Turn your Omni VI "RX Ant" jack into an Aux connector

[N8YX]

(rant)
Why in the heck can't Ten-Tec leave a good thing alone, and why must they go backwards in feature utility?
(/rant)
The Aux RX jack on the Omni V - meant to feed a secondary receiver - was mysteriously disappeared on the Omni VI and the Paragon series rigs. In its place, an RX Ant input was incorporated. Good for the 160M op; bad for those of us who use a second receiver.

Thanks to the seller at Dayton who made me an offer I couldn't refuse, and a couple junked TS-930S signal boards, the "fix is in".

Got several 0-degree, 2-way power splitters (0.1-400MHz) in DIP configuration for the lofty sum of $0.50 each. If I'm adding a receive-antenna output function to a transceiver, I like to use these as they offer upwards of 60dB total isolation between receiver sections...good for keeping each receiver's birdies and digital hash out of the other.

Popping the top cover on the Omni, one finds the Low-Pass Filter Board underneath the speaker shield - right rear of the radio. Remove the shield and a board with Connector 13 and Connector 17 appears. We're going to interface to the Connector 17 jack/coax run and use the RCA plug which is coax-connected to Connector 13 for the receiver feed.

First off, we need to build one of these:

2012-06-15_17-53-06_381.jpg

You'll need to reach for a scrap '930, '940, Icom whatever...anything which has a couple of the push-in cable connectors and a short run of coax with an attached male connector. My junkbox is fairly large and I can source all this stuff from it; the connectors could also be obtained from Digi-Key, Mouser, etc.

Studying the assembly carefully, one sees the center pins of each jack are connected to the two output ports (pins 5 and 6) of the splitter while the grounding "ears" are soldered to the flanking ground pins. Similarly, the coax center is soldered to Pin 1 and the shield to Pin 2. Check your work, then...

[N8YX]

...unplug the coax plugs from Connector 13 and Connector 17. Plug the coaxes into the splitter assembly then plug the male end into Connector 17. A dab of RTV sealant keeps the splitter from flopping around and permits easy removal if you un-mod the unit:

2012-06-15_17-57-00_265.jpg

[N8YX]

Note that Connector 13 is now empty. One could drill the rear bulkhead and add another RX Input jack (or add the desired Aux RX connector to the existing arrangement). There's an inherent bonus in not connecting anything to the port: When you press the "RX Ant" switch on the front panel, the antenna is completely disconnected from both receivers.

Notice that the germanium diode (red component visible on the right side of the splitter, attached to the RCA jack) was left in place. There's a reason for this too: We want to clamp the level of RF which could potentially escape that connector if the PIN diodes in the T/R circuit were to fail somehow. (Never mind that the Omni's receiver - by design - is also susceptible to damage from a failure here.)

When I finished the modification I connected a dummy load to the Omni's antenna port and a scope to the new Aux RX port then transmitted a full-power carrier via the Tune function and observed the level on the screen. About 100mV present - less than would be coupled into a receiving antenna from a nearby transmitter.

The really paranoid could place a 40ma, 12v bulb in series with the coax-to-jack connection. I tried a full-power test with a slaved R-71A (with associated Mute function facilitated by an ARB-704) and noticed no degradation of the receiver front end - even with the preamp enabled.

[KG4CGC]

Way awesome and above board.

[N8YX]

Bump.

When I tried this trick with the Paragons, things got ugly. I noticed around 2v P-P RF at the Aux Ant connector when in full power on 28MHz...and while not harmful to the receiver front end it'll result in a nice, healthy thump from the receiver when the transceiver unmutes it.

The fix was to design and build a multistage muting board which incorporates a small antenna changeover relay. This relay connects to one of the splitter's output ports; when the transceiver is powered up and in receive mode the Aux Ant connector is "hot". When in transmit or when power is removed, the Aux Ant connector is shorted to ground.

I needed to provide an additional 250msec delay between "main relay up" and "Aux Ant hot" to completely eliminate the thumping, so the pair cannot be operated in full QSK CW...but that isn't really what they'll be used for. Both the P-II's and the Omni VI's internal receiver QSK functionality are unchanged so their receivers can be used if break-in operation is desired.

Pics, schematics and more writeup to follow. I have to build two more boards this evening.

[KB3LAZ]

I like the RX antenna and have always found it to be a good idea...I run 80 and 160 primarily though. Also I run separate RX and TX antennas.

[N8YX]

You could still retain that functionality by drilling a hole for another RCA jack but I chose not to at the moment.

Lots of pics and schematics to follow. I'm gonna go play Field Day for a bit.