[Antennas] Link vs Pi

[Chuck O'Neal]

Here is a comparison I did many years ago.

I was given an old class C amplifier with a pair of 4-250's in push pull,
using a B&W butterfly transmitting cap along with the HDVL plug in coils in
the output.  I converted this amplifier to a Pi Network output after finding
the band switching inconvenient. The conversion consisted changing the tubes
to parallel operation including changing the grid input circuit to single
ended and using a B&W 850A output coil with vacuum caps.

I did not see any difference in output power with the same DC input in
either configuration.  All tube operating parameters were identical in both
configurations.  I resonated the link with a series capacitor to get it to
load easier.  I remember being able to load it ultimately a bit heavier with
the link coupled tank for slightly more output before the output started
dropping due to over coupling.  Perhaps this was due to the push pull
arrangement vs. single ended.

Chuck...K1KW

----- Original Message ----- 
From: "Adam Farson" <farson at shaw.ca>
To: "Antennas Reflector" <antennas at mailman.qth.net>
Sent: Monday, January 16, 2006 1:54 PM
Subject: RE: [Antennas] Link vs Pi


> Hi Steve,
>
> As I recall, the pi-network is a little more efficient, as there is less
> coupling loss between the input and output than with the link-coupled tank
> circuit (which is a loosely-coupled air-cored transformer.)
>
> The pi-network was originally adopted for several reasons. It provides a
> better match to a coax feedline if properly designed, and also acts as a
> low-pass filter to suppress harmonics generated in the PA. An outboard
> low-pass filter is usually required between the output of a link-coupled
> tank circuit and the antenna, to ensure regulatory compliance regarding
> spurious and harmonic suppression. The insertion loss of the extra filter
> at the fundamental may be 1 dB or more.
>
> Cheers for now, 73,
> Adam VA7OJ/AB4OJ
>
>
>
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