Now we've moved back
to the UK from Spain, all my wonderful aerials have been exchanged for 4
simple dipoles in the loft of our rented house! The dipoles over 20/17/15
and 10 metres but are also resonant on 40m and 12m. At least I can get back
onto some of the WACRAL nets, but chasing DX may have to wait until we move
in a year's time to our final abode. For the same reasons, the Linear isn't
used, as I wish to be a good neighbour......
The Problem with Grandchildren…..
The
cause of all the trouble was Toby. You have no idea of the things we’ve had
to do because of him. Looking at him – see picture right – you’d never think
he could be such a problem. However, as grandparents, we find that he - and
his brother Thomas - are a joy to be around. So, because of them, we’ve
moved back from Spain to England, after almost eight happy years there.
Living in a rented house means we’re very restricted
with aerials: after doing a bit of research [on the web, with dear old
Google, where else?] I decided to see what could be done by way of “stealth”
or “indoor” aerials, and finally opted for a nest of dipoles in the loft.
I’ve used these before with some success, but not for almost twenty years!
A “nest” of dipoles is simply a number of dipoles with
a common input. As one dipole usually has a high impedance to other,
non-resonant dipoles, they can be fed very simply from a common point. In
other words, a 10 metre dipole won’t cause a problem when using a 17 metre
dipole even though it’s connected to the same transmitter at the same time.
I needed a piece of thick plastic to drill out the
dipole centre connections: but I wasn’t going to be using high power, and
there just happened to be a piece of wood lying on the attic floor…. So a
bit of wood it would be!
The
nuts bolts and washers cost me over £5! No wonder this has become a rich
man’s hobby. That set-up would be enough for 4 dipoles. A couple of spare
holes might be useful for another dipole…. or even to connect the feed line
from the radio.
I stripped the wire out of a short length of old 450
ohm twin feeder, and joined the terminals up. Working in the loft wasn’t
going to be easy, so I opted for butterfly nuts which could be tightened by
hand.
Then I had to make the decision as to how to feed the
dipoles from the radio in the room below. The land lady wouldn’t be too
pleased if I started drilling holes for co-axial cables, so I opted for the
thinnest piece of cable I could find – which happed to be 75 ohm twin: this
is an ideal cable with which to feed dipoles. My dear old friend Richard
G2DYM [now, sadly SK] always used this cable, and he made aerials
professionally. The two wires are so close together, that, in theory, the
current in one cancels out the current in the other: the wire can be bent
and folded and stuck round metal supports, without impairing its efficiency.
It’s very “RF friendly”!
I drilled the tiniest of holes in the ceiling, and
passed the cable through and up into the loft, where I connected it to the
first of the larger bolts with washers, shown on the left hand side in the
picture. The cable came down into the shack three feet above the IC7600
where it was attached to a 1:1 balun, and thence a short run of co-axial
cable to the rig. [An alternative would be a long run of co-ax from the rig
to the dipole centres, and a 1:1 balun to join the dipoles to the co-ax.]
It was time to think of the wire dipoles themselves. I
had plenty of wire from previous aerials and long runs of old earth wire. I
decided to make up dipoles for four bands. Conditions are improving –
there’s DX to be found on most bands at the moment, from 6 metres to 40
metres. So I opted for 10, 15, 17 and 20 metres.
Of course we’re always advised to make each side of
the dipole slightly longer than usual, then, using a VSWR meter, trim the
ends until a 1:1 VSWR is obtained. Excellent advice! However, those who know
me will realise that I am neither the age, nor the shape to go ferreting
around in dark attics with restricted headroom, trimming bits of wire. I
opted for the following dimensions which are for EACH SIDE of the dipole,
and should be resonant more or less in the voice section of the band –
10 metre 8.2 feet
15 metre 11.02 feet
17 metre 12.92 feet
20 metre 16.5 feet
Eyelets were soldered to the ends of each wire.
Crawling around the loft, I separated them as much as possible from each
other [see pic, right].
Then the fun of checking them out began! I have
various ATU options. I can use no ATU at all, just the ATU in the rig, or an
external MFJ “big” ATU.
I started playing on the bands, and found to my
delight that I could tune up, not only on those 4 bands, but on other ones
as well! The table below shows the initial results.
The 40m band was a surprise, but enabled me to get on
several WACRAL nets: however it caused flickering on the TV. Gill didn’t
mind, but I was unwilling to chance causing TVI with the neighbours – even
though they could never guess where it was coming from! So I decided to use
those spare terminals on the wooden block, and cut a dipole for 40 metres.
Each side was 33 feet long. Now this isn’t a huge house, and I had to bend
the dipole through 90 degrees four times [each corner of the house] in order
for it to fit in the attic. The result was a VSWR of 1.9:1 with no ATU at
all, and 1:1 using the rig ATU.
Since installing the dipoles three weeks ago, I’ve
worked stations all round the world, including Australia, Canada, the USA,
plus EA9 and throughout Europe. Of course 100 watts to an indoor dipole
doesn’t compare with 1 kW to a four element beam – and it will take much
longer to break the DX pile-ups. But this simple arrangement has provided
hours of fun for me, and a multitude of contacts. It’s a great pleasure
being possible to get on the WACRAL nets again.
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