Topband: Top Loading

[Henry G. Elwell, Jr.]

This topic had less interest  to the group, or less people
are knowledgeable about the subject; I received three
answers and one request for a summary of the answers from
AA6WG.

Questions 1.: Does a top loaded vertical have less ground
loss than a non-top loaded vertical? (the top loading
produced the current node at its top; essentially zero
current at the base; similar to a 1/2 wave vertical.)
Question 2.:  If yes, did it make radials of less
importance?

Roy, K6XX/0 said "An essentially half wave vertical requires
essentially half wave radials because the current point in
the ground system moves out an additional quarter wave from
the base of the antenna as compared to a quarter wave
vertical element.  Not much is ever free."

Tom, W8JI said: Question 1. - "Yes, it has less ground
loss.  Unfortunately that loss was replaced by the loss
resistance of the inductor ---." He went on to explain why,
and it would be better to leave out the inductor.

	Question 2._ "It would reduce ground losses close to the
base of the antenna by decreasing zone currents (the total
value of currents flowing past an imaginary perimeter a
certain distance out) near the antenna.  It would increase
zone currents further out, and move the losses away from the
antenna base.  This can be good, because the cross sectional
area would be larger, meaning for a given current and
resistivity of the media losses would be reduced.	

	The unfortunate thing is many of the ground losses would
remain, even if base current went to zero!  That because the
fields from the antenna excite currents in the ground that
do NOT have to flow back to the antenna element.  These
currents, coupled to the antenna through the induction
fields and radiation, add losses to the system.  (Think
about a low dipole.  It has no connection to earth yet earth
losses can reduce the radiated field by several dB.  If you
lay down a counterpoise, the losses go away.

	You'd still need a ground system, although the radials
could be slightly less concentrated near the antenna base
except for the requirement of adding a small screen to
reduce electric field losses (from the high base voltage and
its accompanying electric field)."

Bob, VE7BS said: I am sure you know that radials are just as
important with a half wave monopole as with a quarter wave
monopole, so I guess you are asking why.

	Lossy ground will always keep worms comfortable, although
it may be a completely different set of worms (located at a
different distance from the base."

	"I think the difference between quarter and half wave
verticals is that the space between wires has to be reduced
at a greater radius to 'shield the earth' and collect
greater return current out there. When you take far-end
spacing into account it leads to the paradox that the longer
the radials are, the more you need."

	He then quotes from Paul Lee K6TS's Vertical Antenna
Handbook "that current maximum in the radials of a half wave
vertical is .35 lambda out from the base and says '.. The
claim that one does not need a ground system under a half
wave vertical radiator is true only if he is content to
throw away from 40 to 80 percent of his radiated power in
the form of earth losses.'"
	
My conclusions from replies - Question 1 - Yes, but ---.
			      Question 2 - No, radials are important for any
vertical.

Many thanks fellows for you knowledgeable input.

Henry G. Elwell, Jr., N4UH



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