25
Operation cont’d
Peak SWR will show values a little higher than steady-state at times due to the wide dynamic range of the LP-100. As power drops to
below 100 mW during speech, the SWR detector can sometimes grab a higher peak because of the lower accuracy at extreme low
power levels. The worst-case error in this case should be < .10. For best accuracy during measurements, use the fast mode and at
least .5 watts of power. The directivity of the LP-100 can easily be greater than 40 dB as you may have noticed during calibration, even
at low power.
For amplifier tuning, you should switch to Tune mode for fast update of both bargraph and numerical readout. The bargraph sampling in
the LP-100 is about 100 samples/second, and it will display a single dit at 60 wpm, or a string of pulses as with a pulser or keyer set for
high speed. Full accuracy should be attainable down to about 500 mW for both power and SWR. Good accuracy should still be
maintained down to < 100 mW. Note: The bargraph scale is fixed on the High scale in this mode to prevent autoranging from clouding
adjustments. For antenna tuner adjustment, the fast mode is best, or dBm/RL if you prefer peaking rather than dipping.
Normally, the SWR Alarm should be set for 2.0:1 unless you purposely operate with an antenna that is close to 2.0:1 SWR. It is up to
you whether to enable the Piezo transducer, by using JP1. In any case, it is recommended that you loop your amplifier PTT through the
LP-100. This not only helps protect your amplifier, but also the coupler in the LP-100… especially if you have an older amplifier which is
capable of delivering full power into a high SWR load.
Vector Mode
In the vector mode, you can see the impedance of the load in two ways. The top line of the display shows the magnitude and phase of
the complex impedance, and the lower line shows the resistive and reactive components, ie. R + jX. It is important to note here that the
sign of the reactive, or imaginary component cannot be determined automatically by the LP-100.
If you QSY up from your current frequency, and the reactance goes up, then the reactance is inductive (sign is “+”), and conversely if it
goes down, then the reactance is capacitive (sign is “-“). A suitable distance is QSY is about 100 kHz or more. The LP-Plot program has
the ability to determine sign automatically, since it can control your transmitter’s frequency. When it plots a range of frequencies, it uses
the slope of the reactance curve to determine sign, and plots the results accordingly.
It is important to remember that the impedance displayed on the screen is referenced to the coupler LOAD port. This value is related to
actual feedpoint impedance of the antenna by factors relating to the characteristic Z of the line, line length and loss. I plan to add the
ability to display actual antenna feedpoint Z into the LP-100 VCP and Plot programs by providing input boxes for feedline type and
length.
A simple way to provide reasonably accurate antenna Z on the LP-100 display would be to use a feedline which is a multiple of ½
wavelength in electrical length. There would still be some residual error due to feedline loss, but it would give a better representation of
feedpoint Z. I am considering adding a CAL screen to allow selection of feedline loss to compensate for this, and I may also allow the
future entry of feedline length and Zo data. There will be more info on this and other Impedance related subjects in the upcoming
Appendix A.
dBm/RL Mode
Displays power in dBm from +15 to +64 dBm, and load integrity in dB of return loss from 0 to 49.9 dB.
Direct Input/Field Strength Mode
Similar to dBm mode except that it is calibrated to display power from –15 dBm to +33 dBm. There is no return loss in this mode
because it does not utilize the coupler. Power is supplied directly to one of the inputs on the back of the LP-100. This mode can be
used for accurate low power bench measurements, as in checking the output to a transverter or the level of a local oscillator of mixer. It
is also very useful for doing antenna field strength measurements, as in checking a beam pattern. This requires feeding a small pickup
antenna to one of the inputs.
NOTE: The maximum power for the direct inputs is 2W.
Compression Mode
Under development. I started to add this as the LP-100 was nearing readiness for shipment. It will be completed in a future firmware
update. It will show peak-to-average ratio, but I want to work on a steadier average algorithm before releasing it.
Summary of Contents for LP-100
Page 28: ...28 Schematic Page 1 ...
Page 29: ...29 Schematic Page 2 Coupler Schematic ...
Page 36: ...36 Appendix A ...
