Is using a standard parabolic dish as 2.4GHz antenna possible ? - Page 1
Forum section: electronics / rf-microwave

author: whatsthegeek
author_rank: Contributor
timestamp: February 04, 2018, 06:57:48 pm
content: Hi, this is probably a very n00b questionbut:Can I use an off the shelf satelite dish (Without the included recieving head) and mount my own antena in front of the dishto get better reception ?if yes, what type of antena should I mount at the "focal point" ?I'm using an nRF24l01+ with a maximum power output of 0dBm and an input sensitivity of -94dBmHope an RF guru can help me !Thanks in advance

author: babysitter
author_rank: Frequent Contributor
timestamp: February 04, 2018, 07:35:44 pm
content: https://www.everythingrf.com/rf-calculators/parabolic-reflector-antenna-gainGives you a online calculator.http://www.radio-electronics.com/info/antennas/parabolic/parabolic-reflector-antenna-gain.phpTells you that that mysterious efficiency parameter typically can get to 50 or 70%.So at 1m parabolic dish size you get a total gain slightly above 26 dB in the best case.Your direct satellite dish is typically a offset-type (which means its just a part of a bigger parabloic). I guess you should select the feedpoint antenna so that it illuminates the total reflector size (not much less and not much more) and use the right polarisation. Also keep in mind that the direction of gain is, well, offset.Some years ago, Philips sold a receiver for meteosat apt direct reception at 1.7 GHz, they used a disc-yagi-type antenna for the feed.Just google for stuff that has been done to see if it might be the right thing for you.Vy 73Hendrik

author: whatsthegeek
author_rank: Contributor
timestamp: February 04, 2018, 08:13:15 pm
content: Thanks a lot ! Realy usefull

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 04, 2018, 09:03:43 pm
content: https://www.ab9il.net/wlan-projects/wifi3.html

author: whatsthegeek
author_rank: Contributor
timestamp: February 04, 2018, 10:04:13 pm
content: Realy nice too, thanks !I'm gonna go with a helical antena as feed and an old 1m TV parabolaThis is the antena I calculated for my feed:and here are the performance results:Since the beam width is 52° I guess I'll have to set it 1025mm away from the parabola ?I'll use an omni-directional antena as for the transmitter since it's in an RC plane and moves a lot.Does this seem good ?

author: LaserSteve
author_rank: Super Contributor
timestamp: February 04, 2018, 10:39:43 pm
content: The W1GHZ dish design page is a good read.Steve

author: yl3akb
author_rank: Regular Contributor
timestamp: February 05, 2018, 08:58:45 am
content: Quote from: whatsthegeek on February 04, 2018, 10:04:13 pmNo, You must put the feed antenna in focus of the parabola, otherwise it will be defocused and efficiency will drop. And calculate the feed dimensions using subtended angle between focus point and dish edges. 3dB beamwidth should be smaller than substended angle, because You want illumination taper (ideally arround -10 dB) at the edges, otherwise You got spillover and again efficiency will drop. See page 9 here for visualization:http://www.w1ghz.org/antbook/chap4.pdf

author: whatsthegeek
author_rank: Contributor
timestamp: February 05, 2018, 04:55:28 pm
content: oh ok, so, what type of antenna would you recommand for the feed ?

author: yl3akb
author_rank: Regular Contributor
timestamp: February 06, 2018, 09:48:28 am
content: Helical antenna is good choice in Your application (RC plane comm?) because of its circular polarization - You will not have to worry about cross polarization losses in case Your RC plane uses linear pol. antenna. Remember that in case of circular pol. each reflection changes polarization sense, in other words, if Your feed is right circular, resulting pol. will be left circular. This is important if antenna of Your RC plane is also circular polarized.About feed calculations. Do You know the focal length of Your dish (more importantly ratio of focal length to diameter of f/D)? Assuming Your f/D is ~0.35...0.4 (typical for non-offset dish), the substended angle is ~130 deg. For -10dB edge taper required -3dB beamwidth (HPBW) of the feed should be around 80 deg (I am using my own spreadsheet for calculations here).I checked two calculators (http://jcoppens.com/ant/helix/calc.en.phpandhttp://www.daycounter.com/Calculators/Helical-Antenna-Design-Calculator.phtml) with same parameters: f=2.4GHz,N=2,and S=0.25. One gives HPBW 56 deg (edge taper -20 dB) other 74 deg (edge taper -11 dB). Last one would be prefect. I would model the antenna, but in Your case just try to build it. In page 11 of document I posted, we can see that with increasing edge taper efficiency decreases quite slow anyway.You will also have to build matching section because helical antenna natural impedance is typicaly 100...150 ohms.W1GHZ also have article on helical feeds:http://www.w1ghz.org/antbook/conf/Helical_feed_antennas.pdfYou may be interested in 'Deep dishes' chapter.

author: whatsthegeek
author_rank: Contributor
timestamp: February 07, 2018, 05:00:23 pm
content: Thanks for the advice !Isn't there a more efficient antenna type to use as a feed for a standard parabola ?I'm aiming for the highest gain possible(I don't know a lot about antennas btw, I'm just starting with RF ^^)

author: cdev
author_rank: Super Contributor
timestamp: February 08, 2018, 12:57:32 am
content: What is your application? How far does this link have to traverse?Sure, you can use woks, etc. as reflectors and you will get some gain.I am reminded of the old "wok fi" web site - a very old web page which is now gone - that had tons of ideas using USB dongles and various reflectors made from cooking utensils, etc. to establish medium range wifi links in new zealand.https://en.wikipedia.org/wiki/WokFihad a link to it -https://web.archive.org/web/20140817040813/http://www.usbwifi.orconhosting.net.nz/)Note that unless your reflector is mesh its likely to have a lot of wind loading.A yagi would probably be the easiest high gain 2.4 GHz gain antenna to make.If you use a dish, use something like a biquad for your feed..It would likely be the best feed for a repurposed off center feed dish.Look up Trevor Marshall's design.http://www.trevormarshall.com/biquad.htmUsing a helical antenna as a feed for a dish is not optimal because of the phase center issue.Also, just a hunch but the area around the base of your helix doesn't look right. The launching off area is critical for the performance of a helical antenna. You may want a matching stub, some kind of a gamma match.The N connector should not be in the center, it should be at the base of the helix which is not the same thing.Also, take care that the piping you use is low RF loss pipe. Some pipe is and some pipe isn't. I went through several different kinds of pipe several years ago building experimental GPS antennas.Use thin pipe. Very thin plastic pipe.

author: whatsthegeek
author_rank: Contributor
timestamp: February 08, 2018, 09:27:30 pm
content: QuoteIt will be used as a telemetry/command link for the autopilot of an RC plane. If the testing goes well, it should also be used in a model rocket.This means it needs the longest range possible. I already have a gimbal to point the antenna into the correct direction automaticly.QuoteIt's just a normal 1m satelite antenna my neibours threw out. They used to have it on their roof and it held through several storms, so it's pretty solidQuoteyup, from the documents other people here sent, it looks to be the most efficient !QuoteI could be wrong, but, can't I just mount some platic arms to hold the feed right in the center ?Thanks for all the usefull info !