(c) 2020 DX Commander Limited - Page 3
Plate and even connecting to the wrong place on the Upper Spreader. Perhaps colour coding might be cool
so that 40m holes and elements are red, all 20m are yellow etc. Find that label printer!
ELEMENT LENGTHS
Pure copper un-insulated wire has a different
“length” requirement to insulated wire so if you cut your
element lengths according to the maths, you will find they are too long.
For instance, the wavelength for 14.225 MHz is 21.09m long (not 20m). A mathematical quarter wave of this
will be 5.27m. It turns out that the wire we use requires factoring a 93-94% change which is why we can cut
the wire shorter.
Anyway, using the maths, we can extrapolate all the other bands and my IOTA version ended up being:
•
10m
– 2.5m + 5 cms foldback (for the heat shrink loop)
•
12m
– 2.82m + 6 cms foldback
•
17m
– 3.84m + 6 cms foldback
•
20m
– 4.82m + 20 cm (longer foldback compensates for slightly shortened element)
•
30m
– 6.74m + 6 cm foldback
•
40m
– 11.20m (foldback back down pole which will also give you physical 5/8th for 15m)
•
(Optional) 80m - 19m of wire (go up to upper spreader and then throw the remaining element over
the nearest tree using paracord and a small weight. Let it
“droop” a bit – don’t over-tension.
30m band: The pole is not suitable for running another UHMWPE spreader high up, like the Classic. Instead
for 30m, you will have to use the larger diameter tubing (supplied) to secure some shock-cord to with a
stopper knot and connect the element to that. Actually, this was the original design for the classic..!
80m: If you fit an inverted L, cut 19m of wire, go up to the upper spreader and head off to the nearest tree.
Light tension only here. Let it
“droop”. Note that 80m will give you 30m and 17m with your rigs ATU function.
ONLY
Fit your heat-shrink and final carabiners to the shock-cord
*after*
you have sorted all the element
lengths out. This way, once your antenna works to your liking, you can heat shrink the elements and do a
final fit of your carabiners to the paracord.
40m and 15m CONUNDRUM
For purists, this is quite a bit to get your mind
around, so bear with me.
A quarter wave for 40m band has a very wide
bandwidth. It’s huge. It means we can trim our
40m element pretty much anywhere we want and
we’ll always be able to get a great tune.
Here’s the s/w plot with an element cut for a
frequency of 7.1 MHz. You’ll notice, we’re still
under 2:1 SWR from 6.9 MHz through to 7.3 MHz.
In fact, under 3:1 SWR, the bandwidth is almost a
whole MHz wide. In the field, it appears even
better.
USING 40m ELEMENT ON 15m BAND
According to the books, dipoles and verticals resonate on every odd harmonic. This means that according to
the maths, our element tuned for 7.1 MHz should also be resonant on 21.3 MHz. If it were that simple, I
wouldn’t be writing this.
In practice however, the third harmonic resonates higher due to capacitance changes (and other strange
phenomena!) with the longer wire. The easy way out of this dilemma would be just to create a longer
element for the 40m band. A longer element by adding another 10cms would still tune the whole of 40m
band just fine and achieve a perfect tune at 21.25 MHz. And this is what I used to do. It would annoy me that
the wire though was slightly longer than the pole itself and the very top of the element would waggle about in
the air.
Sure it worked, it just didn’t look right – and I was always getting slightly higher than designed SWR
at the top of 40m.
