wire2.art

一个很小和天线计算程序

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