Here are my personal experiences and observations with "BuryFlex".
Back in the 90's, I went through a period of station upgrade,
followed by a period of antenna experimentation, mainly with lower
band receiving antennas. To support all of that work I purchased a few
thousand feet of coaxial transmission line, in a wide range of types
and brands.
For my "main" lines, which needed to be buried to make it
from the house, under the lawn, to the "antenna fields", I chose
BuryFlex. I also used it for my single tower, and around the
rotator. The buried runs are approximately 150 and 175 feet long.
For other purposes, such as test antennas that I would
put up for a few weeks, to arrays of those antennas, I would use
BuryFlex, RG-213, RG-8, RG-8X, RG-58, all sort of stuff. I even
had some 75 Ohm RG-11 which was used mainly as "phasing lines"
in delta loop arrays, and series matching sections.
Before all the dust had settled, I had probably installed
well over 100 UHF coax connectors on cables of all sorts of
lengths, from 1 foot to about 250 feet.
Mechanically, I found BuryFlex to be very rugged, and
whenever I have had the opportunity to check out cables in the
field, including buried, they are in excellent condition. It
does have a high degree of flex, and a small turning radius.
Around the rotator, it worked very well, and after a few
years of turning, it showed no signs of wear at all.
As far as attaching coax connectors, I did find that the
stranded center conductor was a little too wide for a few
of the various connectors I would try to use. I tended to
pick up packs of 10 to 25 male connectors at hamfests and
from the Internet, and they probably covered the range of
name brand, strange brand, gold pins, silver pins, Teflon
insulator, and on and on.
I found that about 10% of the range of connectors
I had would not fit over the twisted BuryFlex center conductor.
Initially, I cut two of the stranded center conductors to reduce
the diameter, but as I accumulated a range of connectors,
I just learned which connectors to avoid when putting ends
on BuryFlex.
As part of my antenna experimentation, I would often
be out in the field, right at the antenna, with a typical
antenna analyzer, such as the MFJ-259, 259B, 269, AEA HF-CIA,
and the Autek RF-1. I would then come back inside
the house, where I had a computer next to the radio. I had
a serial interface for the AEA HF-CIA, so I could capture
SWR graphs on the computer, for a whole range of purposes.
At some point, I picked up a laptop computer, which
made it easy to record antenna analyzer output data right in the
field, next to the antenna, before I entered the BuryFlex
runs back to the house.
An ideal lossless 50 Ohm transmission line should repeat
the SWR found at its load end at its input. The impedance will
be transformed as a function of the length of the cable, but
the SWR should remain the same. Of course we don't have
ideal lines out in the field, we have real lines, with loss.
The impact of the loss is to reduce the SWR at the input (station side)
of the line. In many typical HF situations the reduction is
small.
I noticed that when I overlaid the SWR graphs in the
field on top of the SWR graphs made inside the house, they
did not follow the expected relationship, which is to say the same
general shape, with the inside values slightly lower due
to loss on the cable between the two points.
In some cases, the SWR would rise - I admit, it was
a small amount, but that made no sense to me.
I then performed a test which I must confess I
had not done before. I put a 50 Ohm load on the end of a
length of BuryFlex, and a typical antenna analyzer on the other
end, and swept the frequency across the HF range, perhaps
1 to 30 MHz. I expected to see a flat SWR of 1.0, since
I had a 50 Ohm load, 50 Ohm cable, and a 50 Ohm analyzer.
Well, with BuryFlex, the SWR would swing, as a
function of frequency, between 1.0 and a high of 1.3.
By this time, most of the coax I had once used out
in the field had been rolled up, and stored within
some metal cabinets in the garage. I went and grabbed an
armful of assorted cables, from high quality RG-213, to
pretty darn cheap RG-58 that I used in receiving antenna
arrays. All other 50 Ohm cable, when terminated with
a 50 Ohm load, had a flat sweep across the HF range.
So, something about that BuryFlex was different.
I found all of the lengths that I had rolled up, and
all exhibited the same behavior. I did wonder if I
got a "bad batch", but I do believe that I was measuring
across a set of cables that I had purchased over
a period of perhaps 3 years, and I assumed that meant
that I was looking at different batches and runs and
seasons.
I wasn't sure what was going on, and I wasn't going
to replace that buried BuryFlex, but I stopped using more of it,
especially if I was trying to make useful measurements.
The cable obviously "worked", since I had been pumping
1500 watts through it for year, from 160 meters to 6 meters.
As a few more years passed, and I ended up
interested in higher quality impedance measurements,
as part of understanding and building phased vertical
arrays. I accumulated even more impedance measuring
devices, including old Gen-Rad and Boonton boat anchors.
I did end up with an N2PK VNA, which I believe has
near professional/lab accuracy.
A year or so ago, I got curious about this whole
area again, and now that I had some good quality
measurement equipment, I grabbed some different cables
from the cabinet, and put on a 50 Ohm load, and swept
them from 1 to 50 MHz. Now, what I was really
measuring was the complex reflection coefficient, which
could be converted, by formula, to complex impedance,
SWR, return loss, and all sorts of useful quantities.
When I displayed the data on a Smith Chart, an
interesting pattern emerged. The other 50 Ohm cables spiraled
around the 50 Ohm load. A 75 Ohm cable (with a 50 Ohm load)
also showed a spiral, but it was centered higher up the resistance
axis. The BuryFlex, however, had a spiral which was
centered under 50 Ohms.
Now I'm no Smith Chart expert, especially when it
comes to combining data from different Zo cables, but especially
when comparing BuryFlex to other 50 Ohm cables, to 75 Ohm
RG-11, the pattern is pretty clear.
My conclusion was that the BuryFlex I had here had
a characteristic impedance lower than 50 Ohms, perhaps around 45 Ohms.
After the messages in recent days about BuryFlex,
I went out to the shop this morning, and again grabbed two
different rolls of BuryFlex, a few typical 50 Ohm cables,
and some 75 Ohm RG-11. I wanted to see if I could
duplicate the measurements. All of these cables are a few
years old, and some have been used quite a bit. In other
words, I am not trying to work with new cable right off
of a roll.
I got the same results that I had seen in the
past. I captured two Smith Charts showing BuryFlex versus
other 50 Ohm cable, and some 75 Ohm RG-11. The test
scenario is a random length of cable (more than a few
feet, less than 100), with a 50 Ohm load, and a sweep
from 1 MHz to 50 MHz with 100 KHz spacing. If I were
willing to terminate the BuryFlex with a variable noninductive
resistor, I could probably nail down the actual cable
impedance.
I put the two Smith Charts on a web page so that
they could be inspected by all interested parties.
The URL is:
http://www.seed-solutions.com/gregor...n/buryflex.htm
I certainly can be interpreting this data incorrectly,
and please feel free to correct me if you believe that
I've come to the wrong conclusion.
I do not have an opinion about the "loss" of BuryFlex.
I could make return loss measurements, but for those to
have meaning, I would need to unroll the cable, and measure the
length, and since it's 10 degrees F outside, with 18 inches
of snow, and more coming down, I'm not quite that motivated.
My VNA also stops at about 50 MHz, and the frequency first
mentioned was quite a bit higher (400 MHz). The initial report
also talked about loss as a function of coiling and bending,
which can complicate the test scenario.
For me, and my test equipment here, BuryFlex does
not have the characteristics of any other "50 Ohm" cable
I have measured. I believe that my conclusion that it is more
like 45 Ohm cable as opposed to 50 Ohm cable is correct -
again, for the samples I have here. I have found it to be
mechanically excellent and very rugged. Some coax connectors
don't quite fit over the center conductor.
I am quite suspect, however. I encourage
anybody with access to the cable and appropriate test
equipment to confirm the loss measurements, since I trust
the source of the original claim. Because I don't believe the
50 Ohm specification, I guess it's easier for me to believe that
the loss specification is incorrect too.
If you want to know if your cable is like mine - that's
easy - terminate a length with a good quality 50 Ohm resistive load,
and sweep the frequency while watching the SWR. My BuryFlex
bounces up and down.
Greg Ordy, W8WWV