Can you build a yagi out of plastic? Every element isolated? - Page 1
Forum section: electronics / rf-microwave

author: raspberrypi
author_rank: Frequent Contributor
timestamp: November 21, 2016, 11:55:59 pm
content: I'm messing around with my SDRPlay and started to make a yagi but then realized all the designs have a metal beam down the middle with the directors and reflectors electrically attached to it. I was/am going to make one out of PVC and just stick bits of steel rod across with no direct connection. Goal is to make it adjustable/interchangeable and fit inside my house so I can spin it around in my living room. This gets complicated with a metal beam.

author: Paul Rose
author_rank: Regular Contributor
timestamp: November 22, 2016, 03:57:04 am
content: Yes, a plastic boom for a yagi is fine.Metal boom is a mechanical convenience only.From:https://en.wikipedia.org/wiki/Yagi–Uda_antennaConveniently, the parasitic elements have a node (point of zero RF voltage) at their centre, so they can be attached to a conductive metal support at that point without need of insulation

author: AG6QR
author_rank: Frequent Contributor
timestamp: November 22, 2016, 06:36:22 am
content: I've seen many of them home-built using a nonconductive beam, usually PVC. That works fine.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: November 22, 2016, 09:00:02 pm
content: Conductive vs non-conductive beam require correction during design meaning that dimensions will be different. Some of YAGI design software have this option.

author: metrologist
author_rank: Super Contributor
timestamp: November 22, 2016, 10:45:44 pm
content: I've built several successful antennas, a few Yagis, and one 4 element with PVC boom using info from here:http://www.qsl.net/w4sat/antenna.htm

author: raspberrypi
author_rank: Frequent Contributor
timestamp: January 11, 2017, 09:30:42 am
content: I have built several since this thread all from PVC or wood dowels. They work really well. When I find my SD card Ill post pics and DIY's. Amazing how much you learn just by playing around.

author: slicendice
author_rank: Frequent Contributor
timestamp: January 11, 2017, 02:14:11 pm
content: I built a Yagi for 3G mobile data out of a wooden boom and aluminum rods(got it somewhere in my junkbox). It worked great for my needs.Was planning on making another one using metal only, but never purchased the appropriate boom.The calculated distance between each rod becomes very different if metal only design compared to using non conductive boom. A design with insulated but metallic parts only has yet again very different optimal values.There are a lot of calculators on the internet that supports all 3 types of antenna designs.HEREis one example of a collection of calculators needed to complete the antenna.

author: Spuddevans
author_rank: Unknown
timestamp: January 11, 2017, 05:14:13 pm
content: My Local Ham radio club made a whole bunch of Radio Detection Finder Yagi's, they used 20mm round electrical conduit for the boom, and 3.2mm ali' Tig wires for the elements. It was for the 2m (144-145Mhz) band.To make it easily collapsible we drilled out some M5 nylon screws for a friction grip on the Tig wires, then cross-drilled the 20mm conduit in the appropriate places.Worked really well, and they collapse easily for storage.Tim

author: 4cx10000
author_rank: Regular Contributor
timestamp: January 11, 2017, 09:59:38 pm
content: No problem whatsoever!I have built an array of 8X10 element Yagis for 2 meters with boom made out of wood. Perfect antenna until we had a nasty autumn storm... Here is a link that has some good points on this subject:http://www.qsl.net/yu1aw/Misc/boom_yagi.pdf

author: Ammar
author_rank: Regular Contributor
timestamp: January 14, 2017, 08:06:00 am
content: Quote from: Paul Rose on November 22, 2016, 03:57:04 am+1

author: cdev
author_rank: Super Contributor
timestamp: January 15, 2017, 08:17:08 pm
content: A Yagi antenna is nothing more than a lens for RF. The principle is very similar.Here's a slightly more interesting question, can we build antennas out of materials that partially conduct, i.e. "resistors", and get interesting properties out of varying their shapes and resistive qualities?That is an area that I think has been investigated only just barely.

author: Spuddevans
author_rank: Unknown
timestamp: January 15, 2017, 08:40:03 pm
content: Quote from: cdev on January 15, 2017, 08:17:08 pmYou certainly can build an antenna out of partially conductive materials, but I think you would be disappointed with the results. Think about the purpose of an antenna, to receive (or transmit, or both) signals. With reception, unless you live close to a powerful transmitter, you are trying to get a fairly weak electro-magnetic wave from the atmosphere and amplify the uV's of a signal, process it and then perhaps amplify it enough to drive a loudspeaker.The Antenna needs to have as minimal losses as possible for the frequency desired, adding any additional resistance will add losses, making receiving those relatively weak signals much harder, if not impossible.There's no getting round physics, there are no miracle antennas, yes there are some good designs but all antennas will have limitations, the limits of physics.Tim

author: cdev
author_rank: Super Contributor
timestamp: January 15, 2017, 09:22:24 pm
content: Sometimes, you might actually want loss.

author: Spuddevans
author_rank: Unknown
timestamp: January 15, 2017, 09:32:44 pm
content: Quote from: cdev on January 15, 2017, 09:22:24 pmIt would be interesting to see what cases those would beIf de-sensitivity is the goal, the lack of an antenna would make economic sense.

author: cdev
author_rank: Super Contributor
timestamp: January 15, 2017, 09:50:51 pm
content: Rhombics and other traveling wave antennas may use a resistor to terminate the ends. Another example would be a size reduction of a circularly polarized multi-arm antenna, surrounded by a shorting ring. The ams would either be connected directly to the shorting ring or left open but both approaches have their disadvantages. The optimal solution turns out to be using smd resistors to terminate the ams to the shorting ring. By doing that you can extend the lower frequency coverage of an antenna of a given size by around 25% as well as smooth out the irregularities in the frequency response (its a broadband antenna)Another application for loss which clearly has practical benefits are cavity backed antennas where the cavity is filled with some RF absorbent material.In that application the goal is to completely null out any reflected signals from below. Most of the spiral type circularly polarized antennas are inherently bidirectional, but for a space to ground application if you are on the ground you just want one direction.There are a number of ways to do it, one is something called a choke ring which is a quite frequency selective trap for reflections. (it looks a bit like a circular cake mold) Another is a cavity which can be made to be fairly broadband using resistive materials. That can be built inside of a can that forms the back side of your spiral. Its popular in military broadband antennas too.Also, it seems some people build intentionally lossy antennas (such as verticals with lossy baluns) They clearly have their fans, because of the ease of tuning. A lot of people seem happy to accept a fairly lossy antenna (or balun) because it gives them perceived benefits in ease of tuning. (My memory may be fuzzy on this, there may be other reasons, less need for radials, or some such)

author: Spuddevans
author_rank: Unknown
timestamp: January 15, 2017, 10:25:41 pm
content: That makes more sense, I was misled into thinking you wanted to make an antenna entirely out of partially conductive materials.As you mention, there are many antenna designs that call for less-than-perfect conductors (or even what appears to be a total short, ie loops)The Antenna is a tuned circuit (or part of a tuned circuit), hence you see many designs using Inductors, Capacitors and Resistors.Re: lossy antennas and their ease of tuning; I remember my amateur radio instructor repeatedly telling us that the easiest antenna to tune is always a Dummy load.

author: neil t
author_rank: Regular Contributor
timestamp: January 15, 2017, 11:13:23 pm
content: Quote from: raspberrypi on November 21, 2016, 11:55:59 pmgenerally yagi elements are isolated from the beam, the beam or boom can be non conductive, in my experience steel rod is not that good an element material, copper brass aluminium are far better, depending on the frequency of use even stout copper wire works well especially in the upper bands ie an indoor yagi size limitation. one thing tho is the element diameter is important, the larger the element diameter the better the bandwidth.

author: cdev
author_rank: Super Contributor
timestamp: January 16, 2017, 01:53:48 am
content: A log periodic can be as simple as two zig zag wires fed at the very front.