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Old Jan 15, 2010, 12:31 AM
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sonicj's Avatar
Panama City Bch, FL
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are you using a balun on your inverted vee?
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Old Jan 15, 2010, 10:12 AM
Engineer for Christ
IBCrazy's Avatar
Amherst, VA
Joined Jun 2006
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I don't use a balun. Since the impedance of the TX and inverted Vee antenna is so close I don't need one. Just run it direct.
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Old Jan 15, 2010, 02:59 PM
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ggtronic's Avatar
Montreal
Joined Nov 2005
1,627 Posts
in case somebody want to see, this is the DPCAV 900mghz 3dbi
inside look. What is this kind of antenna IBCrazy ?

brass tube is connected to shield (ground) and the spring is connected
to the center pin:
i only save 3g with this mod go from 14 to 11g...
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Last edited by ggtronic; Jan 15, 2010 at 04:25 PM.
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Old Jan 15, 2010, 07:27 PM
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Amherst, VA
Joined Jun 2006
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It looks like a base fed helical full wave dipole. It might be 3/2 wave too, which would make more sense. How tall is the brass tube? What frequency is it used on?

Basically this combines the helical antenna with a dipole antenna. The helical antenna has a gain of about 1.5 dbi at 1/4 wave, so it is almost isotropic. However, with added length the gain increases. The brass tube would act as a monopole reflector. It will actually direct the signal some.

This looks like a decent antenna design for moderately high altitudes. The radiation pattern would be similar to the 1/2 wave inverted Vee.

-Alex
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Old Jan 15, 2010, 07:37 PM
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Amherst, VA
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Ok, I just checked DPCAV's website. They are calling it a compact dipole with 2 dbi gain.

The real term for this is a helical dipole. It is 1/2 wave not full wave. Impedance is 71 ohms assuming it is perfectly tuned, so you would have a best case SWR of 1.3 which is fairly good. Gain is actually more like 1.5 to 1.6 dbi not 2 dbi. This is very close to isotropic (spherical radiation pattern). A definite good closer range antenna which would work extremely well if you like to fly over your own head. It would also be good for stunt flying and moderately high altitude.

-Alex
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Old Jan 15, 2010, 08:55 PM
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St.Augustine, FL
Joined Sep 2009
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I use this antenna. Love it. I fly with it striaght up I flew it over head and got a lot of static. Once out along side the static when away. I've flown really high at what I think is a good distance and had an awsome picture. But 100 ft directly over head... static.

This what I think is going on. If anyone has anyother ideas please PM me with a fix.
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Old Jan 15, 2010, 08:58 PM
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Amherst, VA
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Flip it upside down. That should help the overhead problem. The brass tube acts as a reflector so the propagation pattern looks kind of like a mushroom.
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Old Jan 15, 2010, 09:07 PM
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St.Augustine, FL
Joined Sep 2009
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Well damn. Its mounted on top or the plane. I was going to test that and fly upside down over my head to see if there was a difference.
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Old Jan 16, 2010, 12:34 PM
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ggtronic's Avatar
Montreal
Joined Nov 2005
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thanks IBCrazy ..
yes i forgot to say its for 900mghz...
brass tube is 67mm and spring=20mm
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Old Jan 18, 2010, 07:02 PM
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Amherst, VA
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gg, that should work well. If you can get that GPS a little farther away from the antenna, that will also help. The orientation of the GPS receiver does not matter. It is receiving signals from many directions, so there is no proper orientation. I usually lay mine in the side and tuck it in the wing or something.
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Old Jan 18, 2010, 07:57 PM
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St.Augustine, FL
Joined Sep 2009
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How do you think it transmits through foam? Say if I were to drill a hole striaght down throught the foam fuselage, slide the antenna through it...
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Old Jan 18, 2010, 09:15 PM
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Amherst, VA
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It will travel through foam like air. Foam is mostly air. What isn't air is plastic... which is exactly what the antenna case is made from. It will travel through thin wood just the same. It is conductive materials that are the problem.
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Old Jan 18, 2010, 09:19 PM
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St.Augustine, FL
Joined Sep 2009
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Well if that is right then I will lay the Vtx across the top of the fuselage and run the antenna straight down. Less drag and more Vtx cooling... Anyone have any information to prove otherwise???
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Old Jan 19, 2010, 07:16 AM
Registered User
Dos Hermanas (Sevilla)
Joined Dec 2007
294 Posts
Hi IBCrazy,
Great information about antennas.
Is it posible to make a "lighter" 1.2ghz antenna for a 500mw LW Tx than the stock one?:

http://www.nghobbies.com/cart/index....roducts_id=357

I am mounting a trex 250 for fpv, the 1.2ghz stock antenna is quiet heavy at the base, so i would like to get or make a lighter one.

Regards.

Patrick
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Old Jan 19, 2010, 07:30 AM
Registered User
Romania, Dolj, Craiova
Joined Sep 2007
14,950 Posts
8 grams. If interested I can post build log, I took pictures.
http://www.rcgroups.com/forums/showp...&postcount=143
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