[TowerTalk] : Phased array radial query

[w8ji.tom]

Hi Dave

The reason this topic brings out conflicting opinions is almost certainly
because of a lack of studies on multi-element array ground systems. That
means the only response possible is conjecture, so I'll conject my two
cents work based on the five or ten systems I've worked with over the
years.

> cross each other at several points. What is the collected wisdom should I
> leave the radials insulated from each other or should I "bond" them at
the
> cross over points?

With a small radial system, all sorts of odd problems can occur when there
are multiple current paths between systems. One is odd resonances can occur
in the ground system. The end effect can be a dramatic increase in base
impedance caused by the radial system impedance being increased by
resonances in the ground system.

I've only measured this problem in small systems with only a few (under ten
or so) radials. I have no idea how that effected pattern or field strength,
because I never bothered measuring FS when the base impedance was "whacky".
Base impedance often has little to do with system efficiency, but it is a
sign that something is a *potential problem* when it not the expected
value.

I also prefer to bond radials for lightning reasons, and to prevent poor
connections that might rectify RF or arc and start a grass fire.

With insulated wire and lots of full length radials, I wouldn't worry about
any bonding other than lightning worries.

BC stations traditionally bring the radials to a common buss strap and do
not "cross" the systems from each radiator, but that is mostly habit rather
than science. In almost 30 years, I've never seen a single FS measurement
comparing bonded and isolated radials in multi-element arrays. 
 
> In looking at some older texts I notice It used to be popular to have a
> small ground rod at the far end of each radial. This practice seems to
have
> vanished from vogue in recent years. Was there any known measurable
> advantage (or disadvantage) to doing this?

RCA's famous study on radials showed that ground rods did nothing (as did
ground screens at the tower base) when adequate numbers of radials were
used. With small systems, they help. The smaller the system, the more they
help.

With sparse elevated radial systems ANY connection to earth at any point,
even through the coax shield, decreases FS. I've measured that effect, and
it is mentioned in elevated radial papers. The feed system must always be
"choked" or isolated to prevent direct connections (like on the outside of
the coax) from carrying current that flows into lossy earth.

The more radials used, the less critical any of this is and the better the
system works overall.

Any opinion on how ground systems affect field strength should always be
formed by direct FS measurements, otherwise it is just a wild guess. Base
impedance or current in wires tells us nothing about effects on radiation.

73 Tom

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