[TowerTalk] Dielectric losses

[K3BU@aol.com]

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

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