[Boatanchors] Link coupled output?

[Gary Schafer]

-----Original Message-----
From: boatanchors-bounces at mailman.qth.net
[mailto:boatanchors-bounces at mailman.qth.net] On Behalf Of
shoppa_boatanchors at trailing-edge.com
Sent: Saturday, September 10, 2005 5:26 AM
To: StephenTetorka at cs.com; boatanchors at mailman.qth.net
Subject: Re: [Boatanchors] Link coupled output?

> I am trying to determine what characteristic impedance transmission
> line to use with my homebrew 1948 transmitter.

I didn't see you specify the must crucial parameter: what band(s)
you're going to cover.

If the antenna side of the link is not resonated with a tuning capacitor,
then the inductance of the link coil has to be high enough that the
impedance of the link at operating frequency is small compared to the
transmission line.  At 40M or 80M the coils become unreasonably large
even for low-impedance (50 ohm) feedlines, and I'm not going to say it's
impossible to do this with 300 ohm line but it's even harder there.

The 50's/60's era handbooks are very clear on this subject.  There's
a couple pages under the heading "INDUCTIVE-LINK COUPLING" in the
HF transmitter chapter.

With tuned output links things are not so hairy.

But if I were you I'd build the transmitter to fit the antenna/feedline
even if this means going to pi-output...

Tim.
_______________________________________________
I don't have all of the original post anymore so I don't remember if it was
a swinging link or a fixed link output.
If it is a swinging link output, try it on the antenna or load you wish to
use it on and see if you can obtain the proper plate current when the
coupling is increased. If you can you are good to go.
If you can't, try a series capacitor in the link  side. Tune it for maximum
plate current at the operating frequency. Then you can adjust the plate
current / loading by adjusting the position of the link. The position of the
link will act as the load adjustment.

Once the series capacitor is peaked and the plate dipped, you will be able
to swing the link in and out to increase or decrease coupling / loading
without having to re dip the plate if coupling is not set up too tight.

Without the series capacitor, if the plate tuning changes as the link
position is changed that means that the link does not match the transmission
line. The plate tuning capacitor is compensating for the reactance in the
load.

If the link is fixed, then the series capacitor in the link side is used as
the load adjustment. If you can not get the proper plate current then you
need more turns on the link or a series coil with the link.

With a flat transmission line, you do not need a series capacitor if the
link matches the line. With the proper number of turns on the link you will
have the proper loading.

73
Gary  K4FMX