[quote=“DM1CM, post:20, topic:9751”]He advises a velocity factor of 0.95 for all his insulated braided wires[/quote]The ultra-light DX-wire UL is twisted strands rather than braid. I wonder whether that makes much difference?
I wonder whether the kevlar-cored wire from Clansman antenna kits has the same sort of velocity factor?
Thanks for the web app. Nice and easy to use. 
– 73, Rick M0LEP
Rick, I imagine 
Peter is doing what most of us would do when considering making a wire antenna - use the standard 0.95 figure, cut long and trim 
to tune. It may well be that he HAS no other values for his wires. And yes - twisted 
is what I meant. Sometimes things go astray in translation. 
to 
… 
Glad you like the app - thanks for the 
!!
I did that with some DX-Wire UL and discovered I’d not quite cut it enough longer than calculated. It’ll do, as it’s only about 100kHz high, and I can get it down that much by putting walking poles to raise the ends and make the “V” shallower…
73, Rick M0LEP
I am not convinced that it is meaningful to refer to the velocity factor of a single piece of wire in isolation. A transmission line has a velocity factor, which is determined by its geometry and most importantly by the properties of the dielectric between its conductors. So if you buy a roll of coax, it’s perfectly reasonable to expect to find the velocity factor in the manufacturer’s specification. But a single strand of wire doesn’t really have a velocity factor which can be specified independently of the context in which it is used. If I were a supplier of wire and were asked to specifiy its velocity factor I think I would have to say “What do you mean?”
The factor by which we have to shorten a half wave dipole to achieve a resistive impedance at the feed point depends on all sorts of parameters of the surroundings, and not just on the properties of the wire.
Martyn M1MAJ
[quote=“M1MAJ, post:24, topic:9751”]velocity factor of a single piece of wire in isolation[/quote]I suspect there are quite a few factors at play. The wetness of the ground, the tautness of the antenna, effects of the feeder, exact heighhts and angles, insulator lengths, etc., will all be having some effect. The type of wire and its insulation will probably be one of the more predictable. Wires with different insulating material and thickness do end up needing to be different lengths for the same frequency in a given arrangement. Would “fudge factor” be a more reasonable name? I’m not bothered what it’s called, but it’d be nice to know how one type of wire relates to another, especially if it helps avoid cut-too-short errors…
73, Rick M0LEP
You’re most probably correct in that Velocity Factor is the wrong name for the effect the insulator has on the wire length as VF is a transmission line term. However, if you say the insulation lowers the VF most people will understand what you are trying to express even if it is the wrong term. It seems that the dielectric affects the distributed inductance hence shortening. It’s a truism that wire-antenna == in-situ adjustment.
I’d use the 95% effect for insulated wire and 100% for uninsulated wire and leave it there. If you build the design using the figures published and the SWR is better than 1:1.5 at the frequencies of interest then the antenna is finished: go out and use it! People do get obsesive about the match quality when really quite bad matches are good enough.
I am convinced that is not meaningful to talk about the velocity factor of a wire antenna, and I got a “C” in my fields and waves class.
Antennas are shorter because of capacitive loading against ground.
They really are shorter than the free space wavelength, but please don’t call it a “velocity factor”. That is for transmission lines.
wunder
Well, I chose the phrase Velocity Factor for this app for the simple reasons that
I’d personally agree with all the comments on VF here, and will try to come up with a phrase that results in fewer hackles being raised - wish me luck!
I can’t think of a snappy term, but it is a correction estimate from free space to the real world.
There is some really nice historical research in this article about the 468/f formula for a dipole length in feet. That formula comes from experiments reported in the October 1926 issue of QST. It was for a 40m dipole at a height of 1/8 to 1/4 wavelength.
Walter, thanks for the link - interesting article and discussion. [Edit: “velocity factor” now renamed length-correction factor in app]
Rob
Hi Rob, That’s a great tool you’ve produced! I was wondering if the number of bands could be increased as I’d like to make a linked dipole to cover 40m plus all the bands 20-10m - making a total of 6 bands and the program only supports 5. Apart from GUI space is there a technical reason why one should not have more than 5 bands on the one linked dipole? Could I infact get the required lengths simply by using your program twice - once for the 20m - 10m bands (5) and then do another calculation for just 40,20 & 10m and add the 20m-40m length to the previous data or is there and impact on the lengths depending how many links one puts in?
I’m sure someone will also want 80m or even 160m - so if you can increase the number of bands, perhaps make it even do up to eleven (if possible) - that should then cover all possible needs - 160, 80, 60, 40, 30, 20, 17, 15, 12, 10 & 6m. I’d be very surprised if someone wanted to make one antenna to cover all bands, but it is a possibility (I suppose).
73 Ed.
You may find that if you make a 10m/80m dipole that the long length of unconnected wire when you pull the link has an effect on the optimum length for 10m. i.e. it gets a bit more fiddly getting it right the more you have. I have had occasions when the unconnected 40m/60m sections on my 30/40/60 dipole have a noticeable affect. Pulling all the links makes the antenna a better 30m antenna but I don’t have conclusive proof of that.
160m dipoles are big and if you have them low then the antenna may be resonant but how well it works is another matter. Similar for 80m. If you want 80 and 160 there are probably better compromise antennas out there than a full size low slung dipole. John G4YSS (@g4yss) uses loading coils to reduce the size of his 160m antenna.
In this case I would recommend clipping the 80m crocodile clip back along the 80m wire at the link. This reduces the end capacitance seen on 10m and thus will reduce/stop de-tuning. Of course if you are not using crocodile clips in your design, you may not have this option.
Ed, you’re welcome. If I could lay claim to any kind of expertise at all, it would have to be on the programming side rather than on the technical aspects of our hobby. So, speaking as a very run-of-the-mill radio amateur, I would hazard a guess that there would not be any insurmountable obstacles to having more than five bands on a linked dipole. [Edit: but read the comments by Andy MM0FMF and Richard G3CWI above]
As you rightly surmise, there would be problems on the GUI side to having more than five bands available in the app - I “design” my apps to look OK on the only computer I possess at the moment - an oldish laptop. Of course, I could grab as much of the screen as I would wish, but forcing the user to scroll up and down (or even sideways) to view various parts of a single-window app would be asking too much of the user.
Why five bands? - simply because I’d seen a few other multi-band dipole apps in various places, and none of them offered more than four: so I thought I’d go “one better”. The question is: where does it end? You want six bands, others might want eight, or eleven.
I really think the best way to go if one needs to activate on a very wide range of possible bands - and this is just my personal opinion based purely on what I’ve read from others’ activation experiences - would be to create/design/build one or more linked antennas with various combinations of just a few bands, and then to plan an activation based on just a few bands. So, the activator might take just one or possibly two such antennas up the hill, and covering just those bands he/she plans to use, rather than haul a (say) eleven-band catch-all monstrosity in the rucksack. Just my $0.02-worth…
Oh yes - almost forgot - if you were to plan to combine results from one run of the program with results from a second run for different bands, then you would find that the apex-angles for each resultant antenna would most probably be different, and therefore that the apex-angle correction on the lengths of the various elements would be different.
Rob
Having just read the news about the 2015/2016 official SOTA Challenge, I’ve added the 6 meter band to the range of possible bands to be chosen by the user in the LDD. Hope this can be of some use…
Rob
I think you could fairly easily compensate for this by adding a long length of end rope for the shorter wave band calculation. Trial and error should then give the same apex angle.
73,
Rod
[quote=“M0JLA, post:36, topic:9751”]
Trial and error should then give the same apex angle.
[/quote]Rod - well, of course! But I wanted to make it clear to anybody who might have thought that results could simply be strung together, that there were other factors involved. I think the vast majority of us would be aware of this anyway.
As you say, one can play with center-support height, end-support height, and extender length in different runs of the app to arrive at the same ball-park apex angle.
Rob
I have used this to design my new linked dipole; thanks Rob. Your Designer does make it easy to see how much (or little) the dimensions change when changes to pole height or end-string length are tried. Not made yet, so maybe I will be back with a question or two for public consideration.
73,
Rod
Hi Rob,
I just put together a 5 band (20,17,15,12 & 10) linked dipole and am currently adjusting lengths to get the resonant points where I need them. 10m was spot on but 12m is off and of course affects the others. This is not a problem, once I get the pully on my flagpole un-jammed (one of THOSE days) and get the antenna down, I’m sure I can get everything right.
The point is however I also just ran a calculation of a three band version (40,20 & 10m) and the 10-20m section is very close to the length of three internal sections on the 5 band design, so I believe it is possible to do two calculations and combine them to get a linked dipole with more than 5 band coverage. As others have said 160m and even 80m may throw this logic, but I hope to have linked dipole for 40,20,17,15,12&10m tomorrow once I add the extra 40m sections on the end of the 5-bander and adjust lengths.
That’s all assuming I can un-jam the pulley of course!
P.S. by the same logic, I believe 30m could also be added into the antenna but as 30m is only CW/Digital in Europe I probably wont be using the band (it’s Phone as well in Australia, so if I were still there, I would certainly include 30m).
73 Ed.
P.P.S. I realise the apex angle changes when you extend the length of the antenna by adding extra bands but it doesn’t look like the length of each section changes very much between configurations, an as we make them +5/10% to allow trimming in any case, I think the small changes caused by the extra bands (as long as one adds them BEFORE trimming starts) should not be an issue. I will see if this is the case or not as I have now taken down the flagpole, fixed the pulley and put it back up so that I can do more tests now.
Hi Ed,
anything’s possible, given enough time and resources 
This LDD is just a tool - and, like any tool, in the right hands can produce a decent job. The USER is the designer, not the tool.
Anyway, sounds like you’re having fun there - be sure to let us know how you get on with the antenna(s).
Rob