This antenna is cut for 145.00mhz
After moving house and a 10 year absence from 6M, 2M & 70CM SSB, I
finally restored my old 2M and 70CM antennas, and now having 2 radios
for 2m SSB with one normally runing WSPR mode 24/7 on 2m and additional
antenna was needed for monitoring and swap-around when needed.
There are some advantages in having a lower gain beam for WSPR when there is a lot of stations spread around the compass.
Using
readily available material from the local hardware shop a 3 element
beam was constructed.
The
design is NBS (National Bureau of Standards) which were developed
around the late 70’s for various sized optimised yagis with
computation charts for various diameter and elements Vs various
length booms.
This
design consists of 3 elements with nominal 7.2dBd gain with elements
of 10mm dia. and a 25mm square aluminium
boom.
The
match device is a cross between Gamma match and a half folded dipole, after using on my previous 6m
build and chosen because if its simplicity and DC grounding of the coax. A
gamma match uses a capacitor to cancel out the inductance of the
gamma rod, whereas in this system the capacitor is gone and driven
element length and
matching point is altered to give the impedance
match and cancellation of reactance.
Requirements:
Pedestal
drill
Battery
drill
1/8” & 10mm drill bits
1/8th
Aluminium rivets and rivet gun
Self
tapping screws 12mm long
1 x
3m length 25 x 25 x 1mm square aluminium,
2 x
3m length x 10mm dia round aluminium tube
100mm
of aluminium strip 12mm wide x 1mm thick to make the gamma
rod/element connecting strap.
Some
25mm wide x 5mm thick x 50mm long strips of plastic to make supports
for gamma rod. You might find a plastic chopping board in the kitchen
to cut a bit off while the boss is not looking. This material is not
real critical as to what you use as long as it is stable in the heat
and weather.
Construction:
Boom
(25mm Square) – Using a bench/pedestal drill and a 10mm twist bit
make 3 holes centrally at the relevant positions for each of the
elements and
cut off the excess length.
Elements - Cut to length as required.
The
short length will be for the director and the long one will be for
the reflector.
Mount
the element centre sections centrally in their correct locations in
the boom using the self tapping screws to hold them secure. Do not
drill/screw into the elements as this weakens them, just let the
screws push tight against them. Put a bead of Silastic around the
tubes where they fit through the boom. This is to prevent wind
vibration eventually wearing through the elements.
Matching rod:
The
2 insulating supports consist of two 10mm holes 30mm apart in a
50mm long 6mm thick piece of plastic and need to be installed on one
side of the driven element section.
The
matching rod has a 10mm long section at the
end where 12mm of the tube is removed, i.e, half round.. This allows the
coax centre attachment screw a bit of space to protrude.
Matching
arrangement - this picture for illistration only:
Bevel
both sharp ends of the tube to allow easy fitting through the plastic
supports.
Mount
all as show in diagram.
The
connecting strap is 12mm wide aluminium 1mm thick. Form it around the
tubes as shown so that 10 mm dia holes at 30mm centres match the
insulating supports.
Feed point:
I
used a SO239
connector fitted through a bit of galvo plate similar as shown except where coax comes thru there is an SO239 socket with very short wire to screw.
I should have
used an "N" connector socket rather than a SO239 as they have better weather proofing qualities.
The galvanised plate bent at right angle as shown, which is secured to the boom
using self drilling screws.
Feed
point -:
All
this is then thickly coated in epoxy resin (5 minute araldite or
similar) to seal the coax and connections from moisture. Cover all
this with tape or plastic to prevent sunlight UV degradation of resin
over time.
Tuning:
Is
done using your favourite method. Adjusting the element length moves
the frequency of lowest point of SWR but should not be required, and the strap position adjusts
for
lowest SWR at that frequency.
These adjustments do interact slightly.
I tuned mine on a short pole at about 2m off the ground so that
adjustments were easy
to reach.
Just make sure that you are reasonably clear of large metal objects
i.e. cars and sheds, and pointed away from same.
Once
satisfied do a check in the final installation location. Seal ends of elements (silastic) as a measure to
prevent water corrosion.
I
tuned mine to lowest SWR at 145 so that it is usable from 144
to 148
VK7DC
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