In a message dated 10/3/01 2:38:44 PM Pacific Daylight Time,
jimr.reid@verizon.net writes:<<
Force 12 spec depends upon 100 feet of coax:
> Sounds like their BW quote depends on there being enough
> loss in the feedline to achieve it.
Some years ago, Dave Leeson, now W6NL posted the
explanation here on Tower Talk. The length of line creates
a double tuned transformer which per theory does, in fact,
increase the "system" bandwidth. It is NOT added loss, it
is a form of matching transformer. I do not recall the details,
but suppose they can be found in the archives. These
posts were at least 5 years ago, perhaps even further back.
Dave is a PhD in EE, and has written many articles/books
in the tech literature; so presume he knows about what he
suggests to be true!
And, yes, I would guess that the "transformer" of a 100 foot
coax would only be optimum for a particular band area;
probably on 15 meters if I remember anywhere near correct, hi.
73, Jim KH7M
>>
Jim: Since 100' seems to be a standard or average length I have this
suggestion. Use the "K7GCO Magic Length" of 91' 2" or multiples which is a
1/2 wave or multiple real close on all bands based on a fundamental base
frequency of 3.562 MHz for .66 VF (1.781 KHz for 182' 4"). Antenna
R measurement can be made in the shack very accurately on all bands with the
simple Antenna Scopes (RF Whetstone Bridge) or the fancier Z measuring stuff
except 30&17M (minor error) for starters. The fancier measuring equipment
also measures reactance also but at resonance it's Zero. Adjust antenna DE
lengths so it is in the middle of the band and/or the GD'ed frequency of the
feedline. It gives very accurate R readings if there is no RF Spill Over and
no PHD degree needed. Factory Specs check out most of the time I've found
with 1/2 wave or multiple coax.
Since all my coaxes are this length or multiples, I can play phasing games
with any coax that happens to be connected to antennas I want to phase. I've
never had all these SWR measurement problems everyone seems to have since the
40's although the longer the coax the lower the SWR. Some SWR actually
improves the match for a higher power input at the end of the feedline for
the 50 ohm output rigs--the desired goal. Pi network finals match a fair
range of Z's and did a good job in the last Century.
If the SWR at the end of the coax was the lowest on a certain frequency for
a beam for example, that's what it was with the bridge at the feedpoint also
or by GD'ing. It never failed so I quit checking it at the beam. Maximum
transfer of power from the rig to the input of the feedline was determined by
the resistive load like 50 ohms it saw there and not at the antenna even if
on another frequency just off a bit. There seems to be something forgiving
and certainly repetitive about this length into "clean matching systems."
Factory specs checked out every time. A little SWR isn't all that bad as it
lowers the SWR and can enhance more RF into the input of the feedline.
One time back at Hy Gain in about 1960 a tribander was tuned up on the
test tower using very good coax and transferred to Andy Andros's tower to
test. A length of Columbia brand new Pollyfoam coax was used--the first run.
The same SWR curves with the same length of coax were not obtained. This
was a very embarrassing
Columbia pulled a huge boo boo. They just changed the solid dielectric
to Pollyfoam. It seems the Columbia Engineers didn't know the basic Zo
formula for coax. The shield OD was the same as was the center conductor OD.
The formula contains the factor of 1/sq root of the dielectric constant "e"
which went from 2.3 for solid to 1.6 for pollyfoam (.66 to .79 VF) and in
order to keep the Zo 50 ohms, the center conductor diameter had to be
increased from about .07" to .1" if I remember right from 41 years ago. If
anyone can look up this article on coax design that was in QST about 40 years
ago I'd like to review it again as mine is packed. When I first looked at a
cross section of this Columbia Pollyfoam coax and since I knew the formula, I
could see without measurement the shield and center were the same and the Zo
trouble was obvious. With a quick calculation in my head I predicted it was
higher. If "e" in the denominator decreases (less dielectric) the Log of
another factor in the formula denominator has to increase for the Zo of 50 to
stay the same for the same OD. The only thing that could change was the
center conductor and it had to increase about .03" and--it didn't. So the Zo
had to go up and it screwed up the SWR readings. That is why the center
conductor of RG 8 Pollyfoam is about .1" diameter to create 50 ohms Zo.
The larger diameter center wire was too large for the ID of the older PL
259 center pins and the ID had to be enlarged and was enlarged from then on.
In a typical Type N Center Pin, 3 strands of the center conductor wire of
Pollyfoam has to be removed so the rest would slip inside the Type N center
pin. I had an article in CQ about 35 years ago about all this in detail
showing cross sections of various Zo's of coax, the center conductor size,
diameters and other differences.
With physical measurements it came out about 60 ohms as did tests with
various resistive loads. Another length of regular coax solved the SWR
change problem. I still have some of the Columbia 60 ohm coax and it makes
great 1/4 wave stubs for matching 50 to 75 ohms. If you ever find some
Columbia RG 8 Pollyfoam, it might be some of the first run 60 ohm
coax--cherish it. I even have some 56 ohm coax from WWII.
In case you didn't know RG8 Pollyfoam coax only has up to .2 dB/100' less
loss at 30 MHz over solid .66 VF dielectric. As I recall the solid has17% of
the loss is in the shield, 3% is dielectric loss and 80% is center conductor
loss. The dielectric % loss of solid dielectric starts to increase a bit
faster over 30 MHz over Pollyfoam and the larger center conductor reduces its
loss. Since the center conductor has the highest % loss by about 4 times at
30 MHz, increasing the center conductor was of fair benefit for a small
diameter change. Pollyfoam's advantage starts to be of value above 30 MHz at
a faster rate with increasing frequency.
Another problem of Pollyfoam is this. The dielectric constant is
unfortunately not constant and can vary the resonant length of it as much as
3' over 100' and alter phasing relationships. In most single coax
applications the variance of VF is not important. I used equal lengths of it
on a beam I have and I could immediately see something wasn't right. I have
a way of changing polarization's of a beam in the shack (you would never
guess how--6 different polarization's) at the end of 2 equal physical length
feedlines 91' 2" long GD'ed at 28.5 MHz. I was not getting the specific
polarization's with 2 equal physical length Pollyfoam coaxes I got with solid
dielectric coax. One was about 36 degrees too long. From my tests I guessed
less than 45 degrees. I had to shorten one Pollyfoam coax 3' (36 degree) to
GD at the same frequency as the other and did so for all matching 2 coaxes
cut from the same 1000' roll. One pair came fairly close. Remember that if
the VF changes--so does the Zo.
2 equal lengths of solid .66 VF dielectric will GD so close I stopped GDing
the second one. It's a good idea to still GD each as I found a piece of bad
solid dielectric coax one time. This would have caused a lot of hard to
detect trouble had I not GD'ed it. I GD'ed only one length of the 2
Pollyfoam coaxes and cut the other one the same length. This is one reason
why the GD'er and knowing how to use it is so important for the competitive
ham.
Would you believe they mix Baking Soda into the dielectric to get it to
foam? It's hard to control the mixture and that's why it varies. Just
kidding. When they make Pollyfoam they blow nitrogen into it and they have
not been able to fully control the variables, therefore the VF varies. For
144 MHz and above I think it's great coax if the VF is not critical. You can
GD it if you know how.
Variances in coax and even SWR bridges can make a well designed beam look
bad. Water in the coax can screw it up also. So the beam Mfgs aren't always
to blame. Open wire line never makes an antenna look bad, has nothing to
saturate and nothing to create an unbalance.
This all points out many problems with coax few are aware of. None of these
exist with open wire line. Any variance of wire spacing alters the Zo very
little. Regardless of any Zo change, if the tuner can match the Z at the end
of the feedline, 100% of the power transfer from the xmitter to the input of
the feedline less the very small losses in a tuner properly designed and
tuned. The same is true matching the Z at the input to a coax cable.
However, a lot more of it gets to the antenna with open wire line for years
and years and years through all kinds of hazards. The Z at any frequency of
the DE is of little concern with open wire line where with coax it's a major
concern. Solder lugs are a lot cheaper that coax connectors by about 20 dB
(new concept of measurement) as is the feedlines comparative cost. There is
virtually no loss in a soldering lug connection ever if properly soldered and
coated with silicone grease. Or in many cases, joints can be soldered for
even less loss and greater longevity. Feedline life is many dB longer for
open wire line and can actually outlive you (I have one that will) with no
increase in loss. You can make open wire line where you can't make coax. If
damaged open wire line is easily repaired with a soldering iron where coax
isn't. Perhaps a more serious look ought to be made of open wire line.
There are no Reflectors where the problems of open wire line properly used
are discused, there are for coax problems on a regular basis on and on and
on. It's not very user friendly at times.
I may actually make some very low loss 50 ohm or higher coax for VHF/UHF with
3M adheasive backed (or contact cement) copper foil covering 2" PVC OD pipe
and about a .8" center conductor foil covered PVC pipe held with slotted
disks (so condensation could drain) for moon bounce antennas. It could
tolerate very high SWR's if needed also. It's also easy to splice
mechanically and electrically. About a .3" center conductor would give about
a 100 ohm coax for certain applications. With 6" OD and a .25" center
conductor about 180 ohms can be created. 2 series connected would give a
balanced 360 ohm balanced coax for "certain applications" for VHF/UHF. K7GCO
List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems,
Trylon Titan towers, coax, hardline and more. Also check out our self
supporting towers up to 100 feet for under $1500!! http://www.anwireless.com
-----
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
|