7dB_COLINEAR_2mts__by_SM0VPO.pdf

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7dB COLINEAR ANTENNA
by Harry Lythall - SM0VPO
Introduction
A little while ago (about 6 years to be exact) the best antenna I had for 145MHz was the 1/4-wave mag-
mount antenna on the roof of my car. So I decided I wanted a bit more gain. I was able to get more by
using
a pair of end-fed 1/2-wave antennas.
This gave about +3.5dBd, which is about 6dB more than the
1/4-wave antenna. The antenna is still nailled to the side of the house and I can access four repeaters,
between Uppsalla and Stockholm.
Now that I have become a bit more active on the bands I want more than just to talk on the local repeater. I
want VHF SSB, so I need a decent horisontal Yagi. I also want to put out a respectable signal on FM,
vertically polarised. Location can be more important than gain, but having a bit extra gain to start with
helps.
This project is about a cheap way of building a colinear antenna for VHF 145MHz, and having about 10dB
more gain than that little 1/4-wave magmount I have not used for 6 years. The actual gain of this antenna is
+7.5dBd.
It consists of five half-wave dipoles, four of which are end-fed by means of a 1/4-wave stub to invert the
phase. The center element is centre-fed at 200 Ohms by means of a 4:1 coaxial balun from the 50 Ohm
coaxial feed cable. Elements and coils are all wound from just two lengths of wire, without the ned for
joints, junctions, soldering or loads of bolts. The most complicated bit is knocking the 12 nails in four bits
of wood.
Materials
The materials consist of 9 metres of multi-strand power cable, with a conductor diameter of about 1.5mm. I
used 32-strand 10-Ampere cable. You also need three sections of 50mm Diameter plastic drainpipe: the
sort that just pushes together with rubber seals. The only other thing needed is four coil formers, so let us
start with those.
The materials needed for the stub coils.
Measure the wire diameter, WITH INSULATION and multiply by 16. Mine is 2.5mm so my coils will be
2.5mm x 16 = 40mm long. My formers are 60mm long, so I have 10mm free at each end when I hammer in
the nails (panel pins).
You need to measure the wire diameter.
The next thing to do is to find the formers. I used 26mm Diameter wooden dowel. This I stole from the
bedroom and guest room windows. Maj-Lis likes those wooden rod curtain supports, and she has not
missed 60mm missing from each of the four windows :-) but if anyone tells her I will kill them!
Construction
The object is to wind a 1/4-wave stub: that is 1/2-wavelength of wire, folded in half, the two ends being the
feed points. The stub will be coilled up, so calculate your 1/4-wave and multiply by 0.8 to get the true
length. With 26mm Diameter formers, 145MHz, this is exactly 408mm, which just happens to be exactly
five turns. So I need to wind five turns, then another five turns in the opposite direction. I shall hammer
three nails into each former at 20mm spacing, to wrap the wire around. It looks like this:
The support nails for the coil, and the coil added.
Bear in mind that you assemble each half of the antenna from one end, winding the coils as you go along.
The stub coils just hang in the wire. The end nails are bent over to form hooks.
Element wire lengths are 96% of a calculated 1/2 wavelength (150000 / MHz = mm). For 145.000MHz this
works out to exactly 1 metre. So all you have to do is to wind the coils at 1 metre intervals, then cut the
center of the centre element to feed the antenna. All the coils then hang on the wire.
The finished stub positioned in the antenna wire.
Feeding the Antenna
I connected the feeder to the antenna by means of a "sockerbit" connector block: the kind you use to
connect light power cables. You need two contacts, but you could use three and use it to couple the three
coaxial braids (shields) together. Note the bit of clear plastic tube around the connector. This tube takes the
weight of the lower element and prevents the feed point and balun being drawn down into the pipe.
The connector feeding the antenna elements.
The balun is composed of a bit of coaxial cable. It must be 1/2-wavelength long, multiplied by the velocity
factor (VF) given in the cable specifications. UR76 has a VF of 0.66 so I need a length of 0.66 x 1040mm =
686mm. You should then short-circuit both ends and use a Grid Dip Oscillator (GDO) to check the balun
resonance is indeed 145MHz.
If you do not have the luxury of a
GDO, then you can build my design.
Alternatively you can assemble the
feeder and balun, terminate it with a 220 Ohm resistor, then check the VSWR across the entire 2 metre
band. If it is best at 144MHz and poor at 146MHz, then you need to cut a bit off the balun coax, reassemble
and test again. The VSWR should be almost perfect at 145MHz.
The completed balun feeding the antenna.
So to take a step back and look at the overall antenna, with all elements, coils, feeder and coaxial balun, we
have a simple antenna with just a few coils. Remember that there is no break in the wire from the top to the
feed-boint, and from the feed point to the bottom.
The complete antenna arrangement.
Supporting the Antenna
The antenna is enclosed in plastic drainpipes. These things just push together and come in 2 metre lengths.
This is bad news because the elements total 2.6 + 2.6 metres. So you need four of these tubes. One for the
upper element, one for the lower element, and one for the horisontal feed and support tube. The fourth tube
is cut into shorter bits to extend two vertical tubes by about 600mm. The bottom tube can be extended by
much more so that it can stand on the roof, or floor, for support.

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