PRC-174 HF Manpack Radio Set

I recently obtained a veteran RT-936/PRC-174 HF manpack radio from a fellow Ham (Thanks!). These were made by Tadiran from a design clearly originating from Hughes, makers of the AN/PRC-74 and AN/PRC-104, radios with similar capabilities and applications.

What I’ve learned and observed so far. I like it.

PRC-174 mounted on an ALICE Pack frame

Some basic specifications:

The set provides USB, LSB, AME, Data and CW comms with 20 watts output (5 watts on AME) . It is fully synthesized from 2 – 30 mc in 100 cps steps and is capable of being operated while being carried. In “Wide” CW and SSB the receiver is 3.5 kc wide, 6 kc in AME, 500 cps in the “Narrow” CW mode. Nice. In the AM (equivalent) mode it runs USB with the carrier re-inserted.

The receiver sensitivity is specified at 0.7 micro volts for a 10 db SINAD. This example is better than that, producing a perfectly usable CW signal with an input below 0.1 micro volts, a measurement that challenged the attenuator shielding in my URM-25 signal generator during the test.

The PRC-174 was powered by a nominal 25 volt, 8 AH NiCad or a 28 volt, 6 AH Lithium Ion battery pack. With a 26.0 volt primary power supply the receiver draw is about 160 ma. The transmitter draws about 2.9 Amps key down, delivering 20 watts into 50 ohms, on CW. That is about a 27% DC-RF power conversion efficiency. (Measured at 3.550 mc.)

Original battery boxes/assemblies are probably rare. This example came with a nicely made riveted aluminum box that fits well with the proper clamps. However it does not seat with the radio’s weather proofing gasket. It is not watertight but is otherwise very serviceable for my purposes. Did someone have a bunch of these professionally made for an after-market? Nice!

This one had an added battery disconnect switch and a mounted 28 volt Mil connector to power it externally or to charge the internal pack.

Here is the box that came with my set. I outfitted it with two 12V, 5 AH Sealed Lead-Calcium batteries in series. Pretty bullet proof; it doesn’t need complex, fussy charging management and is suitable for field expedient charging, including simple solar. Heavy-ish, but together they are 2.3 pounds lighter than the issued TN-1770 NiCad pack. I don’t plan on carrying it very far up the mountain. I’ll radio ahead……

PRC-174 Battery Box

I included an inline fuse holder sized for an auto (low series resistance in a 12 volt system) Mini blade fuse. Five amps is about right, there’s room for spares in here. A previous owner had removed the original radio connector and replaced it, and the battery connectors, with Anderson Power Pole’s. They mounted 3, clipped together for 28 volt service to physically preclude accidentally connecting up some 12 volt load. Good idea. Dummy Proof!

The radio has some kind of battery Save mode which appears at first glance to be a Squelch function. It does not seem to work, I need to investigate/adjust.

The main power switch selects either Receive only or Transmit/Receive (in either LSB or USB). The Receive-only mode is a simple countermeasure against the operator inadvertently keying the transmitter while working under Radio Silence restrictions. Power consumption is the same with either setting.

The PRC-174 uses the standard U.S. peripherals from the VRC-12 and later series radios.

These include the handset, headphones, mic, AT-271 antenna/spring and a modified LS-454 external speaker, plus a CW key, all using the U-229 connectors. The RT (less 9 lb NiCad battery) weighs 10.8 pounds, not too bad for its capabilities.

Note the Whip mount, below. They are apparently somewhat rare; people have been making DIY mounts for their pack sets. I was fortunate to get this one with the radio.

PRC-174 Front Panel Arrangement

A brief History/Speculation:

I have found no online information about the PRC-174 development or specifically its deployment. Apparently all known examples are reportedly from Israeli surplus. Maybe not. There is a PRC-174@Groups.io user site on the Web that has some good current-user information posted.

In the late 1970’s the U.S. needed an “Improved HF Radio” (IHFR), ostensibly to replace the AN/PRC-70 and 74. This need was met with the Hughes AN/PRC-104, a great, long-lived radio/system. I don’t know what the contract competition was at the time.  It reportedly included this radio, also from Hughes, and possibly what became the export PRC-515, designed by Collins Canada.

I’m guessing that Hughes had a “company” model number during the competition much like the AN/PRC-74 which started out as the HC-162. The PRC-174 did not begin with AN/xyz as that designation goes to fielded “official” U.S. equipment with an associated NSN. 

So I further guess that Hughes named their IHFR “failure” as the PRC-174 for potential export. This to denote its father being the Hughes AN/PRC-74 which pre-dated it by about 15 years.  (They are very different radios but with similar capabilities.)

Did Hughes also build some for domestic use or sale to other allies? Anyone know? More research needed here.

The ID tag is missing from this example, no serial number. The mother board sports a hand written “PRC-174 2998”. My buddy has a PRC-174S; the tag label is in Hebrew but the panel and everything inside is in English…

Hughes probably needed a State Department license to allow them to export the design to the Israeli’s to build them at Tadiran for use in the IDF. They did a nice job.

Then Israel may have kept the PRC-174 designation (not an AN/PRC-174).  I haven’t found any info about who else may have used them.  The IDF also used it as part of their HF-700 series, with this as the basic radio.

The HF-700 system included a 100 or 400 watt amplifier/shock mount for vehicular use.  Note the big multi pin connector on the radio front panel for connection to a wired, remote control unit over field wire. This “Hex” pin arrangement seems to be incompatible with the high power amplifier system via a dogbone jumper. Why?

I don’t know how many sets Tadiran made. The PRC-174 or the HF-700 don’t appear in the 1994-95 Janes Military Communications directory.

Design Details:

It is built with a motherboard and plug-in modules. The PRC-174 is VERY complex and compact. I dread ever having to troubleshoot it. A very elaborate test set was available for them.

The August 1976 Technical Manual, OM-107-10, is comprehensive but the one I have from Online sources was an OCR scan of the original and a lot of detail was lost in the translation, especially the schematics. A Maintenance Manual, MA-2124-09503-00, was issued in July 1986 which is pretty good.

PRC-174 Internal Module layout
PRC-174 Motherboard

A portable manpack with a 10′ whip antenna moving around in a constantly changing environment on foot presents a challenge for antenna matching.

The design includes an automatic antenna L-C-L “TEE” matching system that works quite well. After an initial automatic tune initiated by a frequency change, PTT or CW Key closure, the ATU then transitions into a continuous “monitoring” mode.

It then samples the antenna VSWR and then re-tunes the ATU if the measured VSWR exceeds 3:1, reducing it down to 1.5:1. With every transmission. In less than 200 milliseconds. Transparent to the operator. Probably hard to kill it; the Specs say transmitting into an Open or Short will not harm the PA.

PRC-174 Whip Antenna Mount

The detachable whip antenna mount seen above uses a clever design. The mount includes 2 magnets* that signal to the radio that the whip mount is installed. This causes the radio to route the T/R signals from the “Dipole” BNC connector on the front panel to the whip connection. Remove the mount, T/R signals revert back to the BNC connector. No compromise of the environmental protection. Good design.

(* This mount was initially intermittent in signalling its presence to the radio; the magnets had become weak with age. I re-magnetized them with a strong Neodymium magnet, now back in service.)

Like most SSB radios that also work on CW, it generates a suppressed carrier but then drives the mic circuits with a 1 kc tone (non-adjustable) in the lower sideband. This produces a CW signal 1 kc from the suppressed carrier freq that shows in the window display. 

Example: To put a CW signal on 7049 kc you enter 7050.  Then, when you are tuned to 7050 and listening to a signal on 7049, it sounds like a 1 kc tone, a little high for my liking, I prefer about 800 cps but it’s fine as-is…

The front panel includes a 5 LED display. This indicates received and transmitted signal strength, modulation, battery voltage, low voltage alarm, tuning and a tune-fault indication.

PRC-174 Frequency Select

The frequency is entered by push-button. Each push of each digit increments the frequency “up” by one. Not a “band cruiser”, your finger will hurt. These switches look to be the same ones Hughes used in the early PRC-104’s. The switches include integral red illumination lamps, very dim, unless in total darkness with dark-adapted eyes. Bring a flashlight.

Control labels are phosphorescent, no Radium or Tritium. It works.

This example has component date codes ranging from 1980 to 1984, including some with a JAN designation. Almost all the bipolar transistors except the PA and a few PNP types are the ubiquitous 2N2222A, a silicon NPN workhorse – which simplified design and logistics. Many FET’s and latching relays.

There is a lot of standard 4000 series CMOS digital logic on board in the synthesizer and control systems. It does incorporate an EPROM which sets the initial conditions to configure the antenna matching logic.

Thankfully there are no unobtanium microscopic microprocessors or a fragile LCD display waiting to fail. It should be repairable long-term with available parts and not become a shelf queen or a wheel chock.

As received by me, this example was putting out almost 35 watts CW and SSB PEP into 50 ohms. Not wanting to provoke the PA Transistor Gods, I adjusted the ALC circuit to reduce the output back to the specified 20 watts. But it’s good to know that 20 watts is a rather conservative specification. The batteries will also be happier.

The PRC-174S: A variant of this radio was produced with the “S” denoting a digital Squelch system. That variant included some other improvements as well. A panel applique was added to the Mode switch to indicate the new functionality. See below:

PRC-174S Version – Digital Squelch

Not much information out there regarding its use or systems integration but it transmits an annoying high-pitched coded data stream along with the audio. My buddy has one, reports were that it was annoying to listen to as we were initially evaluating his radio. So we disabled that function. A simple “fix” was published online at N3OC’s nice website:

Just adding one ground wire jumper (blue Kynar wire above) on the mother board disabled it; back to “normal”. Note the component designations etched into the copper. Nice!

Initial Field Trials:

Upon arrival a quick evaluation of my new set was in order. Fortunately Winter Field Day 2021 was on the sked so I tested it outside mounted on an ALICE pack frame with cargo shelf.

At the time, and absent a proper 26 volt battery system that fits in the attached box, I wired a pair of 12 volt 8AH SLA batteries in series seen in the external butt pack below. They worked great but at nearly 11 pounds, heavy! Definitely Fixed Portable on this Op.

PRC-174 with external SLA batteries and Whip antenna

As a lifelong, and almost exclusively a CW operator, I appreciate its performance; the 500 cps NCW filter is quite effective if needed. A J-45 Knee Key with connector adapter made it happen. Those H-251A/U headphones also kept my ears warm on Winter Field Day. Win Win!

I took advantage of my kind of field ops (I don’t “do” contests) that weekend to evaluate it while there were lots of like-minded portable stations to be had.  I worked a bunch of stations on 40 meters CW plus the MRCG Net on 3985 kc.  Good signal reports ensued from CA, OR, WA, NV and CO. It’s about 4db more powerful than my usual field set: the AN/TRC-77.

While running CW contacts on 7050 kc with the whip, I connected a 32′ “drag wire” to the ground post and laid it on the dry ground. As a quarter wavelength, low impedance “RF Return” this produced a noticeable improvement in the transmitted field strength. Not surprising.

The ME-61/GRC FSM is pretty handy. Place it far enough away from the radio/antenna that you need to use binoculars to read the meter while tuning/testing. A short piece of hookup wire with an alligator clip to the antenna improves its sensitivity. Hint: Since the lid is non-removable I removed the chassis and rotated it 180 degrees and reinstalled it as shown. Much more convenient to use it that way.

PRC-174 Drag Wire field strength improvement

Of course, operating this radio in any mode besides humping it as a manpack would best be done with a dipole or some other wire antenna versus the simple 10′ unloaded whip. Like then replacing the whip with another 32′ length of wire in the trees making it a Hasty Dipole. That would be a substantially better antenna system for fixed-portable ops.

The radio fits nicely in the main compartment of an ALICE Medium pack, on or off the frame. It will hold all the necessary accessories in the pockets for field ops. Including that non-issued LS-454 speaker which is handy at an FOB/LZ.

PRC-174 in an ALICE Medium pack

The MK-1742 dipole system seen above is the same one as provided with the AN/PRC-74. The radio set deployment drawing in the manual showing the set connected to a slant wire out to a tree is identical to the sketch in the AN/PRC–74 manual, further convincing evidence of its Hughes heritage.

The PRC-174 can also be strapped directly to an ALICE pack frame, the cargo shelf makes it self supporting. As a tall, thin set it would have benefited from a pair of fold-out support legs on the battery box(es) like the AN/PRC-10 has. Probably a weight/cost consideration. The fully rotatable whip mount takes care of the problem; just lay the radio down or leave it upright on the ALICE frame.

With an M-80 dynamic mic and headset or an H-189/250 handset plus an AT-271 whip you’re “Good To Go”. Get to the FOB and rig the dipole, commence operations. Don’t forget the little solar panel.

Aside from my VHF AN/PRC-127ef HT, this PRC-174 is the newest, most modern technology military radio at N6CC.

I’m looking forward to putting it into field use, either “Mudborne” via my LPC’s (Leather Personnel Carriers) or as a fixed-portable setup.

A work in progress, additional Bush Ops evaluations in the near future, stay tuned. As usual your comments, additions, corrections and observations to add to the PRC-174 knowledge/experience base are always welcomed.

BT

NNNN