[Antennas] Link vs Pi

[Adam Farson]

Hi Chuck,

That sounds like an interesting project you had back then. I converted the
strange output tank of an ARC-5 transmitter into a pi-network when I
reworked the rig into a monoband 20m CW transmitter, back in 1962. I was
newly-licensed at the time, as ZS1ZG. The objective was to be able to feed a
ground-plane vertical via a run of RG-58/U. 

I also converted the VFO into a buffer/driver, and built an external
exciter- a 7 MHz Colpitts VFO driving a doubler. The reason for this was
that the South African radio regulations forbade MOPA transmitters in which
a VFO drove the PA directly.

There is a way around the bandswitching dilemma - parallel-feed the PA, with
the tank primary connected to the anodes via a blocking cap and earthed at
the other end. The bandswitch rotor will then be at earth potential. But
then one may as well use a pi-network.

It is interesting how solid-state transmitter technology has brought the
wheel almost full circle. All solid-state linear HF PA's are push-pull, and
transformer-coupled, with a bank of LPF's following the output-transformer
secondary.

Cheers for now, 73,
Adam VA7OJ/AB4OJ


-----Original Message-----
From: Chuck O'Neal [mailto:cdoneal at comcast.net] 
Sent: 16 January 2006 11:19
To: farson at shaw.ca; Antennas Reflector
Subject: Re: [Antennas] Link vs Pi

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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