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G8MNY  > TECH     03.05.22 07:55z 114 Lines 5451 Bytes #18 (0) @ WW
BID : 1300_GB7CIP
Subj: Diamond X500 Colinear Repair
Path: SP7YDD<SR1BSZ<OK0NBR<F3KT<F1OYP<CT1EBQ<CT7AXA<GB7CIP
Sent: 220503/0732Z @:GB7CIP.#32.GBR.EURO #:1300 [Caterham Surrey GBR] $:1300_GB
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                         (Update Mar 21)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

This old 5.2m (17ft) tall high gain 200W dual band colinear aerial has 8.3dB
gain on 2m & 11.7dB gain on 70cms. But my one ended up being internally water
logged, with the water ending up dripping out of the shack coax, after passing
through the N connector. (the only way out!)

Lots of preparation was needed to take it down as it was at 30ft up, on a 12ft
pole lashed to a chimney. Then dismantle it (not loosing which part is which)
and test each component for failure, clean up, repair all the parts etc.

The aerial consists of 3 complex sections:-

        │
________│
MOUNTING█''''''''''''''''''''██───────────────────────▄▄....................
 TUBE   █    BOTTOM          ██_______________________██....................█
────────▀''''''''''''''''''''       MIDDLE                     TOP
        │3
        │Radials

BOTTOM SECTION
This has two aluminium tubes, outer one has the two mast support clamps. The
inner tube is held in position with bolt (unless corroded solid as my one was!)
Inside is a machined up brass N socket connector disk, that screws into the
inner tube. (corroded solid).
The disk supports a PTFE insulation stalk 10mm dia where the 4.5 turn matching
coil is wound. It is tapped 2 turns up by a 10pF C from the N socket, and the
top of coil has a tuning C of 1.5pF to ground & the aerial rod brass ferrule
joiner. The coil tuning capacitance is also to the inner tube! The inner tube
has the bottom fibreglass pole bonded inside it.

                   Radials
     Bolt
-------===================   Fibreglass tube painted white      .----__
      _││Coil> _~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~   _
    N[_ _////_[_]-----------------\\\\---------[]\\\\\\\\\\\[]-------[_]
   con ││ 2 caps __________________________________________________  __
-------===================                                      '----
Outer Support Tube                                           Ferrule jointer
with 2 mast clamps                                            and water seal

The internal 1/8" brass rod aerials has a 70cms 8 turn hard copper phase coil
and longer 2m 61 turn coil on a fibre rod, spaced along the aerial.
The rod joiners are either a stainless clip, or brass ferrule and screw, or a
hex grub screw.
Sponge (sticky draft excluder) is wrapped around any part that could touch the
fibre glass tube as a vibration spacer.

MIDDLE SECTION
This is a thinner tube and has 2 ferules and seals for inter tube joining.
Inside is 2 x 6 turn coils separated by a mechanical capacitor boss and the
rods to a 61 turn coil.

  __.----.      Fibreglass tube painted white                     .----__
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~   _
  ----------\\\\------[c]------\\\\---------------[]\\\\\\\\\\\[]-----[_]
    ___________________________________________________________________
  ~~'----'                                                        '----~~

TOP SECTION
This tube is thinner still, and has a metal top cap. It has 1 ferrule and seals
for inter tube joining. Inside there is a joiner feeding a rod & 2x 6 turn
coils separate by a mechanical capacitor boss, and the long top rod.

  __.----.    Fibreglass tube painted white                          __
    ~~~~~~~~~_~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~│metal 
  ----------[_]------\\\\------[c]------\\\\-----------------------    │top 
    ___________________________________________________________________│cap
  ~~'----'                                                           ~~

THE REPAIR
After undoing the ferrules, the water came out, and inner wire jointing screw
undone to free up the upper 2 sections.

I had to cut a long slot down the outer support tube with an angle grinder, to
free it from the inner tube. And that slot not only freed it, but let me see
the corroded in inner brass disk thread, which I was then able to leaver it out
and undo the hex grub screw in the ferrule, freeing the bottom loading unit
from the rods.

On RF testing it, I found the bottom coil resonated, but above 2m and 70cm
until I put back the inner tube again. Result the wet coil and 2 caps were
still all OK!

I polished up all the copper aerial coils, brass rods, joiners and line caps
with a wire wheel. Then greased up all joint bosses, aluminium soldered up any
loose crimp joints. And I replaced the sponge vibration spacers (turns of
sticky draft excluder stip).

The fibreglass tubes were not cracked, but needed a spot of paint, the faulty
water seal rubber ring was replaced in the bottom ferrule. To stop any further
water problems, I carefully drilled a 2mm drain hole in the brass disk near the
N connector avoiding the 2 small 1kV caps.

REASSEMBLY
Initially done for RF testing to see if it all still worked. SWR 1:12 in the
middle of 2m & OK on 70cms with aerial horizontal 1.5m above ground on wooden
supports. The tests repeated again after spraying anti corrosion metal
protection on the aerial components, and then again over the final assembly.

On re-errecting on taller 16ft pole with new lower loss coax, the aerial was
now @ 35ft in the clear it worked very well, compaired to a backup 9ft triband
colinear in the loft.


Why don't U send an interesting bul?

73 de John G8MNY @ GB7CIP


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