[Milsurplus] [MMRCG] [MRCA] Univac Aircraft Antenna Coupler

Mon Jan 19 18:27:24 EST 2026

MarcPulled the ERA 3001 antenna coupler from it's hiding place and sure enough, there are two Schrader valves on the front panel.  The metal covers hid the valve stems so I thought that they were just ground connections.  A search of the coupler manual finally found that 5psi of dry nitrogen gets it ready for flight.  So I guess it doesn't matter where the coupler is mounted, it is pressurized.Sorry about that!Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy 

    On Sunday, January 18, 2026 at 04:22:22 PM CST, Jim Whartenby <old_radio at aol.com> wrote:   

 Marc
The ERA coupler in the B-47 was mounted behind the pilot who sat behind the aircraft commander.  The pilot was also the rear gunner.  Look at the first enclosed document, the ERA 3000 is item #15.  Since it was mounted in the crew compartment, there was no need for a pressurized container.  I would have to check where the ERA 3000 was mounted in other airframes but I believe that there was no pressurized containers.  I suspect that the solid state ERA 3250 tuner is in a pressurized container.

You can check the "Project Big EVA" document at: https://apps.dtic.mil/sti/tr/pdf/AD0066528.pdf for a rundown of what failed during the accelerated life test for the ARC-21.  The ARC-21 did about as well as any tube powered military aircraft radio, including the ARC-58.  IMHO, where the transceiver was mounted had a great influence on reliability.  See the second enclosed document for a comparison between operational vs. AGREE  testing.  In AGREE testing both the ARC-21 and ARC-58 were subjected to the same shake, rattle and roll testing.  Not necessarily so for operational testing in the airframe.

The ARC-65 operational test and evaluation document can be found here: https://apps.dtic.mil/sti/tr/pdf/AD0690546.pdf  Here the ERA tuner had problems when used with the ARC-65s higher tune power setting but modifications were made.  The ARC-65 manual I have has Change 4 included which is dated January, 1976 so the ARC-65 soldered on far longer then you suspect.

Regards,
Jim

Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy 

    On Sunday, January 18, 2026 at 02:48:43 PM CST, boeing377 at gmail.com <boeing377 at gmail.com> wrote:   

 ERA coupler was used with ARC 65. I do not recall it having a separate control box. ERA was housed in a pressurized case to prevent arcing at high altitudes when carried in an unpressurized area of the host aircraft. 
Spoke with a restored USAF radio tech who maintained ARC 65 systems. He said the ERA was a fantastic HF coupler. Far more reliable than the RT 400 ARC 65 transceiver. 
MarkAF6IM
On Sun, Jan 18, 2026 at 11:22 AM Jim Whartenby via Milsurplus <milsurplus at mailman.qth.net> wrote:

Far from an expert, but thanks Scott!The first generation ERA 3000 (later named Univac then Unisys) coupler was vacuum tube powered, the second generation ERA 3250 was all solid state.  Both are stand alone in that the coupler tuning is controlled internally using the frequency discriminator.  It should work with any transmitter or transceiver.  The first generation coupler has the control electronics mounted in the same box, the second generation is made up of two boxes, the coupler and the control unit.
It is an interesting story of how a computer company became an automatic antenna tuner supplier.  Evidently, USAF wanted a second source for the RCA designed coupler for the ARC-21 transceiver which is also all vacuum tube powered with the exception of a Selenium rectifier.  
The VIP page does not have a decent photo of the first generation coupler.  I sent them a few photos but the page was never updated.  I guess that those responsible for the coupler page are no longer with us.  It would be a shame to harvest the coupler for it's components but a little bit of history slips through our fingers every day.
Below is the first generation, ERA 3000, coupler still mounted on the shipping plywood

Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy 

    On Sunday, January 18, 2026 at 12:11:08 PM CST, Scott Johnson via groups.io <scottjohnson1=cox.net at groups.io> wrote:   

 Jim Whartenby is the resident expert on these couplers, but it is P/O the ARC-65 SSB transceiver used on the B-52 and KC-135 until they were fitted with the ARC-190.  That transition happened in the 1980s, so it is likely that coupler never saw service after it was serviced. Both Jim and I have ARC-65s and ARC-21s (the AM version which was modified for SSB to become the ARC-65 in the late fifties).

Regards,
Scott W7SVJ

-----Original Message-----
From: mrca-bounces at mailman.qth.net <mrca-bounces at mailman.qth.net> On Behalf Of Michael St. Angelo via MRCA
Sent: Sunday, January 18, 2026 10:53 AM
To: mrca at mailman.qth.net; milsurplus at mailman.qth.net; MMRCG at groups.io
Subject: [MRCA] Univac Aircraft Antenna Coupler

Going through some military stuff on a snowy Sunday and found a couple of aircraft antenna couplers I got surplus years ago. They were designed by ERA and this model was built by Univac. The are an HF Coupler mounted in the vertical stabilizer of a plane such as a KC-135 to tune the probe HF antenna. They were hermetically sealed and basically consist of a rotary inductor and vacuum variable capacitor tuned via a discriminator circuit. 

I've attached a couple of pictures. The discriminator section has the cover removed. They were last services at Warner Robbins in 1965. I was told they require 28 VDC and 100 AC 400 Hz. The RF connection is made via a C connector (my favorite RF connector)

I was going to use the components in a tuner but want to get in operational in their original state. Does anyone have any documentation about the tuner or it's controller box?

Thanks in Advance,

Mike N2MS

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