[ARC5] QST articles.

[Dennis Monticelli]

There a variety of matching networks that can effectively transform
the normal low output Z of the ARC-5 transmitters to 50 ohms while
preserving the role of the series L.  I thought the recent article in
Electric Radio did a good job of cataloging them.  The ideas presented
in this thread (which I encourage the authors to develop further) are
sound from an impedance viewpoint, but do vary in the job they will do
in reducing harmonics to levels consistent with current standards.
The series cap trick is popular because it is simple, non-invasive,
and allows enough Z transformation to get some decent power out.  It
provides a modest amount of harmonic suppression because the only help
comes from the series bandpass defined by the variable L and the
series cap.  Other schemes can do better from a harmonic point of
view, but are more complex.  It is relatively easy to determine the
relative effectiveness of various networks in terms of harmonic
suppression by simply noting whether the arrangement is mostly series
L's and shunt C's (i.e. lowpass configuration).

BTW, for the record, I am using Dave's series cap approach (modified
with an additional shunt C employed at the link coupler) and then
passed through the regular station antenna tuner, a MatchBox Jr.  That
combo knocks down the harmonics nicely and gives me 30W out at 500V.
Certain types of antenna tuners (like the Drake MN2700 and MN7) are
inherently low pass configurations.  These tuner types are so good
harmonic-wise that you can run most kinds of older rigs through them
and not worry about radiating excessive harmonics.  The MatchBox is a
bandpass-type tuner which is OK but not terrific as a harmonic
reducer.  On the other hand, use the popular "T" transmatch (with
series C's and a shunt L) and your rig better be clean all by itself.

FYI.  Below is the FCC wording from Part 97 related to harmonics.
Note that pre-1977 rigs get a free ride.  But that doesn't mean we
should be inconsiderate with our emissions......and that includes
addressing key clicks which are inherent in the ARC-5 design. I use an
external key interface box that shapes the make and break.

"For transmitters installed after January 1, 2003, the mean power of
any spurious emission from a station transmitter or external RF
amplifier transmitting on a frequency below 30 MHz must be at least 43
dB below the mean power of the fundamental emission. For transmissions
installed on or before January 1, 2003, the mean power of any spurious
emission from a station transmitter or external RF power amplifier
transmitting on a frequency below 30 MHz must not exceed 50 mW and
must be at least 40 dB below the mean power of the fundamental
emission. For a transmitter of mean power less than 5W installed on or
before January 1, 2003, the attenuation must be at least 30 dB . A
transmitter built before April 15, 1977, or first marketed before
January 1, 1978, is exempt from this requirement."

Dennis AE6C

On Sun, Dec 14, 2008 at 5:10 PM, Kenneth G. Gordon
<kgordon2006 at verizon.net> wrote:
> On 14 Dec 2008 at 16:33, Bob Macklin wrote:
>
>> There is NO HARMONIC suppression built into ARC-5 transmitters!
>
> OK. You're probably right. You obviously know far more
> than I do about the subject.
>
> However, if you read the links I gave you to David Stinson's
> notes on measuring harmonic rejection in the ARC-5s, you
> might come to a different conclusion.
>
> Maybe I am misinterpreting his data, but I don't think so.
>
> In any case, by reducing the loading coil to zero and
> maxing out the link coupling, you would, automatically, be
> reducing any harmonic suppression there MIGHT be there
> to zero.
>
> It has always been my contention that the more of the right
> "stuff" you can put between the harmonic generator and the
> antenna, the fewer the harmonics radiated.
>
> Remove all that, and the result is more harmonics.
>
> Make sense? It does to me.
>
> But what do I know?
>
> Nothing...
>
> Quite obviously...
>
> Ken Gordon W7EKB
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