[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

More information about the AMSAT-BB mailing list