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RE: Question on 1/4 wavelength matching section



Hello Floyd:

> won't odd multiples of 1/4
> wavelength perform the same transformation as 1/4 wave?).

Yes but with reduced bandwidth. Off center frequency, the cumulative (1/4
waves +/- a bit) make it less and less accurate.

>  I don't
> understand why matching section is 72 ohm line.

Each 72 ohm 1/4 wavelength line transforms the individual 50 ohm antenna
feed points to about 100 ohms or so. Two of these antennas plus 1/4 wl
transformers, of 100 ohms each, in parallel brings you back to 50 ohms
combined  at the tee piece.

> I want to use system (A) in that
> diagram .... I'm puzzled by why the 1/4 wave delay line is 95 ohms.

OK, referring to (A) on fig 10.11 the two antennas have native 50 ohm feed
points. each is transformed to ~100 ohms by separate 75 ohm 1/4 w/l
transformers to the tee pieces at the coax relay.

Therefore each T piece has two lots of ~100 ohms coming into it, making ~50
ohms at each of the two relay positions. In the up position, antenna A
radiates 1/4 w/l ahead of antenna B and in the down position vice versa.

The 95 ohm is not there for impedance transformation - that's the delay
section. It's just close enough to the desired 100 ohms and 95 ohm coax is
fairly easily available.

On your original problem, you have a 1" offset. To apply (A) of fig 10.11,
you'll need to add (velocity factor * 1") of 50 ohms delay to the feedpoint
of the foremost set of elements to make them seem 'back in line' (not very
easy: you might like to make it VF * [1/2 w/l + 1"] long to make it
physically easier and switch the sense of RHCP/LHCP).

Sometimes it's easier to consider the power splitter and the CP phasing
entirely separately: example (A) combines both function and so can lead to
confusion, although it is quite an elegant solution IMHO.

73 Howard G6LVB



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