[TriEmbed] Di-Pole Dilemma

Sat May 31 09:49:04 CDT 2014

I'd like to add a few details to Jeff's excellent info in this and his 
previous post.
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.
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).
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.
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.
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.
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.
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.
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.
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.
Gotta go do family stuff, but I thought I'd share some notes.

On 05/28/2014 09:51 PM, Jeff Highsmith wrote:
> Chip,
>
> 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.
>
> 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:
> http://www.qsl.net/wa3yxk/jpole.html
>
> 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.
>
> 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.
>
> 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.
>
> Jeff :)
>
>
> On May 28, 2014, at 11:22, Charles McClelland <chip at mcclellands.org 
> <mailto:chip at mcclellands.org>> wrote:
>
>> Jeff,
>>
>> 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.
>>
>> 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?
>>
>> 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?
>>
>> Thanks,
>>
>> Chip
>>
>>
>> On May 28, 2014, at 10:17 AM, Jeff Highsmith <jeff at jeffhighsmith.com 
>> <mailto:jeff at jeffhighsmith.com>> wrote:
>>
>>> Chip,
>>>
>>> Forgive me for my rusty RF theory, but my understanding is:
>>> . 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.
>>> . Polarization matters. You lose 3dB or so when switching 
>>> polarizations, so make all your antennas horizontal or make all your 
>>> antennas vertical.
>>> . 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.
>>>
>>> At what height relative to the ground are you placing your antennas?
>>>
>>> I hope that helps. Others, feel free to correct me-it's been awhile 
>>> since my ham days.
>>>
>>> Jeff :)
>>> WJ3FF
>>>
>>>
>>> On May 28, 2014, at 9:42, Charles McClelland <chip at mcclellands.org 
>>> <mailto:chip at mcclellands.org>> wrote:
>>>
>>>> To all,
>>>>
>>>> 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.
>>>>
>>>> The Environment:
>>>> - Moteino <http://lowpowerlab.com/moteino/> base and sensor nodes
>>>> - I went with the lowest frequency transmitter 
>>>> <http://lowpowerlab.com/blog/2013/06/20/rfm69-library/> - 433MHz - 
>>>> and the highest power
>>>> - The Moteino site states that there is not much improvement from 
>>>> monopole antennas <http://lowpowerlab.com/moteino/#antennas>
>>>> - In the forums however, there is a posting promising greater range 
>>>> from dipole antennas 
>>>> <https://lowpowerlab.com/forum/index.php/topic,112.msg288.html>
>>>>
>>>> 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.
>>>>
>>>> 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:
>>>> - The first 
>>>> <http://fpvlab.com/forums/showthread.php?3156-433MHz-Half-wave-dipole-for-LRS> looks 
>>>> like what I expected a dipole antenna to look like and could be 
>>>> easily made
>>>> - The second 
>>>> <http://www.taoglas.com/images/product_images/original_images/TI.15.3113%20433MHz%20Connector%20Mount%20Antenna%20290909.pdf> 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.
>>>>
>>>> Any advice on which might be the better choice or whether I should 
>>>> look at other options?
>>>>
>>>> Thanks,
>>>>
>>>> Chip
>>>>
>>>> _______________________________________________
>>>> Triangle, NC Embedded Computing mailing list
>>>> TriEmbed at triembed.org <mailto:TriEmbed at triembed.org>
>>>> http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
>>>> TriEmbed web site: http://TriEmbed.org <http://triembed.org/>
>>
>
>
> _______________________________________________
> Triangle, NC Embedded Computing mailing list
> TriEmbed at triembed.org
> http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
> TriEmbed web site: http://TriEmbed.org

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.triembed.org/pipermail/triembed_triembed.org/attachments/20140531/c62dd36a/attachment.htm>