As usual Chucky, you are defending your position by denial of fact.



Did I say the input impedance of a Marconi quarter wave was 100 ohms
if it was without a counterpoise? Assumption #3, so I guess I did. Not
just at 30 MHz, but at any single frequency for which it was designed
to operate.

And I wish you'd stop using that "ground return path" term you keep
throwing around as if it was something you understood or with which
you even had a passing acquaintance. A conductor placed above a ground
plane forms an image in the ground plane such that the resulting
pattern is a composite of the real antenna and the image antenna. This
phenomenon remains in effect until you have spacing between the
conductor and the ground plane approaching a quarter wavelength of the
operating frequency. Above this, you get into a whole new field of
wave propagation that is outside the scope of this discussion, unless
Chucky wants to go there if he feels threatened. Actually Chucky, I'm
not going to try to explain this to you, why don't you grab a copy of
the US Army Field Antenna Handbook and read it. No disrespect intended
towards the US Marine Corps, but whoever writes their manuals seems to
address the lowest common denominator when deciding to what level of
intelligence its audience is operating, therefore Chucky even you can
understand the theory put forth in the handbook (it even has
pictures). Again, I apologize to any US Marine current or past who may
be reading this thread, you are in no way, at least in my mind, to be
considered functioning at Chuckey's level. It's just that the
DEPARTMENT OF THE NAVY opted not to exclude the likes of Chucky from
its audience when publishing this handbook.



As for the matching networks (antenna tuner) most practical antennae
require some form of impedance matching between the transmission line
and the radiating elements. The implementation of a matching network
can take on many forms, depending on the operating frequency and
output power. There was no sleight of hand Chucky, in this case, it
has always been present in the equipment package of the boat in
question, remember? This was a functioning system and the OP just
wanted to relocate the antenna.



I do apologise for not quoting my sources and resources, which was
rude of me, but then Chucky never does so I thought it was ok.
However, as you insist: ARRL Handbook, Chapter 17, sec "The Vertical
Antenna"; W9UCW "The Minooka Special" Dec 1974 QST; VE2CV "Technical
Correspondence" Feb 1991 QST; ARRL Handbook, Chapter 17, sec "Ground
Systems"; The W1GHZ Online Microwave Antenna Book; The HAM Radio
Operator's Antenna Manual, by Buck Rogers K4ABT; W5DXP's No-Tuner,
All-HF-Band, Horizontal, Center-Fed Antenna; TM 11-5985-379-14&P,
Operator's, Organizational, Direct Support, And General Support
Maintenance Manual; TM 11-5985-370-12, Operator's And Organizational
Maintenance Manual; The RF Transmission Systems Handbook, Ch 15, Radio
Wave Propagation - Gerhard J. Straub; Ch 16, Antenna Principles -
Pingjuan L. Werner, Anthony J. Ferraro, and Douglas H. Werner; Ch 17,
Practical Antenna Systems - Jerry C. Whitkaer



I could go on, but Chucky, you're not really interested in me, you're
just interested in saying "you're wrong" to everyone until you have
had the last post and can claim victory. Therefore, to that end, I say
you win, go back under your bridge and wait for the next traveler,
Chucky. That is what Trolls do isn't it? While your there, with
nothing to do, try reading some theory,
http://ftp.21ic.com.cn/RFDesign/ is a good place to start, you will
find a copy of the US Army Field Antenna Handbook, although maybe too
advanced, as you would have to get you mind around the concepts. My
daddy once told me to never try to argue with an imbecile, "They drag
you down to their level, then beat you with experience" so I guess I
will leave it at that.



P.S. The reason I have all this reference material, is that the
Marconi is my "weapon of choice", you see I am a Road Warrior, one of
those sleazy WarDrivers who use laptops equipped with wireless
Ethernet cards and remote antennae to acquire internet access over
unsecured Wireless Access Points. My antenna is a quarter-wave Marconi
with a 30° counterpoise. I use 30° because unlike 45°, the 30°
slopping counterpoise gives a slight upward tilt to the radiation
pattern's maximum lob without affecting impedance that adversely. The
increase in effective radiated power more than cancels the loss due to
reflection and power transfer losses due to impedance mismatch.



--


"chuck" wrote in message
news | Hello Mungo,
|
| I'd say the first part rates a C-, mostly for credit in summarizing
a
| great deal of material. But did you say that the input impedance of
a
| quarter-wave vertical antenna at 30 MHz is 100 ohms without a
| counterpoise? NO ground return path at all? Sorry, but I think maybe
you
| have fallen prey to Roger's "magic."
|
| Your understanding of what the 23' antenna is and how it works is
sadly
| wanting. It also does not comport with the material in the earlier
| paragraphs. You have not even hinted at the sleight-of-hand
introduction
| of an antenna tuner and why it is needed, and you have introduced an
| antenna without any mention of an RF return path! You are posting to
a
| newsgroup where the readers are generally familiar with the 23' whip
and
| the need for an antenna tuner and an RF return path.
|
| Do yourself a favor: find a book on antennas and read it. An
elementary
| text is a good place to start.
|
| And in regard to the excluded middle, I think I understand now why
you
| have not found it.
|
| Regards,
|
| Chuck
|
|
|
| Mungo Bulge wrote:
| I have to agree with Chucky on at least one point. The postings on
| this thread are humorous. That must be frustrating to those who
were
| trying to follow the thread in the hope of learning something
useful.
|
| The reason I find it amusing is that both the con's and the pro's
are
| to some extent correct in their reasoning all be it flowed to one
| extent or the other.
|
| As I am really old and lacking any substantial short-term memory,
| re-reading the entire thread to glean every single nuance of every
| attempted point would be counter productive for me because I would
| probably forget what I was doing. I will however make some
assumption,
| which I will hold true, for the sake of simplicity. By the way
Chucky,
| the proper use of reduction to the absurd (reductio ad absurdum)
logic
| would require a model theory which is based on the law of excluded
| middle (tertium non datur) which clearly does not apply to the
| statements put forth in this thread.
|
| My assumptions:
|
| 1. The usual antenna system used with marine transmitters
in
| the intermediate and high-frequency bands is the Marconi or
grounded
| radiator, in contrast to the Hertz or ungrounded radiator.
Technically
| the most basic of antenna, it is an "isotropic radiator". This is
a
| mythical antenna, which radiates in all directions, as does the
light
| from a lamp bulb. This assumption is somewhat modified by
assumption
| #5 below, as a reflection (pardon the witticism) of reality.
|
| 2. Depending upon height above ground, the influence of
| surrounding objects and other factors, our quarter wave antenna
with a
| near perfect ground exhibits a nominal input impedance of around
36
| ohms.
|
| 3. Ground losses affect the feed point impedance and
antenna
| efficiency. When mounted on a real ground, the input impedance can
| range from 38 ohms for a well-designed antenna mounted over a
| specially prepared ground, to over 100 ohms for a Marconi mounted
| above poor, unprepared ground that has no radials.
|
| 4. Ground loss reduces the antenna's efficiency, because
part
| of the power being delivered to the antenna is being dissipated in
the
| ground rather than being radiated. The efficiency can be computed
from
| the measured value of input resistance by using the formula;
| Efficiency equals 36 ohms divided by antenna impedance.
|
| 5. The radiation pattern of the Marconi antenna is a half
| doughnut. There is no radiation straight up in the direction of
the
| antenna. The bulk of the radiation occurs at a low elevation
angle,
| which is what is needed to launch a ground wave.
|
| 6. Finally yet importantly, the HF installation shall be
used
| for communications in excess of line of sight. In other words, the
| skipper will want this installation to work when s/he needs to
contact
| S&R in a life-threatening situation, not just to collect the email
| while birthed in some marina.
|
| Ok, now the model part. This antenna is to be mounted on a boat.
So
| what is all this talk about ground? Well let us look at the
Marconi
| antenna. It is actually one half of a dipole antenna. Trust me on
| this, explaining why and how will just get us lost, but the other
half
| of the dipole is needed and is provided by the image produced by
the
| previous mentioned ground. There is that word ground again. Now
| Chucky, this is were some clarification needs to be. This is not
the
| same ground we all have come to know and love when we talk about
the
| ground in an electrical system like your house (safety ground),
your
| car (negative return) or your boat (negative return) this is terra
| firma, real honest to goodness dry land, and for the time being we
| will assume its perfect. Now, by perfect I don't mean a perfect
| conductor, hell it's not even close. By perfect I mean a low
impedance
| to RF currents. The RF ground currents are greatest in the
vicinity of
| the feed point at the base of the antenna up to a distance of ¼
wave
| length from the antenna.
|
| Now we all know there isn't any perfect ground left, the Aztecs
used
| it all to build their pyramids. So how do we make it perfect or at
| least acceptable, we install a counterpoise, a conductor or system
of
| conductors used as a substitute for perfect ground in an antenna
| system. That's were Me's radials or RF ground system comes in. It
| should have the effective radius equal to the height of the
antenna
| (1/4 wave length). I say should have, but in reality the radials
do
| not all have to be the same length and that losses may be
decreased by
| adding extra radials near the feed point. These extra radials can
be
| as short as 1/40 wave length and still be effective. Now, with
this
| added counterpoise, you can pick up this antenna and move it
anywhere
| and it will still function very well with a relatively high
| efficiency. That's a new term I snuck in without you seeing it
coming.
| Cool!
|
| Remember assumption #2 above? I said a Marconi had an input
impedance
| of 36 ohms on perfect ground, well it turns out that with a
| counterpoise it has an impedance of 38 ohms. Trust me, it does.
Now
| let's drag this sucker over to the marina and hoist it onto that
boat
| we have all been talking about. We get it mounted, counterpoise
and
| all. Not easy considering its size. A Marconi antenna for a 2 MHz
| system is 117 feet high (234/2) with a 117 foot diameter
counterpoise.
| Big sucker isn't it? How are we going to keep the counterpoise
level?
| We're not. Let's let it droop, say 45 degrees. I'm cheating here.
Some
| of you may know that a Marconi with a counterpoise set at a 45
degree
| down slop has an input impedance of 50 ohms and just by chance,
that
| exactly matched the radio set's output impedance, thus maximum RF
| power transfer between antenna and radio. Cool!
|
| However, the antenna's efficiency has dropped to 72% and we have
this
| honking big antenna messing up the aesthetics of our nice boat.
Let's
| scale it down. Instead of 2 MHz, let's go to the other end of the
band
| to 30 MHz. That makes the antenna 7.8 feet high with a 7.8 foot
radius
| counterpoise. Before you start screaming about my math, I allow
for a
| "velocity factor" of 5%. It's not as big, but it's still ugly and
we
| need more power to be able to raise S&R when need them.
|
| For aesthetics, let's drop the counterpoise. What happens? The
antenna's
| impedance goes up to about 100 ohms and its efficiency drops to
36%,
| but that isn't the worst of it, the power transfer is not maximum
| because the impedances are not matched. The reflective coefficient
| will be 0.3333
|
| ((100-50)/50)/((100+50)/50), thus a VSWR of 2.0, a return loss of
9.5
| dB which means the power actually reaching the antenna is 11.1% so
now
| I can't reach S&R and I'm going down.
|
|
|
| Me is thinking "I'm vindicated" and Chucky is thinking "Another
nut
| case". You're both right. So why does it work, simple. The antenna
isn't
| a Marconi; it's an industry standard Marine HF band antenna, 2-30
MHz
| bandwidth, 10.8 MHz resonant frequency, 23 feet high and when
| connected to an HF radio set configured to its manufacturer's
| specifications it will perform admirably. If that were not the
case,
| we would have had to have had at the very least five quarter wave
| Marconi antennae ranging in height from 7.8 to 117 feet and we
don't.
|
| That Chucky is the proper use of reductio ad absurdum logic.
|
|
|
|
|
|
|
| "chuck" wrote in message
| ink.net...
| | Well Bruce/Me, I think you need to pull your two "selves"
together!
| |
| | Sifting through the humorous postings, I think your bottom line
is
| that
| | HF/MF vertical antennas will not work well (sometimes I think
you
| mean
| | will not work at all) unless they are (1) over sal****er with a
| return
| | path capacitively coupled to the sea (at least for nonmetallic
| vessels);
| | or (2) over land with 100 quarter-wave radials in marshland.
| |
| | You have labored to persuade us that less-than-perfect marine RF
| ground
| | systems are certain to disappoint.
| |
| | It will surprise you, perhaps, to learn that there are many
| thousands of
| | vertical HF and MF transmitting antennas in operation in the
world
| today
| | that satisfy none of those conditions, and yet enable effective
| | communications activities. Some on land and some over water.
These
| | installations are supported by rigorous theory as well as by
| on-the-air
| | performance data.
| |
| | If you would like to learn more about how this is being done,
often
| with
| | losses of only a few dB below ideal conditions, drop in at
| | rec.amateur.radio.antenna and "read the mail." You'll find some
| | bombastic assertions and opinions to be sure, but also many
reasoned
| | analyses and even quantitative experiments. Hope to see you
there,
| Bruce.
| |
| | Regards,
| |
| | Chuck
| |
| |
| |
| |
| |
| |
| |
| | Bruce in Alaska wrote:
| | In article
,
| | Me wrote:
| |
| |
| | In article
| .com,
| | "Skip Gundlach" wrote:
| |
| |
| | As further background, we have full rails, with the gates
| combined
| | electrically with brass straps belowdecks, attached to the
arch,
| the
| | pushpit and pulpit. We have about 110 lineal feet of 1" SS
tube
| rail,
| | unless you count the inner rails, plus the arch. In addition
we
| have
| | the standard 4" copper strapping leading to a sintered bronze
| Guest
| | plane below the boat, and also connected to a 3x5' plate
under
| the
| | workbench top. I think we have a reasonably good ground.
| |
| | You will never know if you have a "reasonably good ground",
unless
| | you get yourself an Impedance Bridge, and check it at the
| frequencies
| | that you commonly work. Anything that is more than 12" away
from
| the
| | water, isn't going to add "diddley-squat" toward building a
Low
| Impedance
| | Wideband RF Ground System, and anyone who tells you otherwise,
is
| just as
| | uneducated about MF/HF Marine Radio Antenna Systems, as you
seem
| to be.
| | I have seen all kinds of Systems that looked very impresive,
| untill they
| | were evaluated with real insurmentation. 400 Sq Ft of Copper
| Screen in
| | the Cabin Overhead was proffered, as a really good RF Ground,
by a
| well
| | known Boat Builder, 20 years ago. It didn't work any better
than
| | having nothing at all, when tested, in a real radio
enviorment. If
| | you got a Plastic Hull, you are NEVER going to get a Real RF
| Ground,
| | UNLESS the hull builder was smart, (they never are) and put
200+
| Sq
| | Ft of screen under the gellcoat down by the keel. Cellulose
hulls
| | are just as bad, and harder to retrofit that Plastic ones.
| | Like I said in my first reply, Autotuners were invented to
allow
| any
| | "Dufus" to think he install an MF/HF Marine Radio System, and
save
| | himself all that money he would have paid a Compitant
Radioman.
| | SGC Autotuners are some of the worst of the lot, even if they
did
| steal
| | the design from the real inventers. SGC couldn't even copy
the
| design
| | correctly, and "Old PeeAir" couldn't design his way out of a
"Wet
| Paper
| | Bag".
| |
| | Me
| |
| |
| | Geeze Louise "Me" give the guy a break...... He was just
asking
| for
| | an opinion....
| |
| |
| | Bruce in alaska
|
|