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Author Topic: Antenna for robot  (Read 1094 times)
KE7BFE
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Posts: 10




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« on: April 26, 2005, 11:01:32 AM »

I have a robot that I have built that uses ATV for remote control.  I will be using a 70cm video transmitter @ 5 watts max.  I do not have any room for a ground plane for the transmitter antenna.  Does anyone have any suggestions for an antenna?  The antenna will be about 6 feet off the ground.

Thanks,
 Chris
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KE6PKJ
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Posts: 256




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« Reply #1 on: April 26, 2005, 01:27:40 PM »

Expose 6.5 inches of RG-58 coax center conductor for your radiating element, then invert the coax braid back down and over the outside sheath. You could also use a 6.5" brass pipe instead of the inverted braid for a cleaner look. The braid would then be trimmed around the top of the pipe and soldered in place. This gives you an omni antenna with a ground plane to work from. Add a few turns of coax around a 35 mm film can to make an RF choke to stop any residual RF current flowing back on the outside of the coax.

 The downside of this antenna is that it's fairly narrowband. Being that the video signal is 6+ mhz wide you're going to have SWR problems at the band edges, assuming that you've trimmed the antenna to resonance at the center frequency of your choice. This means that something is going to suffer, you could lose some portion of power with the Sync or color information.

 Have you considered using 900, 1200, or 2400 Mhz? The antenna swr to video bandwidth ratio is greater, not to mention the physically smaller antennas.  
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WB6BYU
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Posts: 13485




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« Reply #2 on: April 26, 2005, 01:58:59 PM »

When you say you don't have room for a groundplane, is
that for the whip, the radials, or for a flat plate a
half wavelength in diameter?

There are many different types of small antennas, and a
better understanding of your space constraints (as well
as gain and bandwidth requirements) will help us to
recommend ones that are more likely to work in your
situation.

The most likely approach would be to use a quarter wave
whip sticking out of the top of the robot (or inside it
if the body is plastic) that uses the circuit ground (on
the nearest circuit board, plus stray wires and other
metal parts connected to it) as the ground plane.  Or you
can attach a BNC fitting on the top of the robot with 2
or 3 quarter wave wire radials inside the plastic case
(not needed if the case is metal).  Then you can attach
whatever sized rubber duck antenna suites your needs to
the connector.

Remember, though, that the antenna efficiency usually
decreases as the antenna is made smaller.  Very small
antennas (such as those in pagers) will reduce the range
over which  you can receive the transmitted video.  This
is a trade-off that you will have to evaluate in your
specific situation.
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KE7BFE
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Posts: 10




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« Reply #3 on: April 26, 2005, 03:30:59 PM »

I have considered using higher frequency FM transmitters but have not found any products that fit my needs (at least 2 watts power).  

  I have looked at a few 23cm Comtech based unit that could potentially be connected to an amp but nobody seems to know what the actual power output of those are and that makes finding an amp difficult.  I have tried to email one company (www.13cm.co.uk) with questions but they never respond.

Also I read that AM is better than FM in high multipath environments.  This may not be an issue as I will mostly be using the robot in a wide open space at 1 mile maximum range.  The receiving antenna will be only about 12' off the ground so I think that the transmitting antenna will need to have a fairly low angle of radiation.
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KE6PKJ
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Posts: 256




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« Reply #4 on: April 26, 2005, 03:39:30 PM »

Dale, I wouldn't recommend him using the DC ground of any circuit board or other wiring as the antenna's counterpoise. 5 watts of A.M. video-sync, pulsing at 60 hz is going to wreak a lot of havoc into any circuit board. He should have a couple of wavelengths of coax from the transmitter to the antenna with an RF choke to mitigate any returning RF on the coax entering back into the transmitter. All other electronics should be RF shielded and all wiring capacitively coupled to ground at RF.

 5 watts pulsed RF is a lot of energy to keep out of your onboard electronics, whether coupled directly or indirectly.

73,
Kirk  
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KE7BFE
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Posts: 10




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« Reply #5 on: April 26, 2005, 03:53:50 PM »

Vertically there really is not limit (although I would like to keep the antenna shorter than 4').  The problem is in the horizontal dimensions.  You can see what I mean if you look at this pic:

http://www.robotronia.com/misc/EPIA-BOT06r.jpg

The robot's right shoulder currently holds a Diamond NR-770HA dual band antenna.  I am currently only putting 50-100mW into it so hopefully not having a ground plane doesn't matter as far as potentially causing any damage to the transmitter.

The entire body of the robot is steel but that does not matter unless there is a flat surface right under the antenna right?
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KE6PKJ
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Posts: 256




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« Reply #6 on: April 26, 2005, 04:08:16 PM »

Chris,
You will create far more problems for your onboard electronics using AM instead of FM, even your power supply will be video modulated as the transmitter is pulling current up and down 60 times a second. Go with a 1 watt FM video transmitter and make up that extra watt or two you seek with a gain antenna at the receive end.

Multipath is solved by using circular polarized antennas. Tecom makes some nice omni C.P. antennas. Of course you really don't need to worry about multipath if you're using it in open spaces. A quarter wave antenna for the transmitter and a yagi or barbecue grill/parabolic antenna at the receiver will work much farther than a mile with less than a watt out.

I am waiting on delivery right now for some tiny 1 watt 2.4 ghz transmitter modules, if you need one give me a shout. kadirim@telus.net
 

73,
Kirk  
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KE6PKJ
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Posts: 256




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« Reply #7 on: April 26, 2005, 04:13:23 PM »

Chris, I see we're posting at the same time. That steel frame makes an excellent groundplane at 70 cm. Although I still think FM is the way to go for the aforementioned reasons.

73,
Kirk
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KE7BFE
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Posts: 10




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« Reply #8 on: April 26, 2005, 04:44:09 PM »

I am using 2.412-2.484GHz for the data communications with the robot.  Do you think that running a video transmitter  on a nearby frequency would cause any problems?  

Places like www.hyperlinktech.com have 2.4Ghz antennas for really cheap, they sell them for WiFi applications but do you think they would work for FM too?

One of the reasons I am reluctant to go to 1.2GHz is the expense of the antennas but 2.4GHz is really cheap in comparison.  The only thing is that you get less distance per watt but I should be able to make that up in antenna gain.  

What kind of transmitters are you getting?  I might be interested in trying one if they are not too expensive.






 
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KE6PKJ
Member

Posts: 256




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« Reply #9 on: April 26, 2005, 05:08:18 PM »

Your 2.4 ghz data transceiver is spread spectrum and a video xmtr at 2.4 ghz will definately wipe out your xcvr's front end. You might even find that a relatively high powered transmitter of any kind on any frequency will take out or desensitize your data xcvr. The reason is that data transcievers have minimal front end filtering and high gain FET or MMIC amps allowing any RF to get into and mix with your demodulated data. Only experimenting with power levels, frequencies and antenna placement will determine what will work.

73,
Kirk  
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KE7BFE
Member

Posts: 10




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« Reply #10 on: April 27, 2005, 10:37:27 AM »

If I stick with 70cm do you think that I could use an HT antenna?  The new transmitter I'm looking at buying allows you to adjust the output power from 0-4 watts.  This might make it easier to balance everything out.
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WB6BYU
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Posts: 13485




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« Reply #11 on: April 27, 2005, 01:35:15 PM »

I'd go for minimum transmitter power to a quarter wave
antenna at the transmitter and use a high gain antenna
at the receive end.  The ground plane under the whip
need not be flat.  There will probably be some radiation
from all metal parts of the robot if you use it as the
ground plane, but you'll probably encounter some of that
anyway.  I wouldn't use anything longer than a 5/8 wave
whip in any case, and if the antenna mount is going to
be moving at all then a quarter wave whip is probably a
better choice.  (The problem here is that high gain
antennas have a very narrow vertical pattern.  If the
antenna gets tilted at all, the received signal will drop
significantly.)

The actual amount of power required will depend on the
terrain and the height of the receiving antenna.  Some
possible high gain options for the latter include yagis
(see http://www.clarc.org/Articles/uhf.htm) or plane or
corner reflectors (http://www.cebbik.com/vhf/f432.html).
These will need to be aimed in the direction of the robot
(but you are supposed to know where it is, aren't you?
In fact, you can use them to track the robot if it goes
missing.)
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WA6BFH
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« Reply #12 on: May 01, 2005, 05:26:28 PM »

Make the antenna 1/2 wavelength long. You will not then need a counterpoise ground plane. Use a quarter wave coaxial Q-line to match the impedance at the bottom of the half-wave.
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