[R-390] Differences between the balanced antenna and unbalanced antenna inputs

[Jim Whartenby]

David
The restrictions you mention for the unbalanced antenna are not mentioned in the Navy manual, NAVSHIPS 0967-063-2010, April, 1970.  It does mention Field Change 5 which switches coax connections so that the unbalanced input is now the primary RF input.  Why this was done is not apparent.  Perhaps because of the limited number of antennas abord ship?  I have seen my fair share of fried antenna coils in the SRR series of receivers so a dedicated HF receive only antenna makes sense.  TM 11-5820-358-35 echo the same information regarding the balanced and unbalanced antenna types but it does not mention anything similar to what was done in the Navy field change 5.


>From NAVSHIPS 0967-063-2010: "3. 2. 2 Antenna Circuit. The antenna circuit matches antennas of various characteristics to RF amplifier V201.The BALANCED ANTENNA input (using connector J104) has a characteristic impedance of 125 ohms. Two-wireantenna systems, such as doublets with either 50-ohm twisted pair or coaxial transmission lines or with 50 to200-ohm twin-lead transmissions lines, may be used without serious mismatch. Long wire antennas may also beused if one side of J104 is connected to ground. The UNBALANCED ANTENNA connector, J103, is used for whip,long-wire, random-length, and single-wire antennas. Normally, a balanced antenna is connected via relay K101Acontacts and switches S201 and S202 to the input coil of one of six RF transformers, T201 through T206, dependingupon the frequency being tuned. Similarly, an unbalanced antenna is connected via relay K101B contacts and switch S205 to one of six capacitors within the RF transformers. When relays K101A and B are energized, all antennainput terminals are disconnected from the r-f transformers and are grounded. Relays K101A and K101B areenergized only when an associated transmitter is connected to the break-in circuit and is keyed, or when theFUNCTION switch is in the STANDBY or CAL positions. With the BREAK-IN switch in the ON position and theassociated transmitter keyed, a ground is applied to pin 9 of TB103 to energize relay K601. This break-in relayprovides a ground connection for CR102 which energizes K101A and B. Note that CR102 is also grounded whenK601 is de-energized and the FUNCTION switch is in the STANDBY or CAL position. 


3. 2. 3 The connections of the antenna circuit are changed for shipboard use by field change 5 (figure 3-2). In thiscircuit arrangement, an unbalanced antenna connection is made to J103 but because of the reversal of connectorsP205 and P206, the antenna is connected to S202 and pin 1 of the selected RF transformer primary. Pin 2 of thesame coil is grounded via S201 and a shorting plug on J104."

As for using the balanced antenna as an unbalanced input, I would think that it matters which of the balanced inputs lines is shorted to ground.  Shorting the balanced line that connects to the 7pF capacitor in each antenna input transformer would be the safe bet since the 3 to 12 variable capacitor could still be used to resonate the primary of the antenna transformers along with the ferrite slug.  So the J106 side of the balanced line should be the input that is grounded.  If this is the case, then the Navy field change 5 makes less sense to me.  I would think that all of the above is covered in the "R-390 Pearls of Wisdom."

Regards,  Jim

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

    On Sunday, November 3, 2024 at 03:33:34 PM CST, David Wise <d44617665 at hotmail.com> wrote:   

 I used to think that, but not anymore.  The unbalanced input is meant to be used with an antenna with a specific capacitance, which acts as part of the capacitance that resonates the secondary winding of whichever transformer is selected for your band.  The primary winding that the unbalanced input skips is nonresonant.  The caps there don't resonate, they just establish a center tap for balance.  You get the same number of tuned circuits either way.

Dave Wise
SWL (inactive)

________________________________
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> on behalf of Tisha Hayes <tisha.hayes at gmail.com>
Sent: Sunday, November 3, 2024 10:44 AM
To: R390A <r-390 at mailman.qth.net>
Subject: [R-390] Differences between the balanced antenna and unbalanced antenna inputs

There is one advantage of using the 125 ohm balanced antenna inputs; You
get a little bit more pre-selection through RF transformers T201 to T206.

When using the 50 ohm unbalanced connector (the C connector) it bypasses
those RF transformers.

You can see this on diagram 5-14

*Ms. Tisha Hayes, AA4HA*

*Senior RF engineer (4RF USA, retired)*
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