[ARC5] 18S-4 Now and Later

[Mike Morrow]

I'll throw in some 18S-4 and -4A background info.

Frequency Coverage:  2000 to 18500 kHz in ten channels.  Two crystals may be
installed per channel, doubling the number of individual frequencies that may
be covered, but the two frequencies assigned withing each channel must be within
one percent of each other.  Receiver frequency and transmitter frequency are
both set by the same crystal.  A maximum of 20 crystals may be installed.

The receiver IF is 455 kHz.

The 18S-4 and the 18S-4A are identical except for the Collins 455 kHz mechanical
filter used on the output of the receiver mixer into the IF chain.

The 18S-4 and -4A are very early 1950s vintage.  The AN/ARC-59 JAN  nomenclature
appears to be from the mid-1950s.  The USAF used both commercial 18S-4 and (later)
JAN AN/ARC-59 (18S-4A) systems.  

Side note:  Later the USAF used the commercial 618S-1 like it had used the 18S-4,
with NO JAN nomenclature being assigned.  The USN's RT-311/ARC-38, for which there
is *no* exact Collins commercial equivalent, is *very* different from the 618S-1.


Component Listing                Commercial/Military     Military AN/ARC-59
1. Receiver-Transmitter          18S-4 or 18S-4A         RT-380/AR (JAN 18S-4A)
2. Antenna Preset Match Network  180K-3                  CU-527/AR
       or                              
   Antenna Automatic Tuner       180L-2 or -3            Not Used
3. Remote Control Box            314S-4                  Not Used
       or
   Remote Control Panel          314S-6                  C-1982/AR
4. Receiver-Transmitter Mount    350C-5                  MT-1718/AR
5. Antenna Unit Mount            350D-3                  MT-1719/AR

These systems require only 28 vdc for all components UNLESS a commercial 180L-3
*automatic* antenna tuner is to be used.  The 180L-3 requires 115 vac, 400 Hz.

The website http://www.aviation-radio.com/collins18S4.html that has been cited
has very good pictures of the 350C-5 RT mount and the 180M-1 matching network
test unit used to help preset the 180K-3 antenna preset matching network.  I
suspect these are found only by great effort and even greater luck.  A control
panel is shown with the statement "control box is not original for this setup".
However, the item shown is either a commercial 314S-6 or a military C-1982/AR,
which are totally appropriate.  Perhaps the earlier original 314S-4 *control box"
is desired.

I suspect that it would be easier to both find and use the 180L-2 or 180L-3
than it would ever be to find a 180K-3 or CU-527/AR.  It would also be much
less trouble to use, compared to the preset CU-527/AR.  There are military
equivalents to the 180L auto tuners that show up fairly often:

1.  The CU-991/AR is essentially identical to the commercial 180L-2, including
the same Cannon-type control connector.  The CU-991/AR was used with the
AN/ARC-94 and -102 until the CU-1669/AR became available.

2.  The CU-351/AR is essentially identical to the commercial 180L-3, except that
it uses an Amphenol-type control connector in place of the Cannon-type of the
180L-3.  The CU-351/AR was used with the AN/ARC-38 and -38A.

I have a commercial 18S-4A with a full set of commercial aviation crystals
installed.  As Jeep has mentioned, this set could be tremendous work to place
on any specific frequency unless the sets of coils for specific ranges are available.
After that the PA output network capacitance must be set with a set of fixed caps
for each channel.  Then...the 180K-3 (or CU-527/AR) laboriously preset antenna
matching network would also need adjustment.  I can understand why this set would
have been very much hated by the radio techs anytime a channel's operating frequency
had to be changed.  I don't think there's ever been a set requires as much effort
and custom material, except perhaps the AN/ARC-9 (Bendix RTA-1B), which uses a
very similar approach.  But the RTA-1B is an early 1940s design...the 18S-4 has
no such excuse!

I doubt that the USAF, the main user of the 18S-4 and the AN/ARC-59, ever used
the USN's AN/ARC-38 or -38A unless it was in an aircraft that had once been a USN
aircraft.  The successor in USAF service would have been one of those many 618S-1
units found with a "U.S." on the bottom of the name plate.  The USAF bought two
distinct 1-1/2 ATR Collins sets.  So did the USN.  All four have major differences.
         
           USAF Collins HF Sets
618S-1          2 to 25 MHz   AM/CW       144 channels set by crystal.
618S-1/MC       2 to 25 MHz   USB/AMe/CW  35250 channels set by control panel
                                          CPC-1 using four *alphabetic* dials
                                          (no direct frequency entry).
           USN Collins HF Sets
RT-311/ARC-38   2 to 25 MHz   AM/CE       35250 channels set by control panel
                                          using six *alphanumeric* dials
                                          (no direct frequency entry).
RT-594/ARC-38A  2 to 25 MHz   USB/AMe/CW  35250 channels set by control panel
(RCA designed USB mod)                    using six *alphanumeric* dials
                                          (no direct frequency entry).

There seems to be a lot of similarity between the USAF 618S-1/MC and the
USN RT-594/ARC-38A in the table above.  But the upgrade from the 618S-1 to
the 618S-1/MC is a Collins design while the upgrade from the RT-311/ARC-38
to the RT-594/ARC-38A is a RCA design.  They are quite different in detail,
though both of the awkward non-direct frequency entry systems of the 618S-1/MC
and the RT-594/ARC-38A are pure Collins.