Thread Tools
Jun 18, 2017, 04:30 AM
Registered User
Question

Linear dipole 5.8g - Sleeve length?


I'm trying to hunt down bad reception on my micro quad, and realized now that the antennas I use have a different outer sleeve length than the ones that usually come with the micro cameras (blank one in the photo is what I use, the covered one is the one that came with a camera).
Can anyone tell me how much influence this may have?
Sign up now
to remove ads between posts
Jun 18, 2017, 01:52 PM
Registered User
Looks like they have the same length exposed part on the end, except for one is covered in shrink wrap so you can't really see. I used 12.7mm on my last home made (a piece of coax) linear antenna and it worked great.
Jun 18, 2017, 02:01 PM
Registered User
I roughly measured the covered one and it appears to have 12.9mm just like the other (the wire goes exactly to the edge of the heatshrink). Just the metal shield is quite different between them, thinner and longer than on the other.
Jun 18, 2017, 10:35 PM
Engineer for Christ
IBCrazy's Avatar
It depends on the diameter of the cylinder and the cable going through it. The sleeve is both a balun and a counterpoise. It works by choking out common mode currents in the cable.

They should be ~12.6mm in total length unless you have a very small cylinder in which case it will be closer to 11.5mm

-Alex
Jun 19, 2017, 02:09 PM
Registered User
So if the cylinder and the antenna are each roughly 12.6 mm, that's all good?
Jun 21, 2017, 05:16 AM
Registered User
I think like Alex said, it depends on the diam. of the metal sleeve and the coax cable used. Also it seems the length is a little different if the whole antenna is covered with heat shrink. i gave up on it trying to figure it out because i have no SWR meter for 5.8GHZ.
The picture below shows, although all are suposed to be 5.8GHZ antennas, different lengths and sleeve diam. Some of them i had bought to just check them out regarding length lol. Buth without measuring equipment... There are too much variables in play.
Jun 21, 2017, 05:23 AM
Registered User
The "Lantian" one is the one I use. I figure my issues are mostly down to too much wlan around the park, not directly related to the antenna.
Jun 21, 2017, 05:43 AM
Registered User
Best to have the 5.8GHZ antenna on the quad sticking straight up or down or under an angle so that when you are flying the antenna itself is in a vertical position. Having the antenna horizontal sticking out of the back will give signal loss when in front of you you fly away from you or towards you. On the receiver end don't use a lineair / rubber ducky antenna.. A couple of different lineair directional antennas on a diversity receiver works better in my opinion.
Jun 21, 2017, 10:53 AM
Team AlienWarpSquad
Quote:
Originally Posted by Alias_Hendrik
I think like Alex said, it depends on the diam. of the metal sleeve and the coax cable used. Also it seems the length is a little different if the whole antenna is covered with heat shrink. i gave up on it trying to figure it out because i have no SWR meter for 5.8GHZ.
The picture below shows, although all are suposed to be 5.8GHZ antennas, different lengths and sleeve diam. Some of them i had bought to just check them out regarding length lol. Buth without measuring equipment... There are too much variables in play.
Correct- the tube also is part of the Impedance matching (BALUM) to the TX's Output stage so can vary depending on the EXACT Tx output design. So there is NO easy or quick answer and no easy way to insert an SWR measurement between the TX and the antenna.

the only thing an RC hobbyist can do is Range Test.

Also remember that 5.8GHz is a Shared band and either the TX/Antenna system MUST be FCC Part 15 approved - which means you must accept ANY Interference OR you must have an FCC license to legally operate which means you are responsible to NOT interfere with other operations.
Jun 21, 2017, 04:34 PM
The Dreamer
Pisces's Avatar
tidbit of info on the sleeve dipole. The key word is DIPOLE, meaning it has 2 poles. The dipole referred to here to is a 1/2 wave dipole. Each pole is 1/4 wavelength. This is a balanced antenna and does not use a ground plane ( counterpoise ). The end to end physical length of the dipole is 1/2 wavelength. This is the total length end to end of the 2 poles at the design frequency, corrected for the velocity of the wave ( velocity factor ) traveling along an electrical conductors of each pole ( antenna element ) . In the case of the sleeve dipole the L/D (Length divided by the conductor Diameter) for each of the poles is different and needs to be calculated separately.

And to confuse the issue, the physical design of a 1/2 wave dipole does not need to be center fed. The feed point can be anywhere along thee 1/2 wavelength . The 1/2 wave dipole can even be end fed. The feed impedance of the antenna varies depending on the feed position along the 1/2 wavelength. For the center fed sleeve dipole the theoretical feed impedance is 72 Ohms. In reality for the physical dipole the feed impedance is lower. The sleeve dipole can be designed for a good VSWR match into 50 Ohms.
Jun 21, 2017, 04:58 PM
The Dreamer
Pisces's Avatar
Another tidbit of info on short circuit and open circuit 1/4 wavelength coaxial stubs. For example IBCrazy cross hairs uses a 1/4 wavelength coaxial stub at the feed point. The 1/4 wavelength short circuit stub at the open end prevents the the antenna feed currents from returning down the outside of the feed coax. That is, the open end of of the stub acts as a balun at the feed point. In the case of the sleeve dipole the stub is reversed and the open end is at the end of the antenna element. The idea that the sleeved dipole is an effective balun in a red herring and is not correct.

The sleeve1/2 wave dipole and a 1/2 wave coax fed straight dipole performance is identical.


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Length of 5.8 m dipole elements?? Weanuts FPV Talk 4 Mar 13, 2016 02:58 PM
New Product FQ777 AF957 "GAZER" 5.8G FPV, another cheap 5.8G in the arena SeByDocKy Mini Multirotor Drones 8 Oct 10, 2015 02:05 PM
Discussion uhf 433mhz dipole length jwmachon FPV Equipment 1 May 01, 2015 05:20 AM
Discussion 1.8g Linear Servo at Towerhobbies $14.99 flying-llama Scratchbuilt Indoor and Micro Models 2 May 21, 2011 06:44 PM
Discussion ES18 1.8G Micro Linear Servo RUDDERLESS Scratchbuilt Indoor and Micro Models 11 May 17, 2011 09:41 PM