MULTI-BAND ANTENNA FOR A SMALL SPACE/GARDEN
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Multi-band Home-brew Antenna in a Small Garden or a Newcomer to HF
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Typical Dilemma:
So you have just passed your Foundation Exam and/or you
live in a typical small British garden in an urban or semi-rural setting. You
have no room for a full-sized 40 metre dipole (even in an inverted-v
configuration). Maybe you have tried a vertical antenna such as one fed with a
9:1 UNUN but found it noisy and not a great radiator of your much-prized RF? Or
you are tempted to spend a few hundred pounds on that shiny new multiband (no
radials) vertical that would surely become your solution?
Such all-band off-the-shelf verticals are easily obtainable
and can work well. However, many newly qualified operators are faced with two
major obstacles with these antennas:
1. Price: typically
ranges from around £200 to beyond £500.
2. The need for
radials: Many manufacturers and sellers will maintain that you can work these
vertical antennas without radials. You CAN of course, but just not very well.
An extensive radial system is needed, requiring above all else, space.
If you also consider the tendency for verticals to be
noisier on receive you may be disappointed.
My QTH in West Sussex has a typical small UK plot. My
semi-detached property has a garden measuring no more than 10x8 metres, thus
representing a typical-sized British garden. I have tried the 9:1 UNUN approach
using a 9.5 Metre long wire but found it a noisy antenna, with the noise floor
on 40 Metres typically around S6 to S7. I have the space for a 20 metre dipole
but these are Mono-band antennas and even when forced into another band via a
tuner provide poor RF radiation. My
attention then turned to the Doublet Antenna.
Doublet
Antennas:
There is a plethora of information relating to doublets
on-line. At first glance my quest to use 40 metres at home seemed to be
thwarted as many observers maintain that you need to cut the doublet antenna to
a full half-wave length at the lowest operating frequency. In this case a
length of 20 metres (66 feet), beyond the confines of my QTH. Further reading
however revealed that it would be better for the doublet to be cut to a length
which avoids a full or half-wave length, so that a tuner can does not have to try
and deal with the high impedance which may be presented to them on certain bands.
A total length of 0.4 wavelength of the lowest frequency is often quoted in
online forums. Furthermore, thanks to the work of L.B Cebik (SK) (W4RNL),
another alternative Doublet length came to light. Cebik suggests that for 40-10 metre operation
that a total length of 44 feet (13.37 metres) should present reasonable
performance on 40m, whilst increasing in gain as we rise in frequency. With
this in mind I constructed my 44 foot (13.4m) doublet. Cebik also states that an 88
foot long (26.8 metres) doublet would provide coverage from 80-10 metres. Here
is an outline of the parts and equipment I used at my QTH to construct the
smaller of these two antennas:
· 2 X 6.7
metres (22 feet) of wire
· 1 x
dipole centre
· 300-ohm
ribbon lead – enough to reach your tuner, I needed 11 metres in length (450-ohm
ladder-line is also fine)
· 4:1 or
1:1 Current Balun
· Short
RG 213 patch-lead (shorter the better – no more than 3 metres in length)
· Auto
Tuner
· Antenna
supports (in my case three fibre-glass poles, but trees or other supports can
work)
· Coax
from tuner to Transceiver
Doublet
Configurations:
Strictly speaking, Doublets should be fed directly into a
good balanced tuner in order to present a fully efficient system. This
configuration minimises loss as there is no external balun or coax used as the (extremely
low-loss) ladder-line reaches the tuner itself. For many people however this
presents an issue. Ladder-line is sensitive to being close to objects,
especially metallic, and it is not always possible to route the ladder-line
directly to the shack.
An alternative, as used by myself, was to feed the 300 ohm
ribbon to a good 4:1 current balun and use a short (1 metre long) piece of
RG213 to feed an auto-tuner situated in a convenient shed positioned at the
base of the central supporting pole. From here, I run a length of RG58 coax
into the shack. Whilst this configuration will see a little more loss in the
balun and the short coax between the balun and tuner, it should still see a
very useable multi-band antenna system.
My
Configuration:
My doublet is constructed from 18awg wire as an inverted-v. The apex is supported by an 8 metre tall fibreglass pole and by two four metre tall end poles, again being made of fibreglass. The centre pole was originally a 10 metre pole but with the top two sections removed. The eighth section is still rigid enough to avoid any severe whipping around in 30MPH-plus winds. This centre pole is placed within a 50mm wide gutter pipe which was sawn off to a length of just 3.5 metres. I then dug a square hole which was around 1 metre in depth and filled the hole and the bottom of the pipe itself with chippings to aid drainage, whilst having the pipe held in place. The fibreglass pole was then simply slotted down into the pipe and this has held firm throughout a UK winter.
The end poles are also fibreglass and are
each the bottom (thickest) four sections of two old 10 metre poles. To
strengthen these I simply removed each section out of the base of the pole and
then put some araldite/epoxy glue on the top of the inside of each pole section
whilst slotting the next one back up inside. Adding insulation tape for good
measure I was able to add further waterproofing. Both these poles have proved
to be rock-solid and have provided good support to the end of each doublet leg.
The added advantage of having an inverted-v configuration of course, is that
the two end poles effectively become guying-points for the antenna.
The 300-ohm ladder-line is then taken down the centre pole and then angled to meet the 4:1 current balun attached to a low shed just to the left of the centre pole.
From there, a 1-metre-long piece of RG213 coax feeds into a LDG Z11-Pro ATU, with a short 9 metre run of RG58 into the shack.
The final point to make about my configuration is that strictly speaking I did not have enough room for my doublet to run as a perfect inverted-v. Dipoles/Doublets are quite forgiving performance-wise if up to the final third of each leg is either angled directly downwards or zig-zagged to accommodate the length. In my case, the end of the left hand leg is shaped down the pole for 0.5 metres, with the end of the right hand leg shaped directly down the support pole for 1 metre. An important safety consideration however is to ensure that the end of each dipole leg is above the reach of humans or animals as high voltages are often present during transmission, even at low power.
Performance:
Setting my Icom 7300 transceiver to the middle of the 20 metre band I adjusted my power to 5 watts and selected the FM mode. After a couple of seconds my external LDG ATU tuned a 1:1 SWR. This was the same for each band between 10 and 40 metres. Some installations may have difficulty not being able to tune a specific band. Adding a metre or two of length to your ladder-line feeder may well cure this (there’s always a bit of trial and error somewhere!) but you may well get lucky like me!
The noise levels, when using a balanced antenna such as a dipole or doublet are often lower compared with an end-fed and this is the case for me, measuring S3 for 40 metres and hardly anything on the S-Meter from 10-20 metres (I know I am lucky).
Having used this antenna now for seven months I can safely say it has been a winner. Working Europe is a breeze on 40 metres, often receiving strong S9-plus reports. I have also managed to work DX into North and Central America using 50 watts on 40 metres. For 20-10 metres I have enjoyed success on 20 metres, working North America without too much trouble, plus into South-East Asia. When open (rare I know), even 10 and 15 metres has seen me being able to reach into South America. All running the Intermediate legal power limit of 50 watts.
Conclusion:
We all have different environments to contend with. No one
antenna provides a silver bullet for our HF challenges. However, if you want to
try a multi-band, low space option for HF try making your own multi-band 44
foot doublet. It may save you some pennies and gain you some highly satisfying
and enjoyable radio time.
73
G5TM
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