[AReU] Ground plane VS Counterpoise or what?

[Ronald KA4INM Youvan]

Terry Petrash wrote:
> OK, AReU members, it's time to put on your thinking caps on and
> offer me an "opinion" and possibly a solid correct answer.  

> If you were going to erect an antenna on a 5 foot pole above the
> top surface of a building that is 25 feet above ground and that
> building happens to be a parking garage which is made up of
> steel reinforced precast concrete, would you be looking at an
> antenna that is considered 5 feet or 30 feet above ground in
> considering ground plane reflectivity?  

> I have this delema and am trying to figure it out.  If you don't
> have an answer how about any practical ways to test measure this
> situation to find out, please let me know.   

   This not a problem, I be happy to will let everyone know.
   But you didn't specify the KIND of antenna being discussed here.

   So we will assume (Oh I hate to do that, it's unscientific,
and most frequently just wrong) that you are asking about your
18AT or (some such) HiGain VERTICAL H.F. antenna discussed at
the last AReU meeting.

   A vertical antenna 1/4 Wave Length long (tall) is not an antenna!
It is only part of an antenna.  To operate efficiently with the
CORRECT feed point impedance there MUST be a "counterpoise" known
in HAM parlance as "radials."
   Now, a long piece of transmission line will radiate, but to maximize
our range and to contact the places we wish to work, more efficiency
is sooo much better.  (than less)
   Who wants to burn up 3/4 or 1/2 of their transmitter's power in
the ground under their antenna?

   This antenna can be arranged two ways to operate well.
First is as a Marconi style, with 4 or more radials mounted as close
to the bottom of the vertical radiator as possible running
perpendicular to the vertical part. (the radiator)  1

Second is with two or more drooping radials starting at the bottom
of the vertical radiator as possible running down at a angle to the
vertical part. (the radiator) 2
The angle of the radials will effect the feed point impedance.

Notes:

1 invented by Marconi and the ancients, it has a feed point
   impedance of about 37 Ohms.  If used without a good counter-
   poise the feed point impedance will rise toward 50 Ohms
   offering a much match to modern coax, at the terrible
   expense of lost efficiency.  It is called a "Marconi" antenna.

2 It is called a "ground plane" antenna. it was invented by
   someone at RCA to be used with two way radios, primarily
   for use as a fixed station antenna, but works fine at any
   frequency. It has a feed point impedance of about
   50 Ohms at about 45 degrees of down tilt.
   (and the feed point impedance is adjustable)

3 Both of these antenna are omni directional (non directional)
   and provide a maximum radiation in a direction perpendicular
   to the vertical radiator, or above the horizon, which is above
   ionosphere, above something around 600 to 1,000 miles away
   from your QTH.  Makes for great DX, listen for weak signals,
   that's the DX.
   Adding more vertical radiators allows directionalizing the
   pattern. (driven or passive, a vertical Yaggi)

4 If the radials are mounted low and separated from the bottom
   of the vertical radiator the angle of radiation shall be altered,
   usually to the less good.

5 Anything in between the two will work, no radials will work, no
   antenna will work, but if you want to control the antenna's radiation
   angle and efficiency the close to the above two designs, the better
   it will work.

6 The radials need to be at least 1/4 WL at the lowest frequency
   desired.

7 With the Marconi type of antenna every time you double the number
   of radials from 4 to 8 and from 8 to 16 you will experience a 3 dB
   improvement in efficiency, as for 32 radials, I just don't know.

8 I have no information about improvement in efficiency from increasing
   the number of radials.

9 American AM (.54 to 1.7 Mhz) broadcasting stations are required to
   have 120 radials, (cross connected like a net) unless it can be shown
   some are un-necessary.  (such as antennas built over salt water, on
   a metal structure, or where it isn't practical)

   Did I miss anything?
-- 
       73 (= Best Regards) de: Ron ka4inm at tampabay.rr.com
     100% LINUX, since July, 1997  SENT Time and Date are UTC
        Visit my HAM Web SITE at: www.qsl.net/ka4inm
               NO anti-virus crap used here ever!