THE HAWG AMP
(note: This document was written over a year ago and details construction of a Hawg Amp inside of a CATV tap box. The tap box was included in a kit we made that is no longer available. We regret that the directions have not yet been updated for a new enclosure.
The Hawg Amp project meets the need for a good, inexpensive preamp for ATV reception, which any ham can build. The Hawg Amp gives "the most bang for your buck," in the spirit of the famous Hawg Fence antenna, an inexpensive and effective ATV antenna by Big Shanty's beloved and loquacious 'Kip' Turner, W4KIP. Live television coverage of space shuttle missions from the Big Shanty Repeater Group has been popular in Atlanta. By making NASA Select coverage more accessible in fringe areas we hope to further promote ATV and ham radio. This project is also an example of hardware hacking, taking off-the-shelf equipment and adapting it to our needs (like the ET-phone-home transmitter in the movie.) This design is a modified Ramsey Electronics PR-40 kit. We don't re-invent the wheel; we just customize it a bit. Hawg Amp builders are encouraged to further experiment with and customize the packaging and powering of their amps. **
FUNDAMENTALS OF WEAK SIGNAL RECEPTION
Single minded pursuit of distance by using transmitter power only creates "alligator stations," all mouth and no ears! Most hams progress beyond that view when they realize "if you can't hear 'em, you cant can't work 'em." Modes such as satellite, distant VHF and microwave, and especially moonbounce, demand the highest performance in a receiving system. To receive weak signals, we need gain. The best gain you can possibly get is antenna gain, which works on both transmit and receive. Next to antenna gain, the best receiving gain is a LOW NOISE preamp mounted right at the antenna. A high gain preamp with poor noise figure may make reception worse, by amplifying the noise floor along with the desired signal. For example, suppose your receiver has a 3 dB noise figure. If you use an antenna preamp with 10 dB gain and a 2 dB noise figure, the reception is only improved by about 1 dB (the difference in noise figures), and the noise floor and intermod are increased by about 9 dB. Changing to a 30 dB gain preamp with a 6 dB noise figure, reception will be about 3 dB worse than no preamp at all! The S meter reads higher because you are receiving more noise, but you have less signal. In high school, I cascaded a chain of high gain preamps so I could pick up signals never before heard by mortal beings. I did perhaps hear more roaring thermal noise, howling intermod and screaming parasitic oscillations than ever heard by mortal beings! If you have several gain stages in a receiving chain, put the lowest noise one first, high gain in the middle, and high third order intercept (low intermod) at the end. Of course you will want some bandpass, etc., along the way, but the noise figure of the first stage determines the total sensitivity. This is why noise figure (also expressed as noise temperature) is the figure of merit for satellite TV LNBs. For a good introduction to receiver sensitivity and noise figure, find the section on receiver sensitivity in the ARRL handbook (chapter 12 in my 1990 edition.) Broadband TV amps and "scanner preamps," such as the Ramsey PR-2 kit, use broadband MMICs. These are simple to design with and have a wide bandwidth, like from DC to 2 GHz. They also have 50-ohm or 75-ohm matched inputs and outputs. Unfortunately they DO NOT have low noise figures, usually around 5 dB, or a noise temperature of a few hundred degrees Kelvin. Tuned amplifiers generally have lower noise figures than broadband amps. The ultimate low noise preamps are parametric amplifiers cooled in liquid helium, which can receive a few milliwatts from a space probe beyond the rim of the solar system. Ham budgets usually max out with HEMT or GaAs FET transistor amplifiers, which are tricky to build, since you can easily destroy your expensive FET by just touching it the wrong way. Bipolar transistor preamps similar to the Hawg Amp retail for about $60. The Ramsey kit contained in the Hawg Amp kit retails for $18. **
THE HAWG AMP, CONTINUED
Based on an inexpensive and durable bipolar transistor, the Hawg Amp performs a bit below the more expensive and delicate FET types.
Here are some Hawg Amp specs:
The Hawg Amp will work in 50-ohm or 75-ohm systems. A 5-GHz low noise bipolar transistor forms the heart of the preamp. A modified power feed allows the preamp to draw its DC power from the center conductor of the coax cable, permitting operation of the preamp right at the antenna without running any more wires. Of course, you need a source of DC and some way to get it into the bottom end of the coax without interrupting the received signal path. This first requirement is satisfied by almost any 12-volt DC source; the second requirement is taken care of by a DC power injector. Materials for a DC power injector are included in the Hawg Amp kit; you are left to your own ingenuity for the DC power supply. Cases for the preamp and for the DC power injector are included, also materials for connecting and mounting. Feel free to substitute your favorite DC connector into your Hawg Amp DC injector.
THEORY OF OPERATION
Receive preamps should be located as close to the antenna as possible, usually outdoors at the top of a mast or tower. To get power up the mast, we make the coax do double duty by sending the DC power up on the center conductor of the coax. A DC power injector near your receiver acts as a simple duplexer for two bands: DC and RF. For the basic Ramsey preamp schematic, refer to the diagram in your Ramsey booklet. The input from the antenna is applied through capacitors and inductors to the base of Q1. These input components form a UHF tuned impedance match from the input to Q1. Q1 amplifies the signal. Capacitor C4 couples the amplified signal from Q1 to the output. Resistors R1 and R2 set the bias on Q1 to draw about 7 ma of collector current, which is its best operating point. Capacitor C5 keeps the DC supply clean. Next, look at how the Hawg Amp modified design takes DC power from the coax. We assume the coax center conductor has 12 V DC on it, so R1 connects right to the center conductor. Capacitor C4 lets the amplified RF bypass R1 to the output coax without being attenuated. We no longer need C5 here, and the "DC duplexing" is built right into the collector circuit of Q1. At the indoor end of the coax, C5 keeps the DC clean in the power injector. A choke, L2, couples the DC into the coax while blocking RF, and capacitor C7 allows RF to pass through to your receiver while blocking the DC. (Many receivers don't like to see DC on their inputs.)
Inventory your kit for parts. It should include:
Your Ramsey kit should contain an instruction book, which we WILL NOT follow exactly, plus the parts listed on page 3 of the Ramsey book. You will need soldering tools, small pliers and cutters to build the kit. "Install" generally means to put a part into its PC board holes properly, form the leads, solder well, and cut off the leads close to the board. When soldering, heat the joint enough to make the solder flow freely. Do not "dab" or "paint" solder onto the joint with the iron.
DO NOT INSTALL C1, C4, C5, AND R1 EXCEPT AS INSTRUCTED! You may follow the Ramsey PR-40 booklet and build your kit as Ramsey envisioned, but the Hawg Amp takes a different twist. On the Hawg Amp PC board, DO NOT INSTALL C5; set it aside to use in your DC power injector. DO NOT INSTALL C1, C4 and R1 as shown in the Ramsey instructions; these parts will attach directly to the RF connectors.
BUILDING THE CIRCUIT BOARD
Build the Ramsey PC board, keeping the component leads SHORT for best performance.
[ ] Install R2, C2, C3, C6, L1 and Q1 just like Ramsey says, and solder them in place.
Note that one lead of L1 BRIDGES TWO TRACES, as shown in the Ramsey book.
[ ] R2 100k (brown. black. yellow)
[ ] C2 2.2pF (marked 2 or 2.2)
[ ] C3 4.7pF (marked 4.7 or 5 )
[ ] C6 4.7pF (marked 4.7 or 5 )
[ ] L1 0.04 uH (small two turn coil)
[ ] Q1 2SC2498 (the part with three legs)
Study the PC board and see how it corresponds to the schematic diagram. R1, C1, and C4 will connect into the preamp as shown in the Ramsey kit, except the leads for DC, RF in and RF out will be left flying rather than using the PC board holes.
[ ] Next, insert into the PC board only that lead of C4 that is tied to Q1 collector.
[ ] Insert one end only of R1 into the other hole in the same trace. Solder them in place.
[ ] Insert one end of C1 into the hole tied to L1 and C2. Solder it in place.
[ ] C4 100pF (marked 100 or 101) [ ] R1 470 (yellow. purple. brown.)
[ ] C1 2.2pF (marked 2 or 2.2)
This completes the component installation on the PC board.
R1, C1 and C4 should each have one lead flying. Clip all OTHER leads close to the PC board. Have a friend inspect your installation and soldering. You should have left over C5, .01uF, from the Ramsey kit, and C7, 390pF, from the Hawg Amp kit. These will be used in the DC injector.
MODIFYING THE BOXES
Your Hawg Amp kit includes two CATV tap boxes, each having three F-type connectors. One box will house the preamp; the other will house the DC power injector. The PC boards in these boxes must be removed and set aside with C5. Later, you will strip and rewind a balun core from one of these boards to use as L2 of your DC power injector. We really practice recycling here! The CATV PC boards are held in their boxes by eyelets soldered to the RF connectors. Start by desoldering the one in the center of the board. Go ahead and rip the ferrite core balun off the CATV board if you need more room, but SAVE the balun core for later. There are several ways to desolder the eyelets, including a solder sucker, solder wick or just plain knocking the solder out. The last method is my favorite.
BE CAREFUL WHERE YOU FLICK MOLTEN SOLDER! It can leave burns on carpet, clothing and people! To knock the solder out of an eyelet, choose a work surface you can hit and splash molten solder on, and a powerful soldering iron or gun. Heat the eyelet until the solder melts, and continue heating for a few seconds more. When it is nice and bubbly hot, suddenly rap the box, face down, sharply on the work surface. The solder will all splash out on the work surface. Repeat if the eyelet is not clear of solder. The joint should now break free easily.
[ ] Disassemble one CATV tap box. You will need to modify at least one push-on F connector, depending on the construction options you choose. Take the push-on F connector that held the box and bracket together and pop off the spring sleeve. Cut off the barrel and file it down even with the flange nut, discarding the center pin and dielectric. Re-install the modified connector mounted inside out, attaching the box bracket if you wish.
OPTIONS : If you are not using a mounting bracket or LED, you may omit the third F connector. If you want to power your preamp in Ramsey mode, you can run a flying lead out through the bushing, or you can mount your favorite DC jack in the side wall. You can also seal an LED power indicator into the bushing with RTV silicone rubber. For LED operation, connect a series resistor of 500 ohms to 2k from the DC source to the LED, then the LED to ground. Mind the LED polarity! If you use an LED in the Hawg Amp, provide some RF decoupling to keep the LED from becoming an RF mixer. If you are building the Hawg Amp version, disassemble and modify the other CATV tap box in the same way to house the DC injector. Most of the options above also apply here.
[ ] modify a box to house the preamp
[ ] modify a box to house the DC injector
INSTALLING THE CONNECTORS
Next, we will solder the PC board to a pair of F connector ground lugs. These lugs break if bent too sharply or too often. There are six lugs total in the kit and you need at least three of them unbroken. We will temporarily assemble the Hawg Amp inside out to jig the lugs in place while soldering them to the PC board. Put one ground lug on each connector and install both connectors into the box inside out. CAREFULLY bend the ground lugs around so they lie flat against the back of the PC board while holding the board at about a 30 degree angle toward the connector center pins. This angle allows the completed assembly to slide into the box later. Keep the PC board against the connector body. Center the PC board up with R2 near the connectors and the ground plane against the ground lugs and solder it in place.
[ ] solder the PC board to the ground lugs Carefully remove the PC board from the box, and reassemble the ground lugs and connectors finger-tight. Set your Hawg Amp aside and build your DC power injector.
BUILDING THE DC POWER INJECTOR
It is time to make L2. Rip the ferrite balun core off one of your CATV boards and strip off its windings. Wind two and a half turns of magnet wire through the holes in the ferrite core as shown.
[ ] Fabricate L2, 5/2 turns on ferrite core.
[ ] Assemble the DC injector box with two F-connectors and whatever DC connector and brackets you wish to use.
[ ]Install and solder L3, a piece of heavy bare copper wire from the center conductor of the IN connector straight toward the center conductor of the OUT connector. Cut L3 about 1/8 inch short of the OUT connector. Install and solder C7 to bridge the gap from L3 to the OUT connector. The L3 and C7 should run straight, about 1/8 inch from the metal wall, forming a transmission line.
[ ] L3 install wire transmission line
[ ] C7 390 pF (marked 390 or 391) L2 is wound with magnet wire covered with heat-strippable colored insulating varnish. Use a hot soldering iron and solder to tin the leads of L2 by melting the varnish. Connect L2 from the IN connector to the +DC connector. Connect C5 from the +DC connector to ground, which completes your DC power injector.
[ ] L2 5/2 t ferrite core inductor
[ ] C5 0.01 uF (marked 0.01 or .01 or 104)
You can test your injector with an ohmmeter, by verifying continuity from IN to DC, no continuity from OUT to either IN or DC, and no continuity from any port to ground. You can also test the DC injector by putting DC into the DC port and verify no current draw, DC voltage present at IN but not at OUT. When all is well, assemble the box and your DC power injector is complete.
[ ] DC injector test okay
[ ] DC injector complete At this point you can optionally test your preamp before installing it in the box. Connect a weak signal source to the input and connect the output through your DC injector to a receiver. When DC power is applied, the preamp should draw about 7 ma from a 12-volt supply (plus any current for optional LEDs.) You should also see an increase in signal quality.
[ ] Hawg Amp functional test okay.
Go ahead and mount your preamp in its box. Use a couple of sticky feet to insulate the bottom of the box (covering up the bushing and LED if you included one.) Nip off the back, outside corners of your PC board with wire cutters to get a better fit in the box. Next, insert the PC board with connectors, component side up, into the box and assemble the connectors tight. Carefully bend the PC board down into the box.
[ ] insulate box bottom with sticky feet.
[ ] nip off corners of PC board.
[ ] install PC board in the preamp box. Next, solder the flying lead of C1 to one connector center pin. This is the input. Solder the flying leads of R1 and C4 to the center pin of the other connector, which is now the output connector. KEEP THESE LEADS SHORT.
[ ] solder C1 to the input connector.
[ ] solder R1 and C4 to the output connector.
For best results, tune your Hawg Amp together with your antenna. Best reception is generally not tuned for maximum gain, but for lowest noise figure and best antenna impedance match. If your antenna did not tune up perfectly, here is your chance to compensate! L1 is the tuning element of the Hawg Amp. Spreading apart or squeezing together the turns of L1 will tune your Hawg Amp from about 400 to 500 MHz. Spreading the turns reduces the inductance of L1 and raises the frequency. Fingers and metallic tools near L1 affect the tuning, so use a plastic stick or get clear of L1 to see the results of your adjustments. Assemble and mount everything except the lid of the box. Connect the Hawg Amp to the antenna, DC injector, and receiver. If possible, tune up using the same antenna and input feedline that you will use permanently. Secure the antenna in a stable position away from nearby objects, and provide a weak signal on the air. Apply DC power and tune L1 for minimum smoke ( just kidding ) tune L1 for the best picture. Keep the signal level down to a P1 or P2 picture to get the best peak. Hawg Amp tuning is not sharp or touchy. Verify the tuning does not change when you put the lid on the box. That's all there is to it! If your antenna is not handy, feed a weak signal in through an attenuator of the same impedance as your antenna. Put the attenuator right on the input connector for best results, and follow the above procedure. Your Hawg Amp is now tuned up to the nominal impedance.
[ ] Tuned up and ready to go! **
WEATHERPROOFING AND MOUNTING
For minimum feedline loss, mount the Hawg Amp directly to the connector of the Hawg Fence antenna. Secure the coax to the reflector grid using black UV-resistant cable ties; avoid using ordinary nylon ties that deteriorate in sunlight. Brackets from the CATV tap box also may be used to mount the Hawg Amp. Keep water out of your Hawg Amp by mounting it with the connectors down, and use a cup or container for a rain cover. RTV silicone rubber makes a good rainproof sealant if you leave a small opening at the bottom of the box. A piece of RG-59 cable and two F-59 connectors are provided to make a short jumper for the antenna feed. Optionally, you can use a box with type N or SMA connectors for high performance. You can build a brass or copper box and solder it right to your antenna without any connector, to make an "active antenna." If you built the Ramsey version, just stick the amp inside your portable TV and tie into the TV power bus. **
DC POWER SUPPLY
Whatever 12-volt supply you use to run your other gear will also run the Hawg Amp. A 9-volt battery or any well filtered DC supply delivering 8 to 16 volts at 20 milliamps will work. Omit the LEDs to extend battery life. I do not recommend mixing your ATV wiring with AC house wiring, but the DC injector will mount in a 4-inch square deep wiring box along with a home-brew DC power supply or a DC wall cube supply. Without the plastic face plate, the DC injector mounts on any flat surface. Delete the bracket for tight spots, or eliminate the case totally by building power injector right into your receiver.
IN CASE OF TROUBLE
[ ] The easiest check you can make on your Hawg Amp is to measure its supply current, which should be 4 to 10 ma (plus any LED current.)
[ ] Otherwise, verify that voltage is getting to the center conductor of the preamp output. Power indicating LEDs help here. If you have voltage but no current, Q1 might be zapped. Until power is supplied and Q1 is drawing its bias current, further troubleshooting is futile. Turning the DC supply on and off should change the reception significantly. This shows the amplifier has gain. If not, check for bad solder connections or cracked or wrong parts.
[ ] If you experience intermod or TV interference, check your antenna, boxes, and connectors for loose metallic contacts. Dissimilar metals, such as aluminum and steel, in loose contact can form a nonlinear junction which causes old demon intermod to show up on the scene. Nearby junctions in rain spouts, rusty fasteners, guy wires, metal ladders and wiring can also cause this problem. Thoroughly bond loose junctions by welding or soldering when possible. Otherwise, clean the surfaces to a bright finish, clamp them together under high pressure every tenth of a wavelength, and seal these bonds from corrosion. Nonlinear junctions inside Q1 produce intermod under input overload signal conditions. Operating near transmitters might require extra filtering and shielding to get clean ATV reception. Remember also that nearby CATV leaks can cause cable channels to interfere with ATV reception. **
The Big Shanty Repeater Group hopes you enjoy using your Hawg Amp as much as you enjoyed building it! Try your Hawg Amp on ATV, FM, SSB, satellites or even moonbounce! We hope you will want to experiment with your Hawg Amp and with other circuits. Build your experience with simple mods to things you can afford to trash, like video rabbits. Use the knowledge and experience of your friends, and learn from your failures. Soon, you will find others seeking and benefiting from your knowledge and experience, and then you will realize how much you are learning! Happy hardware hacking! See you on ATV! 73 DE N4YWK ??
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