MY
“EH” ANTENNA EXPERIENCES. At G4ISQ
Firstly I
must say thanks to Ted Hart W5QJR the inventor of this antenna concept(http://www.eh-antenna.com/), and W0KPH for
his interesting article in QRZ.com that got me started on this experiment and
finally Stefano Galastri IK5IIR of Arno Elettronica for his interesting article
on practical methods of construction. Up to now I have built three of these
antenna’s and they have all worked . It must also be stated to understand the
article below you must go to Ted Harts site and read the articles on how the
antenna works. The three frequencies chosen for evaluation were 7.050 mhz,
14.250 mhz and 18.1 mhz. My final practical construction has turned out as
below for 7mhz. It must also be stated that in their articles they explicitly
said that the construction methods they used were for evaluation not final
practical concepts. If you look at the original test designs the components
were on the outside of the pipe and so vulnerable to the weather unless fitted
inside another pipe for protection. If you can get the parts cheap then the
antenna will be cheap to construct but requires the builder to be a good
practical mechanical and experienced
radio technician. In reality a radio amateur fits the bill very well. Looking
at the methods on the other sites the first version of the 7 mhz antenna I
built in their way but it did not stay on frequency as it turned out to be
temperature unstable. This was due to the larger coils and capacitors for the 7
mhz version the capacitors especially changed value by large amounts due to
expansion and consequently moved the
antenna working frequency out of the licensed amateur band allocated. I solved
this problem by making my own variable capacitors out of single sided fibre
glass pcb board and put them with the coils and the cylinders inside the pipe
not outside. This does not mean that if you can find two 100 pf wide spaced
variable capacitors to fit inside the pipe that you could not use these
instead. Very simply they are very expensive to buy if you can find a supplier
and also difficult to find at rallies. The practical problems to solve with
narrow band high Q antenna’s are always the same ie making the components cheap
and stable enough to stay in tune. Any body who has ever experimented with
capacitor tuned small loop antenna’s for example would understand. In the case
of loops motor controlled tuning capacitors are used but in this case because
there are two capacitors which are not ganged then this makes it difficult to
realize in practice. However this is only true for the lower frequencies as the
frequency goes up and the antenna is wider in bandwidth then it is easier to
set up and to keep it set up within legal frequencies giving a return SWR to
the transmitter that the transmitter can cope with. At the end of all this to
make this antenna work you need access to some reasonable test equipment just
to get it into band , after that a simple field strength meter is all that is
needed to get it frying ionosphere. So why does this antenna interest me
……SIZE. HERTZian antenna’s for HF frighten the neighbours, are a pain to take
on holiday with the rig and set up . Work inefficiently mobile ,do not work in
my flat or in my modern ten feet by ten feet garden . etc etc etc. So if this
antenna got me around any of those problems it would be worth its cost. So what
can you do with this antenna if it works.
1.Drop it
in the boot of the car to take on holiday and get an antenna performance
similar to if it was at home.
2.Work
stations with the capabilities of a non compromised antenna ie lack of height
or length in property owned to put up a hertzian antenna properly.
3.From
conversation with neighbour….Your radio is interfering with my Telly. Amateur replies ok give me ten minutes and I will move the aerial away from your
property to the other side of the garden.(Try doing that with a 40meter dipole
or tri-band beam)
4.Taken
from another conversation with a neighbour…How dare you put up aerials in this
posh neihbourhood…reply from amateur … no its not an aerial it is just an
extension on my toilet pipe.
Joking
apart there are probably many other things it can be used for even serious
Dxing that you could not possibly do before…
The parts
list is as follows with UK suppliers where possible
1 metre
long four inch grey (Not Black)plastic marley water pipe from B and Q with
socket on one end as used for toilet connections .
2 metres
of 1.5 mm 3 core solid core mains cable
as used for inter house socket wiring
B and Q
16 swg
enamel covered wire www.farnell.com
50 ohm BNC
socket www.farnell.com
2 off 75 pf
variable rf power capacitors or sheets of 1.6 mm single sided glass fibre
pcb board approx 8 inches by 8
inches(Euro-card size) www.farnell.com
2off two inch 4ba screws or metric
equivalent (hobby shops)
10off 3/8
inch 4ba brass screws or equivalent
30off 4ba
full nuts in brass or plated brass.
10off 4 ba solder tags.
8off 4ba
plain washers.
4 off ¾ inch by 3/8 inch diameter plastic spacers
tapped at each end 4 ba
Piece of
thin plastic sheet 1.5 to 3mm thick 4
inches wide by 1 metre long. In my case
I had a piece of soft pvc sheet but I see no reason for not using melamine
sheet as used on table tops, Perspex or any plastic that is not sensitive to
RF. Fibre glass would be great if you can get the size.
Two metal
pipes 6.5 inches long and 3.875 inches in diameter made out of metal thin sheet
preferably brass shim 10 thousands of an inch thick but copper foil can be used
or even tinplate recovered from tins. If you can find the right size cans or
pipes even better. A pipe of the right diameter made out of aluminium can be
used even if the wall of it is thick as a connection can be made to it by bolt
hole and solder tag.
Piece of
thin plastic sheet 5 inches square to seal the top of the pipe against the
weather or a commercial cap for the toilet pipe(Costs as much as the pipe
itself)
Tie wraps
If you can
not find a pair of capacitors then the most time consuming part is making the capacitors
however this is not as difficult as it might seem. The idea is to make 8 plates
,four fixed ,four rotating out of fibre
glass pcb material. The rotating set is mounted on a brass bolt fixed together
by nuts where the nuts space the plates of the capacitor. The fixed set are
soldered to another piece of rectangular sheet pcb spaced at the correct
spacing. Two rectangular pieces of board soldered again to the base form the
end mounts and bearings that support the rotating section. These are isolated
from the rotating section by making a saw cut through the copper foil in the
middle of the support. It is vital that at all times you do not cut the board
with shears so as to keep the board flat. File to shape or use a fret saw or
coping saw to cut. The rotating set of plates are made by using a pair of
dividers to draw on the pcb material a semicircle of 2 inches in diameter. Each semicircle makes a plate so mark out
eight semicircles. Mark the semicircles with a ¼ inch circle at the
center and then in the center of that
drill 5/32 inch holes on to the straight edge in the center cut out the
plates to size. The other halves of the capacitor ie the fixed plates are made
in a similar way but you need a ¼ inch by one inch rectangle on the outer edge
of the semicircle that serves to solder them to the base plate and to space
them up from the base plate so that the moving set of plates do not touch or
rub against the base plate. Drill 5/32 holes in the same place as for the
moving set of plates. As a point of note here the four ba nuts I used were
0.135 inches thick and the pcb was 0.110 inches thick. You now use one of the
long bolts with nuts as spacers to assemble the fixed set of plates together
one by one at the right spacing then solder them one by one to a base plate
made out of pcb material of length 2.25
inches by 2 inches wide. Set them in the middle of the plate the bolt down the
long way. Remove the bolt and nuts. This is the tricky bit , you need to file
out the 5/32 holes into 3/8 inch semicircles to clear the shaft of the rotating
section. Using nuts as spacers again sandwich the set in a vice to hold while
filing. Next assemble the rotating plates temporarily on the bolt with nuts to
hold them together. Drop the set into the fixed set on the base plate and
measure the distance between the bolt and the base to find out where to drill
the 5/32 inch hole in the end plate. Make the two end plates ¾ inch wide by ½
inch longer then the distance from the base plate to the bearing hole. Half way
up the end plates, cut the copper foil in half with a simple saw cut. Assembly
is as follows on the long brass 4 ba bolt. Put two nuts almost at the head of
the bolt, a washer next, the end plate next, a washer next ,two more 4 ba nuts.
A small space then ,nut, cap plate, nut, cap plate, nut, cap plate, nut, cap
plate, nut. A small space again, two nuts, washer, end plate , washer ,two
nuts. The pairs of nuts are lock nuts either side of the end plates to adjust
the stiffness of the rotating shaft. The whole assembly is now dropped into
place in between the fixed plates on the base plate and centered then the end
plates are soldered to the base plate. The whole assembly should rotate freely
without rubbing as the two plates are
separated by fibre glass they can not short out. The wanted capacitance is 75
pf max, min is about 20 pf. .
Remove the rubber ring from inside the marley
pipe. Note. Lengths of 2.5 metres of this pipe make excellent cheap masts which
you can just drop the antenna onto. Fit into the pipe bottom just above the
flange a 50 ohm BNC socket. If it is a bulkhead type the plastic takes a 6ba or
metric equivalent tap very nicely so no nuts are needed. Fit a solder tag to
one bolt inside the pipe. The inside of the pipe is 4 inches so for a good no
problems sliding fit the dipole elements need to be one eighth of an inch
smaller in size. The tube lengths are 1.5 times the diameter or in this case or
3.875 multiplied by 1.5 inches which is 5.8125 inches, and yes you can make
them 6 inches long to simplify matters.
Metal pipes are no problem but foil home brew pipes need reinforcing. You first
form the pipes out of sheet foil and solder the join down the length of it to
make a pipe which is floppy. Remove the outer insulation from the mains cable
to leave a red and black insulated pair of wires and a plain copper earth wire
. These are the wiring wires for the links to the cylinders and the capacitors
and coils. Make two circles of copper bare wire out of the earth wire from the
mains cable for each cylinder and solder them in the ends of the tubes to
reinforce them. We next need to cut the plastic sheet that mounts everything to
size to fit the inside of the tube. Measure the tube from the BNC at the bottom
to the top of the tube. A piece of plastic sheet is needed of this length and
four inches wide as described in the parts list. After cutting it should slide
in the tube as a nice close but not tight fit. Measure down one inch from one
end and mark a line with a marker pen across the sheet. Go down another six
inches and mark another line. Go down 3.875 inches and mark another line across
and finally go down another six inches and mark a line across. The two six inch
spaces represent where the metal cylinders are going to fit. What you need to
do now is thin down the width of the plastic sheet so that the cylinders slide
down over it for the length marked which is 1 plus 6 plus 3.875 plus 6 inches
or 16.875 inches. Temporarily drop the
cylinders onto the sheet and slide the assembly into the pipe to check the fit
and adjust things if necessary. The same applies whatever type of tube you are
using. Put the cylinders aside now for safe keeping and work on the sheet. For
tools at this point you need preferably a coping saw, an electric drill
,scriber or marking pen, square if possible , ruler and three drill bits
of 5/32 inch dia, 3/16 inch dia and a
drill 0.125 inches in dia or metric
equivalents. The small drill is for the 16 swg wire, the 5/32 drill is 4ba
clearance, and the 3/16 drill is for the tie wraps. Measure down the sheet from
the last line you put on that represents the bottom of the bottom dipole
cylinder 6 inches and put another line across the sheet. You need now to put a
line of ten holes one eighth of an inch in from the edge on both sides of the
sheet edge from this line. The hole size is 0.125 inches. The top coil is wound
on through these holes then the turns are fixed in place with bostik clear
cement. The ends of the coils are fixed to solder tags held onto the sheet by a
short 4 ba bolt and nut. At this point leave the coil winding to later. The
holes are spaced by 0.125 inches. Use the drill bit as the reference to mark
out. There are two coils and two capacitors all spaced by 1/2 inch and the
order from the top is coil, cap, coil cap in my layout method. If you use my
capacitor the board has to be cut out to allow the cap assembly to be fitted in
the middle of the sheet supported on two plastic pillars. One half inch from
the top coil you place the capacitor base plate on the sheet and draw round it
with a marker pen including the two mounting holes. Around the mounting holes
draw 3/8 inch dia circles. Cut out the drawn shape with a coping saw leaving
the round marked holes alone. It should be possible to work the capacitor
assembly through this. You need two plastic spacers tapped at both ends and
four short 4 ba screws to fix the spacers to the sheet and the capacitor to the
spacers. Move on another half inch and repeat the hole set for the next coil.
Move on another half inch and mount the final capacitor. We still have two
coils to fit, these are the isolation coils that connect to the two cylinders.
At the bottom of the top cylinder on either side we need two rows of three
holes on each edge of the plate again as for the main coils. This is repeated
at the top of the bottom cylinder. The coils here are two turns in size and
should be identical as possible. For practice put these coils on first. The
outside of the marley pipe is larger than four inches however use it as the
former to wind the coils. Take the kinks out of the 16swg wire by fixing one
end in a vice, reel out the correct length and then yank hard on the reel. Get
a friend to hold the reel tight and the wire taught and work a soft cloth
tightly up and down the wire to smooth it out. Wind two three turn coils on the
marley pipe cut them off and let them go like loose springs. Remove all
fittings from the plastic sheet. Slide the top coil over the sheet down to the
top set of three holes and start feeding the coil wire into the hole sets
forming it properly with your hands at the same time.until all the turns are in
place. Repeat the process with the next coil down . When you are happy with the
coils set them in place by liberally coating the coils where they go through
the board with bostik clear cement. When the glue is set the coil wire is cut
off to two turns with the correct end lengths for connections. You will have
gathered by now that this is a fiddly difficult job and is more difficult for
the larger coils so patience is needed. The topmost large coil must ALWAYS be
one turn more than the other coil in the phasing network. In my case I found
that 9 turns for the top coil and 8 turns for the bottom coil worked out ok.
Wind these coils and fit them as before. After the glue has set bend the end
connecting wire of the large coils flat against the board for a half inch
either side, cut them off and fit solder tags bolted to the plastic sheet
soldered to the coil ends. The two turn isolation coils are a little different.
One end of the coil is fixed to the cylinder edge the other to a solder tag
held on with a bolt or soldered directly to the connecting wire. They have to
be fixed at the same points on the cylinders at the plastic sheet edge. The
cylinders are fixed in place on the plastic sheet with strips of printer paper
two inches long by one and a half inches wide folded down the middle at right
angles and glued with bostik clear cement to the plastic sheet surface and the
inner wall of the cylinder. Fit the cylinders and lock them in place with four
of these paper strips per cylinder at the ends of the cylinders. When the glue
has set they hold things ok. At the moment you have a very floppy aerial that
really needs supporting by holding it
in the middle of the plastic sheet with a vise. Have a look at Ted Harts three
dimensional drawing on his web site as to how the components are wired. You use
the red and black power wire to connect the components together with the long
wires to the cylinders down the plastic sheet held in place by tie wraps. For
this you drill two holes 3/16 of an inch in dia either side of the wire. Loop a
tie wrap through and around the wire to fix it in place. The body of the lowest
capacitor goes to feeder braid , the bottom of the top coil goes to feeder
braid, use the black power wire for this. The feeder center connection goes to
the bottom of the lowest coil and also to the moving vane of the top capacitor.
The moving vane of the bottom capacitor
goes to the other end of the bottom coil from the feeder connection and on to
the bottom of the top cylinder via the isolation coil . The body of the second
capacitor goes to the top of the second coil and on via the isolation coil to
the top of the bottom cylinder. By
feeder I mean four inches of 50 ohm
coax that can be connected on to the BNC connector when the antenna is in its
marley pipe sleeve. If two solder tags are bolted into the middle of the bottom
of the plastic sheet then all wiring can be fed back to these and the short
length of feeder can connect to the solder tags. The whole assembly should
slide in and out of the pipe freely. What you need now is a metal spacer
between the plastic sheet end at the bottom and the pipe wall so that bolts can
fix the plastic sheet to the pipe. Two holes 10mm dia or so need to be drilled
in the pipe wall at exactly the place opposite the slots of the capacitor
tuning screws so that you can access them with a screwdriver. As I mentioned earlier there is an array of
fittings for marley water pipes including a cap assembly that can be fitted to
the top to seal it against the weather. This needs to be fitted to the top to
complete the assembly. The capacitor holes can be taped up after tuning. Using
the marley pipe as the cover it is possible to make antenna’s for 14 mhz and 18
mhz that look the same but work only on the tuned frequency even though the
cylinder and coil assemblies inside are smaller. This means that you could have
an antenna set up that looks like a set of organ pipes or swap antenna’s at
will on the same mast using marley pipe as the mast. The antenna’s work well at
low levels but again as all antenna’s
the higher the better. Lower frequencies mean another bigger pipe as a cover.
TUNING
THE ANTENNA..
This is
where the problems start. With a hertzian type of antenna you can get the wire
or the elements close to the usable frequency wanted by simple calculation.
Even if the wire or the elements are
way out in size you will get radiated RF that is measurable and possibly a VSWR
of some kind. With this antenna unless you get the coils and capacitors set
correctly then there is no RF and no usable VSWR yet you ALWAYS have a working
antenna at some frequency or other determined by the coil size. If you do not
understand this then go and read the theory and the other articles again. Next
go and read the terms of your amateur license about transmitting out of band.
Very simply once you have cracked the tuning of one of these on one frequency
then you could make ten the same very quickly . It is the first one that is the
problem as finding the right coil size is the difficult part. The easiest way
to set up the antenna is to use a field strength meter and a low power RF
source that you can tune across the antenna. When you get a measurable field
you then tune the capacitors for best performance field wise. If it is being
fed by a 50 ohm source then max field usually means best transmitter match. If
the coils are too large then you have an excellent aerial transmitting out of
band and compromising your license , the same applies if the coils are too
small. At any point out of the bandwidth of the antenna there is no RF. The
bandwidth of the working antenna for 40 meters is 130khz if you are lucky. If
the coils are near to correct you can use the capacitors to get into band
center and the aerial will work there. At the lower frequencies the coils are
large and so are the capacitors so temperature changes change the tuning. If
the components are good then the changes will affect the antenna with approx
1kz bandwidth walk by one degree celcius by this build and construction method
at 40meters. The higher frequency versions have more antenna working bandwidth
and this is less of a problem with the smaller coils and capacitors. From here
on you are on your own as far as test gear, Even if you have an MFJ vswr
antenna analyzer you will find it useless as it is too sensitive to rf on the
co-ax. feeder, but it makes an excellent sweepable test source. A grid dip
meter and a receiver are useful. Experience and test methods get you there but
I can give you some definite don’ts and do’s.
1. Do not
tune it in house rooms with short feeder cables. It can be used in a roof space
but above the house wiring.
2. Use the
real feeder you are going to use for the real final mounting location.
3 tune it
outside in the clear on the top of a post that is at least ten feet above the
ground, use step ladders to reach the capacitors.
4.With a
fixed in band low power RF source ,tune then move away from the antenna, read
the field strength , go back and forwards until set up is achieved and yes you
have to take the antenna apart to adjust the coils then re-assemble it and
start again over and over if necessary.
If you have
a wire HF antenna up already you can use it as the field strength meter source
antenna, but keep the EH antenna away from it.
MAKING
A SIMPLE FIELD STRENGTH METER.
For this you need a 100 micro amp meter
movement , a choke of approx 1 millihenry in size, a germanium diode, a
potentiometer and a couple of 0.1 microfarad disc capacitors. RF picked up
from a source antenna is produced across the choke to earth. This is rectified
by a germanium diode into a 0.1 mic cap . The dc produced feeds a potentiometer
in series with the meter movement to earth that is decoupled by another 0.1
microfarad capacitor. The meter movement can be a multimeter set for current
measurement. Mount everything in a tin or die cast box. If you are using a
separate meter then bring out two short wires for connection. A separate
connector is required for whatever aerial you are going to connect. If you are
using a short wire antenna as a pick up close to the EH antenna then it must be
at a distance away from the antenna so
that it does not compromise the antenna tuning.
After the
article above I have built another simpler version of the 40M antenna that
works very well. It is made in the same way as the other antenna’s below
For my build ideas for the 14Mhz and 18 Mhz versions see the second article….
Brandon S.
Jones (G4ISQ)