Elecraft AT1 41 dB Step Attenuator - Manual (Page 5)

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example, if your transmitter is producing 1 watt and you select 6 dB of attenuation, the power you are feeding to the antenna
is 1 watt divided by 4.0 (from Table 1) = 0.25 watts or 250 milliwatts.
If you are using the attenuator with a transceiver that has a single antenna connection for both receive and transmit, the AT1
will attenuate the received signal as well unless you place all the switches to OUT when receiving.
S-Meter Evaluation
You can determine how sensitive the S-meter in any receiver is to change in signal levels with the AT1. This is best done
with a calibrated signal generator such as the Elecraft XG1 or XG2 but off-air signals can produce usable results as well.
Connect the AT1 between your receiver antenna input and your antenna or the signal generator output.
Tune in your signal generator or a steady on-air signal with a level of about S9 and note the S-meter reading. Now switch in
attenuation until the signal level drops exactly one S-unit and note the attenuation needed. Most S-meters are assumed to
represent about 6dB change per S-unit, but this can vary widely from one receiver to another. If, for example, your S-meter
does indicate a drop of one S-unit for each 6dB increase in the attenuation, you can see from Table 1 that a station who
changes its power from 5 watts to 20 watts (ratio of 4.0 from the table) should show exactly a one S-unit increase on your
meter.
Add attenuation to see if the same amount is required to reduce the signal additional S-units over the range of your receiver’s
S-meter. Making a note of this response will help you evaluate other station’s signals when they experiment with changes.
Receiver Weak Signal Performance
If you have a calibrated, low-level signal generator such as the Elecraft XG1 or XG2, you can use it with your AT1 to better
understand the weak signal capability of a receiver. Connect the AT1 between the signal generator and the receiver antenna
connector. Start with a signal level of 1 µV from the generator and set all of the attenuator switches to OUT. Tune in the
signal, then start adding attenuation on the AT1 until the signal drops to the threshold of your ability to hear it. For example,
you might find that you can just hear the signal with 22 dB of attenuation switched in. Looking up 22 dB in Table 1, the E/I
column (for volts and current since we’re working in microvolts) shows a ratio of 12.6. Dividing 1 µV by 12.6 yields 0.079
µV for the signal level at the receiver antenna connector. Note that you can hear weaker signals with a narrower receiver
bandwidth. That’s because there is less noise being passed through the receiver to compete with the signal.
Troubleshooting
The most likely causes of trouble with your AT1 will be:
1. You’ve installed a resistor incorrectly.
2. A resistor has been damaged, usually by overheating if more than 2 watts was applied to the AT1 for any length
of time. Overheating typically will cause a resistor to increase in value.
3. A solder bridge occurred between pads or a joint was missed, producing an intermittent or high-resistance
connection. A missed solder connection may not show up until some time after you built the unit, as oxidation
slowly accumulates on the surfaces.
Check all the solder connections on the bottom of the board with a strong magnifier. Look especially closely at the 10 solder
pads for each switch. Make sure all of them are properly soldered and that there are no solder bridges.
You can test each attenuator position for the proper resistance as follows:
Connect a DMM set to measure ohms between the center pin and ground at one BNC connector. Make sure nothing is
connected to the other BNC.
Set all the switches to OUT, then switch each one, one at a time, to IN and compare the resistance shown on your DMM
with those below. The ranges shown below are what the tolerances of the resistor may allow. The actual possible error may
be greater than shown, depending upon the accuracy of your DMM.
1 dB: 456.4 ± 22.8 ohms 3 dB: 154.4 ± 7.7 ohms 6 dB: 83.6 ± 4.2 ohms
10 dB: 62.6 ± 3.1 ohms 20 dB: 51.4 ± 2.6 ohms