EMRFD Message Archive 1736

Message Date From Subject
1736 2008-06-21 23:37:17 Rick Link Coupling
I use link coupling frequently within systems, but not as a way to connect an active device
to an antenna. Wes hit the nail on the head with his harmonic suppression comment.

Link coupling is an elegant way to connect a high Q tuned circuit to a relatively low
impedance, balanced load. It is particularly attractive with a high impedance device such
as a vacuum tube connected directly across a tuned circuit. The method was widely used
on the output of transmitters in the days before television, when there was little need for
harmonic suppression. The amateur frequency allocations were harmonically related, so
that the undesired outputs of ham transmitters would only interfere with other hams.

These days, a better approach is to divide up all of the functions that must occur between
the output transistor and antenna, and tackle them one at a time. Not only does that
result in a transmitter with outstanding performance, but each function may be designed,
tested and optimized separately.

Here's a short list of the things that have to happen between the output transistor
collector and the antenna lead:

Feed DC power to the transistor

Present an RF load to the transistor using the familiar approximate formulas

Transform impedance at the transistor to 50 ohms

Filter the harmonics so the output of the transmitter is at a single frequency

Match the 50 ohm output of the filter to the impedance at the antenna lead

It is common to combine some of these functions, particulary in simple transmitters
designed to meet minimum performance requirements. Our more recent designs have a
few more components, and much better performance. A complete list of components to
achieve each of the above functions is:

A wideband RF choke to feed in DC, and a DC blocking capacitor so that only RF goes to
the transformer.

A wideband transformer between the transistor load impedance and 50 ohms

A multi section Pi-Network low-pass filter to get rid of the harmonics. (Since I built my
spectrum analyzer, I usually use one more section than any of the handbook designs, so
that I can't measure any harmonics.)

A matching network to connect the 50 ohm transmitter output to whatever impedance the
antenna presents

By separating all the functions, I can optimize each independently. For example, I can use
any transmatch circuit I want, without depending on it for harmonic suppression. By the
time the signal gets to the transmatch, the harmonics are below the spectrum analyzer
noise floor.

Most importantly, by designing, building, and experimentally optimizing each section
separately and then connecting them all together, I don't need a spectrum analyzer to
know my signal on the air is clean.

Best Regards,

Rick kk7b