To Alan, Peter and Mal,
Tnx for your comments. I have added my reply in red.
Alan, G3NYK, wrote:
Hi both, could I ask if the said insulator is fitted with an "anti-corona"
ring. Severe changes in direction of the wire can produce quite high voltage
gradients, even when not at the end of the run. Another thought is to series
connect....ie put a "egg" in the short rope length. This will be effective
if it is a "leakage" current but not if it is corona, which can act rather
like a small blow-torch. This is why it pits the surface of the insulators.
No Alan, it is not a matter of corona. I have never observed it. Moreover
the rope failed where it passed over the pulley. If it had been corona I
expect the burn would have been near to the "hot" end of the insulator.
An "egg" would increase the leakage path slightly but the path will be
increased already by some 16 cm due to the added "Radio Kootwijk"
insulators.
Peter, G3LDO wrote:
I have had this trouble even when using Dick's "Radio Kootwijk" insulators.
It only seems to happen where there is a short length of insulated rope
between the insulator and the support. What seems to happen is that the
insulator eventually becomes dirty (in my case covered with a thin film of
salt). This, together with wet weather conditions, produces a conduction
path that can carbonise the short rope support. The fix is to use copper
wire between the insulator and the support fixing point.
After a storm at sea I have seen blue spark tracts running along the salt
coated insulator but this arrangement has not failed. However the insulator
should be cleaned because such sparking causes minor damage to the surface
of the insulator.
The problem reported by Dick doesn't seem to happen where there is a long
rope from the insulator to the support structure.
Peter, I think the situation at your location near the sea is even worse
than at my place, being some 15 km from the coast.
Copper wire between the insulator and the fixing point is not possible with
me because the insulator must be connected to the hoisting rope. I hope
that the insertion of the "Radio Kootwijk" insulators next to the existing
pyrex glass ones, plus using new rope that is less hygroscopic (is that
English? It is not in my dictionary) will improve the situation sufficiently.
Mal, G3KEV wrote:
So far I never had a rope or insulation failure. No ignition or sparks or
any wire burn out.
I use marine grade rope available from any marine or yacht supplier. The
rope comes in various sizes and the most useful for me is 2mm up to 15mm.
Mal, do yo know what material the rope is made of? Does it absorb water?
I made some measurents on remains of the aerial. The T-aerial consisted of
a 2 x 20m dipole with 11m open line feeder with the feeder wires connected
in the shack.
The inverted L-aerial is just half of that, one 11 m feeder wire, connected
to a 20m topload.
T-aerial Inverted L-aerial.
Capitance 385pF 210pF
Total R of
aerial-earth system 35 ohm 50 ohm
Aerial current 3.2A 2.3A
Part of the increased resistance is due to the larger loading coil
required with the Inverted L-aerial; but the major part must be caused by
the decreased "foot print" as Laurie calls it.
Perhaps I will measure the ERMP of the Inverted-L to complete the picture.
It was 140 milliwatt with the
T-configuration.
73, Dick, PA0SE
The transmitter I use on LF was constructed by Harry Grimbergen, PA0LQ. It
is a semiconductor affair but with 4 valves PL519 in parallel in the PA. At
an input of 0.9A at 500V it produces 300W output.
At my request Harry has provided an extra input so the TX can be driven
from an external frequency stable source instead of the built-in VFO.
Yesterday I collected the modified transmitter. The aerial consists of a 2
x 20m dipole with 11m open line feeder without spreaders. The feeder wires
are connected together in the shack and fed against earth, converting the
dipole to a T-aerial.The two halves of the aerial can be hoisted separately.
Starting transmitting the aerial ammeter swayed wildly and I heard a
disturbing noise outside. It appeared that one half of the aerial had come
down.
It turned out that one hoisting rope had been burned through where it had
passed the pulley. Between the pulley and the "pyrex" insulator had been
some 15cm of rope. But it had been raining for several days and the rope
bust have been soaking wet. See the attachments; the top end of the rope
was on the pulley (sorry for the resemblance...)
I intend to replace the rope and to insert extra ceramic insulators of the
dog bone type as I have supplied to several British amateurs (they come
from the closed down "Radio Kootwijk" commercial PTT-station).
The rope was at least 10 years old and white when new.
There has been some discussion on types of rope on the reflector recently
but at that time I was not interested in the subject and rather clumsy
deleted the messages.
So may I open the subject again and kindly request your opinion as to the
most suitable type for my application. As you can see in the attachment the
type I used may absorb water rather easily and that obviously converted it
into a conductor for a strong enough LF current to burn it.
I have to wait for some better weather before tilting-over the mast and do
the repairs. Perhaps I will first insert an extra insulator in the
remaining half of the aerial (that can be done easily at ground level) and
use it as an inverted-L aerial on LF. If that results in a reasonable
aerial current I may even do some beaconing at night.
73, Dick, PA0SE.
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