Measuring things is fun!  This article is about the components of the UHF repeater that was recently donated to the Wilson County Amateur Radio Club.

First, let's take a look at the duplexer.  It is currently tuned for a receive frequency of 466.800 MHz and a transmit frequency of 461.800 MHz.  If you're familiar with duplexer theory, feel free to move on to the measurements.  Meanwhile, I'll offer a quick explanation:

Since a typical radio repeater is a full-duplex device, it must receive one specific frequency on the antenna and simultaneously retransmit the signal at a different frequency through the same antenna.  Transmit power may be 100 watts (+50 dBm) for example, but receive power may be as little as 0.001 picowatts (-120 dBm or 100 quadrillionths of a watt).  That's an output-to-input power ratio of 10,000,000,000,000,000 to 1.

No matter how clean the signal is, all transmitters will spill some energy over into adjacent frequencies.  At UHF, we separate the TX and RX frequencies by 5 MHz, but even at that split, there is more than enough spillover energy from the transmitter to destroy the receiver.

That's where the duplexer comes in.  In this case, it consists of a set of resonant cavities that are extremely selective.  Their operation is not unlike blowing air across a bottle top to produce a tone -- the internal volume of the bottle determines the resonant frequency that you hear.  A cavity filter works on the same principle, but at radio frequencies.

So here we have a pair of cavities that very selectively allow a 466.800 MHz signal to travel from the antenna to the receiver and another pair that allows 461.800 MHz to travel from the transmitter to the antenna.

Duplexer Data Plate
Duplexer Data Plate
(click for large view)

If we connect a spectrum analyzer to the antenna port and sweep a range of frequencies across the transmitter input port, we can measure the energy that the cavities allow through to the antenna port.  Then we do the same from the antenna port back to the receiver output port.  The signal generator is built into the spectrum analyzer and is coordinated with the analyzer's frequency sweep.  It is "normalized" by directly measuring the signal generator through all the test cables without the duplexer to calibrate out any effects of the test setup.  This way, we measure only the duplexer and not the effect of the test cables.

This graph shows the transmit path with the purple trace and the receive path with the yellow trace:

Duplexer Frequency Response, TX & RX
Duplexer Frequency Response, TX & RX (click for a large view)

The analysis tells us that the 3 dB bandwidth of the transmit filter is 1.3 MHz and the bandwidth of the receive filter is 1.5 MHz.  Those bandwidths are indicated by the blue lines & arrows.  The numbered markers are described in the data table and tell us several things:  The transmit filter has 2.10 dB of loss at 461.800 MHz and the receive filter has -2.95 dB of loss at 466.800 MHz.

The #2 and #4 markers show cross-channel isolation of -57.36 dB and -56.39 dB, but the real values are much lower than that because at this setting, the bottoms of the lines are down in the noise floor of the instrument.  To measure the real isolation, we need a different setup using a strong signal and a very sensitive setting.  So far, I've measured it to at least -82.4 dB, but I know it's better than that.

Next, let's measure the duplexer's return loss.  This tells us how well the device accepts energy by measuring whatever it reflects.  This is also done with a spectrum analyzer, tracking signal generator and a return loss bridge.  In this configuration, the bridge passes energy from the signal generator to the duplexer's antenna port and diverts any reflected energy to a separate measurement port.  Again, all measurements are made relative to a calibration measurement, so we only see the duplexer's performance and not the test gear's.

Here is the setup and the measurement:

Return Loss Measurement Setup
Return Loss Measurement Setup
(click for a large view)
Antenna Port Return Loss Measurement
Antenna Port Return Loss Measurement
(click for a large view)

The left-hand cable coming out of the spectrum analyzer is the tracking generator output which feeds into the return loss bridge.  The bridge is connected to the duplexer's antenna port and the cable that comes back to the spectrum analyzer's right-hand port carries the reflected signal.

In the graph, the yellow line shows return loss, which is directly proportional to SWR as depicted by the green line.  From this, we see that the duplexer exhibits 1.04:1 SWR at the transmit frequency and 1.10:1 at the receive frequency.  Optimum SWR frequencies are slightly different (markers #3 & #4), but at our frequencies of interest (markers #1 & #2), it's still excellent.

We can measure return loss and SWR on the TX and RX ports as well:

Return Loss, TX Port
Return Loss, TX Port
(click for a large view)
Return Loss, RX Port
Return Loss, RX Port
(click for a large view)

That's what I have for now!  Also check out my Preamplifier Measurements article.

Comments   

0 #1 KB5EO 2014-08-28 10:52
UPDATE, 8/28/2014:

I measured the isolation between the transmit & receive ports (with a 50 ohm calibration load on the antenna port) as 98.27 dB.  I did this by connecting my HP/Agilent 8648B signal generator to the spectrum analyzer's input port and adjusting the level to read a 0 dBm peak at 461.800 MHz.  Then I inserted the duplexer into that path and remeasured the peak at -98.27 dBm.  By using an RMS detector, the analyzer's built-in preamp and a resolution bandwidth of only 100 Hz, I was able to minimize the noise floor.  I believe this method to be reasonably accurate to around -130 dB.
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