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Re: Separate RX feed for FT847



Phil VK6APH wrote:
>I thought you might be interested in how I solved the Tx problem with my
>FT847.  I did not want to make any internal changes to the radio so my
>solution is completely external. See the attached circuit.....

I use an arrangement almost identical with this.  However, here is an important note of caution when using the 12V that appears on the FT-847 antenna socket in RX mode:

After having blown yet another 144MHz GaAsFET masthead preamp recently, I took a good look at the timing of the RX-TX and TX-RX transitions on my FT-847.  With the aid of a scope looking at (1) the RF power output at the VHF antenna socket, (2) the DC level on that socket, and (3) the PTT line (on separate traces), I noted:

1.  On FM or SSB, pressing the PTT, switching from RX to TX, there is an 8 ms delay between the 12V going to zero and the RF output starting.

2.  On CW, on the first key down, the delay is 4 ms.

3.  On FM or SSB, releasing the PTT, switching from TX to RX, the 12V appears on the antenna socket after 0.4 ms, but some RF power continues for a further 2 ms!  On FM mode, for 1 ms it is still almost at full power!

4.  On CW, of course, the delay in TX to RX switching is a menu item (mine is set to 100 ms, which my measurement confirmed).

Measurements were all on 144 MHz at about 10W output.

The delays noted in 1 and 2 are fine, and give enough time for a relatively fast relay to switch.  But the delay in note 3 is a real killer, and almost certainly the cause of my fried 3SK97.  

It seems to me that if you are protecting your downconvertor with an external relay arrangement like VK6APH and I both use (and others, as I recall from earlier 
discussions on this BB), then to be absolutely safe, the relay switching time must be between 2 - 4 ms.  If it switches slower than 4 ms you can squirt RF power into your downconvertor if you press your CW key (if you don't have a key then you have up to 8 ms if you press the PTT on SSB/FM).  If it switches faster than 2 ms then you can do the damage on phone when you let go the PTT.

Another interesting behaviour I observed while measuring these:  with the front panel power control set for 10W output, in FM mode there is still a full 50W RF output burst for about 0.5 ms when you press the PTT.  It takes nearly 10 ms for the power to fall to the 10W level.  On SSB or CW the same happens but the peak power is less (about 20W).

Neill
G4HLX

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