Radials for Vertical (length) - Page 1
Forum section: electronics / rf-microwave

author: @rt
author_rank: Super Contributor
timestamp: October 31, 2016, 10:49:47 am
content: Hi GuysI’ve picked up somewhere, such as radio club, that when using wire radials as the ground plane for a vertical antenna,that the radials should be 1/4 of the wavelength (or at least 1/4 wavelength).From this I assume the ground plane being improvised is a circular shape 1/2 wavelength in diameter?Cheers

author: Ammar
author_rank: Regular Contributor
timestamp: October 31, 2016, 11:17:52 am
content: Yes, a quarter wavelength vertical wire against a ground plane will make for a nice antenna. This is called a monopole antenna.I am not sure where you have got the idea about the size of the ground plane though. Ideally, the ground plane would be infinite. A ground plane creates a kind of "mirror image" of what is above it. Therefore, your quarter wavelength monopole is a sort of sneaky dipole antenna, with slightly different characteristics (eg. half the radiation resistance).

author: mojoe
author_rank: Frequent Contributor
timestamp: October 31, 2016, 02:49:10 pm
content: Quote from: @rt on October 31, 2016, 10:49:47 amIf you are asking about using wire (usually four or more) to make a ground plane, then yes, the wires should be 1/4 wavelength long (or longer). It also depends on what frequency antenna you are making and where you are mounting it.For a typical homemade VHF/UHF 1/4 wave groundplane antenna that is mounted multiple wavelengths above ground, you use four stiff wires (brazing rod or brass rod is common) for the groundplane and angle them down about 45 degrees. The reason to angle them down is to raise the feedpoint impedance, to better match the 50 Ohm impedance of the radio. The exact length of the four wires is not critical. The length of the radiator is and will need to be trimmed for the desired frequency.For a vehicle mounted 1/4 wave or 5/8 wave VHF/UHF antenna, the metal body of the vehicle is the groundplane. This is why fiberglass bodies present a problem. You either need to add a sheet of metal for the groundplane, or you use a "no groundplane antenna", which is simply an end fed 1/2 wave antenna (like a dipole, only fed at one end instead of the center).If you are talking about a ground-mounted HF antenna, then you want as many ground wires as possible and you want to bury them, if possible. With a ground mounted vertical, or one mounted close to ground level, you get very high ground loss. In other words, a lot of your RF signal is wasted by heating the earth.If you can mount your HF antenna well above ground level, only four wires for the groundplane work as well as many wires at ground level (like the VHF/UHF antenna above). Of course, with the longer wavelengths, mounting a vertical HF antenna high in the air can be a problem. On the upper HF bands, it is more practical.All of the above assumes we are talking about an antenna for single band use. Multiband antennas get more complicated.For more details and even formulas, look into any edition of the Amateur Radio Handbook, or one of their Antenna Handbooks. With some Googling, you can also find this information on the net.

author: @rt
author_rank: Super Contributor
timestamp: November 01, 2016, 02:02:08 am
content: Hi againThanks for the replies, but I also really forgot to ask my questionWhere does the 1/4 wavelength size come from? (Or at least 1/4 wavelength)I understand the measurement is not critical, but I'm trying to understandWhy a bunch of ceramic GPS antennas don't work as expected.I have made good ones, but they have smaller than the recommended 40mmground plane, and I can't relate 40mm to the wavelength for 1575 MHz.I have more than a passing interest in radio, but my antenna hereis a multi band (trapped) dipole for HF, so it's not really about that.

author: mojoe
author_rank: Frequent Contributor
timestamp: November 01, 2016, 04:46:55 am
content: Well, you began by asking about 1/4 wave antennas and ground radials. This is a good beginning question. Jumping into GPS patch antennas before you understand the basics isn't going to get you anywhere fast. You should look into the references that I mentioned. Everything starts with an explanation of the dipole antenna and builds from there.

author: @rt
author_rank: Super Contributor
timestamp: November 01, 2016, 05:51:25 am
content: The first post was supposed to include another paragraph relating to the GPS antenna, but it was lateMy understanding so far of the ceramic antennas is that the size of the ground plane will affect resonanceespecially since the distance from the signal element is constant with the thickness of the ceramic.That makes it easy for the manufacturer to make recommendations, as they know the resonant f won’t be draggeddown lower due to closer proximity to the ground plane.but I am stuck at why a particular measurement of 40mm relates to the frequency of the antenna (if it really does at all).There is very little transmission line, as I have cut the SMA connector right down to the butt and mounted it directly to the copper ground plane.My question doesn’t arise from any problem with making a good antenna, only that my best ones have a smaller ground plane than recommended.Below two examples where the first one according to manufacturer recommendations, and barely works at all (it does get the time).The second, the ground plane is supposed to be at least 40mm, but with ground plane 28mm, works a lot better, and about as goodindoors as far as time to fix, as a QFH antenna.

author: Ammar
author_rank: Regular Contributor
timestamp: November 01, 2016, 10:07:21 am
content: Quote from: mojoe on November 01, 2016, 04:46:55 amAbsolutely does