You have now set the record straight, about Antennas, and well done, the whole tale was about 6 inches over most heads on this forum.
Jerry Miller, SOSS
At 01:03 PM 8/17/2004, Simon Van Leeuwen wrote:
It pisses me off that some people think (and try and dictate) that when they
have heard enough of something...so has the rest of the community; I trust this
will put things in order (probably not);
Keeping any antenna away from other devices that conduct as far as possible is
a good thing especially if the other conductor approaches a similar length. How
far will depend directly on your particular aircraft.
I will repeat, every install (without exception - even two identical models in
every respect back-to-back) will demonstrate a unique RF signature. The ONLY
way to determine whether your intallation will be adequate is...hang on...a
rigorous range test.
What I, and a couple of others are "suggesting" (you can do nothing else - as
in lead a horse to water...) comes from formal education, 3 years of antenna
design (in my case anyways), and real world experiences and testing (in and out
of the lab - on some of the R/C electronics we use in sailplanes and gas
engined/jet turbine aircraft).
It is painfully obvious that antennas are running next to some sort of conductor
(s). Separation is a valid factor, double the distance between the nearest
conductor and the potential extraneous noise (if there is any) drops off by a
factor of four. I will refrain from elaborating, trust me. Tough to do I'm
sure, as there is enough misinformation bantied about on these lists to keep
folks from the truth. Why? Egos. Also moving the antenna out of the same plane,
away from other conductors will add sginificant range (most times). However
this is difficult to achieve (there are two vectors associated with radiated
energy - 90 degrees opposed). If we always flew upright, we could stand the
antenna veritcally and match planer geometry by having the TX antenna vertical
as well.
A dipole is NOT the be-all to end-all, as it in itself has excellent null
points off each end which we have used to actually locate interference. That
is, when the interference we were attempting to locate disappeared, we knew it
was directly off either end of the dipole (and it was - I used this procedure
to locate a bad overhead AC transformer which was splattering the 72MHz band,
actually from DC up into the GHz).
The fact that the pseudo-dipole works so well in our R/C circumstance, is the other half...the GND.
Some of you may be familair with antenna propagation plots, essentially the
radiation/reception pattern can be logged and displayed in 3D. The UUT (unit
under test) is incrementally moved through all 3 planes on a fixture and
stopped say, every 5 degrees and a measurment is taken. This occurs in all axis
(X, Y, Z) then rotated another 5 degrees and repaet the process. Go here for
some examples of 3D plots (I did these):
http://www.murandi.com/services/plots.html
As you can see the fatest area demonstrates the highest reciprocity (equal
ability to transmit or receive a signal), and the nulls are demonstrated as the
area where the sphere has been sucked in. The plot to the right show just how
reciprocity deteriorates to a null off either end.
some other information: http://www.ewh.ieee.org/soc/es/Nov1998/13/PATPLOT/PATPLOT.HTM http://www.poynting.co.za/software/files/SuperNECA3.pdf http://www.djmelectronics.com/articles/emc-antenna-parameters-p2.html
The fact that the servo wiring (GND specifically) plays a part in offering up
the other half of the pseudo-dipole makes for a rather well balanced 3D plot,
where the area of lowest sensitivity is still "relatively"(!) acceptable. The
physical location (the layout) of the GND wiring is what makes the OEM single
lead antenna on our Rx's more than acceptable for it's current role...to allow
the aircraft to move through infinite planes of flight-attitude and still
receive a valid signal.
HOWEVER...this area of low receptability can be fairly big, therefore the
amount of actual headroom is marginal. That is, say somewhere off the end of
the antenna, behind the aircraft (antenna running down length of fuse). We can
at least agree the worst sensitivity occurs somewhere in this area (exclude
carbon and it's spacial influence for now).
Try and imagine a conical section off the back of the aircraft making up the
null zone. In other words, walking into this zone with your transmitter
immediately reduces effective range significantly. The size of this null zone
(or conical section) can extend 45 degrees..or from the RX's POV it sees a
relatively weak signal from 45 degrees aft off starboard through to 45 degrees
aft off port. In this scenario, flying while the aircraft has it's ass pointed
at you is causing the RX to work overtime trying to understand what you are
commanding. My point here is this: the zone can be so big that although the
aircraft can swing through 90 degrees in both X and Y axis, you never recover
adequate signal before impact.
Poor/improper ground range testing will not pick this up. The direct result of
the aircraft sitting on the ground (it being a huge GND plane itself)
alters/moves that null zone, just as it moves the worst transmission point off
the end of the TX antenna I spoke about a few days ago to somehwere else in
space. Simply walking away from the aircraft, reaching some point where servos
start to jitter IS NOT an adequate range test. You may have just confirmed the
area of greatest sensitivity, that's all.
The above is just one example related directly to our models, and how things change when antennas are involved and other dynamic factors play a role.
Add to this a carbon fuselage and that cone may move around and envelope the entire aircraft, making the existing antenna element extremely directional.
There ARE no hard and fast rules, follow recommended procedures as outlined in
your systems manual, learn what works and does not from your peers,
and "attempt" to discern between truth and bullshit here on the lists. If
someone makes an all-encompassing statement...chances are it has bovine
ancestry.
To answer your question Ed, place your antenna, test it, and see if you get
adequate range. Move the aircraft around, not the transmitter. Leave it in
someone elses hands and "you" wlak the aircraft around him/her. Find the area
of least reception, then move the aircraft in all planes looking for yet again
the worst reception. Then, and only then will you have an accurate picture of
what to expect when you launch that puppy.
As far as what constitutes a rigorous range test, the archives should have plenty of meanderings from yours truely...and a very knowlegeable Mr. Chan.
Quoting Ed Jett <[EMAIL PROTECTED]>:
> Nope. It is not going away yet. I keep seeing statements like this below
> in all of these discussions and yet, I do not understand how we can run a
> dipole antenna system (or half dipole for that matter) out the wing or down
> the fuse and stay away from other conductors (wire, pushrods, CF rods, servo
> leads, etc.) I guess I would like a definition of "stay away" in inches or
> cm.
>
> EJ
> ----- Original Message -----
> From: "Doug McLaren" <[EMAIL PROTECTED]>
> To: "Bruce Hobbs" <[EMAIL PROTECTED]>
> Cc: <[EMAIL PROTECTED]>
> Sent: Monday, August 16, 2004 6:03 PM
> Subject: Re: [RCSE] Soaring #4125 / antenna ad-nauseum
>
>
> > Stay away from any other conductors like wires or carbon fiber, and keep
> >the wires going straight out.
>
>
>
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Radius Systems Cogito Ergo Zoom
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