[K6BW] Re: long wire

[Bill Smith]

I thought I'd send this to the reflector as general information.

Great, Ken.  You are making progress.

You should be able to measure power output in watts, with a separate SWR
measurement.  An SWR of 2 is barely usable.  It is ok, but you should be
able to do better than that.  We'll have to discuss SWR and what it means at
the clubhouse, or you can read about it in the ARRL handbook or antenna
manual.

The frequency formula is:   antenna length of a 1/2 wave dipole in feet =
468/MHz.

example:  a dipole for 3870 KHz (3.87 MHz) length = 468/3.87 = 120.9 feet
long.  It would be cut in the middle with wires attached to the shield and
center conductor of a 50 ohm coaxial cable.  Both wires should be the same
length for best operation.

There are many factors that affect the impedance of a dipole, a major one is
height above ground.  Wire size can also have a minor effect in the "Q" or
sensitivity of the antenna SWR to frequency.  As a consequence, the 120.9
foot length is only a starting point, one must trim the antenna by adding or
subtracting additional wire to arrive at minimum SWR.

One way to do this is to cut the antenna length about 5% long (it is easier
to cut off wire than it is to add it back) and then put the antenna up in
its operating position.  It is critical to raise it to its full operating
position because of the effect ground has on the antenna.

Hook the antenna up to your transceiver and while transmitting at a low
power, "sweep" the frequency of the transceiver to obtain the lowest SWR
measurement.  It should fall to nearly 1:1 at some frequency.  If not,
something is disturbing the antenna.

Use the formula backwards to arrive at a new constant.  For example, let's
say that you cut the wire to 130 feet.  That would in theory make the
antenna resonant at 468/130 = 3.6 MHz.  But when you measure the antenna, it
actually is resonant at 3.72 MHz.   Use this measurement to obtain a new
constant by multiplying 3.72Mhz X 130 feet = 483.6.

Now use this new constant to more closely approximate the desired length of
the antenna.    483.6 / 3.87 = 124.96 = 125 feet.  Cut the 130 foot antenna
to 125 feet and it should be resonant very close to 3.87 MHz, the frequency
you have chosen for the antenna.

You will find that transmitting antennas are quite critical when compared to
receive antennas.  You will also find that a resonant antenna is
tremendously better for receive that a random length wire.  A random wire
appears to work when hooked up to a receiver, but you'll be amazed when you
compare the signal to background noise of a signal received with a resonant
antenna.  The signal seems to "pop out" of the noise!  We can demonstrate
that with the various antennas we have at the clubhouse.

73 de Bill, AB6MT
hbco2 at sbcglobal.net