Measuring SWR at 1296 MHz - My Way..

14 October 2010

Have you gone looking for an easy way to measure SWR (or RF power ) at 1296 ??? One thing is for sure, it isn't easy - simply because the "normal" range of SWR meters most of us own do not go up to 1300 MHz.

Of course, it is different if you own a Bird RF power meter (eg a model 43 ) with a plug-in element that covers that part of the spectrum - but many of us don't have that type of equipment available - at least at a hobbyist level. Diamond Antennas do make such a meter - their SX1100 - which covers as follows : Frequency range: S1 (1.8 to 160MHz) S2 (430-450MHz, 800-930MHz, 1240-1300MHz) - where S1 and S2 are two different sensor ports. By the way, this particular meter is available here in Australia from Ross at  Strictly Ham. Ross also handles the Daiwa CN-801G meter made specifically for the 1300 MHz band. Telling you where to buy such a meter makes this process simple - except I didn't attack it that way. 

Last year I spotted a KDI CA-523 Directional Coupler on eBay AU ( seller rf-buy2008 from China - still listed now at around $US26 posted free - a year later, though The-Test-Equipment-Store (USA) lists them too - at a hugely-inflated price), specifications nominally 30dB covering 500 MHz to 2 GHz, with SMA connectors. My original thoughts were to simply use this device as a standard piece of test equipment coupled into the output cable of 23cm transmitting gear to sample frequency,  look at spectrum purity etc..  I duly won the bidding at about $US16 and received the coupler about 2 weeks later.

 

 

 

Frequency Maximum Insertion

Frequency Coupling Flatness Minimum Maximum Loss Above Maximum Maximum

Model Range (Note 4) (Note 4) Directivity Input/Output Coupling Loss Peak Power Avg. Power

No. GHz dB dB dB VSWR dB kW Watts Outline

CA-523 .5-2.0 30 ± 1.2 ±0.75 18 1.20 0.2 3 50 2

  

It worked at 1300 MHz into the frequency counter but my 'experimeter-type' thoughts turned to the creation of an SWR meter slash power meter utilising it.

A bit of web searching allowed me to locate an article titled "SWR Meter for 144-1296 MHz Bands" by Jack, OK1TIC. Original article PDF is here ( Local server copy here).  Jack uses a home-made directional coupler where I have a commercial one but the ideas behind the detector impressed me. In particular, the method he used to detect both positive and negative half-cycles and thus improve detector efficiency - and output voltage - is a vast improvement on many "conventional" approaches. He refers to G7EYT's contribution on the FRARS web site, as per G4RFR, article here.

 

Jack's schematic

 

I set about testing that style of detector circuit using a few different diode types with RF from the sig gen at +10dBm and very quickly found out that most garden-variety/readily-available diodes mostly fail to operate well above 400 to 500 MHz. I suspect that it is the junction capacitance, even though quite low (typically below 1-2pF ) - that kills the efficiency.

One other thing I realised is that if I wished to use both coupler output ports then I would have to make up two identical "detectors". These days that is relatively easy because of the advent of surface mount parts and easy-to-create PCBs.  I proceeded to make up a layout to suit a few different diode styles and then printed two detectors side-by-side on one piece of PCB material. I also placed a trimpot in series on each detector so that I could adjust their sensitivity to be the same - something I hoped would make the process easier since I was going to use just one 50uA FSD meter movement and switch it across each detector, using a single "FSD calibration" pot.

 

Developed & etched PCB

 

Then came the crunch - what diode type was I going to use ???  It is hard to find good microwave SMD diodes available at short notice these days but RS Components currently have an offer to send web orders free so I did some searching on their web site and came up with 3 possibilities :

 

RS Stock No.		Description
610-9197		Diode,RF,Schottky,detector,microwave,dual,CC,SOT323,HSMS-286F
544-4865		Diode,RF,barrier,Schottky,dual,SOT143,BAT15-099
621-8831		Diode,barrier,Schottky,dual,series,SOT23,HSMS-2822

 

These were all marked available as local stock to Australia so I ordered 5 of each - their minimum quantities - with a next-day delivery assured.

The first one was a Common Cathode style so wasn't really going to help in this case but would be good for other projects where low power RF switching was required. Of course I could always mount two of these diode arrays - one each way - using only half of each package. The latter two seemed promising - the BAT15-099 and the HSMS-2822.

The PCB construction was very easy, very quick. I used only one 0805 SMD 560pF capacitor as the integrator on each leg where Jack had used a 100pF plus second cap, a 560pF. I used parts that were from the same component bags for each side with the hope that they would turn out to be virtually identical in sensitivity.  I decided that the best (and easiest) diode to use was the HSMS-2822 as these had two diodes in the package, in series - so the common pin was the cathode of one diode and the anode of the second.

The next step was to measure both detectors independent of the directional coupler. That meant checking each with a range of RF inputs from the signal generator. Happily, both detectors actually outputted the same meter reading with the trimpots at minimum resistance with 0dBm input. The main thing I had to remember to do afterward was to set the trimpots so both detectors read the same on the meter at the same RF input level. 

Let's do some maths at this point. One ( 1 ) watt = +30dBm less the coupler loss of 30dB = 0dBm output to the detectors. Ten ( 10 ) watts = +40dBm so +10 dBm to the detectors. Twenty ( 20 ) watts = +43 dBm so +13 dBm to the detectors. My sig gen will output +13dBm up to 1040 MHz so I might be able to calibrate for actual power along the way. Of course, this assumes that the 30dB coupling loss is (1) correct and (2) stable with frequency. The manufacturer's specs above say a coupling of +/- 1.2dB of 30dB with a flatness of +/- 0.75dB so it is possible to calibrate it for RF power accurately if you go to extra effort.

With the directional coupler in circuit, the meter did not read at all on +13dBm ( 20mW) when fed into the main RF input port - as expected. The transverter was fired up with FM selected on the driver transceiver with a 50 ohm dummy load on the through-output of the coupler, and the meter went over-scale.... Even with full series potentiometer resistance in circuit, the meter was still being supplied with too high an input. 

The series FSD-set potentiometer that I used initially was obviously too low a value and I ended up revising that to a 1 megohm linear style so that it could handle the output power level of the 23cm transverter.  That solved the over-scale issue.

Next, would it actually indicate relative SWR ?? Yep, it did.  Setting the switch to the Forward position, meter to FSD then across to Reverse and the meter read just under "8" on the 0 - 50 scale. That "8" (or 16%) can be interpreted on the scale below. { These values have been calculated in Excel using the standard formula for SWR. }

 SWR AS A PERCENTAGE OF FULL SCALE DEFLECTION

 

Meter Scale %	SWR Value
0	1.00
1	1.02
2	1.04
3	1.06
4	1.08
5	1.11
6	1.13
7	1.15
8	1.17
9	1.20
10	1.22
13	1.30
15	1.35
17	1.41
20	1.50
25	1.67
30	1.86
33	1.99
35	2.08
40	2.33
45	2.64
50	3.00
55	3.44
60	4.00
65	4.71
67	5.06
70	5.67
75	7.00
80	9.00
82	10.11
85	12.33
90	19.00
95	39.00
100	Infinity

 

 

 

 

Ok, that meant it was about 1.4:1 at 1300 MHz... Given the connecting cable and the load were an unknown factor at the frequency, it was probably approximately correct.

Next step was to connect my "normal" shack dummy load to the output, calibrate to FSD - and the Reverse reading was now around "5" ( or 10%) corresponding to a SWR around 1.2:1.  Again, that seemed reasonable. I do have a 2 GHz RF load packed away in my WIA Prac Assessment case but it didn't seem to be worth the hassle of finding that particular load.

There is one further set of tests that should be made at this stage - the infinity tests. This is testing the meter with (1) an open circuit load and (2) a short circuit load - if you can call these two conditions a "load".  The precursor to these is that the meter needs to be fed via an in-line power attenuator ( 3, 5 or 10dB ) so that the source device (transmitter / transverter ) is not damaged. The short and open circuit tests should both have a 100% ( or full scale) Reflected reading after calibrating for FSD on Forward. This corresponds to an infinity / infinitely high SWR value. Unfortunately I don't have one of these available so these tests aren't possible at this time.

I then used a piece of double-sided tape to hold the PCB on the 'bottom' of the jiffy box, formed the wires and screwed the lid on because, at least for now, I was finished.  

The photos below show how I built mine.... { Mouse over images to see larger }