Diverters are used to shield side-mounted antennas.
They are horizontally polarized and are placed above
and below the antenna to be protected. Distance above
and below usually isn't critical as the diverters are not
in the radiation pattern (e.g. horizontal vs vertical
polarization or if a beam the diverters are parallel to
the boom rather than the radiating elements) but within
a few feet should do it; they extend past the antenna by
a few feet too.
Lightning comes down from the sky in ~150' increments
called step leaders. As the step leader approaches the
earth, streamers head upwards toward it from chimneys,
trees, towers, people, etc. If the streamer and step leader
complete the path (known as the return stroke), you have
a lightning strike.
The tower provides some protection. In your mind, rotate
a ball with a diameter of 150' all around the tower. That's
the area with 96% protection. A tower taller than 150' will
need diverters for side-mounted antennas for obvious reasons;
a tower lower than that will not unless the side mounted
antenna is right at the top of the tower. Any antenna above
the tower can be struck.
The purpose of a lightning rod (Franklin terminal) and the
"porcupines" is to protect by providing the streamer and
taking the resultant return stroke (actually a series of pulses)
to a good lightning ground. By making the rod the highest point,
the likelihood that the rod's streamer will be the one chosen by
the step leader is increased. In other words, you rather than
Mother Nature determine the path, insofar as much as possible.
Note, however, that without a good ground system and preferably
good surge suppressors (which are really diverters), the lightning
rod will not complete its job. This is the reason the porcupines
got a good reputation as preventing strikes. They in reality did
not (and it's a matter of public record - more on this later), but
as an improved ground (or a ground to begin with) was installed
simultaneously, damage ceased; so, voila, strikes are prevented!
Oh, well.
I suggest you contact PolyPhaser and see if they'll send you the
tape done by the PBS TV stations in Tampa and Denver. It's
an eye-opener. It shows porcupines guaranteed not to be hit
being hit. Also, contact Steve K7LXC for further info on this
subject (including the rolling ball theory).
Surge suppressors are needed because surge energy will be
impressed on the outer conductor of the coax and induced on
the center conductor. The former will be taken care of by
grounding as the coax enters the building. The surge suppressor
diverts the energy of the center conductor to ground. Suppressors
for telco, power, etc. work kinda sorta in the same manner.
73,
Bob AA0CY
former PolyPhaser Sr Appl Engr
----------
From: brunet@us.ibm.com[SMTP:brunet@us.ibm.com]
Sent: Thursday, September 09, 1999 9:18 AM
To: towertalk@contesting.com
Subject: RE: [TowerTalk] lightening and uhf/vhf triband vertical on top of
mast
Bob, Can you tell me more about the diverters? If the tip of the 2m-70cm were
about 10' down from the yagi would that help shield the vertical?
Thanks, Pete
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