Topband: GAP Vertical Question

[Tom W8JI]

Hi Don,

>
> Then, why do broadcast stations that use vertical towers at approximately 
> a half wavelength, purchase valuable real estate and spend thousands of 
> dollars for the copper to install from 120 to 240 or more radials,  each 
> usually a half wave or more in length?
>

They almost always do not.

First, they would be fools or have idiots for engineers to have more than 
120 radials. The only real reason they use 90 radials or so is it sometimes 
gets them out of expensive proof-of-performance measurements.

The exact FCC text is down the page here:

http://www.w8ji.com/counterpoise_systems.htm

Second, nearly all stations except the old clear channels on the low end use 
towers around 1/4 wave, often just around 0.2 WL.

> See G. H. Brown: "Ground Systems as a Factor in Antenna Efficiency", IRE 
> Proceedings, June 1937 p. 753.  Brown demonstrated that the distribution 
> of earth currents and ground losses is such that the region of maximum 
> current and loss occurs at a distance of about 0.35 wavelengths from the 
> base of a ground mounted half wave vertical antenna, which was verified 
> experimentally.
>


But the field is so weak at that point that there really isn't much to be 
gained when the radiator is 1/2 wave tall. Plus broadcast stations have a 
huge investment, and overkill is not an issue for them. That's why the 
transmitters are so conservative.


> There is zero loss at the base of the antenna itself, since there is no 
> base current because the antenna a fed at a current node.  An rf ammeter 
> inserted in the ground lead, as well as one inserted in in the antenna 
> lead attached to the insulated base of the radiator will read zero.  >

That is absolutely wrong.

A typical half-wave BC tower is in the several hundred ohm range because of 
tower thickness. They are almost never over 1000 ohms.  Halfwave BC towers 
at exact resonance are typically about 1 ampere per kilowatt.

There isn't an end-fed half wave in the world with infinite impedance and 
zero current, even the very thinnest lossless wire would not be zero.


<<The ground losses occur farther out from the base of the antenna. Low 
effective earth resistance provided by a good ground system is ABSOLUTELY 
NECESSARY for vertical antennas of ANY height if one expects good radiation 
efficiency. The claim that no ground system is needed for a half wave 
vertical is nothing more than a long-standing popular misconception.
>

Actually we do need a ground system, that much is correct, but requirements 
are much relaxed from very short antennas. With a half wave end fed wire 
element it is pretty tough to lose more than one or two dB in ground loss, 
but it certainly needs something there to allow it to be fed. It cannot be 
fed without some counterpoise.

Thick towers are another matter. The surge impedance of the tower is so low 
that end impedance never gets all that high. This is why thick antennas are 
wide bandwidth, and why extreme power SWBC stations use thick cage elements. 
They do that to keep the impedance extremes, even at the open element end, 
down.

<<
This topic prompted me to dig out and review an anecdote I recall reading in 
my decades-old copy of CQ magazine's Vertical Antenna Handbook, by USNR 
Capt. Paul H. Lee, K6TS (1974). He reported receiving mail from a ham who 
had made the "discovery" that he could tune and operate a half wave vertical 
without a ground system, feeding it by a parallel tuned tank circuit whose 
lower end is grounded.  Since an rf ammeter in the  ground lead showed no 
current, he could dispense with the ground system and its loss.  He 
suggested to the Capt. that he should "discover the new world of half 
verticals with no ground system".
>

Current is not zero. It cannot be zero at the feedpoint of any end-fed 
antenna. That is an absolute practical and theoretical impossibility. It is 
a myth or a pretty gross misconception.

While Captain Lee had a lot of good stuff, he was also over the top with 
some things.

It is quite easy, with a thin vertical half-wave element, to get into the 
80% efficiency range with a very minimal ground screen.

People think of losses as current flow problems, but a significant portion 
of losses in a half wave ground independent vertical are electric field 
induced losses at the antenna base.  The same would be true for a thin 
element and no radials, and just a ground rod or several ground rods.

Thinner half wave verticals are pretty efficient with a minimal ground. When 
they get extremely thick the base current increases, and conduction losses 
can be a significant factor.

73 Tom