Hello Brian,
Yes I believe it does contradict Bruce's own position as he stated it.
The relative performances of the various marine RF return path
techniques is a fascinating, but complex, area in which virtually all
the data are either theoretical or anecdotal. The theoretical materials
don't focus much on RF grounds on small, non-metallic cruising boats.
Imagine the problems one would encounter in order to objectively and
quantitatively compare two competing RF ground systems. An RF impedance
bridge could be used to measure the ground loss resistance of the
competing systems, but the mesurement would have to be made first with
one system on the boat, and then with the other system on the same boat.
If the system being mesured involves copper mesh in the hull while the
boat is built, that is obviously going to be difficult. The measurements
would have to be made in the same location and with the same antenna and
at the same frequencies.
Then there is the problem of knowing what is being measured. For
example, suppose the RF ground system is connected electrically to the
prop, or the rudder post, or a metal keel. How much of the ground loss
reduction is attributable to these items and how much to the RF ground
system itself? It matters, because these are techniques being advocated.
As you can see, it becomes very difficult to make generalizations
(except this one, of course). But boaters want to know these things.
They want to know what RF ground system performs best, and they want to
know trade-offs. If A performs 5% better than B, how much more does it
cost? Most non-ham boaters are not comfortable with the idea of a one
decibel change in transmitted or received signal strength. It is
difficult for them to translate radio performance into those terms so
even if we know how much better A is than B, it is difficult to
communicate that. Even more so when we recognize that there are several
dimensions to performance: galvanic issues, lightning protection, noise,
frequency, etc.
Amazing as it seems, I am not aware of any serious, repeatable,
quantitative comparison of ANY two RF ground systems for cruising boats!
What has been published in boating/cruising magazines does not qualify.
So boaters wind up instead subjected to a barrage of opinion of
questionable quality. One constant emerges from all of this: many
different approaches are in use and some measure of operational success
seems to be obtained from most of them.
My belief is that the better one understands the general idea of an RF
return path (and this applies in the air, on land, and on the water) the
more likely it is that a "better" (cheaper?) RF ground system can be
developed. Unfortunately, this discussion has revealed that this is the
one ingredient most conspicuously absent from the strong overstatements
of opinion we are finding.
Keep up the good work, Brian.
Regards,
Chuck
Brian Whatcott wrote:
On Sat, 30 Jul 2005 21:45:27 GMT, chuck wrote:
Well Bruce/Me, ///
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.
///
Regards,
Chuck
I see that he realises that airborne trailing wire antennas can work
well at HF with just a tube fuselage as a ground reference - miles
away from ground!
This just about completely contradicts his earlier posts about ground
references needing to be in close proximity to the ground for
successful HF work, wouldn't you say?
Brian Whatcott Altus OK
|