Need a multiband vertical ? , here's some ideas for you
The quest of being on the air, all the time, bad weather no excuse.
Like so many others i have a 14 meter crank up tower which carries all my HF antennas.
The challange with this arrangement is that it has to be lowered when storms are present.
So i have, for some time been thinking of ways to keep me on the air when the tower is down.
A freestanding, self supporting, 33 feet, 4 band vertical to the rescue.
(Component values and tubing dimensions is listed further down.)
Well this might seem like an easy job, just put up some aluminium and stick a tuner at the
bottom. But i would like to be able to run legal limit power into it, and auto-tuners in that
category are pretty pricey. So i decided to go another route, by LC matching 4 bands.
I decided to put up roughly 10 meters of vertical alu. that makes for a handy 1/4 wave
antenna on 40m. I then thought about what other bands were of interest. When the
tower is down, i can still work 20m reasonably well with the tribander on top of the
tower some 6 meters up. But using the vertical on 20m would give me the benefits
of a 1/2 wave antenna, with all the nice low angle radiation that comes with it.
The idea of being able to work bands on the vertical that i dont have antennas
for on the tower, seemed pretty cool to me. So i planned my LC match design for
40m, 30m, 20m, 17m and heres what the finished matchbox looks like.
Remember that if you duplicate any of this and you want to work this design on 20m
,the antenna, being 1/2 wavelength long on 14 Mhz, is extremely high impedance
and you have to use components that will withstand the voltages that comes with it.
these can be in excess of 3-5 Kilovolts, depending on your power !.
The obvious advantage on 17m where the antenna is 5/8 wavelengths long.
On ebay you can get some really cheap 4 channel remote controlled relays and
i decided to use one of them, together with 8 signal relays, to switch in the band
i wanted to work. This way i would have 4 individual LC matched inputs to work with.
In the below image the remote module is on the left. The 4 relay outputs from the
remote control relays are routed through the copper shield plate and controls the
band selection via the 4 x 2 relays on the right.
The relay-sets to the right is 40m (the cable piece) then 20 - 17 - 30
At the bottom of the image is the bias-t circuit which allows DC to be routed via the
coax from the shack to the antenna. The DC is extracted via the 100uH inductor
and some capacitors. RF continues through the orange 10nF mica cap's to the relays.
Here's the bias-t circuit i used. There's many ways to do this with the same result.
In my case, the DC present on the coax is the supply for the remote control module.
You can buy these of the shelf, but hey, they take like 15 minutes to make yourself.
Heres the remote control, just a normal keyfob, range for this set is 200 meters.
The antenna itself is constructed from 3 different diameters aluminium tubing.
the bottom 1 meter section, taking most of the bending forces, is 50mm diameter.
The middle section which is 4 meters long is 40mm diameter.
the top section is 4.75 meters long and 30mm diameter. This makes for one very
sturdy vertical structure that will take a lot of bad weather before failing.
So total height ended up being 9.75 meters. Just experiment with your own
setup and make it resonant on 40m, that will dictate the length you need.
The dimensions i used in EZNEC to simulate the setup was not the exact same
lengths i ended up using because trees and brush next to the antenna will
have some impact on real world dimensions.
This is how i build the setup with the fibreglass insulator between the antenna
and the bracket.
On 20m & 30m the antenna impedance is fairly high so the match section is
build like this.
On 17m C is placed before L, and on 40m it's fed straight through with a piece of cable
between the relays.
Here are the values i ended up with in my design. Use these as a starting point.
40m - straight through (Relays still needed to switch)
30m L: 2.5 uH C: 120 pF (L = 11 turns on 25mm form)
20m L: 2.3 uH C: 30 pF (L = 13 turns on 21mm form)
17m L: 1.7 uH C: 60 pF ((L = 10 turns on 21mm form)
And of course, as with any vertical, a good groundplane is needed !!!
Here's a grab of the impedances on the 4 bands seen from the coax side.
LLast, but not least, be sure to use materials for your insulator that are suitable for RF !.
I bought a 700mm long, diameter 40mm piece of DELRIN..or so i thought.
It turned out to be NYLON. The carbon content of nylon makes it very
UN-suitable for use with RF signals of any significant power.. here's why
My amplifier kept going in to protection mode when i turned up the power.
no wonder, as the arcing is clearly visible here. It must have looked pretty
spespectaclular in the dark cold winter night when this happend :-)
The proof is in the pudding as they say.. heres a quick test run during poor propagation.
Good luck with your projects, and if you fail, keep trying.
It's the only way to learn and improve !.