Dipole with uneven elements - Page 1
Forum section: electronics / rf-microwave

author: @rt
author_rank: Super Contributor
timestamp: May 30, 2019, 04:58:26 am
content: Hi GuysI have a Comet H-422 multiband dipole that is supposed to be assembled so that one 40m element is always 200 or 300mm longer than the other.The 200/300mm depends on wether or not it’s assembled straight, or in V configuration.I’m wondering why this is?The element with the longer of the 40m extensions is supposed to connect to a particular terminal on the supplied balun.I’m assuming this extra length for the other element is somehow made up for electrically inside the balun.The reason I ask, is because at some stage the supplied balun was changed (to CBL-2500), which has no markings to distinguish either of it’s antenna terminals,and wether or not the 40m elements should now be adjusted to the same length.Cheers, B.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 30, 2019, 05:57:03 am
content: It extends the bandwidth by using your feedline as an active element.In other words, a dipole (as such), it ain't.Tim

author: @rt
author_rank: Super Contributor
timestamp: May 30, 2019, 08:38:18 am
content: Ok, but I’m no wiser what to do with the balun that is now supplied with the antenna.The original one had a marked terminal, but this one doesn’t.Should I even up the elements for this one?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 30, 2019, 07:22:07 pm
content: Impossible to say. If it's a current balun, it will partially defeat the imbalance, and reduce shield current on the feedline. If it's a voltage balun, it will dutifully drive all three elements.Can you remove the cover and see how it's wired?Tim

author: tkamiya
author_rank: Super Contributor
timestamp: May 30, 2019, 07:23:30 pm
content: It may be that old balun was a voltage type and new one is a current type. Voltage type has one side common to the ground. It is possible it could have some effect. Current type doeesn't care. Either way, it is not going to matter much. In my younger days, I did extensive (but unscientific) experiment with dipoles. I never saw any difference. If you like, wire it one way and wire it the other way, to see if there are any difference.I have heard of experimental antenna by varying antenna length, trying to compensate for current imbalance caused by LACK of balun. I never heard of commercial product using such a dubious mt hod. Personally, I would make it the same to start. Then adjustment process will determine its (their) final length. Since this is a rotatable dipole, as you turn it, VSWR will change, too.Antenna assembly dimension is just a starting point. Depending on installation environment, you will have to adjust it, especially for highly loaded bands, like 40 meters.

author: @rt
author_rank: Super Contributor
timestamp: May 30, 2019, 10:03:06 pm
content: Hi,There’s a picture of the balun I have now already pulled apart here:https://ce4wjk.files.wordpress.com/2012/08/balun-cbl-2500.jpgAt first glance I thought the ground side was connected straight through, but not quite.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 30, 2019, 10:06:47 pm
content: Looks like a triple winding, TLT style, so, voltage mode it is.Can always put a CMC on the feedline just beside it. Preferably with a ground return to the nearest (larger) structure, e.g. metal tower, to sink remaining ground current.Tim

author: tkamiya
author_rank: Super Contributor
timestamp: May 30, 2019, 10:11:42 pm
content: http://www.benelec.com.au/pdf/1738301_Datasheet.pdfData sheet agrees with you. Voltage it is.But TOROID? Clearly, it's a bar/rod ferrite.

author: tkamiya
author_rank: Super Contributor
timestamp: May 30, 2019, 10:15:05 pm
content: There is nothing special about this balun. A typical voltage type. So if I were to put it up, I'd make the length the same, wire it up and start tuning.

author: @rt
author_rank: Super Contributor
timestamp: May 30, 2019, 11:54:33 pm
content: ThanksIf I’m seeing it right, it’s the driven element that is connected straight through.This is the top half of the case, so the antenna driven element terminal would end up to the left when looking at the balun’s antenna terminals.This is consistent with the original supplied balun, which had the dot marking on the left terminal, and I have read it’s the driven 40m element that is supposed to be longer.I actually cut off the extra 300mm from the element to even them up and see what would happen.It’s not like I can’t just go and buy a length of the same aluminium tube if it was important.The sweet spot on 40m moved up in frequency, which I suppose is to be expected since the element has been shortened.It does look like I’ll need to sleeve the cut part back on to get it back around 7.15 MHz, but could also try adding length to both.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 31, 2019, 12:04:17 am
content: Alternately just add a little loading L to each element, at the balun.Tim

author: @rt
author_rank: Super Contributor
timestamp: May 31, 2019, 12:11:22 am
content: Just coil the flying leads between the balun and antenna?