External HF/VHF Auto Aerial Changeover Relay Unit
This simple unit is fitted inside a small metal box and automatically selects the correct aerial for HF or 4/6 Metres. The unit houses two SO239 sockets for the HF and VHF aerials, a short lead with a PL259 plug which connects to the transceivers aerial socket, a 2.5mm standard coaxial dc socket for the 13.8vdc supply and finally a short lead connected to Pin 7 of a 13 Pin plug which interfaces to the rig's accessory socket which now supplies the band changeover voltage.
This modification requires the fitting of a resistor, diode and capacitor inside the IC-7300 and unless you are confident in doing so "do not attempt it" I accept no responsibility for your mistakes while doing this.
After modifying the rig a dc output of about 1.8v appears on Pin 7 ( previously unused ) of the 13 Pin Accessory socket when it is tuned to any frequency in the range 33 to 74.8Mc/s which is used to turn on a transistor in the new external relay unit which drives the HF/VHF changeover relay. The circuit below shows the additional parts fitted within the IC7300 and below that pictures showing the physical location of the parts, NB, Pin 2 on the socket is ground. It can be seen on the circuit that the 68K resistor and 1N4001 diode offer complete isolation so that if Pin 7 is accidentally earthed or connected to the 13.8v supply for example no ill effects will be noticed with the operation of the transceiver.
Regarding the circuit below, the 68K resistor should be fitted close to L860 and the 1N4001 diode and 1nF capacitor fitted to the 13 Pin socket, see pictures.
NB, a fuse of about 500mA should be fitted in the 13.8v DC supply lead.
The pictures below show the new external aerial switching unit
External Switching Unit Circuit
It can be seen from the circuit above that as a simple but very useful addition to the circuit an oscilloscope monitoring point has been fitted which samples a portion of RF from the HF aerial socket, (R2/R3/SKT4).
R1 10K Resistor, 0.25w
R2 10K Resistor, 0.25w
R3 1K Resistor, 0.25w
R5 3K9 Resistor, 0.25w
C1, C2, C3, 1nF Min Disc Cer Capacitors
D1 1N4148 Diode or equivalent
D3 LED (Red, indicates VHF operation)
TR1 TIS90 or equivalent
RL1 Suitable 12v Aerial Changeover Relay
SKT1 SO239 Socket (HF Aerial)
SKT2 SO239 Socket (VHF Aerial)
SKT3 2.5mm dc Coaxial Socket (13.8vdc Supply, Fused @ 500mA)
SKT4 Min Banana Socket (For Oscilloscope monitoring purposes)
PL1 PL259 Plug with short connecting Lead (To IC-7300 Aerial Socket)
PL2 13 Pin Din Plug with short connecting lead (To IC-7300 13 Pin Accessory Socket)
FB1, FB2 Ferrite Beads.
Any sort of small metal box can be pressed into service for this project but I used a cheap (CB Type) low pass filter as it was just the right size and already had the two SO239 sockets fitted so I just needed to drill the extra holes for the other sockets and leads.