LF world record:equipment details
Wed, 18 Nov 1998 10:06:16 -0500
Steve Rawlings wrote:
> Andre' Kesteloot wrote:
> > Steve Rawlings wrote:
> > > the distance to my
> > > locator (IO81PP) would appear to be 1913 km.
> > Steve: well-deserved kudos are in order.
> > Rumor has it that you are now pumping 400 watts in a 16 meter vertical? How
> > do you generate the 400 watts in question? What do you have as a hat on top
> > of your vertical? details please, as we wait, breathlessly, on this side of
> > the Atlantic...
> > 73
> > Andre'
> Hi there, Andre'!
> Isn't it fantastic? The world distance record is still to be confirmed. But a
> great distance anyway. Actually it's a 12 m high vertical - although the total
> length of the radiator is more than 12 m. [...]
> About my vertical:
> The main part of the vertical uses a 10 m fibre glass pole to which insulated
> wire has been taped. At the top of the wire, a loop of about 6 cm in diameter
> has been made by folding the wire back on itself and soldering it in position.
> The purpose of the small loop of wire is to help prevent corona discharge from
> the top of the vertical. The base of the fibre glass pole has been lashed to a
> length of wood, and the wood 'U'-bolted to a section of aluminium ladder. The
> ladder is then secured in an upright position to the frame of our children's
> garden swing that I made out of wood about 15 years ago.
> When mounted on the ladder, the vertical is about 12 m tall. About 4 m from the
> ground, the insulated wire leaves the fibre glass pole and slopes down to the
> top of the loading coil. The loading coil is positioned about 1.5 m from the
> back wall of the house, or about 10 m from the garden swing. The coil former
> was made by my son, Trevor, using the two plywood sides from a cable drum that
> were screwed to broom handle 'spacers'. The result is a wooden coil former of 0.4
> m diameter by 1.0 m tall. The base (cold end) turns use 75 ohm data coax with the
> inner and outer joined together. The upper turns are of multi-strand cable. The
> total resistance is about 4.5 ohms;
> the inductance is about 5 - 7 mH (I still haven't measured its inductance). On
> top of the coil sits a variometer with a range of about 1.6 - 2.4 mH.
> The main earth is achieved by a connection to the outside water tap. A wideband
> matching transformer (using an isolated, tapped secondary winding) feeds the
> loading coil against earth. Note: No link winding!! (Transformer: 58 mm 3C85
> core, 10 t primary; secondary taps at 50/100/150/200/250 ohms. 150 ohm tap is
> 'best' (lowest SWR) in dry weather; 200 ohm tap is best in wet weather.)
> About the amplifier:
> I haven't yet drawn out the circuit, but it's a standard configuration, rather
> like the QRO Class B Modulators of the 1960s. The grids need about 8 W of
> drive. I use a centre-tapped secondary winding on a 25 mm OD 3C85 core to feed
> the grids, with a 50 ohm primary winding. I found that I needed lots of padding
> on the grids to keep the valves from oscillating. [During bench-testing, I
> generated some very colourful displays - both inside and outside the valve
> envelopes - until I finally managed to stop the self-oscillation mode.]
> The anode (output) transformer uses a 58 mm OD 3C85 core. 160 turns,
> centre-tapped, on the primary; 9 turns as a 50 ohm secondary. I've still got lots
> to do to
> complete this project, but it is already a real pleasure to use. The built-in
> SWR bridge; LPF; and tx/rx c/o still need to be completed and tested.
> About the output transformer:
> Before winding the output transformer, Finbar EI0CF alerted me to the problem of
> using high voltages with ferrite or dust-iron cores and suggested using "high
> voltage tape". I think what he meant was that the conductive core could become
> a short circuit path to the RF voltage developed across the anode winding. Dave
> G3YMC suggested using PTFE tape and I knew I had a couple of reels of PTFE tape
> of the sort used by plumbers. So I wound a few layers of PTFE tape around the
> toroid. I then held the PTFE tape in place by a few layers of white PVC tape.
> Then I wound on some more PTFE tape, then some more PVC tape. I then wound the
> 160 turns of 26 swg enamelled copper, anchored with nylon tie wraps, leaving a
> spacing of about 14 mm between the start and finish points. More PVC tape was
> used to wrap the centre (but not the extreme ends) of the primary winding. The
> secondary winding used wire from stripped-down mains flex. The turns were wound
> over the centre part of the winding which had previously been covered with
> PVC tape. The transformer was mounted on some 3 mm brown board of the type
> that used to be used in the manufacture of tag strip (resin bonded something
> something, I believe). The ends of the windings were sleeved with neoprene
> sleeving and terminated on terminal posts fitted to the board.
> I hope the above is of interest. Thanks for being so good at keeping in touch.
> With best regards,
> Steve GW4ALG