"
href="http://fpvlab.com/forums/showthread.php?3156-433MHz-Half-wave-dipole-for-LRS">first</a> looks
like what I expected a dipole antenna to look
like and could be easily made</div>
<div><span class="Apple-tab-span"
style="white-space:pre"> </span>- The <a
moz-do-not-send="true"
href="http://www.taoglas.com/images/product_images/original_images/TI.15.3113%20433MHz%20Connector%20Mount%20Antenna%20290909.pdf">second</a> would
be much better for packaging and use on a sensor
that may be moved often - but it does not look
like a dipole antenna to me.</div>
<div><br>
</div>
<div>Any advice on which might be the better
choice or whether I should look at other
options?</div>
<div><br>
</div>
<div>Thanks,</div>
<div><br>
</div>
<div>Chip</div>
<div><br>
</div>
</blockquote>
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