                             TLFE.TXT


When  a  receiver on the frequency f (wavelength w) gets an interference
of the frequency f'(wavelength w')  and,  if  the  ratio r between f and
f' (r = f/f') is suitable, one can  eliminate  or  atenuate  strongly  the
interfering  signal  with  line  transmission  filters without affecting
significantly the operation on f (reception and even transmission).
One of the possible methods  is  to  put  that line in parallel with the
equipment antenna conection (using, normally, a T connector).  Any
good quality  line  may  be used, but coaxial cables are more commonly
used with radio equipment.
Indeed, there are 2 methods:
1 - using a line  length  equal  to  an  odd  multiple  of  one  quarter
    wavelength  (w'/4)  with  the  free end opened (infinite impedance),
    reflecting a zero impedance on f'.
2 - using  a  line  length  equal  to  an  integer  multiple of one half
    wavelength (w'/2) with the free end shortcircuited (zero impedance),
    reflecting a zero impedance on f'.

The  idea of the two methods is to reflect an impedance on f as greatest
as possible to do not affect the  operation.  If this impedance is not much
greater than that existing  on  the  connector  (normally  half  of  the
antenna  impedance  because  this  is  in  parallel with the radio input
impedance, equal  to  the  first  one).   If  this  condition  cannot be
achieved, the method is not usable.  So, tests must be  carried  out  to
verify that.  It's important to  note  that  better results are got with
higher line surge impedance (this DOESN'T need to  be  equal to
the antenna impedance).
At first we must try the method 1 because, normally, it leads to shorter
line lengthes.  If the results are not good enough, we try the method 2.
The program TLF.EXE is divided in 2 parts.  The  first  one  acomplishes
those tests for both methods  (the  best  results method must be choosen
for the implementation).  The second part calculates the physical length
of the line to be prepared. The results are shown in meters and in feet.
The program is presented  as  TLF.ZIP  and  may be freely distributed to
hams.  When exploded, TLF.ZIP creates the files TLF.EXE, TLFE.TXT  (this
text) and TLFP.TXT (this text in portuguese).

I hope it will be useful.

Luiz Amaral - PY1LL/PY4LC

