R8600 Interesting Antenna Selection tidbit

[N9JIG]

So I have a (relatively) poor performing attic antenna connected to the HF ("Ant-2") antenna port (SO239) on my R8600. I have my new scanner antenna and multicoupler system connected to the V/U ("Ant-1") port (N-connector). While twiddling the knob (on the radio, get your mind out of the gutter!) today I disconnected the HF antenna but was still able to hear some HF traffic, including WWV on 10 and 5 MHz while Ant-2 was selected.

When I disconnected Ant-1 as well I lost all reception.

What my keen, professional powers of deduction deduced was that there is poor isolation between the Ant-1 and Ant-2 ports. While not a real problem, it did surprise me.

 

[Token]

Let me make sure I understand what you are saying. You normally have your radio configured so that Ant-2 is HF, and Ant-1 is VHF/UHF. You were tuned to an (multiple) HF frequency, with no antenna on Ant-2, and you still heard signals, obvious leakage from the Ant-1 port.

Looking at the schematic and the parts used, I can see this happening.

When tuned below 30 MHz the Ant-1 path, via the Converter Switch (CONVS NC) and V/U Switch (VUS NC) defaults to a path directly to the NO contact of ANT1S (RF Unit RL1).

When Ant-2 is selected that HF path is ANT2S NO (RF Unit RL2) to ANT1S NC (RF Unit RL1).

Note that in this condition (of tuned frequency and port configuration) both paths end up at pins of the RF Unit RL1. Both signals paths end up on two pins of the same relay. So in the conditions you describe the only isolation would be the NO to NC isolation of a single relay (relay ANT1S, part no FTR-B4CA009Z). This relay is a fairly generic slim line low current relay, and not a specifically designed RF component, so I can understand the isolation being not so great.

If you tune above 30 MHz (but below 1100 MHz) then the VUS relay should change from NC to NO, adding more isolation (from the Ant-2 / 3 path). And tuned above 1100 MHz both the VUS and CONVS relays should go to NO, adding a third relays isolation.

T