160/75/40/20-Meter Dipole in 130'
Ken Bessler KG0WX
April 4, 2007
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KG0WX 20/40/75/160 trap dipole
This antenna is the culmination of a weekend's worth of work with
the goal being to make a physically small 160m antenna with the addition
of the extra bands being a bonus. The target was to make an efficient 160m
antenna that fits into a 75m antenna's space, is coax fed and requires NO
tuner or ground radials. Total cost was under $40.
130' of antenna wire, plus extra for adjustments
16" of 3" id schedule 40 PVC pipe, cut into 4" lengths
6" of 2" id schedule 40 PVC pipe, cut in half
4 4'x.25" hardwood dowels (cut into 2" pieces) & sealer
36' of Tramflex RG-8X coax (25.9pf/ft)
5 brass coat hangers, hooks cut off, straightened & varnish removed
28 solder eye lugs
14 #8-32 machine screws
14 #8-32 nuts
Center & end insulators
Soldering gun & solder
Single edge razor blades or x-acto knife
Drill with 1/4" & 1/16" bits
Saw for PVC & dowels
Grid Dip meter or MFJ analyzer with dip coils
Heavy wire cutters
Google for "Coaxial Trap Design" freeware
Start with the 75m traps - cut 2 4" lengths of 3" id PVC. Drill a 1/4" hole 1"
from the end (make the hole angled so the coax doesn't kink). Cut 44 2" lengths of 1/4"
hardwood dowels and wrap the middle of the PVC form with them (like a belt). This
extends the O.D. of the PVC pipe to 4" diameter, which nets a better performing trap.
Cut a 9' length of coax and remove the jacket from the last 2" of one end. Insert
this into the PVC and wrap 1/2 turn around the form then tack the end in the hole
with a drop of superglue. Set aside to dry and repeat for the 2nd 75m trap.
7.86 turns of coax around the dowels and mark/drill for the second hole. cut the to
coax to length so it sticks through the hole by 1-1/2 inches or so, trim the jacket
again to fit the second hole. Lay a small bead of Gorilla glue around the dowels
and wrap the coax tightly around the form then insert the end into the second hole
and tack with superglue.
Separate the center conductor and braid on each end of the
coax. Solder the braid from one end to the center conductor of the other end. Drill
1/16" holes in the ends of the form (4 holes per trap) for the coat hangers. You
straighten and strip the coat hangers then use half a hanger per trap end. Insert
the hanger and fold it up on the outside (you will end up with the hanger in the
same shape as it was in the beginning, just smaller and with the trap threaded onto
the bottom of the hanger). Solder the coax to the hanger where it passes through the
trap - Braid on one end and center conductor on the other end. Solder lugs to the
ends of the coat hangers. NOTE: Don't just depend on the solder to keep the lug on
the hanger when the antenna goes up - I did and the lugs just pulled off when I
raised the antenna. I used spare antenna wire to tie the trap to the antenna wire
like a safety wire
Repeat for the 40m traps, with no dowels though and wind 5.46 turns. The 20m traps are a different monkey, though - the coil form is only 2" id and the resulting trap is sensitive to the
number of turns you use - I found a 1/2" addition to the coax length changed the
resonant frequency by 200 kHz! I wrapped just under 4.4 turns.
I used a Heathkit GD-1B Grid dip meter to verify that the traps all worked - this is
optional but you could also use a MFJ analyzer with dip coils, too.
Build your center feedpoint (I didn't use a Balun) and attach 17' of antenna wire to each side.
Attach the 20m coils to the wire - note - you MUST orient the traps so that they
all "point" the same way. That is, the end of the traps with the center conductor
soldered to the coat hangers must all face the feedpoint (or the reverse) - just
keep 'em all lined up. Now, with 17' of wire on each side of the feedpoint and the
20m traps on the ends of the wire, raise the antenna and trim it for where you want
on 20 meters. I found this section of the antenna very broad-banded - I only had to
trim 2-1/2" off each leg to cover all of 20m
Now to the part where you have to start "guessing" - with my Tramflex brand coax, the
nature of my traps as loading coils on the lower bands determined how much wire I
needed for the lower bands. Your traps may be different! Here is what I ended up with:
20 meter section: 16' 9-1/2" each side
40 meter section: 9' 9" between the traps
75 meter section: 14' 8-1/2" between the traps
160 meter section: 33' 4-5/8" on the ends.
Total antenna length is just under 130'
I installed the antenna as a flat dipole @ 25' up. Here are the RF results I got:
20 meters: 476.2khz bandwidth centered on 14175.0 (Trap Freq 14017.8)
40 meters: 151.4khz bandwidth centered on 7207.0 (Trap Freq 7010.2)
75 meters: 97.5khz bandwidth centered on 3930.75 (Trap Freq 3891.4)
160 meters: 47.2khz bandwidth centered on 1877.0
Now comes the fun - making a few contacts! For reference I have a Cushcraft R7000 vertical ground mounted, a Mosley TA-33M @ 25', a ladderline fed 40m inverted V @ 25' and a 75m linear loaded inverted V @ 25'. Normally, the R7000, 40m inv V and the Yagi trade off as to which one works better - It's a matter of take off angles and target distances. The 75m linear loaded dipole was my only 75m antenna so I assumed it was OK. Now, the new antenna on 20m is always at least as good as the R7000. On 40m there is no contest - the new antenna beats the R7000 by 1-2 S units and 1 - 1-1/2 S unit over the ladder line fed inverted V. On 75m the new antenna beats the linear loaded by the same amount. I don't have another 160m antenna for reference but after a few evenings on the band I can tell it's working equally as well as the rest of the antenna.
OH - one more thing - I don't own a tuner but I might get one to extend the useable range on 160m....
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160/75/40/20-Meter Dipole in 130'
by IEP on August 21, 2009
Mail this to a friend!
This antenna construction reads great!
As a newer HAM and novice antenna builder it would be great if there where pictures to illustrate the text.