How the radios interact (or interfere) With Each Other
Radios Working Together
Motorola radios:
The XTL 5000 radios are all of the mid power (50W) variety, all with remote control heads. I chose W3 handheld heads, because they are cheap on eBay, and the all-in-one mic+head combo is a huge space saver, and dash space is at a premium considering I fit 6 radios/scanners onto it somehow.
I do NOT have the correct TIB (transceiver interface board) on the radio units (the ones for W3 heads are very rare and expensive), so the radios are 100% functional except for the power button on the heads, which I bypass by forcing the radio to power on (jumper the BATT+ and IGN pins in the acessory connectors) and use a high-amp relay to cut main battery power to the radios with a switch on the dash.
To further save dash space, I made a dual-radio setup with the UHF and 900MHz radios. This allows one control head to control two radios, with a button to switch between them. The cables for this are all DIY: the W3 heads use standard RJ45 connectors internally, so I used standard CAT6A cable and RJ45 couplers to run the heads from the trunk of the vehicle to the dashboard. The dual radio setup was accomplished by running the head cable to one radio accessory port, then jumpering 7 of the 8 control wires (everything except for the power wire, only one should be attached to power the head, doesn't matter which) to the other radio's acessory port.
XTL radios DO NOT have speakers built in, and the speaker outputs are NOT grounded, so all 3 radios have their speaker outputs run through small audio transformers, then to a simple resistive summing circuit, which feeds an audio amplifier board, that powers a speaker in my dash. This allows indipendant volume control of all 3 radios.
HF Radio and CB Radio
Since the HF rig and the CB radio both operate in the HF bands, they can interfere with each other. To solve this, I made a custom interface box. The Yaesu HF rig uses an 8-pin DIN connector to drive the auto tuner, and I hijacked the PTT signal from the Yaesu to strategically kill the CB radio.
The custom box has two RJ45 ports. One plugs into the radio, one to the tuner. The ports are a 1:1 passthrough, so the tuner will work even if the box fails. The box taps the power lines for... wel.. power, and the PTT line as a signal input, and powers the status LED's on the RJ45 ports to indicate different statuses (input voltage OK, 5V bus OK, PTT active.)
The box internally has an RF relay, a 5V linear regulator, and a simple op-amp and diode-based OR gate driver for the relay. The relay common and NC contacts are run to two BNC ports on the box. There is also a third BNC port connected to one of the OR gate inputs. Finally, there is a circular DC power jack that is used to interface with the CB radio.
The CB/Scanner radio antenna cable is run throught the RF relay in the custom box. Normally (unpowered), the relay is closed, allowing RF to pass. When an external signal on the third BNC port is received, OR the 5V PTT line on the HF rig is pulled to ground,the OR gate triggers the relay and shorts the CB/scanner cable, disconnecting the antenna. When the 5V PTT line from the HF rig is pulled to ground, it also activates the OR gate output, which triggers the RF relay.
In addition, the OR gate output is tied to a transistor, which shorts the DC barrel jack on the side of the box. The CB radio has been modified: the microphone on the CB has an "RX" line, and when that line is broken, the receiver in the radio is shut off. I added a port on the back of the CB radio, with a cable run to the DC port on the box. When the DC box's port is shorted, the RX line internal to the CB is cut, completely disabling RX (this is done because the HF rig is very powerful, and presents an interference risk even with the CB RF port shorted.)
RF tap for the CB radio:
As mentioned above, the HF and CB rigs interfere with each other when transmitting. The HF rig interference has been solved, but how about the other way around? The CB may only put out 5W of RF, but the bands are very close, and it's easy for the HF rig to pick up the CB.
To combat this I built custom RF taps: all-aluminum project boxes with two RF ports and a single RCA port on the side.
Inside, there is simply a direct line from one RF port to the other. I pulled the center conductor and insulator out of a short length of RG-58 cable. An insulated, thin guage (22 AWG) wire is wrapped around the insulator to make a pickup loop. I needed about 90 turns in 3 layers to get a good enough signal at the CB's power level. The loop is connected to a high speed diode, and a ceramic capacitor, making a half-wave rectifier. This rectified signal directly drives the base of an NPN BJT transistor. The Collector and Emitter of the transistor are tied directly to the RCA jack on the side of the tap. The function is simple: when RF is present in sufficient quantity, the transistor turns on. I installed this tap in line with the CB antenna. The wiring is not complete, but I installed another RF relay, this time in line with the HF radio antenna. When the CB radio keys, the RF tap transistor turns on, which will trigger the relay and short the HF rig's RF port, limiting interference from the CB radio.
RF Tap for the Motorola Radios:
The Motorola radios have a similar interference problem: when they key, the RF gets into the CB/scanner antenna, and can interfere with the scanner. To solve this, I made a second RF tap, identical to the one for the CB radio, but with fewer turns on the pickup loop, since the higher frequencies makes the tap more sensitive... I guess? Don't check my math there.
I installed this tap in line with the tri-band antenna for the Moto radios. I ran the trigger port to the third BNC port on my custom HF box, which allows the RF tap to be an external trigger for the CB/scanner circuit mentioned above, and short the scanner's RF port when any of the 3 Moto radios key.