quagi_6_element_pmr446_ENG.pdf

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Homebrew quagi antenna for the PMR band (446 MHz)
Antenna from a few bucks
Date of the last modification: 26.03.2010.
Written by Andor Rábay („andorpapa”)
e-mail: mecsekpmr@freemail.hu
License (click on the picture below):
Why should we build our own antenna?
There are very few antennas on the market in Hungary, which are portable and cover the
PMR446 band. The cheapest one is the polish made Dipol ATK-5 which has 7.5 dBi gain at 446
MHz. The ATK-5 is a good „backpacker” antenna: its length is only half meter (1 �½ ft), its weight
is 0.5 kg (1.1 pounds) and the price is only 1800 Ft + shipping (about 11 EUR). An other
manufacturer, Carant has 7 element yagis for this band, they have gain between 9 and 10.5 dBi,
their length is 70 cm (27 �½ inches) and weight is between 0.5 and 1.5 kg (1.1-3.3 pounds). But
these are more expensive models.
The DIY antenna manufacturing has two advantages: you can build a cheap antenna, exactly
with the parameters you need. If you want to build an atenna for backpacker DX purposes than you
can construct it with few elements and lightweight materials, or if you're planning a stable one up to
the roof than you can build 2-3 meter (6-9 feet) long antennas, focusing on the weatherproof design.
The quagi
My plan was to create an antenna what is easy to build, cheaper than the ATK-5, but has at
least the same gain as the yagi. Hence I choose the quagi. The quagi was developed in the early '70s
by two radio amateurs. Imagine a standard yagi with ordinary boom and directors. Now replace the
dipole and the reflector with quad elements, and you've got a quagi! The quad radiator is a square
shaped element usually made from wire or thin rod, each side has the length of λ/4. The reflector
has the same shape but it has a little bit longer sides. The directors are the same what we know from
yagis. The boom made from a non-conductive material, wood or fiberglass.
What are the benefits of the quagi?
–
–
–
–
–
cheap materials
easy to build, doesn't need advanced tools
50 Ω feedpoint impedance, so doesn't need impedance transformer
easy to tune
has the same gain as the yagis
Well, the quagi has disadvantages too, for example a yagi equipped with a folded dipole has
much wider bandwidth and there are some pitfalls through the construction, but I will explain them
later. Let's start with the required materials!
–
wooden slat/rod/chairleg etc (105-110 cm which is around 3 �½ ft) with arbitrary cross-section
(mine is 25x25 mm which is 6
1/3
inches)
3 mm (1/8 inch) brass rod (3 meter, nearly 10 ft)
22-23 cm (9 inches) long wooden stick (2 pieces)
screws
–
–
–
That's all! Most of these stuffs are already available at home, except the brass rod, which
costs 290 Ft (1 EUR) per meter, so the material's price could be kept under 1000 Ft (around 3
EUR). An additional cost reducing method is to replace the brass directors with cheaper metals. An
antenna mounting part (shackle) is required too, it's about 300 Ft (1 EUR).
Dimensions
The developers of the quagi specified the dimensions for 446 MHz, so this is a good origin,
but don't forget that the thickness and optional insulation of the elements has strong influence of
dimensions. The original description mentions directors with 3 mm (1/8 inch) diameter from any
metal (copper, brass, aluminium). For the quad elements it suggests the #12 TW house wire which
is an insulated copper wire with 2.8 mm diameter. Since I used 3 mm thick non-insulated rods for
the quads, I had to increase the length of the elements. The new dimensions can be found in this list
(on the internet most of the dimension tables are containing the original dimensions based on the
2.8 mm thick insulated wire):
R
DE
D1
D2
D3
D4
712 mm
680 mm
289 mm
287.5 mm
286 mm
284.5 mm
R is the length of the reflector, DE is the driven element and D1-D4 are the directors.
The distances between the elements:
R-DE
DE-D1
D1-D2
D2-D3
D3-D4
173 mm
130 mm
272 mm
144 mm
215 mm
So the overall length of the boom is 934 mm (36
1/3
inches) but it is recommended to leave some
space for the mounting section. I left 10 cm (nearly 4 inches) at the end of the boom for these
purposes. But you can mount the quagi not only at its end, but between D1 and D2. In this case try
to avoid metal support, use wooden or fiberglass supporting methods.
Construction
I recommend to start with the brass rods. You can cut them with saw, wire cutter, tooth,
nails or anything. At the first round cut a little bit longer pieces because it's allways easier to cut
more later than extend a too short rod. Every director is shorter 1.5 mm (1/16 inch) than the
previous one so precise, accurate work needed. But don't be afraid, it doesn't require special tools. I
cutted 3-4 mm longer pieces compared to the proper length, than I refined it with a minor angle
grinder. Remember: don't cut with the angle grinder but refine! Believe it or not with a small angle
grinder you can set the elements length with 0.3 mm accuracy. Finally use a file or rasp to achieve
the perfect length. Do the same with quad elements.
Practical tip: put stickers on the directors with their number after you've cut them because
they've got almost the same length and it's easier to identify them if they are labeled (you can see
this on the following pictures).
1. picture
– The labeled directors
2. picture
– The difference between the directors' length
Let's prepare the boom! I used 25x25mm (1x1 inch) wooden slat because I found it in the
backyard and seemed to be good for this project. You can use slats with different cross-section too.
After cutting the proper length of wood, mark the place for the elements. If you want to mount the
antenna at its end, don't forget to leave enough space for the mounting hardware! The original
description talks about directors going through the boom which is an easy way to mount the
elements – if you can drill straight hole into the wood. I can't, because handiness is not my strength.
So I chose another way: terminal block. I fastened the brass rod into a piece of terminal block and
screwed it to the boom. If you want to mount the elements through the boom, drill holes with the
diameter of the directors. Push the rods through the boom and fix them with a drop of glue.
The quad element will be a harder job. You have to not only bend them but to construct
support wooden support for them. The quagi's driven element has its feedpoint at the middle of one
side, so you have to bend symmetrical the brass rod. The polarization depends on whether the
directors and the fed side of the quad driven element stays horizontal or vertical. On the PMR band
we need vertical polarization, so we use directors staying vertical and feed one of the perpendicular
side of the quad element.
Bending the brass maybe the most difficult part of the quagi construction. Because you
have to consider that the folded part necessarily has a bending radius and during the bending
procedure the brass lengthen a little bit which results longer quad elements overall. You can bend
the quads with everything you have: chuck, edge of the table etc, but try to keep the bending radius
minimized. Remember: after bending the rod to U shaped, the whole element must be planar.
3. picture
– The folded brass rod
After bending the brass rod to U shape, let's create the quad support wooden element. Drill
two holes on the wooden stick and push through the legs of the U shaped brass on the holes. The
last step is to „close” the quad, so you have to bend the U shaped legs to form a square. Yes, a
square with equal side lengths, not a rectangle. You can solder the 2 open ends together for the
reflector element, or you can use terminal block to clamp them. This method has an advantage: with
the terminal block you have an about 1 cm (2/5 inch) long tuning section for the reflector.
The driven element's open ends are the feedpoints. Solder the coax cable's inner lead to one
end, the outer lead to the other. Use a small amount of glue to fix the brass rod to the support wood.
You can connect the quad elements through their support stick to the boom with the same method
as the directors. You can screw or glue them. Don't forget the proper distances between the
elements!

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