[Fwd: LF: FT140-43 impedance transformer]

Andre' Kesteloot akestelo@bellatlantic.net
Mon, 07 Dec 1998 17:53:12 -0500

Peter Bobek wrote:

> Hi Rik/ON7YD, hi OMs,
> When matching a (resonant !) antenna to a 50 ohm generator I always prefer
> the L-network
> as suggested by Bob/ZL2CA.This statement is based on 25 years of
> experimentation with
> antenna matching systems for 160m. The L-network is a simple network (2
> components)
> and is easy to build. It also attenuates harmonics!
> A step-up transformer for matching 50 to 200 ohms would need an inductance
> of approx.
> 1 mHy (at 137 kHz) on the high impedance side (200 ohms) to avoid other
> problems.
> This means you need quite a few turns. Another disadvantage at higher power
> levels is
> the high(er) voltage at the high impedance side which could easily cause
> arcing over
> when you are not designing the transformer properly. Teflon insulated wire
> would be the
> best for that purpose! Also wrap the core with electric (or teflon) tape to
> prevent damaging
> the wire insulation.
> I did some quick calculations for Rik´s L-network. All he needs for the
> L-network to step up
> from 50 to his 200 ohms is a (low loss) coil with 100 microHenry and a
> capacitor having
> approx. 10 nanoFarad.
> Coil in series with the 50 ohm tx output and then the 10 nF cap connected
> to ground. At this
> same connection (coil/cap) you connect the 200 ohms of your (resonant!)
> antenna system.
> Just make certain the cap is not a cheap ceramic bypass cap! Mica would be
> the best
> material followed by polystyrene and similar stuff. Connect several
> capacitors in parallel
> to handle the rf-current. Attention: peak voltages across the cap at the
> 500 watt level will be
> in the kilovolt range!
> As Hajo Brandt/DJ1ZB also pointed out in a previous mail caps should have
> higher voltage
> ratings (1kV DC-rating does not mean the same at 137 kHz!). This, however,
> up to a certain
> extent depends on the type of cap being used. General rule: connect 3 caps
> in series then
> you are on the safe side. (Total capacitance will then be one third!)
> 73, Peter / DJ8WL