wrote on Sat, 5 May 2007 13:03:32 EDT:
Wayne Lundberg wrote:

Back in the mid 50's on patrol in the Formosa Straights I had daily
challenges to read code being jammed by the Chinese. I knew then, that only
Morse code would be a viable means of communication. Even as we, in the
Navy, were installing the new teletype systems. A single ping of
interference on the teletype signal would cause an error. Then came Single
Sideband, and then digital.

What I'm getting at is what I read about the recent battle between Israel
and the Iranian sponsored terrorists and how they were able to almost
disrupt communications through jamming techniques, but the Israeli's
prevailed through the use of line of sight technology.

Virtually all military radiocomms nowadays uses spread spectrum techniques
to avoid jamming and interference.

For voice an SSB frequency hopper is simple and reliable. Digital stuff
tends to use broadband code sequence modulation.

The military aviation band of 225 to 400 MHz uses conventional
AM but stations there can use a variety of voice-frequency
Keyers* for security. Jamming fast-movers tooling at 0.8 Mach
requires enormous RF power to stay within their range, something
not normally done by all sides. :-) There are "Have Quick"
modifications to military avionics but I don't know for sure what is
public data on that.

* capitalized 'Keyer' refers to its nomenclature and does not
have any direct reference to a morse code keyer.

Land forces can use the AN/PRC-104 family manpack HF
radio which has its carrier manually dialed in. The same
Keyers can be used on that SSB voice frequency radio
as with military aviation. Going operational in 1985, the
"104" is essentially QRP and includes an automatic
antenna tuner side-by-side in the pack to optimize the
whip antenna. Mobile and fixed variants use the same
R/T with external power amplifiers for up to 400 W. The
104 family is the first of the IHFR or Improved High
Frequency Radios for the U.S. Army. A replacement is
due to be selected soon. "Green" radio collectors might
want to start looking for surplus 104s by next year...makes
a good HF transceiver for portable work.

Prime use for all land forces is the 30 to 88 MHz band, FM,
whether on foot, vehicular or airborne. The keystone in that
is the AN/PRC-119 family called SINCGARS (SINgle
Channel Ground Air Radio System). The "119" uses
digitzed voice (even though sent over FM) in-clear, can be
connected to a variety of data terminals for sending-receiving
that. The salient feature of SINCGARS is the ability to both
frequency-hop and/or "scramble" the digitized voice
according to an entered "hopset" done by the operator on
a touch-screen on its front panel; it has no direct manual
carrier frequency controls, only the touch-screen input.
In the FHSS mode the hop rate is ten per second. The
SINCGARS family includes mobile, airborne, and fixed
variants with RF power amplifiers to boost the manpack
R/T Tx output of about 10 W. There are two types of HTs
operational which are SINCGARS format compatible and
Harris Corporation is making a SINCGARS compatible
transceiver for the UK military. The SINCGARS general
format has been adopted by NATO members. The
PRC-119 went operational in 1989, saw some use in the
first Gulf War. A later "SIP" or Sincgars Improvement
Program dropped the PRC-119 size and weight in half.
As of late 2006 the total number of SINCGARS radio
sets hit a milestone of 300,000 units made, the maximum
number of radios of one kind made anywhere, any time,
most produced by ITT division in Fort Wayne, IN.

SINCGARS requires precise timing and updating to
maintain a net contact in full encryption mode. This is
initially provided by a TCXO and further precision of
time-of-day updated by connection to an AN/PSN-11
HT GPS receiver (older 119s) or by an internal GPS
receiver (newer SIP 119s). A nice feature of FHSS
mode is that many networks can be operational within
the same 30 to 88 MHz band without interference.
As to DF-ing techniques in the field, that is extremely
difficult even with sophisticated test equipment; the
encryption code for voice and data is very robust and
cannot be broken in the field for any tactical advantage
of "the other side." To effectively jam them over an
octave of bandwidth requires enormous amounts of
broadband RF power, enough to render "the other
side's" communications useless.

Although a few PRC-119 (old version) have been seen
on eBay for sale, those are only the cases and chassis
frame. Some of the internal ICs are custom with no
commercial replacements. It is not promising for "surplus."

Standardization of the JTRS or Joint Tactical Radio
System is underway by all U.S. military branches. That
uses the entire spectrum from 30 to above 400 MHz, will
be compatible with existing formats and offer the flexibility
of software defined radios of extreme versatility.

All of the preceding information is obtainable from public
sources.
----
While I was in the Army not the USN during the early
1950s, (sending RTTY across the Formosa Straits, not
myself), the encryption method used with old-style
5-level TTY was robust enough that there is no
evidence that any other side was able to break it. It
took direct physical capture of the USS Pueblo and its
TTY encryption equipment to effect an intercept. While
I can't vouch for USN radio equipment of the 1950s,
Army fixed-station RTTY circuits used better-than-
average demodulators for 850 Hz shift RTTY that could
effectively ignore transient noise spikes on receive.

73, Len AF6AY