Antenna matching for a dipole - Page 1
Forum section: electronics / rf-microwave

author: enemra
author_rank: Contributor
timestamp: April 04, 2025, 02:51:11 pm
content: Hello friends,I am working on a research project involving antenna design for a HAM payload on a deployable drone. The drone includes a deployable dipole antenna. After antenna deployment, the dipole antenna spans out like shown in the picture (red lines). The question that I have is about the antenna matching to the RF input line.The setup is as follows.There is a 433 MHz-438 MHz transceiver terminated at 50 ohm. The transceiver output is connected to 50 ohm:73ohm Balun, and from balun to two dipoles. 73 ohm balun is chosen because the impedance of each dipole element is 73 ohm.The schematics is shown in the picture. J5 is the 50hm connection to the transceiver, and J4,J6 are connected to the dipoles via 75 ohm RF cables.The question is whether the approach is OK or am I violating the design for dipole? The two elements of dipole are around 6cm away from each other. I am following 50ohm matching until balun and after that 73 ohm everywhere.

author: Andy Chee
author_rank: Super Contributor
timestamp: April 04, 2025, 07:46:42 pm
content: If the drone is only communicating with ground stations, I’m inclined to use a quarter wave ground plane antenna mounted upside down underneath the drone.

author: A.Z.
author_rank: Super Contributor
timestamp: April 04, 2025, 09:32:42 pm
content: Quote from: Andy Chee on April 04, 2025, 07:46:42 pma rubber ducky antenna should workalso since with a Lambda of 70 cm the antenna would be pretty high and would offer an almost spherical pattern

author: vk6zgo
author_rank: Super Contributor
timestamp: April 06, 2025, 02:14:25 am
content: Quote from: enemra on April 04, 2025, 02:51:11 pmIf you are operating both dipoles in parallel & you have 1/2 wavelength 75 ohm feeder on each, you need to match to 37.5 ohms Z not 73. 37.5 ohms is a reasonably good match to 50 ohms.

author: antenna
author_rank: Frequent Contributor
timestamp: April 08, 2025, 08:13:05 am
content: I'm going to assume that the two elements form a single dipole. In that case, you should be feeding 73Ω from one coax to both elements with the center conductor going to one, and the shield going to the other. But there is a problem with that....The gap between the two elements must be spanned by wire, and that too becomes part of the antenna. You then have the highest current part of the antenna passing right over the drone control circuitry. Not sure what power you are running, just something to think about. Routing RF to the elements with coax does nothing to solve that problem as there is no ground to connect the shields to at the feed points, so common-mode currents will double back anyhow.I would second Andy Chee and A.Z.'s suggestion, ground plane/rubber duck upside down off the bottom.Regarding your matching system. There is no need for PI networks, simple L-networks will do fine here. PI networks are used where broadband matching is needed, but you are using a single frequency. I would also be tempted to try a high pass match instead of low pass match. High pass puts the inductor to ground which will prevent any static buildup from the prop wash zapping the RF front end.With some copper foil tape on the bottom of the drone arms for a ground plane, A short wire with a weight on the tip holding it down would make a good antenna, and there would be no need to retract it when landing greatly simplifying the implementation. Gravity holds it vertical during flight.

author: A.Z.
author_rank: Super Contributor
timestamp: April 09, 2025, 04:47:26 pm
content: on a second thought an horizontal full wave loop may fit well and offer better performances

author: antenna
author_rank: Frequent Contributor
timestamp: April 09, 2025, 08:18:03 pm
content: Quote from: A.Z. on April 09, 2025, 04:47:26 pmWith a loop there will be nulls. The main lobes are a little wider than a dipole, but that also makes the nulls sharper. It would probably require the ground stations to match polarization too, which creates another problem in knowing where the drone is.. Drone deployable suggests not always knowing where it is launched, so the ground stations would not know which way to orient their antennas.

author: A.Z.
author_rank: Super Contributor
timestamp: April 10, 2025, 07:55:13 am
content: Quote from: antenna on April 09, 2025, 08:18:03 pma HORIZONTAL full wave loop, check the pattern AND consider BOTH the wavelenght (Lambda) AND the distance of the loop from groundHTH

author: A.Z.
author_rank: Super Contributor
timestamp: April 14, 2025, 06:58:14 am
content: Quote from: A.Z. on April 10, 2025, 07:55:13 amhttps://practicalantennas.com/theory/loop/full-wave/#:~:text=Azimuth%20radiation%20pattern%20of%20a,about%201%20dB%20of%20gain.hopefully the above will helpnotice that wth a Lambda of 70cm the antanna (the drone) will be at several WL from ground

author: antenna
author_rank: Frequent Contributor
timestamp: April 14, 2025, 11:26:23 am
content: A.Z., A horizontal full-wave loop in free space is directional You are suggesting the use of a directional antenna for a situation that requires omnidirectional coverage. If you are you implying that a full-wave horizontal loop is omnidirectional in free space, that is wrong. A loop is a directional antenna similar to a dipole, even in free space. The ground changes the lobes due to reflections from the ground, but its not what gives an antenna it's basic directivity. I don't understand what you are trying to tell me.Edit:I think I do now, that the drone will be high enough for the ground stations not to be along the loops radiation axis. The issue there is that omnidirectional coverage will require heights in excess of line-of-sight. Almost twice as high for the expected coverage radius. And there will still be a cross polarization issue.

author: A.Z.
author_rank: Super Contributor
timestamp: April 14, 2025, 02:56:02 pm
content: if you don't like the loop, an eggbeater antenna may fit as well as two crossed dipoles (horizontal)notice that I'm assuming that such external antenna won't be used to "pilot" the drone but for some other type of traffic, heck LoRa may fit pretty well

author: antenna
author_rank: Frequent Contributor
timestamp: April 14, 2025, 07:20:06 pm
content: Its what enemra likes that counts. Eggbeater would work well, but it is bigger and little more complicated to make.

author: A.Z.
author_rank: Super Contributor
timestamp: April 14, 2025, 07:31:26 pm
content: Quote from: antenna on April 14, 2025, 07:20:06 pmsure and either a horz loop or two crosssed dipoles won' t be difficult and would fit quite well allowing the drone to takeoff/land normally

author: A.Z.
author_rank: Super Contributor
timestamp: April 15, 2025, 12:40:04 pm
content: willing to absolutely go for a vertically polarized antennahere is a sleeve dipole which may fithttp://w5sdc.net/duck.htmlbut I think that a crossed dipole or "turnstyle" antenna would be betterhttps://www.researchgate.net/figure/a-A-crossed-dipole-antenna-above-a-finite-square-Pec-surface-b-The-3-D-radiation_fig4_282776048