[Milsurplus] TCS-9 Output matching?

[Thomas Adams]

At 11:33 05-05-11, Roger Basford wrote:
>Hi All,
>
>I'm putting together a TCS-9/TCS-12 station and I read a mention of a
>simple mod to allow the TX to use coax-fed antennas. Has anyone got a
>copy of this? I'm getting  about 1.5 amps RF into a BC-375 dummy load
>but not had much success with an end-fed 66ft inverted L on 80 metres.

Too low an impedance, obviously.

What worked here on 160 meters was a half wave wire, fed Windom style
(a single random length wire feeding it 14% off center, driven against
ground).  Worked beautifully; according to the books, that's about 300
ohms feed point impedance.


>Also, the handbook I downloaded has an addendum showing a mod to the
>antenna current meter, moving it from a ground lead to the feed to the
>antenna terminal, which seems more logical to me. Any thoughts on this?
>With an external RF ammeter in the feed to the dummy  load I get the
>same reading as the panel-mounted meter.

Moving from the antenna terminal to the ground lead SHOULD produce
an identical reading; all you're doing is rearranging the circuit components
slightly.

If nothing else...  moving the RF ammeter to the ground lead clearly and
effectively proves to Newbies that the ground system IS a necessary part of
an antenna system.

The ground lead arrangement DOES have a slight advantage.

In AM broadcasting antenna arrays, RF ammeters are normally jumpered
with shorting straps when not being used to protect them from destruction
by a lightning strike on the antenna. While that's not as critical a 
factor here,
the addition of a simple switch to short the hot terminal of the meter to
ground is very easy to do.

Also...  it's my feeling that the readings of an RF ammeter in the ground lead
are slightly more accurate than those in the antenna lead. In the antenna lead
configuration, part of the RF to the meter is SHUNTED AROUND it by capacitance
between the meter movement and the panel it's mounted in!

Again, a broadcasting / higher power analogy... in bigger rigs than 
the TCS you'll
frequently see RF ammeters mounted on insulated subpanels. If this isn't done,
there's danger that the shunted around current can become so large that the
voltages between the meter movement and a metal panel can build to the point
where the case of the meter will literally BREAK DOWN and flash to ground!

Just a few rambling thoughts on RF ammeters and their applications.

73's,

Tom, W9LBB