Detailed Operation Cont’d
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-100A 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-100A 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 either one of the inputs on the back of the LP-100A. 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 or
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. Selecting this mode automatically returns the Peak/Avg/Tune mode to Average. NOTE: The
maximum power for the direct inputs is 2W.
Peak-to-Average Mode
This mode lets you determine the average power in a signal by taking 40,000 samples/second, and compares this to the peak power in
the signal. The result is displayed as a ratio in dB. I provide a couple test tones, which are available on my website at
http://www.telepostinc.com/Files/two-level-tone-loop3.zip and http://www.telepostinc.com/Files/loud_tone.zip. The loud tone is used to
set the maximum power and proper ALC range (with processing OFF). The two-level tone is used to determine the peak-to-average
ratio of the output signal. It can be played back on your PC, or converted to mp3 and played on a portable player. It can be played over
a speaker into the microphone, or directly into the mic input. The two-level tone provides alternating loud and soft tones with 20 dB
difference in level. This tone should provide the following Peak-to-Average ratios vs. effective compression ratio.
I plan more test tones with different characteristics in the future, which is why I decided to keep the display as Peak-to-Average as
opposed to Compression, which would only be accurate with one test signal. I will provide additional tables such as the one above with
the additional test signals.
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