Topband: QST Jun 06 RX Loop

[Doug Waller]

> I feel that a non-"shielded" loop will work just as well, plus it would
> be easier to construct and to feed.

Before discovering the phased endfire loops for reception on 160m, I tried most kinds of loops known to man.  In space, shielded and non-shielded loops work about the same.  Near the ground, coupling with other objects and non-homogenous ground unbalances the non-shielded loop, taking the sharpness and depth from the null, or even destroying it.

The loop shield is a shield of sorts, providing equal magnetic coupling to surrounding objects, provided the shield gap and ground wire connection are substantially opposite each other.  The shield "is" the antenna, with the received signal's RF current flowing around the ends of the gap, inducting signal to the internal loop wire by transmission line action.

The main advantage of a loop is it's nulling of vertically polarized ground wave noise sources off each side.  Loops receive propagated waves omni-directionally, so once the loop is set for maximum local noise nulling, further rotation will not significantly benefit propagated wave reception.

A second shielded, tuned loop advantage is that it is extremely sensitive, to be such a small antenna.  Resonating the loop also makes it much less responsive to strong nearby AM broadcast signals, which can introduce background noise.

>My experience is that loops, tuned or untuned, made from hard-line with its
>solid shield and greater diameter, have always greatly outperformed those
>made from RG-58 or RG-59.

This is my experience.  Woven coax shields, insulation and excessive capacitance introduce unacceptable loss, compared to hard-line or copper tubing.  My coax loops were great disappointments, with an absence of a deep, sharp null, and a lack of sensitivity.  Rotating these loops with a rotor will verify their poor performance.

If a loop is all you can accommodate, shield it for balance and resulting deep null, use hard-line or copper tubing for efficiency and resonate it for higher sensitivity and out of band, strong signal rejection.  RF choke the feedline shield near the feedpoint and ground the feeder shield 15 feet (3m) away.

73, Doug / NX4D