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