On 2 feb, 09:37, "Antonio Vernucci" wrote:
Wim,

the story began when, a few days ago, I was going to replace a trap of my HF
yagi. Not to make mistakes, I consulted the antenna assembly manual where I
found a big banner: do not invert traps otherwise the antenna will not work.

So, I thought, this is a case in which a bipole cannot be inverted.

This is clearly due to the fact that the external body of the trap (an aluminuim
can about 2 feet long), which contains two coils resonated at different
frequencies by means of built-in capacitors, is effectively part of the antenna
radiating element. So, the trap is a bipole not only comprising lumped elements,
and that is the reason, I believe, why it cannot be inverted.

73

Tony I0JX
Rome, Italy

Hello Tony,

So it isn't a bipole.

The tube containing the trap is a (short) common mode transmission
line that has effect on its environment (like a metal case with its
capacitive effect on ground). The trap inside the metal tube is in
series with the end of the short transmission line.

The ground in this case is the boundary where the near field is no
longer dominant w.r.t. to the radiating field. Think of a circular
ground at 0.16lambda from the structure. So one end of your bipole
has more ground to this virtual ground and is therefore a 3-pole
device.

For the limitation on lumped circuit approach, it is not important
whether or not it contributes to the overall radiation of a
structure. When you make a floating ground out of 4 quarter-wave
radials (monopole?), this structure has minor influence on the far
field pattern of the quarter wave (or better half wave) radiator above
it.

Best regards,


Wim
PA3DJS
www.tetech.nl
without abc, PM will reach me