| |
VK5QX > INFO 13.07.02 08:07l 179 Lines 7791 Bytes #999 (0) @ WW
BID : F90279VK5QX
Read: DB0FHN GUEST
Subj: CATENARIES/CABLE INSTALLATION
Path: DB0FHN<DB0ZWI<DB0CHZ<OK0PKL<OK0PPL<RZ6HXA<SP7MGD<7M3TJZ<HA3PG<VK3AVE<
VK5BRC<VK5SPG<VK5LZ
Sent: 020712/0720Z @:VK5LZ.#ADL.#SA.AUS.OC #:38923 [Elizabeth] $:F90279VK5QX
From: VK5QX@VK5LZ.#ADL.#SA.AUS.OC
To : INFO@WW
OTHER USES.
Harry, M1BYT, pointed out the meanings and a use for the word "catenary",
(and did provide the correct spelling too.)
_______________
" A catenary (not sure of the correct spelling) is a wire or rope
suspended above. In rail traction terms this would mean a wire hanging
above and along the legnth (sic) of the rail-track, from which the power
is
drawn to drive the train. The old trams were driven by electric motors
supplied by a catenary wire over the tram track. "
_______________
As described, a catenary refers to a cable, length of chain etc., which is
suspended with the ends located in two different vertical planes. Embodied
within the definition is also the description of what is known as a
"catenary angle", however there is no reason for this unusual geometric
aspect to be considered in the current context.
Several bulletins on the subject have indeed referred to the use of
catenaries for the purpose of providing (electric) power for such as trams
and trains.
Another application is with regard to the actual supporting of cables and
wires where a "catenary" wire is used.
This is an important facet where it comes to the matter of towers and
antennas and yet one that rarely seems to get a mention in the various
handbooks devoted to Amateur Radio.
It seems that many Amateur Radio operators either ignore or are not aware
of the benefit of using a "catenary" wire to carry the weight of coaxial
feeder cables and control cables installed between a tower and a building.
A combination of coaxial feeder and control cables can mean that quite a
deal of weight is involved together with resultant strain and damage.
Simply allowing the cables to dangle and to carry their own weight,
sometimes over quite a distance, can mean that faster deterioration of the
cables and wires can occur, in some instances to the extent that the inner
conductors can actually fail altogether.
A far better approach is to install a suitable strength supporting
catenary to which the cables are tied or lashed for the length of the open
air run between tower and building.
Probably the best material for a catenary of this nature is stainless
steel wire rope. This is an excellent weather resistant material and is
also very flexible.
Such material is commonly available from stores which supply boating
requisites and is usually obtainable in various sizes. (Thickness)
The same source can also provide suitable wire rope clips which allow you
to bend and clamp the wire rope into a loop, thus providing a simple way
of fixing to a leg of a tower or to an eye bolt. This latter is usually
screwed into a suitable place near the building entry point for the
cables.
In some instances it may also pay to have an adjustable turnbuckle,
preferably also of stainless steel, which allows you to set a suitable
tension on the catenary cable.
The various cables can be attached to the catenary by wrapping plastic
insulation (PVC) tape around both the cables and the supporting wire.
Alternatively, they could also be lashed with plastic ties or tied with a
suitable weather resistant cord. (Please note that plastic ties are
invariably subject to deterioration over time.)
Whilst on the matter of cables between towers and buildings, I might
mention that there are undoubtedly better ways to install feeder and
control cables than by running them through the air, so to speak.
This is to install the cables so that they are routed down the full height
of the tower to ground level. In this way the cables can be supported,
strapped or tied, to a leg of the tower for most of their length.
They can then be taken through a suitable protective ducting either along
or under the ground. The ducting would presumably be made from some
convenient diameter PVC piping.
(Perhaps one should also remember to include a "pull-thru" rope where
ducting is used, so as to cater for addition of cables at a later date.)
I was fortunate to be able to obtain some suitable lengths of semi-rigid
50 ohm foam dielectric cable which featured a termite proof outer cover.
Such a cable is quite suitable for burial in the actual ground itself and
did away with the need for a ducting type pipe.
The type of installation at my location, as described, is ideal where it
comprises stacked rotary beam antennas.
The feed cable for each of the antennas is terminated at the top of the
tower in an "N" type connector. From there a more flexible length of
coaxial cable, RG58-U or similar, is run to the actual feed point of the
antenna.
Adaptors from "N" type to "BNC" type connectors are used and the whole lot
is well waterproofed using self-amalgamating tape. (This material is
really marvellous and strongly recommended for use in such situations.)
This flexible length of smaller cable is long enough to allow somewhat
more than a 360 degree rotation of the antenna(s).
The antennas for the lower frequencies, wire dipoles for the 80 and 40
metre bands, are installed in the "inverted vee" fashion with the centres
of the antennas attached to the top of the tower, thus allowing a similar
approach to feeder connection.
I might add that, where horizontal wire dipoles are concerned, it is
usually the case that the feed cables will have to be made self
supporting.
In such instances it is usual to have a centre insulator system that
ensures that the actual (electrical) cable to dipole connections do not
carry any weight. Various methods are adopted to achieve this condition
and such could be left to individual ingenuity. In these cases the weight
of the feed cable is carried by clamping, in some way, to the outer casing
of the cable.
If anyone is interested, I could describe in some detail the method that I
employ which embodies the use of a triangular piece of flat material,
preferably light and strong, such as fibreglass sheeting.
P.S.
Here are a few practical considerations that can come into play where the
connection of feeders using smaller flexible lengths of cable, as
described above, are concerned.
In very many instances antennas (commercially made) do not include
provision for a plug in type connection. Single terminals for connection
are commonly provided generally using some type of connecting lug.
In these cases it is necessary for the terminations, and resultant cable
"pigtails" at the end of a coaxial cable to be carefully insulated and
weather-proofed.
In situations where a fault may exist it can be a difficult and time
consuming task to remove the weather-proofing to gain access and
disconnect at the actual connecting terminals.
There can come a time when one encounters a fault condition where it would
be most convenient to be able to easily isolate the antenna from the feed
cable.
If a requirement eventuates where the antenna has to be removed for
maintenance purposes, it can be seen that it is an easier matter simply to
unplug a cable connector than go through the trauma of disconnecting as
just described.
The alternative would be to remove the antenna and have the added
inconvenience of also carrying the whole assembly down from the tower with
all the feeder cable still attached.
It is certainly much easier to conduct a test of the feeder cable by
simply breaking the plug type connection and placing a dummy load, or
other appropriate test gear, at the end of the feeder cable.
You may also note that having a relatively short length of reasonably
flexible cable, with a suitable plug type connection as described, usually
leaves little to worry about, even where reasonably high output power, up
to a kilowatt or so, is concerned.
Regards,
Ian
__________
73 de Ian, VK5QX
@ VK5LZ.#ADL.#SA.AUS.OC
12 July 2002
Read previous mail | Read next mail
| |
