How To Make A Cavity Filter For 10 Mtr
Build a two-Meter Duplexer
W alk upward to any ham who has already put a repeater on the air and enquire him, "What's the toughest technical problem you had with it?" He will probably say that obtaining sufficient signal isolation betwixt the transmitter and receiver was the toughest. Satisfactory isolation can be gotten, just ever for a cost. Many of the solutions to this problem compromise receiver sensitivity or transmitter power output. Other solutions throw off the balance betwixt receiver and transmitter coverage areas. When a duplexer is used, insertion loss is the compromise. Merely any insertion loss is more than outset by the use of one antenna for both the transmitter and receiver. Using 1 antenna assures equal antenna patterns for both transmitting and receiving. Duplexers have been in use by many commercially operated repeaters for many years. A large number of these systems use frequency separation of two percent or more between input and output channels. Amateur two-meter repeaters have a separation of just 0.four percent, or 600 kHz. A decrease in frequency separation makes the job of providing adept isolation fifty-fifty more hard. A duplexer must come across some basic requirements. It must attenuate the transmitter carrier, this prevents receiver overloading, which in plow would reduce receiver sensitivity. A duplexer must also attenuate whatever noise or spurious energy from the transmitter on or near the receive frequency. In addition, a duplexer must provide a proper match between transmitter, antenna and receiver. If you lot are having trouble visualizing these functions, Fig. thirteen volition help. Transmitter output on 146.94 MHz going from betoken C to D should not be attenuated. However, the transmitter free energy should be greatly attenuated between points B and A. Duplexer department two should attenuate whatsoever noise or signals that are on or near the receiver input frequency of 146.34 MHz. For good reception, the noise and spurious point level must be less than -130 dB (0 dBm = i milliwatt into 50 ohms). Typical transmitter noise 600 kHz from the carrier frequency is eighty dB below the transmitter power output. For 60 watts of output (+48 dBm), the noise is -32 dBm. The duplexer must make up the difference between - 32 and -130 dBm, or -98 dBm. At present, permit'southward talk about the received signal. Start of all, the received signal must get from point B to A with a minimum of attenuation. Section one of the duplexer needs to provide enough attenuation of transmitted energy to forbid receiver overload. For an average receiver, the transmitter betoken must be less than -thirty dBm to meet this requirement. It is the job of duplexer department 1 to make upwardly the difference between the transmitter output of + 48 dBm and the receiver overload bespeak of -30 dBm. One thing that many duplexers have in common is the use of loftier-Q coaxial cavities. The loaded Q of a crenel is affected by electrical electrical conductivity and dielectric losses. Surface loss tin be reduced past silver plating, although clean copper is acceptable. Air dielectric duplexers cavities are the most practical for amateur duplexers.
The Excursion:
A quarter wavelength resonator was selected for this duplexer pattern. The length of the center usher is adjusted by turning a threaded rod, which tunes the cavity to frequency. Free energy is coupled into and out of the tuned circuit by the coupling loops extending through the tiptop plate. The crenel functions as a series resonant circuit. When a capacitor or inductor is connected across a series resonant circuit, an anti resonant notch is produced, and the resonant frequency is shifted. If a capacitor is added, the notch appears beneath the resonant frequency. Calculation an inductance will brand the notch appear to a higher place the resonant frequency, and the value of either component will determine the spacing betwixt the notch and the resonant frequency. Fig. 14 shows the band-pass characteristics of the cavity with shunt elements. With the cavity tuned to 146.94 MHz, and a shunt capacitor connected from input to output, a 146.34 MHz signal is attenuated 35 dB. With a cavity having an inductance across it, and tuned to 146.34 MHz, the attenuation at 146.94 MHz is 35 dB. Insertion loss in both cases is 0.four dB. Three cavities with a shunt capacitor are tuned to 146.94 MHz and connected together with curt lengths of coaxial cablevision. The attenuation at 146.34 MHz is more than 100 dB, while insertion loss is ane.5 dB. Response curves for a 6 crenel duplexer are given in Fig. xv. The schematic diagram for the complete duplexer is shown in Fig. 16.
Construction:
A small lathe for metal work will machine the brass acme plate, the threaded tuning plunger bushing and the Teflon insulator bushing. Fig. 17 shows the completed top plate assemblies, i with a capacitor, one with inductor. The dimensions of these parts are given in Fig. 18. All the other parts can be made with mitt tools. "DWV" copper piping is used for the outer conductor of the cavities.
See Fig. 20 for a consummate overview of an individual crenel. The wall thickness is 0.058 inch (1.5 mm), with an outside diameter of 4-1/8 inches (105 mm). When y'all buy the pipe, borrow a tubing cutter big enough to handle this size. The wheel of the cutter should be tight and sharp. Make wearisome, careful cuts then the ends will be square. The outer usher is 22-1/2 inches (571 mm) long. The inner conductor is made from type "M" copper tubing having an outside bore of 1-iii/viii inches (35 mm). A six inch length of one inch OD brass tubing is used to make the tuning plunger. Soft solder is used throughout the assembly. Unless you have had experience with silver solder, practise not use information technology. Eutectic blazon 157 solder with paste or acid flux makes very good joints. This type has a slightly college melting temperature than ordinary Tin-Pb alloy and has considerably greater strength. First solder the inner conductor to the top plate. Then finger stock can be soldered within the lower end of the inner conductor, temporarily held in place with a plug made of aluminum or stainless steel. While soldering, do not let the flame from the torch to overheat the finger stock. The plunger bushing is soldered into the tuning plunger and a twenty inch length of threaded rod is soldered into the bushing. Cut 6 slots in the superlative of the outer usher. They should be five/8 inch (16 mm) deep and equally spaced around the tubing. The bottom end of the 4 inch tubing is soldered to the square lesser plate. Because the center conductor has no support at one terminate, the cavities must be mounted vertically. The size and position of the tuning loops are disquisitional. Follow the given dimensions closely. Both loops should be 1/8 inch (3.two mm) away from the center conductor on opposite sides. Connect a solder lug to the ground cease of the loop. Then fasten the lug to the top plate with a spiral. The free stop of the loop is insulated by Teflon bushings where it passes through the top plate to connect to the BNC fittings. Before final assembly of the parts, clean them thoroughly. Soap filled steel wool pads and hot h2o work fine. See that the finger stock makes a house contact with the tuning plunger. The top plate should fit snugly in the superlative of the outer conductor. A large hose clamp tightened around the outer conductor will continue the peak plate in place.
Tuning:
Await until the cavities have been checked for band-laissez passer characteristics and insertion loss before installing the anti-resonant elements, C1 and L1. See Fig. xiv. Information technology is preferable to use laboratory test equipment when tuning the duplexer. A second method uses a low ability transmitter with an rf detector and a VTVM. Both methods are shown in Fig. 19. With the exam equipment connected every bit shown in Fig. 19A, adapt the signal generator frequency to the desired repeater input frequency. Connect a calibrated pace attenuator between points X and Y. With no attenuation, adjust the HP-415 for 0 on the 20 dB scale. You can check the scale of the 415 by switching in different amounts of attenuation and noting the meter reading. You might note a small-scale error at either high or very depression signal levels. Next, remove the step attenuator and supplant it with a crenel that has the shunt inductor, L1, in place. Adapt the tuning screw for maximum reading on the 415 meter. Remove the cavity and connect signal X to Y. Set the bespeak generator to the repeater output frequency and adapt the 415 for a 0 reading on the 20 dB scale. Reinsert the crenel between X and Y and adjust the crenel tuning for minimum reading on the 415. The notch should exist abrupt and have a minimum depth of -35 dB. It is important to maintain this minimum reading on the meter while tightening the lock nut on the tuning shaft. To check the insertion loss of the cavity, the output from the signal generator should be reduced, and the scale of the 415 meter checked on the l dB expanded scale. Apply a fixed i dB attenuator to make certain the error is less than 0.one dB. Supercede the attenuator with the cavity and read the loss. The insertion loss should be 0.5 dB or less. The procedure is the same for tuning all half dozen cavities, except that the frequencies are reversed for those that have the shunt capacitor installed.
Adjustment with Minimum Equipment:
Try getting a tube transmitter when using this method of adjustment. You will want a minimum of spurious signals, otherwise, there will be a lot of false indications. The VTVM should be capable of reading 0.5 volt or less, total scale. The rf probe should be good to 100 MHz or higher. Sections of RG-58/U coaxial cable are used as attenuators. See Fig. 19B. The loss in these 140 foot sections is nigh x dB and volition help isolate the transmitter in instance of mismatch during tuning. Set the transmitter on the repeater input frequency and connect P and Q. Obtain a reading between 1 and 3 volts on the VTVM. Insert a cavity with shunt capacitors in identify between P and Q and conform the cavity tuning for minimum reading on the VTVM. It should read between 0.01 and 0.05 volt. You can calculate the rejection in dB by the formula 20 log V1/V2. It should be -35 dB, minimum. Bank check insertion loss past putting the transmitter on the repeater output frequency and noting the VTVM reading with the cavity in and out of the circuit. You tin make a 0.five dB attenuator from seven feet (2.13 one thousand) of RG-58/U. This 7 foot section can be used to check the scale of the detector probe and the VTVM. Cavities using a shunt inductance tin exist tuned the same mode merely with the frequencies reversed. If you try to melody two or more cavities continued together, transmitter noise tin cause the rejection readings to be depression. In other words, there will exist less attenuation.
Results:
This duplexer is conservatively rated at 150 watts input, but information technology should withstand up to 300 watts. Silver plating the interior of the cavities is a proficient idea if the input power volition exceed 150 watts. A duplexer using plated cavities has an insertion loss of under 1 dB, and a rejection of more than - 100 dB. Unplated cavities should be taken autonomously every ii years, cleaned thoroughly, and retuned.
Miscellaneous Notes:
ane) Double shielded cable is a must throughout the system.
2) The VSWR from the antenna should non exceed 1.2:ane for proper duplexer functioning.
3) Good shielding of the transmitter and receiver at the repeater is essential.
4) The antenna should have four or more wavelengths of vertical separation from the repeater.
5) Conductors in the near field of the antenna should be well bonded and grounded to eliminate noise.
half-dozen) The feed line should exist well bonded and secured to the tower or mast.
7) Feed lines from other antenna in the near field of the repeater antennas should exist well bonded and every bit far from the repeater antenna as possible.
viii) Individual cavities or pairs tin can be used to amend the performance of split antenna or split site repeaters.
9) Private cavities tin be used to help solve intermodulation bug.
This article is part of the "Repeater Builder's Technical Information Folio."
Original article appeared QST magazine as described by W1GAN July 1972.
The article likewise appeared in the ARRL Antenna Volume, and in the FM & Repeaters Manual.
Copyright ARRL, All Rights Reserved.
This web page created September 1999 and is © Copyright 1999 by Kevin Custer W3KKC, All Rights Reserved.
This spider web folio, this web site, the data presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.
How To Make A Cavity Filter For 10 Mtr,
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