[amsat-bb] Re: Test Gear for 1.2/2.4Ghz Equipment
John P. Toscano
tosca005 at tc.umn.edu
Fri May 30 21:29:00 PDT 2008
John Henderson N4NAB wrote:
> Need some advice on test gear, signal source, swr measurement for L and S
> band. Just got 1.2 board for 910H and have unit on order for 2.4 from SSB.
> Home brewing antennas and need some way to check/adjust. My MFJ is only good
> for up thru 70cm.
You might want to consider some surplus HP equipment.
The HP432a power meter will cover any frequency you are likely to need.
It requires the additional purchase of a thermistor head. The 478A head
is good to 11 GHz, and 8478B head is good from 10 MHz to 18 GHz. You
also need the thermistor cable.
This setup covers a power range of 10 microwatts to 10 milliwatts full
scale. So you would also want a few precision attenuators so that you
can measure power levels higher than 10 mW without destroying the
thermistor head. A -30dB, 10 watt attenuator would bring a 10 watt
signal down to 10 mW, or full-scale at the maximum power range of this
gear. I also found a -40dB, 50 watt attenuator that brings 50 watts
down to 5 mW or half the power rating of the thermistor. If you were
brave (or foolish) you could put 100 watts through this attenuator very
briefly and the thermistor would be happy at 10 mW maximum. But
precision attenuators don't retain their precision qualities if
overheated with excessive power, so go easy there.
Now, this still doesn't get you the ability to measure SWR, or measure
forward and reflected power into/out of an antenna. To do that, you need
one more piece, which is a directional coupler. Power goes into the
directional coupler and its output is connected to the antenna or other
device under test (DUT). You connect your power meter's thermistor to
the forward or reflected coupling port. By measuring forward and
reflected power, you can determine the SWR.
Sure, the Bird 43 wattmeter is convenient -- power in one side, antenna
connects to the other side, and you can read the SWR right off the main
dial. The "problem" with the Bird is that you have to buy additional
"slugs" for every frequency band and every power level of interest. With
the HP system described above, you only need one meter, one cable, and
one thermistor to measure power at any frequency between 10 MHz and
either 11 or 18 GHz. Although some precision attenuators are frequency
specific, you can easily buy only attenuators rated to 18 GHz and you
are good for any frequency that the meter can measure. Likewise,
directional couplers have highest precision at specified ranges of
frequencies, but in many cases, you can calibrate one with the above
setup plus a good 50 ohm dummy load. By calibrate I mean that if the
dummy load is near 1:1 SWR, the forward power coming out of the coupler
at the frequency of interest can be easily measured with the dummy load
on the output, and you now know what the coupling level of the forward
port is, whether -10dB, -30dB, -16.2dB, or whatever it turns out to be.
Then you physically rotate the device (put power into the output port,
put the dummy load on the input port, and measure the forward power
coming out of the "reverse" coupler port). Now, even though the coupler
might not have been designed to work at 2400 MHz, if the coupling is
measured in this way, the coupler is usable unless the coupling you
measure is extremely poor.
That may be a bit more of an answer than you were looking for. But this
setup seems to be much more versatile to me than a Bird 43 with a box
full of slugs. I actually bought a couple of HP432a meters, thermistor
cables, and 8478B thermistor heads, and 18 GHz attenuators of -10, -20,
-30, and -40 dB, plus a two-way directional coupler. With these pieces,
I can measure power from microwatt levels to tens of watt levels at any
frequency for which I have equipment, as well as forward/reflected power
and (indirectly) SWR. No, I don't have any 24 GHz equipment and this
setup would not test it if I did, but you'll not find a 24 GHz slug for
a Bird 43 either! On the other hand, it is possible to buy a K486A
waveguide thermistor for the 432A and measure 18 to 26.5 GHz too!
If you're interested, the following HP/Agilent operator's manual for the
432A would be a good place to start reading:
73 de WØJT
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