[TowerTalk] Vertical dipole other choices?

[Jim Brown]

On 10/27/2020 11:56 AM, Rob Atkinson wrote:
> By the way, I don't know if this was pointed out or not, but with a
> very good low loss ground system, creating a highly efficient vertical
> monopole, the power density is almost twice as high as that found with
> a vertical dipole radiating the same amount of power.  "The result is
> a directivity or gain that is twice that for a double length dipole."
> See _Encyclopedia of RF and Microwave Engineering_, Wiley 2003 v. 4
> pp. 3238  - 3244 entry by O.P. Ghandi, G. Lazzi, and C.M. Furse.

NEC models clearly show that a radial system has a rather small effect 
on the far field signal strength for ANY half-wave vertical dipole. A 
major advantage of a half wave vertical is that it raises the location 
of the current maxima, which lowers the vertical pattern. This is also 
true of a loaded center-fed vertical, and of a quarter-wave base fed 
vertical. See http://k9yc.com/AntennaPlanning.pdf which ran in National 
Contest Journal several years ago.

Antennas that have been shortened by loading, even when the loading 
itself is capacitive and relatively low loss, produce less field 
strength than a full size antenna. We learned this when designing and 
building loaded 80M verticals county expeditions and Field Day.

> Unrelated:  In the Nov. QST in the Doctor Is In column, the author
> Joel Hallas W1ZR, states that a 180 degree base fed vertical monopole
> needs no ground system, only "short counterpoise rods, which look like
> radials but are usually only 4-6 feet long, to provide a connection
> point for the shield of the coax."  That is utterly incorrect.  It is
> voltage fed, but it is still a monopole, and is still unbalanced and
> induces currents in the earth.  It needs a ground system.

Ground losses are I squared R, and for a half-wave vertical, both ends 
are a current minima.

On 10/27/2020 12:17 PM, Paul Christensen wrote:
 > If a 180-degree base-fed radiator at ground level didn't
 > require an adequate radial field to capture return current, AM
 > broadcast stations with 180-195 degree vertical radiators like WLS,
 > WSM, and WLW have all wasted a lot of money over the last 90 years.

Broadcast antenna systems are part of the license, and historically were 
dictated by the Commission. Also, some of the stations currently using 
half wave radiators started out as 5/8 wave. Most  were Clear Channel 
stations, 50 kW with antennas designed to cover half the country, and 
protected from interference in that protected coverage area. That all 
changed 30-40 years ago, when these stations were protected for only 
about 700 miles. I suspect that change may be the reason that 5/8 towers 
were shortened -- the 5/8 tower has a broad, strong lobe that peaks at 
about 55 degrees and may fill in skip zones better than the narrower 
vertical pattern of a half-wave radiator.

Also, AM broadcast radial systems were historically pretty much cookie 
cutter designs. NEC modeling I've done on half-wave radiators show that 
radials have relatively little effect, but NOT zero, and they become 
increasingly significant as the antenna gets LONGER than 180 degrees. 
When these stations were built, FM broadcasting didn't exist, and AM was 
TV today, so stations were going for every fractional dB to increase the 
potential audience they could plot on coverage map to show advertisers.

73, Jim K9YC