Fig,Mounting feed throng h capacitors.

boxes ih^t come with more than lour screws.

If you are either \er\ intrepid, or a bit of a fool tike me. then you might want to consider making your own boxes;. One of the neat things 1 learned to do in junior high school metal shop was operate a bench brake. This tool allows you to clamp a piece of sheei metal between two surfaces, and ihen using a pair of handles, route one surface upwards, producing a bend up to 9(' degrees in the met Lit. B> properly planning the box "in the flat" and then bending it. you can make your own box of any shape.

Other forms of metal box

A number of times in my build ing career I've needed specially sized or shaped boxes that were not easily available on the market <although that might change now ,t see ihe SLSCO.M discussion below), tf you go to ihe type of hobby shops that cater to model builders, you w ill often find adis-ptay of bra*s sheet metal strips, solid rods and hollou tubing These can be formed into squares* rectangles or oiher shapes using ordinary tools. Bending can be done on a bench vise, or using pliers.

If you ha\e access to a jeweler's supply store <or lapidary shop), then you also have access to a number of special tools that arc also of use to amateur radio constructors These tools are used b> sHxersmiths, amateur and professional. A jeweler's sa\s. for example, is a kind of jig saw thai can be used to make very intricate cuts, as well as standard straight-across cuts. If you buy one of these saws. I also recommend you bu> a book on making silver jewelry in order to learn how to use the saw.

Jewetr) supply stores also have tiny drills that can be used to drill fine holes in printed circuit boards or metal chassis. I've used both the Fordham Hex-Tooland the Dremel Motofool™ for both jewelry and radio constructing. 1 he Dremel (which I actually own) has a large number of accessories that can drill, burnish, grind, cut (you've probably seen the TV ad ... it'll do anything bui hammer).

Another little-known but terribly useful tool used by jewelers is the parallel-jaw pliers, These pliers look tike ordinary heavy-duty flat-jaw pliers, except that the planes on the inside of the jaw s remain parallel as the handle is squeezed. It von try to make a bend in a piece of sheet metal w ith ordinary pliers, then >ou will find that it skewed to one side. The parallel-jaw pliers eliminate this problem.

Plastic boxes

Plastic ' utility boxes" have a loi of applications in electronic construction. I've used them for a lot of instrument and power supply projects. However, for most RF projects they are not too useful—the) do not have am shielding to offer!

Plastic utility boxes are easy to work, and have some really attractive features (especially where insulation is needed on the outside, or where appearance is an issue). If you absolute!) must have both shielding and a plastic box. ihen there are a couple of alternatives. You could spray paint the inside with conduct»\e copper paint (not all copper-colored paint

Fig. 4a. Flange bo.x.

isconductive.l. Second, you can buy thin copper toll and cement it to the inside of the box. Very thm foil can be bought irom the type of hobby shops thai cater to dollhouse makers. They use the foil to simulate the copper roofs that w ere once popular on houses. B\ the way, I've used such copper foil for making shielded small loop antennas.

Passing a wire through a shield

11 you make a project thaL has internal shielded compartments it might be necessary to pass a DC power suppl) or signal wire from one compartment to another tl the wire carries DC« then a feedthrough capacitor could be used, but for signals a feed-through capacitor could short the to ground. \ sometimes use the smallest si/e grummets to do this job iFifc, 5k It works quite nicely. Drill a hole in the partition ti) accept the grommet's barrel diameter. There arc three diameters on a grommeL The nutside diameter ro d.) is the overall diameter of die part, while the inside diameter {i,d. i is the diameter of the hole through the diameter. The barrel diameter ib.d.) is the diameter of the center portion that fits through the hole jFig. 61. One of the mistakes made by some constructors i including me) is to improperly si/e the hole lor the grommeL 1 match die shank end of a drill hii to the b.d. of the grommet to make the selection.

Some neat boxes from SESCOM

From time to time 73 advertiser SESCOM ^ends me samples of their small metal boxes for electronics projects. Most of the time, 1 buy some more for my own because they are so useful. If you've been disappointed with ihe quality of ho\es sold in certain chain

73 Amateur

Fig. 4h. Fkin^e box u ith additionid screws.

outlets, or even from some major mail-order outlets, then y ou are going to be delightfully surprised at the SESCOM offerings.

hirst, lei me suggesi you contact SESCOM for their product brochures: I) Constructor's Hardware, 2) Audio Construction Made Easy, 3; Audio Solutions, and 4) Lab Box-It™. Let s take a brief look at iliese products.

For a number of years V ve used the SESCONf SB-series Rl shielded boxes * see Constructor's Hardware ). After SLSCOM provided a sample. I ordered more than $ 1 IK) worth on my own lor projects in "Joe's Basement Mental Therapy Laboratory(where t let the w ind out of my head by doing electronics projects! j Ihe SB-series boxes are tin-plated steel with covers that have RF-style "finger flanges.* SESCOM also sells low-cost Rf: feedthrough capacitor^ to allow DC and low frequency lines to pass in or out of the box without spraying the internal RF all over the place. When you compare die SESCOM price for these capacitors w iih what you find in the major suppliers' catalog v you are gonna be impressed!

The latest product samples SESCOM sent are their Lab Box-It^ (LAB-x) and Mini Box-It™ iMPB-x) aluminum boxes. These boxes come in "kit form" and you assemble them yourself. Each kit has two end pieces, top and bottom pieces, two side pieces and four extruded softeners for the corners Tim sheet metal screws arc used to hold the assembly together by fastening the end plates to the

Fig. 5. Using a grommet to pass a wire through a box. Radio Today * November 1997 51

& Beyond

C. L Houghton WB6IGP San Diego Microwave Group 6345 Badger Lake Ave San Diego CA92119 []

QRP transceivers for microwave operation

This month I would tike to describe how the San Diego Microwave Group has cranked up the QR I* micruw av e construction mill and produced a variety of low power transceivers. In doing so I want to get into some of the cor side rations needed to make a transceiver function using low

VHF and Above Operation frequency drivers, such as two-meter or similar multimode rigs.

The bands that rigs were constructed lor include 12%through 10 OH/.. The equipment was all home-constructed ¡from "found" surplus or some of the excellent surplus electronic materials we obtain from Quakomro Electronics,

We are fortunate that Qualcomm has allowed our ¿roup access to surplus material for amateur radio use only. Our group breaks down the material to prevent the original equipment from being recycled hack into commercial r

Fig, 6 Shifulard ^rouwict.

e\iruded stiffeners. A standard miniature Phillips]V screw driver will do ihe irick. 1 he side and top/bouom panels are held in the stiffeners by sliding them into the slots on the stiffeners.

I here is a wide variety of sizes of both forms of box, so most constructors should be able to find what the> need As an experiment* however. 1 decided to make a custom size box from one of the LAH-x boxes, 1 used a jeweler's saw io cm the extruded stiffeners to a custom length, Ji was a bit Lrick\ to ensure thai all four cut stiffeners were the same length, but a little work w ith a line Swiss file did the trick (recommendation: cut them a millimeter or so over length and then File them down). The sides, top and bottom panels are made ol aluminum, so the) cut nicely with a pair of straight sheet metal shears (don use ihe curved-stylc sheet metal shears).

One of the reallj neat things about the LAB-x series of SESt'OM boxes is the variety of end plates that are available. The boxes come w ith blank end plaies, so you can drill your own holes. Bui a number of different pre-cut end plates are also available. Included in the list of "specials'* arc:

AI 0.25-inch for RCA phone jacks.

Bj 3/8-inch for quarter-inch phone jacks and standard bushing roLuy su itches and potentiometers,

D> Two 5/16-inch banana jack holes,

KjSO-239 chassis mount "I IfF" coaxial connector,

F) 3/8-inch shaped for BNC connectors.

G) l/2-iiieh "D" for isolated BSC connectors.

H) DB-9 and DB-25 computer connectors (ever tried to cut one of Ihose w ith a Swiss file?).

I) Special holes for male and female \LR audio connectors.

Not all styles of special endplate are available on all boxes, but the assortment is broad enough to allow most builders to save a lot of time.

Connections l can be reached at the address listed at the top of the column,

SLSCOM: 2100 Ward Drive. Henderson NV 89015-4249; I-800-551-2749 (orders), or (702) 565-4828.

use. Smarts that are usable to the amateur radio community are made available. The main unit to be briefly discussed here is the svmhesizer our group and many others have u\ed as the basis for a transceiver converter for many of the rnicrow ave bands.

The easiest rig or microwave transceiver thaL can be constructed using these parts is a QRP rig for 2304 MR?. While we arc able lo obtain materials from Quale* >mm to assist us in this construction project, other sources must also be used. The main ingradient obtained from Qualcomm i> the synthesizer/local oscillator. The basic synthesizer operates at 2620 MH/ and is converted lo 2160 MHz for mixing with a high-frequency IF radio, A two-meter radio t IF) was selected, as this gives the fillers a chance io remove the lower mix product 2016 MHz (2160 -144 MHz).

Many pans need to be assembled to construct a complete convener. The hasic components are the same tor every one. They include an RF preamplifier, transmitting amplifier, mixer. Oiler. and RF/IF switching- The converters I will describe will be constructed to operate from 12 voits DO and he driven with a tw o-meter low-power HT or similar transceiver, Because the transverter is linear in operation, it can be used on any mode of transmission but is especially suited for narrowband IM, SSB, andCW A I! modes of transmission are supported in an identical fashion, as you onlv have to change the mode on the driving HTorrig to change modes. No circuitry changes in ihe transceiver are required for lhis operation,

Normallv SSB is the preferred mode of transmission lor most operations* FM is reserved for stronger signal paths or those such as mobile operators who often have verv distuned SSB speech because of Doppler paths arriving at varying limes. In this case, narrowband h'M can cut through the clutter of the Doppler and make communication-^ readable—prov iding signal strength is strong.

This type of trails verter is normally driven by a multimode two-meter source. The only requirement is thai the driv ing source be ol low power on transmit to prevent burnout of the nans verier'\ mixer, If the two-meter rig can output high power levels lo the miver. a piece of "unohiaimum"" could be destroyed. lo prevent this, my two-meter rig, an old-style two-meter transverter, was modified by removing the power outpui stage to prevent an;. possibility of high power feeding the mixer

This is a drastic step and those with expensive, newer two-meter radios might not want to take it. The alternative is to placc an at tenualor in the coax to reducc the power lo acceptable levels for injection to the mixer. Normally, a power level 50 mW or so, near the +10 to +15 dBm level, is perfect, It is tow enough that ii will not cause mixer burnout and vet still w higii enough to provide proper injection to the mixer. You can push this drive level with higher level mixers to something near + L7 dBm as a maximum drive level.

In receive, you will have lo switch out the attenuator to have maximum sensitivity on receive. If you were to leave in a 20 dB attenuator thai would be required for use on a JO-watt rig, it would attenuate the receive signal by 20 dB as well. B> sw itching out the attenuator on receive, you allow the receive section to operate with maximum sensitivity. This switching circuit can be cither an automatic sela> selection circuit or simple toggle switch affair,

II you choose to use a toggle switch, be sure to throw it to transmit prior to actually transmitting, as ihe 20 dB protection attenuator is not in [he transmit path unless the toggle switch is operated first In manual operation with a several-wait rig for the two-meter transceiver» it can be an easy slip of the hand to get out of sw itching sequence and destroy the mixer with high power, it is indeed an expensive price to pav for not using mixer switching protection, since mixers for the microwave bands are not inexpensive.

The best remedv to avoid this problem is to incorporate some sw itching protection lor the mixer. Several variations of this scheme can be used to great success 1 he circuit that can be used for protection from a 10-wait transceiver (two-meter) can be constructed from a few extra parts. These pans are added io the mixer port io prevent direct contact between the transmitter and mixer port.

What the circuit consists of is a basic MM1C op amp driving a fixed resistive attenuator, t he type of MMIC is not critical, as the frequency of operation is quite low —in the 145 MHz range. The gain of the op amp is determined by the value of the attenuator required to limit transmitter power to the required drive leveU For instance, my Yaesu FT-480 two-meter multimode rig puts out 10 watts of power. Inserting a 20 dB attenuatorreduces the 10 watts to 100 milliwatts (10 watts = -M0 dBm, 1 watt = +30 dBm, and 100 mW = +20dBrn),

In the transmitter path, a fixed 20 dB attenuator would be switched into the transmitter path towards the mixer when transmitting. The op amp and another 20 dB attenuator are connected in the normal receive path. This combination lias zero gain, so on receive its action is transparent. Should the switching fail, transmitter power will be attenuated by the receive 20 dB pad and feed the op-amp output backwards. Feeding the op amp backwards is additional protection against power getting to the mixer.

This receive attenuator can be made from two-watt carbon resistors. The transmitter's 20£IB attenuator should he capable of handling the full 10 watts. In this scenario, there is no damage to either the mixer, op amp. or attenuator.

This changeover can be automatic, derived from an RF-detect circuit on the IF coax path, or it could be nothing more that a simple manual toggle Switch. In either case, with this power protection circuit the transverter's mixer is protected from certain switching errors preventing high power to the mixer, and replacement costs for a new mixer.

There have been many different switching circuits labeled failsafe that would switch attenuators in the transmit path prior to power output from the IF driving transceiver. Our Microwave Group has found several two-mater ri^s that even when in the low power mode will output high power for a small fraction of a second. This short burst of power is too quick to be observed on a normal metered power meter. So it's better to assume that all rigs can present a danger to a costly mixer. Protect it!

11 you are a dedicated micro waver and have the microwave frequencies in your blood, you will probably do as I have and remove the high power capability of your driving radio. No one iikes to cut into the circuitry of any new radio to modify it, especially since the cost of a inultimode radio today is quite high However, if a used multimode were to be modified, there are several approaches that can be taken to minimize the permanent modification to the radio. These are alternatives Lo the RF switching protection circuit.

Most of these radios have a low-power switch which will limit power to less that two watts or so. Don't depend on this switch* since being a switch it can be depressed—and you re in trouble with too much power to your transverter. The solution I took to this problem is to open up the radio and determine from the schematic or measurements what function the switch is performing, Then duplicate it in hard wiring, making the sw-iich function permanent. Now if you accidentally operate the low-power switch, you're still permanently switched to low power and can't go to high.

The beauty of this simple mod if tea lion is that with minimum effort you can return your radio to normal function in a matter of minutes by clipping out the switch wire strap.

Another possible modification is to remove DC power lo the final RF stage. This trick was discovered when low power was reported as trouble on a friend s radio. In this ease, the actual trouble was that the final transistor was blown open possibly through poor SWR or whatever took the device out (if the transistor has failed in the shorted mode, this trick won't work).

In retrospect, the driver was putting out less than a watt with the final transistor in this Liblown1 stale. Another creative way to reduce maximumpowerpuiptit is to do what

Kerry N6IZW did to a old ICOM IC-245 t ig. He modified a pre-driver RF amplifier stage that was emitter-biased with a 10 ohm resistor lo ground. Changing this resistor to one of about 1 k£l permanently powered dow n the amplifier strip to about 100 mW,

This prc-driver stage was now just tickling the driver with minimal RF to barely turn on the driver and final with insufficient drive for full power output. This w as just what the doctor ordered: a permanent condition forcing the transmitter on low power of 100 inW output all tile time with minimal modification. To restore this conversion to normal, jusi short the I k resistor in the emitter circuit with a 10 L'l resistor and the transmitter is back normal. What could be belter than limiting drive to the prc-driver and driver/final amplifier string? Power w?as adjusted to obtain just under 100 milliwatts of outpuL power, by trimming the I k resistor to the proper value for the power output required.

In this manner, the modification took a few-' parts and some surgery but still allowed the radio to be retained in nearly intact condition, I plan to modify my IC-245 in the same manner. 3 also want to incorporate a power switching circuit to switch he-twee n transceiver driver circuits and norma] 10-watt operation, 1 w:ill investigate just what kind of switching method can be used to allow bullet proof, fail-safe switching between power levels.

I know what 1 said about the dangers of unreliable power switching (toggle or slide switch), bur I'd like to sec if something can be done reliably—'possibly the addition of a plug connector or a key-actuated toggle switch on the radio. When the plug is removed or the key is inserted and turned in the switch lock, the radio is in permanently-low, 100 mW power output, inserting the plug or turning the n e w sw itch off switC he s the bias circuit back to the 10 watt position. At least that or something similar to that position rs what 1 am pondering and will tinker with,

A last word about two-meter multimode radios. I am not too hot about one feature ol' the ICOM TC-245 radio for portable work. The problem lies in the red LEDs in the frequency display. When used in portable operation (out in direct sunlight), the display is quite hard to read. Even in most outdoor conditions, you have to shade the display and squint to see a faint display, Et's not the radio's fault but instead just the design of the frequency display and the use of the red LEDs.

This problem is quite prevalent in early two-meter multimode radios of this period. The exception to this rule is the ICOM IC-202, which has a velvety smooth VCXO-controlled dial with operation limited to (LSB) SSB only. Another possible radio is the YaesiE FT-480 multimode rig, which uses a green fluorescent display and is quite bright in direct sunlight. It's another example of older multimode radios that can be made quite valuable; as drivers for microwave conveners.

New radios are available; with the most inexpensive multimode model being hy Yaesu, the FT-290R. This radio is still being offered by Yaesu for under $600 brand new, Ham Radio Outlet.™ and other retailers have it. I am sure, The rig sports full multimode operation and an LCD display that can be observed in direct sunlight. It will output 10 watts from 12-voli installations but with [he optional battery pack will put out two watts for portable stations, Sounds just like the thing to think about if you want to start with a new-1 multimode radio, It's not overly pricey, has multimode capabilities, and has a display that can be used in daylight or direct sunlight.

Wei), that's it for this month and some of the approaches we have put to use in converting our low-power two-meter radios. High power is not always the answer. Indeed, it's very interesting how low-power applications can be quite satisfying. With power levels under a couple of watts on most converters, very interesting propagation and just good old fuii are always available with QRP operation. _

73 for now; Chuck WB6XGR

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