Wire antennas and knots - Page 1
Forum section: electronics / rf-microwave

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 26, 2016, 11:07:20 am
content: Hi! This has been nagging me for a while, and any antenna design resource I've consulted doesn't really seem to cover it.When making a wire antenna, one can't avoid having a knot at the ends. likely at the feed point as well, if it's a dipole or such.How do you calculate wire length, then? do you count the wire in the knot, or loop, if U-clamps are used, or does the knot have so much choke effect, that it can be disregarded all together?Of course you could just make the wire a little too long, and then test it, and trim it for optimal length, but since I'm mostly dealing with receiving antennas, it'd be nicer to have a rule-of-thumb or two behind me.Thanks in advance!

author: JoeyG
author_rank: Regular Contributor
timestamp: September 26, 2016, 12:57:26 pm
content: What frequency?At 30MHz (10metre wavelength) the knot is <1% of the overall wavelength,At 3GHZ (0.01metre wavelength) the knot is more like 30% of the overall wavelength.http://www.mobilemark.com/engineering/antenna-terminology-defined/https://en.wikipedia.org/wiki/Electrical_lengthhttp://www.pulseelectronics.com/download/3248/antenna_basic_concepts/pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 26, 2016, 03:43:52 pm
content: If the knot is tied in bare conductor, it matters very little. The extra width will manifest as end-effect, making it a little longer than the total physical dimension.If insulated, then the folded-back bit will lag behind the incoming wave slightly, giving a longer length error. But not equal to the total length knotted up, not nearly.There's no way to calculate it, but you'll probably be trimming length anyway, so that should be fine. It's also in the safe (longer) direction.Tim

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 26, 2016, 03:53:10 pm
content: Why aren't more people building microwave wire-antennas?Nah, Sorry, to clarify, I was talking about low HF bands, maybe 160-20m.But I get that the error in length will be tiny compared to the overall wire length, thanks!

author: Ammar
author_rank: Regular Contributor
timestamp: September 30, 2016, 12:48:47 am
content: Like several have pointed out, if the knot is small compared to the length, it really will not make a difference.The theory says that a thicker wire will make the antenna electrically longer than its physical length. This is denoted by a correction factor k. Typically, a half wave dipole will be 0.48 of lambda rather than 0.5 lambda to correct for this. This is with regards to the thickness of the wire.To be more specific to your question, conductive lumps at the end of the antenna will create capacitive "hats" and increase the capacitive reactance. A technique that can be used intentionally for impedance matching. But in your case, this increase may be insignificant.

author: tautech
author_rank: Super Contributor
timestamp: September 30, 2016, 01:21:12 am
content: There's knots in wire and there's good knots in wire.Here's some I use on my farm:http://www.cyclonewire.co.nz/file/fencing-facts-brochure.pdfOne we never use for wire is the reef knot.The best end strain knot and has the least amount of wire used (turns) is shown on P2 as the strainer tie off knot.We commonly use this knot with 2 final wraps and it has tremendous holding strength.I'd happily use it with any wire at any strain around any termination.

author: German_EE
author_rank: Super Contributor
timestamp: October 02, 2016, 11:17:42 am
content: I prefer the Albright knot:http://www.animatedknots.com/albright/index.php?Categ=fishing&LogoImage=LogoGrog.jpg&Website=www.animatedknots.com

author: tggzzz
author_rank: Super Contributor
timestamp: October 02, 2016, 12:21:46 pm
content: Quote from: tautech on September 30, 2016, 01:21:12 amI'm not surprised the crimp sleeve has the highest breaking strain. We use those, with two sleeves (one at each end of a 6" overlap) when splicing the cable use to winch-launch 500-1000kg gliders. Winch launching involves accelerating that mass to 50mph in ~4s, so the tension is significant - and it ought to be relatively easy to calculate. Yes, the splices part company, but only after noticable parts of the copper sleeve have been abraded.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: October 02, 2016, 12:47:33 pm
content: Don't worry about the knot. Unless you do a really sloppy knot it is a smaller effect than the sag in the wire which is also usually neglected.

author: tautech
author_rank: Super Contributor
timestamp: October 02, 2016, 05:52:05 pm
content: Quote from: tggzzz on October 02, 2016, 12:21:46 pmThe alloy crimps we use for HT wire have grit within for additional grip and for a tidy joint we normally do a wrap either side with the unused tail, then roll the tail back over the crimp while under tension so it snaps off clean. In the early days when these crimps first saw use it was considered necessary to do so in case they slipped. They dont.