wire4.art

一个很小和天线计算程序

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