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1994-05-01
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3KB
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60 lines
SUBJECT: Wire antennaas - part 2
In this posting, we'll look at long wires and random wire
misconceptions.
Placing an undertermined length of wire out to a tree or
support mast is *not* a long wire antenna. This is a random
wire antenna which most of the amateur community calls a 'long
wire' antenna. This misconception leads to many discouraging
ideas about long wires. A long wire antenna is an antenna that
is at least five wave lengths at the lowest frequency. After
five wave lengths, the SWR becomes insignificant to the xcvr for
a properly tuned long wire.
Five wave lengths you say. Yes, five wave lengths. So, at
3.75 MHz the length of a minimum long wire would be (984/fMHz) X
5 or 1312-ft. Not an antenna you can slap in most back yards
unless you are a rancher.
When it comes to VHF and above, the longwire antenna can be
easily outclassed by numerous other designs, but at low HF
frequencies the other designs become too cumbersome to use and
the longwire fills the niche.
Let's take a look at the random wire antenna used by so many
of us hams. A tuner is a must for any work with a random wire
antenna. These antennas usually consist for a wire stretched
between here and there. Dissapointed hams soon find that the
easy way out isn't the answer. That 75-ft wire is not going to
get them the performance they invisioned. Random wires are
good for SWLing provided a tuner is used with the antenna.
They're cheap to construct and cheap to buy. But a 75-ft
wire in a standard size lot is not much of an antenna for
amateur radio use. Granted, it maybe all you can afford or
be able to set-up at your QTH, but a ham can do better for
transceiving.
---------------------------------------------------------
Longwires can provide nearly 3dB of gain over a dipole at
4 wave lengths provided the antenna is sufficiently placed
above the ground and the wave angle of the antenna is correct.
Five wave lengths is minimal for best results.
The formula for longwire length is:
Length in ft= 984(N-0.0250)/fMHz where N is
the number of wavelengths.
The longwire is a fixed antenna, so care should be taken
where the antenna is to be placed. Your favorite RF stomping
grounds such as Europe or the Far East, etc should be kept
in mind.
Next posting will have some additional information on
longwires and other wire antennas for standard size lots.
-WS