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SMP linked dipole designer


I was investigating recently various websites and software providing dipole calculations and, since they didn’t do quite what I was looking for, I decided to play around with some ideas. Here’s what I came up with: http://www.sotamaps.org/extras.php - I don’t know if it’s of any use to anybody here…



Brilliant Rob, thanks for making this available!
Roger MW0IDX


Thanks Rob I will give the SMP LDD a try . A very handy tool .
73 de Ian vk5cz …


Brilliant! Thanks for sharing Rob. I can retire my spreadsheet now.

73 Richard G3CWI


Hi Rob,
Interesting. I like the way it works.
I note that your calculated lengths are shorter than the 95% of free space length rule. Any comment?

Also have you chosen a particular wire diameter and is it bare or insulated?



He is obviously calculating the effect of having the elements sloping downwards (about 3% for every 5 Degrees)


Well, as Colin has correctly guessed, the program attempts to correct for the sloping angle of the antenna, in addition to the rule-of-thumb 95% velocity factor for wire. Values used are derived from a program-internal lookup table, rather than on an algorithm: I’m considering making this table editable as a user option.

As to wire diameter and bare/insulated, the correction factors for these are small, but nonetheless highly variable and depend greatly on wire material and physical characteristics, and on the type of insulation used. Such corrections IMHO are therefore hardly worth taking into account in such a program - I leave such considerations to the discretion of the user. Much greater influences on tuning/antenna length would be soil conductivity both in the near and far field, vegetation, rock formations, man-made structures in the vicinity of the antenna, summit topology and - well, I hardly need to “teach my grandmother how to suck eggs”.

From my perspective, the program tries to give a good estimate of the major dimensions of a linked dipole antenna, which may be followed when actually building one. In addition, the graphics should give a good intuitive grasp (it’s to scale) of how the various elements in a linked dipole relate to each other - potentially rather useful for one such as myself, who has never actually seen one, but who may be planning to build one sometime in the near future.



Richard, your spreadsheet was one of the starting points for my investigations, so hearty thanks for having provided it!



Nice tool. Thank you Rob.



Now added options to export in plain text file, or CSV file formats in addition to PDF.



Well done Rob, a great tool. Would it be useful to list what assumptions the designer is making, eg PVC insulated wire of n mm diameter?

73 Andrew G4AFI


Andrew, thanks for the flowers!

See my reply to Ron VK3AFW above. One finds quotes online such as the following:

  • “for thicker wires the length of the antenna must be increased slightly”, and
  • “an HF dipole will be SLIGHTLY shorter when built with insulated wire due to the dielectric effects of the insulation - a common figure is 3%”,

Trying to quantify these accurately in a program, unless it’s NEC, would be a mostly pointless exercise in frustration. You’re welcome to try this yourself, but for me, and for SOTA purposes, it’s a non-starter.

One factor which people often forget in building wire antennas is they often use a soft-copper (non-hard drawn) wire which they may have lying around in the workshop. After it’s been strung up a few times, the copper wire will be found to have stretched considerably, making a mockery of hyper-accurate antenna length calculations. So, hard-drawn copper wire or, better, copper-clad steel wire would be a better choice [Edit: for stability of length over time this may well be true. But see Richard G3CWI’s comment below on the poor handling characteristics of such wires.]

Anyway, just cut your wire sections a little longer than calculated, add a bit more for your connector(s), string the antenna up over “average ground :smile:” and tune each part of the antenna in turn by cutting off small pieces until you’re happy with that section. Start from the center section and work outwards. It ain’t rocket science - I hope!



…not necessarily Rob. The handing characteristics of hard-drawn wires makes them more difficult to use in the field.


Hi Rob,

I wasn’t suggesting the designer would have options for different kinds of wire - just what you have assumed. It may help produce more repeatable results. But, yes, I cut longer and then trim using my antenna analyser.

73 Andrew G4AFI


True enough, Richard, true enough!


See my reply to Ron VK3AFW above, where I stated in broad terms the assumptions of the program:

So, for the same starting conditions, and the same lookup table, the program produces repeatable results. The results may not agree exactly with other dipole calculators to be found online, but one often has little idea of the assumptions or calculation methods used by their authors.



Hi Rob,
I’ve looked up a trusted reference and for an additional 1% reduction in length the wire would need to be about 1% of a half wave thick so your assumption it can be neglected is endorsed.

The nominal 5% end effect must however be influenced by any insulation that has significant thickness as say the DX wire, hook-up wire or electrical wiring wire. I find that the spacing between the link clips when not connected makes a measurable difference to the resonant frequency.

However in practice antenna proximity to vegetation on a summit and I guess also soil/rock characteristic variation all cause similar or greater effects. While I have used my link dipole without an ATU, to optimize the radiated power I think it is warranted to use one, especially if you use both SSB and CW…

So it would probably suffice to make the antenna according to your excellent designer and use an ATU to sort out the inevitable site effects. This would also help compensate for detuning by stretching over time.



Very neat Rob. Tnx
Marc, W4MPS


Program now includes the option to set results for different wire types, insulated, un-insulated, etc. Use the “Settings” button to set wire type. Velocity factors used are probably :wink: close enough for SOTA purposes.

Differences in results are most evident for the lower frequency bands.



Re the velocity factors used in the “LDD” calculations: I notice quite a few people are using antenna wire from DX Wire in Germany. I contacted Peter Bogner, who runs DX Wire to ask for some information on velocity factors for the various types of antenna wire offered by his company, and he was kind enough to provide me full information. He advises a velocity factor of 0.95 for all his insulated braided wires; this is incidentally the “standard” factor used most everywhere.

Nonetheless, and as previously stated, the user of the LDD can choose from a range of velocity factors in the application.