dipole antenna issues - Page 1
Forum section: electronics / rf-microwave

author: m3vuv
author_rank: Super Contributor
timestamp: September 29, 2019, 02:41:07 pm
content: hi all,im trying to string a dipole up in the loft for 28.300mhz,ive made both legs at 2.52 meters long,which should be in the ball park,its made of pvc coated speaker wire,on im using a vk5jst analyser,that says its 1.3 to 1 swr,however my swr meter says its over 10-1 swr,also my rigs atu wont match it either,its clear of any metalwork and wireing by about 8ft,any ideas why this is.ps.all the coax checks ok,thats rg-213about 12ft long,cheers m3vuv.

author: tautech
author_rank: Super Contributor
timestamp: September 29, 2019, 08:32:04 pm
content: 28.300mhz seems an awfully low frequency, don't you mean MHz ?If you do I suggest you get the designator correct as this IS an engineering forum !

author: fourfathom
author_rank: Super Contributor
timestamp: September 29, 2019, 09:04:12 pm
content: It could be a connector or adaptor problem. I had some UHF-BNC adaptors that were intermittent and would sometimes give me off-the-chart SWR readings. If your analyzer says it's good, look at your interconnections.

author: m3vuv
author_rank: Super Contributor
timestamp: September 30, 2019, 02:08:07 am
content: i suggest you get some new eyes or glasses you twat!!

author: vk6zgo
author_rank: Super Contributor
timestamp: September 30, 2019, 02:49:40 am
content: Quote from: m3vuv on September 30, 2019, 02:08:07 amSorry, but you are the one who needs glasses.Small "m" means "milli", big "M" means Mega.It ill behoves someone seeking help to get "aggro" at a mild comment like that.We don't have to help you, after all!

author: tautech
author_rank: Super Contributor
timestamp: September 30, 2019, 05:05:40 am
content: Quote from: vk6zgo on September 30, 2019, 02:49:40 amYes the OP is very lazy poster with few capital letters ever used and one might think since he registered in 2016 correct protocol of usage for units and designators in an engineering forum might have been learnt by now.Any Prof or lecturer worth their salt would scribble all over their work in red pen !That nobody has seemed to picked up on this is a little surprising yet just a single mention seems to have hit a raw nerve.Anyways it's not that the OP's forum involvement is at question only the sloppy way he/she uses Hz, V, A, F and numerous device names.Is it wrong to set expected standards of electronic engineering forum participation ?Back to the OP's question, he/she has made a dipole that gives unexpected results fed by coax and apparently without the prerequisite matching balun or even ferrites to control coax braid currents.Some good reading here:https://www.eevblog.com/forum/rf-microwave/yagi-uda-antenna-driven-element-impedance-question/

author: fourfathom
author_rank: Super Contributor
timestamp: September 30, 2019, 06:28:55 am
content: It sounds like there's some history here, but I don't care about any of that garbage. Yes, it should be "MHz".Moving on, it is common to connect a piece of coax directly to a dipole, without a balun or choke. You can still get a usefully low SWR in most cases. Yes, there will be shield current and pattern imbalances, and even a little impedance mismatch, but a 1.5:1 SWR is typical for this. It will work as a transmit and receive antenna. I've done this myself when I'm in a hurry or don't want to be bothered with the finer points.a 10:1 SWR has nothing to do with not having a balun or a choke. Instead, it means the antenna is significantly mistuned, or we have a cable or connector problem. Since the impedance analyzer says the SWR is 1.2:1, then the problem isn't the antenna or the coax. That leaves a connector or interconnect cable problem. Or a broken SWR meter. Or misreading the SWR meter. Or something else, but it's not the lack of a balun.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 30, 2019, 07:13:07 am
content: OP, where did you connect the analyser?If it was directly at the feedpoint of the antenna, the logical next step is to connect the coax, then check again at the coax input.Did you still have the SWR bridge on circuit when you tried to get the internal ATU to match?Of so, remove the SWR bridge from circuit & try again.Such devices aren't "bulletproof", so one or both sides of the transmission path may be open circuit internally.

author: radiolistener
author_rank: Super Contributor
timestamp: September 30, 2019, 05:13:04 pm
content: Probably there is an issue with feeder routing. The feeder routing must be strictly perpendicular to the dipole.