You asked for it, here it is:
To refresh the memory, here is the original W8JI statement:
>>To top this all off, there is virtually no difference in performance
using a foam dielectric. The vast majority of loss is conductor
resistance related, and has nothing to do with what dielectric is
used in a coaxial cable except as that dielectric affects the size of
conductor you can use for a given outside diameter to have a given
impedance.<<
The ARRL Antenna Book 17th edition, page 24-16 says:
"The power handling capability and loss characteristics of coaxial cable
depend largely on the dielectric material between the conductors...."
Fig 22 on the same page shows attenuation vs. frequency. If one looks at coax
like RG8 "regular" dielectric and RG8 foam dielectric curves, one can see
that slope of the curves for both coaxes is different, indicating that while
they have similar or close physical dimensions, at higher frequencies
"regular" dielectric coax has steeper slope, therefore loses increase with
frequency. Curves (lines) for most foam coaxes closely follow slopes of air
dielectric coaxes or lines. Polyethylene "regular" dielectric has a steeper
slope indicating increasing loses with frequency.
Most of us know that when coax gets contaminated, insulation between the
conductors deteriorates and this shows as sometimes significantly increased
loses. This is clearly caused by change in the quality of dielectric and not
by thinning of the wires or increased resistance in them.
Velocity factor in the cable (due to dielectric properties) has effect also,
more "slowing" of the wave in the cable, more wavelengths needed to pass
through the lengths, more loss. Goes higher with frequency, more "waves"
present.
So W8JI "gospel" that there is "virtually no difference" is dumbest statement
of the year #2. ("Explanation" by W8JI to follow, dancing around the what
"is" is. :-)
I was glad to see Tom W6XR "correcting" W8JI statement about "inferiority" of
Force 12. Contesters and DXers I know, have nothing but praise for F12
antennas. F12 bunch has done remarkable amount of work designing and testing
their antennas, and managed to beat world record in CQ WW with bunch of
verticals on the beach. My hat off to them and they gave me inspiration to
explore and take advantage of the phenomena. Judging by W8JI statement, he
knows nothing about antennas. (Using his style of posting "gospels" :-)
I appreciate the postings and advice provided by Tom, but I object to some of
the "gospel" statements that are false and being presented with "authority."
I would be more careful in wording and less sure about things I am not sure
about. Some might remember Tom's attacks on some of my postings about my
findings about 160m propagation, etc. Typical modus operandi is, Tom states
something (wrong), it gets rebutted, he tries to dance around, eventually
loses the argument, goes quiet for a while and later emerges as a guru on the
subject. Has happened before with other decent hams like KM1H, AMP reflector
etc. If the Rauchian followers consider him "RF Saint" than follow him and be
happy. I just can't stomach his wrong statements after the beating I got from
him on reflectors for being right. Sorry if that offends some of you, I can
too fade away behind KM1H.
BTW on the "sloper thing" the poster had factory configured antenna switch
which is normally open for unused connectors, so the unused slopers were open
(disconnected 1/4 waves), therefore "transparent" to the driven half-sloper.
(re Dumbest statement of the year :-) So he was wrong. I make just short
remarks because I am very busy with work around N2EE QTH - Mountainview
Resort, and I am getting tired of educating him and then his surfacing as a
guru.
73 and good coaxing!
Yuri, VE1BY, VE3BMV, K3BU
If you care to read comments on the coax subject by one of my mentors who was
removed from the reflector, here it is:
He claimed "the lower loss of polyfoam is more due to the larger center
conductor than the polyfoam dielectric without any qualifications such as the
frequency--the major factor." A blanket statement without the necessary
qualifiers is "technically irresponsible." When I read this I immediately
remembered an article on Low Loss Coax I read about 35 years ago in QST
sitting in this very chair describing the characteristics of polyfoam
dielectric that he obviously didn't read. I called QST and got them to look
it up that was 36 years ago (1 year off) and it was sent to me. I remembered
everything in it even as I do articles I read even in the 30's and give the
summary of it below based on a "% Loss Chart." It shows how the "solid
dielectric loss" starts at about 2% at 1 MHz and at 10,000 MHz finally equals
the "total copper % loss" at 50% and the center conductor loss at 50% about
8,000 MHz. I immediately knew the % loss figures for polyfoam would alter
this substantially. The graph I just got from Times Wire&Cable of % Loss of
Polyfoam is based on a .85 VF and verifies this.
It's true that the lower dielectric constant polyfoam requires a larger
center conductor to retain the same Zo (for the same outside diameter) which
is the highest loss element. The formula is Zo=138/SqRoot of dielectric
constant (2.3 solid, 1.6 Polyfoam) Log D/d. But the better dielectric came
first. It becomes a problem the higher you go. At HF Cu loss is the primary
loss. I'd use polyfoam on 6M and up.
Even at 10,000 MHz the dielectric loss for polyfoam only goes now to 15%
(very very low) and it's only 2% at 100 MHz. The center conductor loss is
55% at 10,000 MHz (3.7 times larger than the foam loss). The larger center
conductor in foam coax even has a higher % loss than with solid dielectric.
The "Technical Disinformation Pipe" he smokes continuously has bad stuff in
it. de K7GCO
________________________________________________________________________
Where do you get ICE bandpass filters & beverage matching boxes? The
same place that pays for the hosting of this list: The eHam Store.
Order online at http://store.eham.net.
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