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R.F
Cables
UR67
or RG213
Whatever
the type number, both of these cables are equivalent. This is really
the minimum quality cable that should be used for 23 cm. The only exception
to this is for very short runs within equipment, or between say a transmitter
and an amplifier, where the flexibility and convenience of a smaller
5.5mm diameter cable such as UR43 may be useful.
An alternative
such as 0.141" diameter semi-rigid cable could be used, although
this and the necessary special connectors are more expensive. Returning
to UR67, this is 10.3mm diamter cabl that is quire flexible, and is
thus suitable for making turning loops around rotators. The cable has
a plastic oversheath, beneath which is a thick layer of braiding. A
thick ploythene dieelectric reveals a substantial inner conductor made
up of 7 stiff strands of wire. Many different manufactureers produce
cable to this specification. It is a very popular cable in the Amateur
Radio faternity, and is widely used on the 2 m and 70 cm bands.
However,
at 23 cm it produces a loss of 7.3 dB per 25 metre length. This is rather
high, and in view of the better performance alternatives now available
for very little extra cost, we would not recommend its use. Being 10.3
mm in diameter, standard N plugs were made to fit it. It is also possible
to find BNC plugs to fit it directly, although these are still rather
expensive.
H100
This cable is produced
by the dutch manufacturer Popes, and is available from the well known
Amateur Radio cable supplier W.H.Westlake. It is slightly smaller diameter
than UR67, 9.7mm. This is one of its major failings, since standard
N connectors are not a very good fit, and special connectors to suit
it do not seem to be generally available. Beneath its plastic oversheath
is a very light braid covering a layer of copper foil. Beneath this
is a semi-airspaced dielectric covering a solid centre conductor. The
overall construction is significantly less flexible than UR67, and the
copper foil screening is liable to fracture if flexed excessively. Hence
it is definitely not recommended for turning loops around rotators.
The good news is that the loss is much less than UR67, at about 4.2
dB per 25 m and it costs about 20p per metre more. However, in view
of the difficulty of fitting connectors to it, and the availability
of an alternative, it is once again not particularly recommended.
WESTFLEX
103
This cable is a special
one again available from W.H.Westlake. As its name implies, it has a
diameter of 10.3mm, thus fitting standard N connectors with the exception
that the inner conductor is slightly larger than UR67. This can be overcome
by filing it down a little, or by using the special connectors which
are now available from Westlakes (price about £5.00 each ). Westflex
103 is also quite flexible. The loss characteristics are very similar
to H100. It is thus a combination of the better qualities of UR67 and
H100, and the cost is about the same as H100. Hence it is the first
cable which i would recommend for 23 cm use.
ANDREWS
HELIAX
Andrews Heliax is
manufactured by the American Company Andrews, who produce a wide range
of RF equipment for professional use. Being a professional product,
the new cost is probably beyond that which most amateurs can afford.
However, it is the practice of many professional radio equipment installers
to replace lengths of cable on a strict time basis, regardless of wether
it is faulty or not. These secondhand lengths of cable often appear
on the market at rallies for a fraction of the new cost, and are a good
buy. However, do not buy any such lengths if they exhibit dents in the
sheath, serious mechanical damage or puncturing, or any sign of internal
dampness. Now lets consider the cable itself. Generally, Heliax cables
are constructed from a gorrugated solid copper sheath with a plastic
outer covering. The dielectric is composed of a rigid white foam, and
this covers a solid inner conductor. It is avilable in a variety of
sizes, the most common of which are LDF-250 (3/8" nominal, 11 mm
diameter) and LDF-450 (1/2" nominal, 16 mm diameter). Special Andrews
connectors are needed for all types, and be warned, they are not cheap
! However, these are often available "surplus" at rallies
for much less than the new cost. This cable is not very flexible, a
fact recognised by Andrews who also produce flexible Heliax cables if
this property is needed. However, i have never seen these flexible Heliax
cables available at rallies. The RF performance is excellent, LDF2-50
producing a loss of 3.3 dB per 25 m, LDF4-50 2.2 dB per 25 m. These
cables are thus highly recommended for fixed runs, provided you can
obtain some at a reasonable cost. Brand new, the cost may be somewhat
prohibitive for most Amateur budgets.
JAPANESE
FB SERIES CABLES
These cables with
their distinctive white oversheath are imported from Japan by the Portsmouth
based firm of Nevada. The construction comprises the aforementioned
white plastic oversheath, beneath which is a thick layer of braiding.
This covers a foil screen and a rigid white foam dielectric. The inner
conductor is a solid core. The cable is available in 3 sizes ; 5D-FB
is 8.1 mm in diameter, 8D-FB is 11.1 mm in diameter and 10D-FB is 13
mm in diameter. It is just possible to use standard N connectors with
8D, but the other sizes require special connectors which are about twice
the cost of standard types. 8D produces a loss of 4.2 dB per 25 m, whilst
the best type 10D is 3.4 dB per 25 m. Hence 8D has similar loss characteristics
to H100, but far superior construction. It is just about flexible enough
to be used in a turning loop around a rotator. It costs about 50% more
than H100 or Westflex 103. 10D offers very similar performance to LDF2-50
Heliax, at about twice the cost of H100. Even though the essential special
N connectors for it cost about twice as much as standard types, it is
a high performance cable that is affordable on an Amateur budget. The
largest size 10D is best suited to fixed runs, since it is starting
to become a little inflexible. In conclusion , these Japanses cables
are highly recommended.
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