A so-called random wire antenna is an end fed antenna. As typically
installed, it is a compromise antenna but great for portable use because it
is easy to pack and easy to install. One end goes straight into the rig,
often with no feedline, and the other end in the air attached to something
as high as you can find. It is important to use a counterpoise. A standard
recommendation for the 40m band (see QST, March 1936, p. 32, "An Unorthodox
Antenna") is an 84' long end fed and a 17' long counterpoise (6.5' for 20m).
While these lengths have been shown to work well on many bands, which
is helpful if you're in a hurry to get on the air, read up on the topic
and experiment. Here is an ARRL
random wire page.
When on the trail, at Field Day, or whenever a good antenna is
unavailable, a random wire can save the day, but it is important to
understand its properties. At multiples of half wavelengths of the
transmit frequency the impedance is so high that tuners in most rigs
often can't match it. The trick, therefore, is either
a more capable tuner - see AA5TB's
excellent page for EFHW info - or, addressed below, to cut the wire
so that it is not a halfwave multiple of any frequency you want to use.
NOTE: Rewording the previous paragraph, if you have a capable enough
tuner the colored bands below are the best lengths as long as your
wire is at least 1/4 wavelength. For typical tuners built into a rig,
these are the worst lengths and you want to be as far from colored
bands as possible. (Center fed is so appealing because you avoid this.)
Feeling a bit too lazy to calculate that by hand, however, I wrote
programs to do the dirty work. The python code is the newest, simplest and
probably best of the three. The graph ranges from 1/4 to 1/2 wavelength by
default since you must use at 1/4 of longest wavelength and ideally there is
no need for more than 1/2 wavelength of wire. An option is shown to graph a
full wavelength, however. You can move your mouse over the graph and the
length is displayed as 'x=###' in the upper right.
Modify bands and
frequencies for your country or favorite modes. For example,
here are values for CW portions of the USA
40-30-20-15m bands for my 4 band Elecraft K1 qrp rig.
Save program output in file named 'f', and use
this
gnuplotconfig file by typing gnuplot rw.gnu.
A png image named g.png will be created.
Again, the bands are USA, so change them
for your country and favorite modes of operation. Even easier than
the C code, in Matlab or Octave get a plot by simply putting the
bands you're interested in in a vector like the example below, by typing
>> rw([40 30 20 15])
>> rw([40 30 20 15], 'cw')
Use a slide rule! It's an efficient way to get all the multiples
doing the initial 468/f on C and D scales. Multiples are on C and
answers on D - one setting.
The idea
is that for a given ham band there are min and max frequencies which
are easily converted to half wavelengths by the famous 468/f generally
used by hams for dipole lengths. (QST, Oct 1926, "The Length of the
Hertz Antenna" shows in line 2 of the table that a similarly installed
random wire should use 423/f. W3EDP's results in the above referenced
1936 article, however, fall in line with 468/f and so is used here.
Change the program to use 423 if desired.) In addition to multiples,
wire lengths less than 1/4 wavelength of the lowest desired frequency
are blocked out because an antenna should be at least that long.
Lengths to avoid are not always the same as
ones
recommended on VE3EED's page because I use all halfwave
multiples, and of course different band combinations yield different
results.
Wire Lengths for Various Band Combinations
The graph below shows lengths to avoid for different collections of bands.
The fewer bands, the fewer high impedance regions to avoid. You also
want the antenna to be at least 1/4 wavelength long for each band you
plan to use. For instance, to work 40m be sure the antenna is at least
10m or 33' long. Use only the white gaps for your antenna lengths, and if
you're using different ham bands than in these examples, modify and rerun
the program as I did, above, for my K1. If you want all the bands from
80m up, W3EDP's 84' antenna, 17' counterpoise is probably the way to go.
Notice from column f that there is no length giving low impedance for
all US ham bands. You have to give up a band or two, like 60m and 6m, as
shown in the column e.
