Lts Transverter

The 1296-MHz amateur pand, 01 23 centimeters is is frequency called has long been a dark mystery to most amateurs. The log'strcs of gett-ng something \Q oscillate ai lhat frequency in j siabie manner. as well as amplifying [he resultant signal and modulating II. have served Co discourage all but the most technically competent amateurs from ever operating □n the band Add the problems of designing .i high-gain, low-noise front and. and the diltlcullies of getting all of Ihis stuff to worn with piles of inierconnitciing chassis and cables without a decree m electrical engineering, antf you have the situation that existed not too many years ago on 23 cm

Wiih ih* advent of transveriers usrng tow-noise front ends and the 'eductions in ensssit 5iZ+ made possifi'e e f ioFid Siati " ■ rr jit& 12^6 began ;o be seriously considered by many UHF operators. There were still the problems with power, but even receivers with 5dB noise figures could pun a weak drifting CW signal out of the ether using a homemade disk constructed from chicken wire or door screening on an old TV antenna, Coffee-can feedhoms were the order ol the day, and for the adventurous it was the loop yagL A bifl Slep irom Ihe old days, but ^¡till not as convenient as a 144-MHz muthmode transceiver.

Two development have finally brought 23 tm down to earth for the casual operator. the Gallium Field Effect Transistor ¡GaAsFETi a no ne* hnes of high-gain ■ T0-

JB- power transistor^ tor grounded-Pase operation at up to 3 GHz ft was inevitable that some manufacturer .vouid marry (he two in a high-performance 1296 transverler What was surprising iS that the manufacturer *as from Germany—nor Japan! Enter SSB Electronics of tserlohn. West Germanyr and the LT23S transverter

AI first gtance, the LT23S is an attractive. functional piece of equipment II measures 11V* 'wide (30 cm) fry 8 V*" deep 122 crn) by 3% * high (0 cm) The case is a hard aluminum shell wilh a plastic ring surrounding the froni panel A heal sink protrudes about IV {A 5 cmi from the back panel

The Froni pane?; contains the following controls; From the ;elt. a switch sheets e* ¡her of twocrystai i-f frequencies The sup' aiied crystal will downebn^ert Ihe 12961298-MHz band to 144-146 MHz. Many serious contefiters obtain a second Cryslal To shift the deseed band segment up so mat the conversion at t*o meiers is now 146- T48 MHz. This eliminates any urjssi-bio fcedthrough from strong nearby stations on 114 MHz during a contest (In Europe, the iwo-rneter allocation ends at M*5 MHz, so shifting th^ conversion fre quency up ma^es sense i Tne formula for this crystal is detailed in the owner s man uaJ.

Next is a transmit switch This does exactly that and disables the receiver while setting the idling bias tor the driver and final transistors. This function is paralleled by a rear panel RCA type phono lack One merely grounds this Jack and the unit goes into the transmit mode. The rent switch is for power and controls the feed from tfte rear-panel dc connectors. Finally, tiim* Is a powu* molar that indicates oulput in Watts. On the fear panel connections arc-made tor mpuVoutpu! to a 144 MHz trans ceiver, 1296 receiver input from the an tenna and 1296 rf oulput claimed 10 Watts serosa 50 Ohms| Three binding posts are supplied dc mpul [13 8-145 volts del ground and a second red post that supplies dc voltage in receive and cu(s ol' while ir. ^ansmir This >s to provide tor a mast-mounted preamp it one is used.

Ali m all the LT23S breaks new ground on 23 cm by offering the user a simple to-use transverte* One merely adds an an tenna relay, antenna, power, and multi mode 144-MHz transceiver The from end provides sufficient gain to work rnofii signals encountered on the band, whhte the 10 WüUs win carry a distance. Thai is, as sumlng the user has connected low loss Iransmission line and a good gam antenna to the U23S! Remember ihat con-venitonai RCi-ß U has about i0 dB of loss per 100 feet at this frequency, so something more along ihe lines ol Beiden 9Qi3 Vj * or even V,- hardline is in order But havmg i\ an n one case instead oi c-n 3 or 4 separate chassis with a myriad number oi connecting cables can t be beai

Nov* on to the meat and pot^toe* of ttns review How weft does it work? The first LT23S sample made its way biiek from Ihtf VHF Shop in Pennsylvania as 1 was in the midsi Qi frantically assembling a 432-MH? station for the Slide Mountain expedition (see the November, 1985, issue of 73), .-ind consequently it sal on IhfcSheJf for about 3 weeks unlil U was able to atari making qualitative lests, problem which surfaced immediately is that the on board crystal oscillator drifted severely, lo the tune of about J00-20Ü Hz per minute1 This was unacceptable, and on air tests with Tom Waldron KG3R. th2 proprietor ol the *HF Snu[ convinced him mat it A-as indeed galloping up the banü

Another unit ¿vas exchanged for the test unit anj i? loo suffered frorr. sam« malady, and almost at the same ratef Sub sequent conversations with the factory in Germany, Rick Connor WB2NPE, l*ara Lauzums KC2PX, and other LT23S users resulted in many sotutrons to the problem Tom K03R suggested using a higher-

Photo B. in aide the L T23S (top viewy

grade crystal and sard he will equip all mocefs imported into (hr USA with citais from lnteTriationai Crystal in Okia homa Rick WB2NPE suggested rounding the crystal case and snortrng the unused crystal socket pins m the second oscilla tor together, Both mods worked, but the ultimate correction .Mme via Ivars KC2PX Ihrough an unnamed amateur in Oklahoma, who removed ihe coupling capacitor from the unused oscillator to the llrsl multiplier stage That permanently fixed Ihe problem ituj J recommend io all LT23S owners tne following modification

The covers and jnside board must be re mo^-eo by lot^seninç afl screws around the ca.^e and pwiei on the ^ront This gives ac cess Co the underside o+ the mixer i f^ sitch ng roar : Loci^ thei.^-p? capacitor from the unused second oscillator and desc-ider it out of the circu t Replace the cover and all will be well Note that the Supplied crystal Is rn channel 2. or "F2n as labeted on the front panel. The unit exhibits excellent stability mot measured) after this modification Should you desire lo use this oscillafor again, merely replace the capacitor, or better yet, switch the crystal m ihe ,JF2" socket.

Photo B shows lhi> main chassis layout First-ctass workmanship is evident herf\ One unique feature ot (he LT23S is lhal unlike other transvf rters.. you need no I ust an attenuaior to cut dc^n the d^ve from your muitimodi1 radio lassuming the maximum you ar^- supplying is about 10 Watisi. Two ! kkJ vaJue ?esistors forrn a 5G-Ohm 10 War: swamping network to ' bur- up ' tne excess power For those- us ing multimodes with adjustable output I recommend turning the drive at 144 MHz all the way down to the minimum of 1 or 2 Watts. The ^wamptftQ network will take care of the excess A clearly marked po tentjometer. "P IN," controls the drive level, and for youf i^irticular radio you should set this pot fully counterclockwise before applying drive. Carefully Increase the sensitiviiy until Jhe 1ront-panel meter just apOLii pms.

This tunngs us to a note regarding ac curacy of that same meter: Using a Bird Model 43 wattmeter 25 VJatt î.1-i.a GHz slug, and QiîlI d'-, dum=T, load, tne measured when ten Watts was indicated on tMe LT235 ; actually & 5 Watts Tn ^ measurement ais made usrng a 14-volt power supply ind when 10 Watts ¡s ic lually measured on the Bird the LÎ23S meter Is pinned io The right The sampling circuit m ine LT23S Usés an HP 2SOO no: carrier diode with a 5Q-0hm terminated coupler lths likely thai the response of other 2800 diodes could vary all over the place, so it s not worth worrying about the accuracy ot the meter |r ■ s help-.ji as a relative output indicator and iF you are really a nit picker, it could be re car prated againsi a laboratory-standard wattmeter

SSB recommends using a t4.5-volt supply for the trarisveru?r How inuch of a difference does this make? With a 13.S-volr suppty, maximum oulput was 7.6 Watts. At 14 volts, it wasB.2. And at 14.5 votts, it was indeed 10 Watts. That's how much? If you are using an external amplifier, the difference between 7 5 and 10 Watts might not cause much consternation, but il you plan on running the ur.r barefoot crank tho output on your suppty up Mosi commer clalfy-mads pov.-er suppi es can e^s ■. be a l ussed for higher output 'tti an - n^-ia, pot or ^ensr between the regulator a d ground.

The rece /er front end *35 tested for noise figure using a Hewietl Packard model 340A noise-fEgure meter. On this equipment, the noise Figure turned out to be 2.0 dB. SSB claims t.8 .JB, so the measurements are close enough for govern-meni work, as rhey say, The HP^340A is about fifteen years ols jnd the discrepancy could exist there It wa^ not possible lo measure (he 1-dB comprt^Sfon poinl as only aboul - 20 dBm of s-gnai could be generated on the available test equip men- This rs a very strong signal for 1296 anj couid be likened to *or*ing a station rtith about 1-kW erp aboui a nait m ie a^a^. or closer No detectable compression occurred a1, this point Similar tests On oth?- SSB ^296 preamps indicate tne actual 1<J8 compression pcim to be about 0 dB or slightly belter, so ^sume thai is the case here

Receiver conversion gain Is specified at 24 dBP The measured value 1 s Ifl dB. *vhich is adequate, bui it would be nice to have Ihe additional 6 dB or so, especially when using an older muttimode 01 two-meter converter with a medtocre front end, In receive the unit consumes 180 mllliamps ol current. When in standby, the value is 350 mJlhamps. and key down with 10 Walts it's 2,5 Amperes. So a small power supply of 3-4 Amps will do the JOE Another nole of caution: The firidf transistors, Phillips BLU39S, are not swr ,votec(ed Be careful not lo abuse them by transmitting into suspect loads ot unknown loads They are not cheap to replace and not easy to come by Under normal operation an swr of 2.1 cm be totaled without difficulty

Well: Enough of (na: Let s shut off the iignal generator, unhook the spectrum analyzer, and engage in some on-the-air tests. The LT23S perform-id admirably in the recent CO WW VHF WPX contest, where the NV60/2 group nelled 1(1 QSOs In Ihe AflRL September VHF GSQ Party, 10 stations were also worked from this QTTH. Reports were of exceptional linearity of the audio waveform, and listening to other LT23S us era confirmed this Speaking of which, it appears to be the hot setup around here in northern New Jersey, as nearly half of the stations I worked claimed to be using one! It doesn't take long, for good news to spread, apparently At Ktae. I use the LT23S to drive a single 1CXI00. yielding aboul 70-80 Waits of outnut. This Feeds 60 leei of '/6 Sptroline ana then drives 4x23 F9FT 23-cm yagis The prevrous setup, using a Microwave Mod ules 1296V144 and SSB PA2510 amplifier, worked well, but the receiver in the LT23S gets me edge, as the noise figure In the MMT 1256/144 is ¿bout 2.5 dB or so. One confusing aspect was learning to wire the antenna ftlay backwards—that is, energized in "Receive" and out in the "Transmit position, i nad to remember ic reave the power switch on. otherwise the ampli her went Into standby with an idling cur rent ol 45 mA a modified Dow Key relay did the trick. The modification consisted of replacing the UHF connectors with type M. since 1 lost 1.5 dB on transmit using The UHF type!

The exciter in both -cases a Kenwood

TR-9000, which has a laiily good from end but can scan memories and change Ire-quencifjs at a very rapid rate. When I heard activity on part of the band, I programmed It into memory and scanned until I heard a station 1 needed during the activity hours. The LT235 does not have rf-detected switching and must be hard-keyed through the RCA jack on the back, A foot switch takes care of the problem, or you can use a keying iacik on your multlmode if it's there I installed an internal reed re lay *n the TR 9000 and trial did the tnck. This was the way to go on t296 during the contests! I had a ball with the LT23S and have one very minor complaint. The earth

{negative) connection on the rear panel suffers from a condition where it comes loose and Moats. Youll try making a se cure fit with the power supply leads arid go crazy. Apparently the binding post comes loose insider and substitution of a iypcial American-made post cures thai problem

All in all, a nice piece of work irom SSB Electronics. The LT23S sells m the S650 price class ana the sole US importer me VHF Shop., 1G S Mountain Boyievara Mountaintop PA 1870?.

Peter Putman KT2B Morris Plains NJ

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