Topband: Quarter wavelength stub as trap

[Tom Rauch]

> What do you think about using a quarter wavelength shorted
stub, made with
> coaxial cable, as trap for an 80m + 160m inverted L?

It is terrible.

On 80 meters it looks like a 3000 ohm resistor.

On 160 meters it looks like a 50 ohm inductance with a Q of
only 25, which is 2 ohms loss resistance.

> 15.6m of RG213, shorted at one end, will block 80m and
will represent about
> 50 OHM of XL on 160m or it will only invert the phase on
80m and work like a
> lossy inductive reactance on 160m?

3000 ohm resistance at the high voltage point will be lossy
on 80 meters and allow considerable leakage. Very bad for
80.

It will be a lossy reactance on 160. You will have
insulation problems on 160 if you place it inside the
element, because the voltage at the shorted end is higher
than at the top of the 1/8th wave and placing it inside the
mast brings that voltage back near the base. Placing it
inside the element will also add more dielectric losses on
160 (since there is a large voltage difference between the
shield and the vertical and a strong electric field through
the vinyl jacket), so the real losses will be higher than
the stub Q of 25 might make us think.

Much better to use a regular trap of even modest components.

73 Tom