Dear Ian, LF Group
You wrote:
The antenna current is now 2A plus or minus the accuracy of the system...
I'm not sure which band this applies to, but in an earlier mail you said
the TX puts out 205W at 73k and 100W at 136k. For Iant = 2A, this works out
to a total loss resistance, including that of the loading coil, of 51 ohm
at 73k, and 25 ohm at 136k, which in either case is very respectable - so
you are probably getting near optimal efficiency for a fairly small antenna
already, and I would be surprised if a major improvement were possible on
what you already have.
Withe the substantial ground system you describe, experience shows that
even quite large additions or subtractions from the grounding won't make a
major difference to antenna efficiency. I'm not sure why it should make a
huge difference if the antenna end is earthed directly, or via a torturous
route back along the coax, but it seems to me the main effect would be to
add inductance in series with the earth connection, which, depending on how
the ground connections are made in the antenna circuit as a whole, might
affect the impedance transformation produced by the tapped loading coil -
since you are matching to a very low impedance, it would only take a few uH
of distributed inductance added into the circuit to significantly modify
the matching network behaviour. If this is true, it should be possible to
achieve the same antenna current with or without the direct ground
connection by changing the tapping point on the loading coil to suit each
connection. Another related possibility is that, with the ground connected
at the loading coil end, part of the amplifier output current returning to
the amplifier earth terminal will flow through the ground, rather than via
the coax braid. It may be that this path is subject to greater resistive
losses than if the current flows exclusively in the coax braid. In either
case, transforming the PA output impedance to 50ohm as others have
suggested would reduce the problem. Or you could leave it as it is with the
ground wire disconnected...
As regards the fuses blowing when a matching transformer is used, does your
PA have a DC blocking capacitor at the output? Normally, a loudspeaker
connects directly to the output so that no bulky audio coupling capacitor
is required. However, with no input signal, the PA is bound to have a DC
offset at the output of up to a few 10s of mV. When the load is a speaker
on the end of long leads, this is not much of an issue, but if an RF
transformer with a few turns of heavy wire as the primary is connected
directly across the output of the amplifier, several amps of DC will flow,
which may be what is causing the problem. The DC offset at the output may
also be increased if the amplifier is driven into non-linearity,
which probably will be the case when trying to get a reasonable RF output.
A DC blocking capacitor of several uF worth of polyester capacitors in
series with the output will prevent this being a problem.
Cheers, Jim Moritz
73 de M0BMU
|
