Yaesu FT-897: Interpreting Meter Readings

Yaesu FT-897: Interpreting Meter Readings (PWR, SWR, ALC)

The “scales” at the bottom of the panel seem to be “permanently” calibrated in “S” units for receiving – no matter what you have the meter reading set for (i.e. PWR, SWR, ALC, etc.) That is the first 9 ‘ticks’ correspond to S9, the next tick must be 10 dB over 9, and the last highest tick must be 20 dB over 9 {or something like that}. The scale is linear and the first 9 ticks are equally spaced. The total scale length appears to be 14 ticks. You can read 0-9, 11.5, 14. These are the “tick marks” which you use to interpret ALL other readings – not just “S-readings”. Hit the “F” key once and set to menu ‘i’. Should see the headings over buttons A, B, C – “MTR”, “PWR”, “DISP” (hit button “B” until you see “PWR”).

DEFINING THE TICK SCALE:
‘0’ scale appears over the ‘R’ in “MTR”, ‘5’ over the ‘P’ in “PWR”, ‘11.5’ over the ‘D’ in “DISP” and ’14’ over the ‘P’ in “DISP”. You can see the scale is linear by plugging in an analog 0-1 ma meter into the 1/8″ meter jack just under the AF, SQL/RF knob. Set menu 061 to read “SWR” or “PWR” so that on transmit the meter reads ‘SWR’ from 0-1 ma. Anyway this just shows that the on the ‘tick scale’ 7 ticks (i.e. 7/14 = 50% scale) corresponds to 0.5 ma (half scale on analog meter). The tick scale is “linear”.

POWER SCALE CALIBRATION:
For my psk-31 setup I did a “power test” using my dummy load. Make sure you have menu ‘i’ and button ‘B’ reads “PWR”. Now select menu 075 and turn down the power to 5 W. Leave the panel showing menu 075. I used CW mode and my CW key to transmit on 3580.6 kHz. Transmit into the dummy load and note the “PWR” meter reading in ticks. I cranked up the power until I got a higher “tick scale” reading of either a whole number or half number (because its easier to read it this way).
Results were:

Power Ticks
5W 2.5
7 3
10 3.5
15 4
19 4.5
23 5
27 5.5
32 6
36 6.5
41 7
44 7.5
49 8 (max. for psk-31 mode for 100 W transmitter)
56 8.5
65 9
84 11.5 (9+2.5)
100 14 (9+5) (this is full-scale).

SWR SCALE CALIBRATION:
Select menu ‘i’ and press button ‘B’ until it reads ‘SWR’. I used a 0-1 ma analog meter plugged into the meter jack to see what was going on here. I got the idea for this from VE3GK, Gerry King’s review of the FT-857 in the 2003 July/Aug “TCA – The Canadian Amateur” magazine. He indirectly implied the “tick scale” is 15 units but I my own observations show that it is 14.

The “SWR” scale does not actually read “SWR”, instead I believe it reads the ratio of V-reflected/V-forward. If we define k = V-reflected/V-forward then k ranges from 0-1 (i.e. what the 0-1 ma analog meter would read) or 0-14 ticks on the FT-897 LCD scale. The SWR in terms of k is:

SWR = (1+k)/(1-k) where k=0-1 = “ticks”/14

Using “t=ticks” directly, SWR=(14+t)/(14-t) with t=0-14

Note that SWR = 1 (1:1) when “t=0″.

Anyone who has built an old style SWR meter from ARRL handbooks etc. will recognize the above formulas especially when using a 0-1 ma meter. The meter was set to 1 ma on “forward” and then the reading taken on the “reflected” setting. That’s why these old SWR meters are calibrated to show SWR=3 at mid scale (i.e. where k=0.5) since:

SWR=(1+0.5)/(1-0.5)=3

I checked the above formula against my external SWR meter and my external tuner and dummy load. I “mismatched” the tuner to increase the SWR and watched the “tick scale”.

Results using whole “ticks”.

“Ticks” SWR=(14+t)/(14-t)
0 0
1 1.15
2 1.33
3 1.55 (Power starts to fold back here)
4 1.8
5 2.11
6 2.5
7 3 (here the high SWR “HSWR” starts to flash)

So at just “3 ticks” the SWR is 1.55:1 and power begins to “fold back” at the higher power settings. My “forward” power on the external SWR meter started to take a nose dive just before 3 ticks. On lower powers like 5-20 W the “fold back” is not as noticeable since we are already at low power. I use the FC-30 add-on tuner and have found through considerable portable operation that the tuner will tune as long as the SWR is below “7 ticks” (i.e. SWR<3). This makes sense since the tuner is rated for SWR’s < 3.

Anyway I stopped using external SWR meters since I found the "calibration" could be way off anyway. I now just use the FT-897 "tick scale". The important thing is to keep the SWR below 3 ticks (for no fold back) or below 7 ticks (to avoid High SWR of 3).

Also, certain coax lengths can reduce the SWR that is seen by the FT-897 (i.e. get low SWR tick readings) but the true SWR on the coax may be higher. So I believe that the FT-897 SWR tick scale indicates what the "FT-897 thinks is the SWR" and may not be the "true" SWR on the coax. Also, as mentioned by others in this group, the SWR indication is sensitive to the power setting. That is the SWR can go up as power is increased if you do not have a perfect SWR=1:1. So a low power tune up at 5 W may indicate an SWR of <3 ticks but 100 W full power may indicate >3 ticks. So I readjust the tuner to lower the SWR to below 3 ticks at high power. It’s not just “possible RF on the outside of the coax” that causes this because I’ve used balanced dipole antennas with baluns etc. and this increasing SWR behavior still happens.

THE “ALC” METER SCALE:
For psk-31 I adjust the sound card volume so that there is no ALC indication. I adjust the volume into the rig until the “PWR” “tick scale” shows a maximum of 8 (49 W) when the rig is set for 100 W (menu 075). There should be no ALC indication.

Reference:
FT897 Yahoo Group

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