I got my Sotabeams bandhopper in the mail today, and saw in the instructions that it should be mounted at 6 meters elevation, even if the pole is higher. I’ve always heard that “higher is better” when it comes to antennas, does anyone have an idea why this should be at only 6 meters? Is the limiting factor the length of the fastening ropes / risk of having the wrong angle?
Correct. It is designed for that height. They will work at other heights of course but the angles will not be optimal and the resonant frequency will change somewhat.
The SOTABeams Band hopper is a very forgiving antenna. As Richard said it is designed for 6 metres AGL (above ground level) at the centre but I have operated it on a 10 metre pole, I have also had it lower.
The angle of the V is however often decided by the space you have to get it in. I have the 80m version, but I removed the 80m sections as I use it mostly on 40 & 20m - so the angle will be steeper than planned. I have now added my own 60m extensions to the antenna, so again the angle will be different.
I can’t always get the antenna out in a straight line either - so it becomes a V in two directions - it still works fine.
Like I said this is a very forgiving antenna - in my experience it simply works!
Thanks for the quick response. I’ll try it out in some various configurations and see what works with my mast here. I’ve been running on a 12m mast until now, so going down to 6 meters seems a bit strange
Not really. They will still be in a common plane; just not a vertical one.
I was just describing how it looks - the wire wont be straight when looked at from above. In any case this is normally an NVIS antenna so the direction it is laid out is not particularly relevant.
Well, actually, it will - from one particular angle somewhere above it. This is exactly the same as if you want it to look “straight” from the absolute plan view (which I believe is what you aspire to - but not sure what the benefit of that might be) - even in this case, there is still only one exact angle of observation from which the wire looks “straight”.
None from above, unfortunately - I don’t have a drone to take photos on activations, however there are plenty of ground level photos of my antennas on my activations under my blog page at vk2ji.com or dd5lp.com (same site).
In any case, the angles don’t seem to be critical in any case, which was my point. There “may” be a loss, but in my experience, in operation these are not significant enough to affect the activation.
Only if the legs are pulled so tight that they do not sag, i.e. if they were individually straight. Or if the wind is blowing hard and steady enough, and from a particular direction, to force any sag in the wires to lie in the plane defined by the three attachment points of the antenna, and your observation point.
Quite right Rob - my point exactly! Although I prefer the two intersecting lines defining a plane rather than the three points. Entirely equivalent though of course…
Only if the lines are straight…
Which they are - by definition.
In pure math certainly, but we’re discussing wires held at attachment points at their ends, and sagging under tha action of gravity.
Oooo! Oooo! I know this! I know this!
Catenary and it’s given by y = cosh(x). I learnt that when I was 16 and wanted to know what the hell cosh, sinh and tanh buttons did on my new calculator!
In 34 years of engineering, I cannot recall ever using a hypbolic trig function. Lots Euler and de Moivre’s stuff but not hyp trigs.
I wrote a program to solve for equation roots using Newton’s method on my TI59 programmable calculator when I was just 18.
Can I have a prize? Or at least a round of
You get the Big Clap(s) you deserve. But I remember a time last century when I was a machinist in a factory asking another fellow what the cosh, sinh, etc. functions were on the calculator he was using (I was just finishing my A-level Pure Maths at the time). He gave me a very condescending look and told me he used those all the time, and couldn’t be bothered to explain them to me. Uh-huh, said I…
PS - if you like catenaries, this might interest you: https://en.wikipedia.org/wiki/Taq_Kasra
“No wind” is a very rare occasion at my QTH near the North Atlantic coastline. It will probably occur once after doomsday. Generally I only use guy ropes on one side of the antenna, ropes on the back arent needed for short time use.
@M1EYP By definition ropes (that aren’t vertical) are never straight, they will always sag. The sag may not be noticable to a human, but the sag is still there. There are even a lot of nice formulas to calculate this. We spent a few weeks in the mechanical engineering classes on these lovely things.
@MM0FMF You get a prize for using a TI calculator My TI-83 programmable calculator from high school is still my everyday calculator.
Can I suggest a change of diet in that case!
My TI-59 still works. The new NiCds are just about dead now! Card reader no longer works. But apparently it’s the magnetic medium on the cards that has failed with 38 years exposure to the air. The same thing has happened to all those 9track tape reels people saved from their PDPs and VAXen and System 370s.
you get a round of for that too