We are not just doing this because it works better than anything FAA approved that you can buy.  I have, personally, lost track of the number of times, flying to College Station to teach, daily, where I was IMC under positive control, and ATC put another airplane close enough for me to read a serial number off of one of its components.  "Radar" just doesn't work, period.  Neither the procedures in use, nor the equipment.  Its a matter of you being responsible to save your own life.

Of course, we do have a better solution, but the FAA will not act on the certification - they claim no money in the budget to come out.  So...
 

Here is one deal, what use to put this old DME in the panel that you don't even look at (because you have your moving map, and it measures distance).  In the interim, it might be a long while, fit that problem, as a solution, to the problem of what to do because the FAA still hasn't certified TailLight (which will display "there is each and every airplane within 30 miles of you" and where and how it is moving on any display), and everybody doesn't yet have the TailLight transponder chip put into their transponders, to fix the transponder from "C74c is wrong".  Need an interim solution to problem "B", which you can just go use, that you make with small changes to problem "A", and that works with target transponders as the FAA has messed them up.  Below continues the "Heath Kit" instructions for do-it-yourself.

That DME is an AWACs radar, really, for ATCRBS transponder equipped targets, but just has some minor problems preventing it from working that way.  The instructions below are about removing those preventers, but doing that will not allow it to work as a DME no more (did you want one anyway?).
 

Here is the other deal, for a transponder.  Just exchange the transmitter and receiver frequencies, and put a custom IC in there to do the "DME" job, just with a different pulse modulator, pulse width discriminator, pulse gaps, reply dwell, ...  This aint a do-it-yourself, can only be done with our custom chip, so we might as well do the whole thing.

It costs the same as an AT-150 transponder.  It's actually cheaper to build, because almost everything in there, except the power supplies, comes out, and our IC chip goes in.  New cavity.  And the TailLight receiver (you also get the AIS-P receiver, so you don't need the following.)

Now, if you happen to have a dual Narco Transponder installation in your airplane, slide those two transponders out, and slide in our converted (contains our "fixkit") also AIS-P talking transponder (the upper box) and this here TCAS (the lower box).  That gives you terrain avoidance, obstruction avoidance, runway incursion avoidance, collision avoidance from other TailLight transponder equipped aircraft, and the TexasRanger protection from plain old FAA goofed up transponders.  All you need to do is hook up the GPS to your display device and the converted transponder, and the suppression wire between the TCAS and the transponder.  QED:

That bottom unit is the TexasRanger TCAS with the TailLight receiver.

In case you get ramp checked, there are two options:  The first has 7 segment displays that live under the spin wheels, that show through when the unit is working.  The second has the spin wheels removed, and the 7 segment displays are mounted there.  Just say it is the new Narco panel, so you have the original plus the new one.  The spin wheel knobs do nothing.  On the function switch, OFF is, well, you know.  SBY lights the ident "warm up lamp" and displays 1200.  Use that to impress the inspector, is he gets curious about the second option.  Don't worry, it isn't a real transponder.  You can also test the segments by pushing the ident button to get 8888 (don't show the inspector).  ON is Mode 3A (squawk) interrogation, and the display shows range to nearest target, the bulb shows target presence.  If you push the button, display shows squawk code of nearest target, and the bulb shows if he is identing.  ALT is Mode 3C (altitude) interrogation, and the display shows range to nearest target, the bulb shows target presence.  If you push the button, display shows pressure altitude of nearest target, and the bulb goes out.  There is a sonalert that honks if the nearest target is in attack mode (range is closing), doesn't honk if nearest target is a friendly (range receding).  The unit automatically follows the closest target to your aircraft, even if that means exchanging airplanes.  However, just to make sure, if you want to, you can manually start the scan over again (that is what the spring return TST does) to look for the nearest target (in case one just turned on at that airport under you).  It's like the "clear display" button on a storm scope - allows you to start over.
 
 
 

Now, for those Alphius KN-60C conversion instructions I promised:

The interrogation is on, twiddle the channel selector frequency, and it isn't talking to transponders.  Well, empty the transmitter crystal carrousel, use the crystal marked 66.80 MHz, which is really 66.8017 MHz, solder it in permanently at Y101, to get 1/2 of the local and transmit beat oscillator.  Special order one other crystal, on 19.0316 MHz, solder it in permanently at Y121, to get the other 1/2 of the beat oscillator.  Poof, always transmits on 1030 MHz to transponders, crystal controlled; always has local oscillator for 1090 MHz received reply from transponders, crystal controlled (they are the same beat oscillator).  AWACs transmitter frequency.

The DME receiver IF, intermediate frequency amplifier, where most of the receiver gain is, is on 63 MHz.  Because VorTac stations always have a 63 MHz offset between receiver and transmitter.  That way, the receiver local oscillator is also the transmit oscillator (save the cost of another oscillator chain).  Retune the receiver IF to 60 MHz (3 coils).  That way, you can keep the same trick and use it on transponders.  Poof, always receives on 1090 MHz, crystal controlled.  The local oscillator (the first one if the design is dual conversion) is already at 1030 MHz.  AWACs receiver.

The King KN-60C DME is a dual conversion receiver design (most DMEs are, to eliminate the image frequency from being received, which would now be at 970 MHz).  Either retune the second IF over 3 MHz (this is not practical at the 8.47 MHz 2nd IF strip frequency, where most of the receiver Q is), or special order a second local oscillator crystal 3 MHz from the one you now have (don't have to move the 2nd IF strip frequency).  Know what, the 68.4683 MHz crystal removed from the carrousel can be used for Y301 (don't need to special order a new one, close enough to 68.47 MHz).  Remove old, solder in borrowed (no longer need it for its original job).  KN-60C dual conversion AWACs receiver.

The whole frequency conversion of the receiver and transmitter, at the cost of one 19.0316 MHz special order crystal, nothing else but retuning labor.  Poof, AWACs/TCAS R/T unit.

Also, in the KN-60C, get rid of those "slide wire" inductors - unreliable.  Add three turns to each of three coils.  Tune.  For now, put up with one slide wire coil, solder it in place, or make sure the wiper is at least "clean".  When I get the time, I'll figure out what L117//C136 is doing, and replace the slide wire coil part.

The only thing left is to clean up the pulse processing:

VorTac pulses coming in the DME receiver were 3.5 microsecond duration, the transponder pulses will be 0.45 microsecond.  Stagger tune the 8.47 MHz IF stages, for a wider bandwidth.  Get it as wide as possible, then crank up the IF gain in the AGC circuit at R346 to make up for some of the loss of sensitivity.  Don't need the full DME receiver sensitivity, can suffer some S/N degradation, because transponder transmitters are high power, will also be very close when you need to know.

The VorTac pulse pairs coming in the DME receiver were spaced 12 microseconds.  The old analog signal processing in the latter part of the second IF strip will not identify transponder replies to interrogations.

One option is to use our Actel FPGA on our little circuit board, to replace and improve the detector analog delay line, with a digital signal processor transponder reply detector.  If you do that, it can also provide the data and drive a display of the transponder information.  We thought it cool to see the squawk code and altitude and ident status of the target aircraft, but we soon found out it was usually 1200, and doesn't appear on the ATC "radar", because they remove 1200 squawks from the controller's display in IFR IMC (else they get too busy, and they don't reliably see 1200s anyway).  ATC gets upset if you tell them about stuff that they don't see.

You don't need the special Actel FPGA component to complete your AWACs project, anyway.  We had removed Q308 and replaced DL301 with a digital delay and digital coincidence detector and digital 3.5 microsecond coincidence output pulse extender.  That, and the intermediate registers to capture the data, was that special Actel FPGA part.  But you can keep what is there, and just remove and exchange DL301 for any form of analog 20.3 microsecond delay line.  Might should also put a monostable aft of TP303, before the range board, so that the coincidence pulse is the expected 3.5 microsecond duration rather than just 0.45 microsecond (that old range board it feeds aint none to swift [kind of like the FAA]).

The interrogation pulses going out our transmitter, now on the right frequency to talk to transponders, will be 3.5 microsecond duration.  That isn't going to work well, because some transponders might ignore the interrogation.  Change the modulator circuit to narrow them down to 0.8 microsecond.  Keeps the "long pulse" detector in the target transponder happy with our now standard interrogation (it thinks we are a real "radar" or "TCAS").

The interrogation pulse pairs going out our transmitter, now on the right frequency and right pulse width to talk to transponders, will still be spaced 12 microsecond.  Change the modulator circuit to stretch that to 21 microsecond, else shorten to 8 microsecond, leading edge to leading edge.  You want to be able to read the altitude from your target (optional), so favor 21 (which didn't work with the KN-60C).  Or 8 microseconds for squawk and ident (which did).  Maybe two of the second, followed by one of the first, and use that recovered reply code to run a display to say target squawk and target altitude and ident status (well, that was the option we had in mind, when we started; why we did the Actel chip).

That's about it.  The DME will "slide" this timing window over range from us, looking for almost always transponder replies to our interrogations, as it interrogates at 150 per second (search mode).  When it finds them, it will "lock" that timing window on the almost always reply, and shift to a 25 per second interrogation rate (track mode).  It already has the circuitry to handle "what if I don't get a reply to every interrogation".  There is a delay to hold the timing window, before it goes back into search mode from track mode.  It already has the circuitry to handle "what if I get replies to not my interrogations".  DMEs are built to put up with a string of asynchronous "radial determination" pulse pairs, because it was listening to a VorTac.  So, your system will work in a heavy radar environment just fine.  The timing window derives distance to target, and there is already circuitry in the KN-60C to feed a meter for that.  Just need correct for a transponder dwell time of 3 microseconds, instead of a VorTac dwell time of 50 microseconds (adjust the meter zero pot setting).

Suggest add a momentary button to the front panel, to allow the pilot to double check for a sudden appearance of a close in target (just took off under you as you overfly an airport).  Normally not necessary, when flying cross country, not bad to have flying near airports.  The equipment normally changes lock from one target to another in order to follow the closer target, when two exchange range threats, regardless of azimuth.  If one target overtakes another target in distance from you, regardless of direction to each, the TexasRanger will always switch to track the closer target, to follow the most danger.  It's already built that way as a DME.  Just parallel your front panel momentary push button with the "channel change" momentary contacts.

The idea is the same market there is for the original Proximity Detector (prevented by incomplete FAA certification at the Fort Worth region for 12 years now) - warn of a close target, plus say range to target.  Except the TexasRanger is an active interrogator.  Option for identity and altitude of target and ident status of the target (on a display by using our FPGA), not really necessary.

Change the time scale to recycle the search at a maximum range of, say, 4 miles - Don't need to be listening 100 miles out.  Another pot adjustment.  Maybe put a label on the meter to remind you of range marks.  The FAA ATC is supposed to always keep all targets 5 miles away from you, so it should never alert (just keep sweeping).  You get a range indication of every ATC failure with a closest target range lock.

Maybe add a horn - sonalert, selected for slightly below the threshold of pain.  When it locks a target, honk the horn to alert the pilot.  Add an On/Off switch on the sonalert.  Horn can get tiring in a busy area, pilot can occasionally glance at the meter/display if turned off the horn.

The KN-60C is particularly well suited to the conversion, has a 40 watt transmitter (because it has vacuum tubes), the needle "sweeps" when there is no target lock, and indicates range when there is one.

Lookie here, "Texas Ranger" (I live in Texas, this thing ranges the closest proximate target) for the cost of your old DME that you didn't use anymore anyway.

Though interrogation rate is low, and though interrogation power level is low, this thing will generate interference with ATCRBS (ATC "radar").  It pollutes 1030 MHz with more interrogations, and that causes 1090 MHz to be polluted with more replies from everything around you to those extra interrogations you made, just like TCAS.  As a DME, it claimed to have a 100 mile range.  Only one thing can work at the same time, TCAS or ADS-B or radar, so...  In testing, we have noticed "jamming" effects from this equipment to "radar", just like TCAS & ADS-B does (but TailLight doesn't).

Now, mathematically, we can easily prove that the deleterious effect to the National Airspace System ATC "radar" is less than that produced by products such as the B.F.Goodrich "Skywatch", a $25,000 TCAS.  This is done by showing its numbers and interrogation rate as compared to the TexasRanger behavior.  The Skywatch numbers have been added to the NAS through FAA certification.  So, it's only a "brand name" difference, but all are jamming to ATC "radar".  Also, that Skywatch jamming level is not anywhere near the extent of radar jamming that the mode S based airline TCAS I & II systems interfere with ATCRBS.  And that airline TCAS interference level is much lower than what ADS-B will interfere with ATCRBS once it is fielded.  All tremendously worse, all FAA certified, all flying in the NAS legally.  So, TexasRanger is much less, so what the...

Now, the FAA might cause you trouble, anyway.  They had M.I.T. Lincoln Labs conduct a study, to have the official finding officially registered, that all of this jamming on 1030 MHz and 1090 MHz from TCAS has absolutely no effect on the ATCRBS packets getting through.  That was necessary, for their certifications of TCAS I and TCAS II, even though it is a technical farce.  So, if you get caught, and are accused of jamming "radar", just get that official FAA finding through the Freedom of Information Act, and use it in your defense in court (you are doing much less, and the more doers cause absolutely no problem, so, therefore...).  However, always remember, what goes on in court has absolutely nothing to do with reality.

It's your life, and you need to protect it.  "Radar" doesn't work, the FAA killed C74c, then threw up jammers, you can't depend on it, that is adequately documented by FAA ATC employees, even official FAA documents from FAA engineering at Atlantic City.  The FAA already says they can live with the unannounced coast mode, dolts haven't even noticed that ATC radar has been broke for quite a while due to the C74c TSO that they broke, not likely they would even notice the difference in jamming (horrendously bad going to ever so slightly worse with TexasRangers in the air).

And, the FAA can always recover by completing the certification on AIS-P.  TailLight has been in FAA certification for years, giving you a moving map and collision avoidance and terrain avoidance and obstruction avoidance and runway incursion avoidance, for a $500 transponder mod or a same usual transponder price for a new transponder (also fixes the broke C74c, so "radar" gets fixed), and you don't even have a need to do transponder biennials to keep it working; and for a $500 receiver that feeds your choice of Windows based display (if you want a display).  Wouldn't you really rather have all of that, instead of just a meter and horn (so the TexasRanger interference to ATCRBS could go away)?  So it shouldn't take the FAA long to finish certification, once they wake up (to Skywatch & even more TCAS & ADS-B) and put a priority on it.  Remember, TailLight is cheaper and generates even less interference to ATCRBS (and also fixes it, so that you have dual independent collision avoidance systems), and you would want all of that more, anyway, if they let you have the choice.  It's their fault.

Follows is the general idea of the technical data, specific to the conversion of the King KN-60C DME into a TexasRanger aircraft active interrogator AWACs ATCRBS transponder range finder.  This page is old, and was created before the development process got fully under way (we learned more in the process of the doing).  As we progressed, and from builders using these instructions, we find better ways to do things, so, much of the below is earlier than optimal, but still gives the valid ideas and methods.  Rather than have our webbermeister constantly update with new improved procedures and scope trace data, we put downloads of detailed construction.doc and tuning.doc files for MicroSoft Word for Windows, complete with current Tektronix THS-730A scope traces captured through Wavestar software, procedures and techniques that we can keep editing.  We reserve the right to update, as more builders provide better suggestions and data.  If you really want to know the best and latest.  To complete the tuning of your kit, you will need an oscilloscope (suggest 200 MHz) and a signal generator and spectrum analyzer (capable of 8.47 and 60 and 1030 and 1090 MHz).  If you and your avionics shop don't have these, visit a local amateur radio enthusiast.  Having the King KN-60C DME service manual also helps (your avionics shop should have that).
 
 

Step 1: Final Amp modulator needs pulse spacing at 8 usec
    We can't do the mode 3C interrogation (we find out later)

Transmitter "RF output" pulse, which is also the CR206 voltage
  Anode to ground
    Original design of "12 usec with 3.5 usec" is a DME
        C215 = 0.0033 uF
        C216 = 0.033 uF
        L207 = 1-2 uH
        C217 = 0.22 uF
        C218 = 0.33 uF
           search
 

That does the job
    Modified design of "8 usec" pulse spacing correction
        C215 = 0.0033 uF
   C216 = 0.015 uF
        L207 = 1-2 uH
        C217 = 0.33 uF
        C218 = 0.33 uF
           search
 

Step 2: Why is the second pulse "anemic" in this trace?
    Try to keep 1030 local oscillator chain "pumped" for 28 usec
        Goes into high power "driver mode" when about to interrogate
        Enough time to cover a transponder Mode 3C interrogation, plus

Transmitter "hold power up" pulse, which is also the suppression pulse
    blu/wht wire of modulator to ground
    Original design of "20 usec" to cover DME 12 + 3.5 + tolerance
        C203 = 0.33 uF
        C204 = 0.33
        C205 = 0.22
        C206 = 0.22
        C207 = 0.33
           top is lock on
           bottom is search
           circuit is obviously optimized for 12 usec pulse spacing
           second pulse is "in the dip" (why anemic)
 

New                              Comparison @ search

Modified design for "28 usec" so we could do a mode 3C interrogation
    C203 = 0.33 uF                 Elevated trough slightly
    C204 = 0.33                    Extended pulse length slightly
    C205 = 0.33                    This aint a gonna do it
    C206 = 0.33                      (!@#$%!)
    C207 = 0.33
        top is lock on
        bottom is search

L208 is optimized for 12 usec, adding cap doesn't do it
    Can't fix this without mod L208, which wouldn't fit
    Can't fix this without add another cap, which wouldn't fit
    Can't run KN-60C at 21 usec (P3 pulse wouldn't exist)
    Can't get target to say its altitude using KN-60C R/T mod
        (Why we did #1 at 8 usec)
 

Let's try to balance this drive for the shorter 8 usec interrogation

Modified design to "8 usec" - settle for squawk interrogation
    L208 short middle coil
    C203 = 0.33 uF
    C204 = 0.33
    C205 = 0.33
    C206 = 0.33
    C207 = 0.33
        top is lock on
        bottom is search
           L208 is now optimized for 8 usec, by remove middle section
           Gonna have a real nice interrogator power out
 

Let's look at relative timing of interrogation within "drive" window

Step 3: Final Amp pulse width for 0.8 usec
    Now we need to fix the transponder interrogation pulse width

Let's look at actual RF power amp plate voltage
    It's 40 watts only because it has a vacuum tube

  P P P    P P P

1800 Vdc plate line to 10 meg to 1 k to ground, 1k bypassed with 3.3 pF
    Expected two thin pulses
    Got 6 thin pulses which combine to make two thick pulses

It looks like this L201-CR206 is intended to keep pulsing V101 for 3.5 usec
    Q keeps V101 oscillating with "pulse feeds" to restore energy

We fix:

     P1       P3                    P1        P3

Remove C208, C212, short where removed L203, L206
Keep 50 ohm transmission delay line C209, L204, C210, L205, C211
    L201 = 3.3 uH
  R206 = 6.8 ohm
  L202 = 10 uH
  L203 = 3.3 uH   removed shorted
  C208 = 0.047 uF removed Oscillator at 404 KHz
  C209 = 0.1 uF
  L204 = 4.7 uH   50 ohm
  C210 = 0.1 uF   transmission
  L205 = 4.7 uH   line delay
  C211 = 0.1 uF
  L206 = 3.3 uH   removed shorted
  C212 = 0.047 uF removed Oscillator at 404 KHz
    search
 

A closer look
    Boost voltage and Final Amp plate voltage, each P1 P3 pulses

 P1                              P3 8 usec later

Timing looking good

Transmitter "hold power up" pulse, which is also the suppression pulse
    blu/wht wire of modulator to ground
    the "flat" line
1800 Vdc plate to 10 meg to 1 k to ground, 1k bypassed with 3.3 pF
    the "pulse" line containing RF cycles

Pulses nearly same amplitude
Pulses nearly same 0.8 usec at "power width"
Pulses nearly spaced 8.0 usec for mode 3A interrogation

R/T mod complete, add chip with its PCB to complete the digital logic, tune some pots on the range board, remove the speed board.  QED.

Certification is by FTFAA method WTDKWHT.