[RTTY] Antenna and radiation issue

[Tom McDermott]

> By necessity my shack is on the second floor and I am currently 
> using a long wire antenna (175 feet) and an old (AEA-3000) tuner.
>
> Everything loads up fine and I am able to work SSB without any
> difficulty. RTTY is another matter. Every time I key up, the
> station locks up... the computer freezes, the TNC has to be
> turned off, reset, and reinitialized.
 
Hi Phil,
 
Every antenna has two halves. A long-wire antenna is one that tries
to pretend that it has only one half. Unfortunately, it means that whatever
radiation occurs utilizes also the other (missing) half. Usually that's the
feedline of the coaxial cable.  It means that you have invited the other
half of the antenna right into your shack.
 
A counterpoise has to be really low impedance in order that most
of the current for the pretend half goes to it rather than the coaxial cable,
and at RF that's really quite difficult to do. Supplying the (missing) half of
the antenna (in whatever form you can manage), and providing a
common-mode choke (current balun, or coiled loop of feedline for example)
to try to make sure that the outside shield of the coax has as high an impedance
as possible (to encourage the antenna current to go elsewhere) is a good start.
Sometimes capacitive coupling from the antenna to the feedline can partially
bypass the choke, then a second choke closer to the antenna tuner can help. 
 
Finally you may be so close to the antenna that you are directly in the radiation
field, then all the cables (including AC power) pick up a lot from the antenna. In
that case you may need to either relocate the antenna further away, shield the cables,
minimize couling loops (such as power or ground loop diameter), etc.
 
  -- Tom, N5EG
 
 



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