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Number 16 on your Feedbackcard ATV Trans a Oven! Low-cost high-power microwave operation has arrived.

by David Pacholok KASBYI

netic field having a secondary effect on this. I First, I modified the magnetron cavi- ty to couple RF to a instead of to the oven compartment. I removed the interior radomelsplatter cover, field stirrer blades, and mag- netron output matching section. Next, I shorted the waveguide open end with a plate (Photo A) and installed an E-field probe to couple the RF to an N-connec- tor output jack. (Photo B shows the details of the construction of the E-field probe.) Magnetron current, , and fre- quency were measured and plotted in- dependently to quantify performance in he majority of the amateur spec- this modified cavity. In power output T trum allocation lies above 1300 vs. cathode current measurements, for MHz, yet when you scan those bands, a power out range of 50-400 , and you rarely hear anything but band a cathode current of 50-250 mA, I . Hams have let these regions lay found a very linear relationship. See fallow because of the idea that mi- Photo A. Waveguide shorting plate, to prevent the mi- Figure 1 for the vs. current crowave equipment is complex, expen- crowave RFffom entering the cooking chamber, and to curve. This data suggests that: sive, or just unavailable. reflect this energy back to an E-fieldprobe. 1. The 2M189A magnetron is a cur- To be sure, there are concepts unique I rent-operated device. The anode-to- to microwave design, but they are nit neces- cathode voltage changes only about 1 per- sarily harder to grasp than those in lower cent, with a 2: 1 change in cathode current Ik. frequency RF design. And, as microwave 2. Power output is a linear function of Ik. applications find a larger place in society, as 3. Output frequency is a non-linear (but with ovens, and satellite TV, affordability monotonic) function of Ik, with increased and availability of surplus microwave equip- current causing an operating frequency in- ment constantly increases. crease. The average frequency "pushing" coefficient is about 0.1 MHzJm.4, with a use- Project Features ful frequency swing of about 20 MHz. The goal for this project was to provide an inexpensive, relatively simple high power What Mode To Use? microwave using a microwave The above conclusions ruled out AM oven as the foundation. This project meets the double- , because of the large following goals: incidental FM that would result. On the other *Low cost-less than $200. hand, an FM deviation of 2 MHz would cause *High power output-250 Watts minimum. Figure I. Graph showing frequency versus Ik incidental AM of only 15-20 percent, so I *Parts readily available from consumer elec- for the magnetron. This shows that output investigated wideband FM video trans- tronic supply houses. ffequency is (non-linearly) related to current mission. *Emission type compatible with standard to the magnetron. To check compatibility with existing TV low-cost BIW receivers. receivers, I used an FM video-modulated sig- *Frequency of emission in the 2390-2450 *The basic transmitter scheme is adaptable to nal generator as a source for an MDS MHz amateur band, compatible with Multi- other emission modes, such as narrowband downconverter and a 5-inch monochrome re- point Distribution System (MDS) TV down- FM, with phase-lock circuitry described ceiver. I got a fair quality picture with the converters. (Historically, these downcon- below. television adjusted for IF slope detection, and verters have been misused to "pirate" with sync and vertical lock achieved at devia- television movie distribution at 2156 and Modification Description tions of 700 kHz to 3 .O MHz. The best picture 2162 MHz. They have been widely sold A magnetron is a self- quality occurred at 2.2 MHz deviation. through magazine advertisements and elec- contained, crossed-field power oscillator. tronic flea markets, so there are tens of thou- Built-in cavities primarily determine oscilla- Modulator Circuit Description sands of them in existence.) tion frequency, with anode voltage and mag- The modulator serves two purposes. First, 54 73AmateurRadio July, 1989 it is a high-voltage current source with high ed-grid voltage with a current gain A floating screen supply of about + 100 open-loop gain, setting the magnetron cur- of unity, with enough voltage capability to volts is provided, with R28 included to limit rent to a known value, and establishing a drive the magnetron. However, to an input screen dissipation. The floating supply al- frequency and power output. See Figure 2. voltage at U1, a transconductance amplifier lows only plate current (magnetron current) U2, a 7805 5-volt regulator, establishes a is formed, with transconductance given by: to be included in the control loop. Additional reference voltage adjusted by R5 and R6. components with functions are: This voltage is applied to the non-inverting .R3, R14, and R15, which prevent parasitic input of high-speed op amp U1, which drives oscillation in U1 and V1. source follower Q1. The output of Ql, plus *R12 and R13, which aid current sharing in R9 and R7, provide negative to U1 of this amplifier must be suffi- Vla and Vlb. in the ratio 5.7: 1. At equilibrium, Ql's drain/ cient for the modulator's second purpose- source current produces a voltage drop across video . This must be 4.5 MHz, if *D3, which protects Q1 in the case of V1 Rl1 that equals 5.7 times Ul 's non-inverting you want to include the audio . arc-over. voltage. Frequency response measured with a current *Conventional rectifiers, fil- Temporarily ignoring screen grid current, probe in the plate leads of V1 was down 4 dB ters and bleeders. plate current equals cathode current in V1 at 4.5 MHz. Adding C6 (1200 pF) provides a (a,b combined). Since Vl 's cathode current pole for this frequency, flattening the re- WaveguideICavity Operation I equals Ql's drain current, VD rises or falls sponse to beyond 6 MHz. C1 and R8 serve to The waveguide circuit is deceptively sim- until the V1 grid 1-to-cathode bias causes couple an external 4.5 MHz subcarrier gener- ple: The oven's TElo waveguide feed (from Ip = Ik = ID = IS. V1 is therefore a ground- ator to the modulator. tube to cavity) is shorted with a copper plate.

Figure 2. Transmitter schematic. 56 73Arnateur July, 1989 Figure 3. NBFMphase-lock system schematic. 58 73AmateurRadio July, 1989 (See Photo A). This is analogous to of RG-174 that comes off pin 6 of a coaxial or microstrip short, the LF357 IC. Attach this to pin 2 where wavefronts are reflected of U1 in the transmitter circuit back with a 180 degree phase in- (Figure 2). Before doing this, how- version. At a quarter guide wave- ever, be sure to remove the 4.5 length from the short: MHz audio subcarrier at R8, and A the video input. Ag'd(~l~c)2- 1 You have now converted the mi- crowave oven transmitter to use where Ac = 2X guide broadwall with NBFM (t-5 kHz) voice mode! dimension. Now adjust the magnetron cavity The reflection is in phase with probe length and R6 until the mag- the incident wave from the mag- netron locks up at all times during netron, and an E-field probe (see the magnetron anode warmup (5-7 Photo B) is inserted at this voltage minutes). \ maximum. Ordinarily, maximum power transfer occurs when this Transmitter Improvements probe is A14 in length. Deliberately for NBFM shortening the probe introduces a .Bypass D5 and D6-Dl7 with reactive mismatch at the magnetron output wire this circuit into the transmitter unit. 0.0005 to 0.001 pF 3kV minimumcaps. This port. After an unknown number of degrees Refer to Section A on the schematic-the reduces "hum bars" in the picture and low- rotation within the feed structure (Matsushita overtone VXO circuit. The entire unit should level audio buzz in the NBFM mode. would not provide tube data), this causes the be temperature controlled at 70°C by "crys- *Isolate the metal case of the 0.74 pF (on TI) magnetron to be pulled lower in frequency by tal ovens," or something similar. The oscil- from ground with plastic blocks, some 25 MHz from its design frequency, lator drifts at around 100 Hz per degree, nylon screws, or other means. This will also ensuring legal amateur band operation. causing about 1.6 kHz per degree for the reduce hum bars and buzz. frequency out drift. Stability is traded off for *Using insulated standoffs, isolate T1 lami- Floating Operation simplicity in this design. nations, and frame from ground. This will One important feature of this conversion is Refer to the 151.85 MHz crystal in the further reduce hum bars and buzz, and will the modification of the high voltage power VXO circuit. Choose this crystal after you result in better insulation in TI after mods. supply for floating operation. The original build the oven video transmitter and measure *Disconnect the magnetron filament power transformer had one end of the sec- the stable operating frequency range using feedthrough from ground! Otherwise you ondary grounded to the frame. I lifted this end one UBP585 and a 600 MHz counter. won't get full video bandwidth, and the and attached it to a high-voltage lead wire. Now refer to the tank NBFM mode PLL filter won't work (no This modification eliminates the need to float coils, to the upper right of the crystal on the phase margin). See Photos C and D . the entire modulator above ground, which schematic. You fabricate this by winding six also requires video-bandwidth opto-isola- turns of #24 wire on a 3.3 k %W carbon Performance tors. Hi-pot tests at twice the rated voltage resistor. Then, wind one turn of feedback Spectrum analysis indicated the perfor- confirmed that the modification was reliable. winding, tightly coupled, and one turn of mance of the transmitter. The 1st Bessel null output winding, loosely coupled. display (Ik = 160 m.4, Vp = 3500V, Mod. EME Anyone? Now look at Section B, the connection be- index = 2.4, Mod. freq. = 1 MHz, and Narrow band FM (f5 kHz deviation) re- tween the VXO and the IOC. There is about center freq. of 2.431 GHz) shows that the quires a clean RF source low in noise and 0.6V PEP for 300 Hz VXO deviation, which modulation is primarily FM. incidental FM. You can use the phase-lock or results in about 5 kHz of magnetron devia- frequency-lock loop, as shown in Figure 3, tion. The VXO deviation is linear up to about Additional Comments and Observations with the non-inverting input of U1 equivalent + 10 kHz output (magnetron) deviation. The following notes may or may not apply to the varactor control voltage in a conven- In Section C, the IOC is cheap 'n' dirty, but to the system if the NBFM phase-lock system tional VCO. plenty effective. The Handbook has a bet- is installed. The following notes discuss sections of ter-and more complicated-version of this. Warm-up drift is significant over the first the phase-lock circuit, and tell how to Finally, at Section D, find the two-foot lead ten minutes of operation, representing about

Photos C, D. Disconnecting the mugetronfilament feedthrough from ground. Drill out the rivets (Photo C) andpushfeedthrough 3/16" into sheet metal box of magnetron, and then epoxy in place (Photo D). 60 73Arnateur Radio July, 1989 supply. This effect is caused by the 60 Hz job. Also, retain the door interlocks (I in- switching of the diodes, varying the capaci- stalled the modulator in the now-unused tance to ground at the magnetron cathode. cooking cavity.) Antennas can easily have These transients are out of the control loop. high gain at this frequency-DO NOT Grounding the power supply and floating the POINT THEM AT PEOPLE OR OTHER modulator at high voltage is a solution, as is LIVING BEINGS! floating the magnetron and cavity. Either Although this is not a "high performance" would increase circuit complexity and in- , it represents a low- crease exposure to hazardous . cost effort to achieve significant power out- As with any non-locked oscillator, a put at microwave . change in system load impedance will change Readers interested in finding out more the frequency of operation. A high power about this project can contact the author for isolator is one solution. albeit an exvensive details, at Creative Consultants, Photo E. Microwave leakage detector-a one. I used a stretch line to measure the load 1815 W. Higgins Road, Sleepy Hollow, IL must for this project! pulling effects of a 1.5: 1 VSWR over all 601 18, Telephone: (312) 428-5676. phase angles. The frequency changed F6 15 % of the available tuning range (2.5 MHz). MHz as the phase angle varied. At the design Article materials, except the phase-lock sys- Avoid magnetron "moding ," appearing frequency of 2430 MHz, all modulating tem, were drawnfiom the March 1989 issue on a spectrum analyzer as a comb instead of a products should remain within the amateur ofRFDesign. CW signal. This can be caused by a VSWR band. This is not a trivial problem, greater than 1.5: 1, or by operation below and may require line trimming or line about 50 rnA. If low power operation is de- strctchcrs to place the phase angle in a sired, raise the filament voltage to 3.4 - 3.6 stable region. Thc lowest possible an- V, since internal RF contributes to proper tcnna VSWR is the best solution to the filament (cathode) temperature in normal op- line-pulling problem. eration. If used with a true FM television receiver, Beware! such as a modified satellite TVRO unit, the Remember, for this project, simple pre-emphasis network shown on the SAFETY IS PARAMOUNT! This schematic diagram will improve video SIN transmitter has 4 kV DC and high by up to 10 dB. Also, TVRO receivers use power microwave energy present. greater than 20 MHz IF bandwidth, greatly- - Use a microwave leakage detector to reducing the effects of warm-up drift. check the integrity of ;he modified Photo F. The complete microwave oven ATV transmit- Small "hum bars" are visible in the pic- unit (see Photo E). You can buy an ter unit. The transmitter circuit is located in the oven's ture, due to the floating high voltage power inexpensive detector suitable for the cooking chamber.

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. -~ - - - -- CIRCLE 53 ON READER SERVICE CARD 73Arnateur Radio .July, 1989 61