Topband: Receiving Loops

[Tom Rauch]

> Id be especially careful to choke the feedline at the loop - esp on a
tower
> with all that metal so close else you're loop will be a great noise
magnet.

Loops belong on or near the ground. They certainly do not belong on the side
of a tower or mast, unless you want to use the tower as the antenna or part
of the antenna.

Choking the feedline is almost impossible because the choke impedance would
have to be many dozens or hundreds of kilo-ohms to have an effect. If the
tower or any other metallic object runs past the choke and to the area of
the loop, you negate any advantage of the choke.

In order to balance the loop by itself, the inner conductors MUST leave at
the bottom of any shield. The shield on the loop from that point up MUST be
exactly symmetrical and exactly above the tower. If the shield is not
exactly symmetrical and grounded to the tower at the bottom, and the tower
is not exactly symmetrical electrically the loop will "see" the tower.

The shield in a loop actually is the antenna that picks up the signal. The
conductors inside the shield only couple to the shield by transmission line
effects. The conductors inside are not an antenna at all.

This is why using a shield, if not perfectly symmetrical, is often worse
than no shield at all. This is why an unshielded loop (when properly
constructed) has the same electrical response and balance and immunity to
close by electric field dominance as a shielded loop.

A shield simply makes the antenna easier to "balance", but only if the
shield and conductors inside the shield exit the shield properly. If, for
example, the conductors exited the bottom and the shield was "split" to one
side of the bottom (grounded on one side and floating on the other of the
internal wire exit point) you built an antenna that almost perfectly acts
like an extension of the feedline and whatever the antenna is grounded to.

A final important point is that about 1/10 wl away from a small loop the
response is electric field dominant. It is only very close where the
magnetic response dominates.

73 Tom