2.4 Ghz DIY dipole Antenna  - Page 1
Forum section: electronics / rf-microwave

author: soligen
author_rank: Contributor
timestamp: October 12, 2017, 03:01:45 pm
content: I have been looking for instructions for making a sleeved dipole antenna for 2.4 Ghz for use on my DIY RC receivers. I haven't found anything that clearly specifies the all the specific lengths of each part of the antenna, I imagine because there are so many variables.Although I don't really have the kit needed to evaluate an antenna design, I have been experimenting and this is what I came up with. I am looking for feedback if I should adjust any lengths, and if someone with the equipment to properly test this antenna would like to help be out by testing it, that would be wonderful.Desired center frequency 2.425 GhzCoax: RG-178 (velocity factor .69)Sleeve length: 26mmElement length : 28mmCoax feed length: 170mm (2 wavelenghts x velocity factor)The sleeve was made by folding the braid back and flooding it with solder. The primary element ends up with 2 layers of heat shrink (over the dielectric) and the sleeve ends up with one layer. Pic attached showing 3 different stages of the build.My tuning was done with my Taranis RC transmitter which has a RAS indicator which is supposed to be SWR. I had the same good reading at 27, 28, and 29mm, so I took the middle length.For reference, here is my RC receiver project:https://github.com/soligen2010/RC_RX_CABELL_V3_FHSS. I have made a number of receivers that work well, but when I try to send telemetry from the RX back to the TX, it is way less reliable that the signal from TX to RX. I am thinking the best antenna I can make would help this.Thanks

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 12, 2017, 05:04:55 pm
content: Make a biquad for the ground station. It is both compact (unlike a dish or cantenna) and high gain.

author: soligen
author_rank: Contributor
timestamp: October 12, 2017, 07:48:37 pm
content: Quote from: NiHaoMike on October 12, 2017, 05:04:55 pmThanks for the suggestion. I have tried a 5 DBi antenna on the ground station, but not a biquad - I am concerned it will be too directional. I'll look into this once I am happy with the RX antenna.For this thread, I really want to focus on the RX antenna (in the plane) which needs to be omni-directional and reasonably small

author: Gribo
author_rank: Frequent Contributor
timestamp: October 12, 2017, 09:26:11 pm
content: Your dipole is a bit on the short side. Wavelength in air @2.4GHz is ~12.5cm. The exposed parts of the coax should be ~6cm, not 3.

author: TheSteve
author_rank: Unknown
timestamp: October 14, 2017, 05:26:16 am
content: Quote from: Gribo on October 12, 2017, 09:26:11 pmHe built a dipole which should have 1/4 wavelength radiating elements so his measurements are fine.

author: soligen
author_rank: Contributor
timestamp: October 16, 2017, 02:04:06 pm
content: Correct, and since my elements are not in free space, my understanding is that this should shorten things as well. They are different lengths, but the bottom is fatter then the top, and I also read somewhere that fatter elements should be shorter too (Can anyone confirm?). So, intuitively these seem reasonable to rookie me, but I don't fully understand antenna design, and I don't own the gear to properly evaluate it, so any info from people with experience on 2.4 antennas would be appreciated.I had also tried making the sleeve from 1/8 inch (just over 3mm) O.D. brass tubing. I had the same results testing on my Taranis TX. I went with the folded back braid just because it required less materials to make the antenna.BTW I has started with 31mm both top and bottom, but that antenna performed very poorly.

author: cdev
author_rank: Super Contributor
timestamp: November 06, 2017, 12:46:38 am
content: Why not make a turnstile or quadrifilar helix antenna? (circular polarized) antenna. Or maybe a clover leaf style antenna. They are circular polarized so are particularly good at rejecting fading. Just make sure both antennas are identically polarized.(stick with either RHCP or LHCP)

author: soligen
author_rank: Contributor
timestamp: November 07, 2017, 02:57:57 pm
content: Quote from: cdev on November 06, 2017, 12:46:38 amThanks for the suggestion. On an RC plane or quadcopter I am a bit concerned that these would be too fragile. Cloverleafs are used at 5.8 GHz in RC for video, but they are a lot smaller than 2.4.How do you think mixing these antenna types would work out. A simple dipole on the plane (actually 2 dipoles on the plane as it has diversity) and one of these others on the transmitter.

author: cdev
author_rank: Super Contributor
timestamp: November 08, 2017, 12:03:48 am
content: You should ask people who fly RC aircraft a lot.I have never flown one. The big advantages of circular polarization is freedom from fading and rejection of noise. Its likely better than diversity in that respect if there is line of sight between the two stations.The explanation is fairly technical but Wikipedia explains it fairly well.https://en.wikipedia.org/wiki/Circular_polarization#AntennasThe best way to use a circular polarization antenna is to communicate with a receiver with another circular polarization antenna of the same sense. In that situation there are a number of advantages over linear polarization which apply especially to communication between a ground station and an aircraft or space vehicle.You could use two circular polarization antennas on receive in diversity, sure. Would that be more reliable? Its hard to say. Diversity usually offers an improvement in signal reliability, but in this case I am not sure if it would add anything.Having the opposite polarization would not buy you anything unless you were trying to receive a circular polarization signal that had reflected off of a flat conductive surface an even number of times (like once)I would not expect adding another, linear polarized antenna to be more than marginally helpful if the transmitting antenna was circular polarized.So what I am getting at is diversity and circular polarization are two ways of attempting to solve functionally similar but not identical problems that make the "sum" of using the two together likely to not be as large as one might think. Or so I suspect.Ask the RC community.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 08, 2017, 05:03:46 am
content: One reason to use two antennas of opposite polarization is MIMO, which is not your case.