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wire4.art
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1994-05-01
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3KB
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84 lines
SUBJECT: Wire antennas - Part 4
Both of the following antennas are using 12 guage wire.
Wire Dipoles: Use the formula 468/fMHz to cut the dipole.
String it up in the area you have planned. Attach your coax
and a balun. Check the SWR. If it is within your spec's it's
ready to use. If not, do the following:
o Check the SWR from the bottow of the band to the top.
don't step on anyone having a QSO. Use the least amount
of power to do this. I'll use 40m as an example.
7.0 1.7:1
7.05 1.8:1
7.1 2.1:1
7.15 2.2:1
7.2 2.5:1
7.25 3.0:1
7.3 3.5:1
This indicates that the antenna is too long and the
resonate frequency of the antenna is below the band.
This could happen because of the following:
o A thicker diameter wire was used.
o You mis-measured.
o Something near the antenna is resonating with it.
o Too low to the ground.
Solution: trim off about six inches either side and try
again.
o Case two: SWR Readings;
7.0 4.0:1
7.05 3.5:1
7.1 3.3:1
7.15 3.0:1
7.2 2.4:1
7.25 2.0:1
7.3 1.8:1
This indicates the antenna is too short. "But I keep
cutting off wire and it's still too short." (Raised eyebrows.)
All is not lost here. Solution: add telescoping elements
to the ends. Aluminum tubing, 3/8-in & 1/4-in dia.,secured
with a screw should do the trick. 3/8-in dia. = 1-ft, 1/4-in
dia. = 11-in in length.
__________________ wire attaches here
=============__________________
Now you can adjust the ends of the dipole until the SWR reaches
the desired level at the desired frequency.
This problem can happen when the guage of wire used is smaller
than the recommended size.
Remember: Thicker => shorter; Thinner => longer.
Here's some Dipole measurements for HF:
BAND LENGTH-FT DESIGN FREQUENCY
10m 16' 5 1/2" 28.45 MHz
12m 18' 9" 24.94 MHz
15m 21' 11 1/4" 21.335 MHz
17m 25' 10" 18.11 MHz
20m 33' 0" 14.2 MHz
30m 46' 3" 10.12 MHz
40m 65' 5" 7.15 MHz
75m 123' 2" 3.8 MHz
80m 130' 0" 3.6 MHz
160m high 240' 0" 1.95 MHz
160m low 252' 10" 1.85 MHz
Have fun!
-WS