> 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
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