IC-705 power measurement

Andreas Krüger, DJ3EI, , Source.

The setting

They have brought various gear. Among these, a “Rohde&Schwarz CMA 180 Radio Test Set”.

Among the things I brought: My IC-705, serial number 13001835.

A good occasion to figure out how much output that rig produces.

Setting

We start the measurements at 7038.600 kHz USB-D. The rig is driven by the WSJT-X program, which in turn is set to WSPR mode. (This is just what we happened to have used for a previous experiment.)

The rig is driven by a by a trusty old linear DC power supply that claims to deliver 13.8 V. We measured it to actually deliver 14.0 V (without load).

The rig is connected with the measurement setup via a rather short length of RG 213/U coax, about 1.5 m long or so.

We use the “tune” functionality of WSJT-X.

0 %

At 10 W max power

The IC-705 RF Power meter is set at 0 %.

ALC engages at -28.8 dB pwr in WSJT-X. Here and in what follows, “ALC engages” means a setting where a single small blue ALC mark is just appearing at the left end of the ALC bar. At that level, the rig produces 53 mW. Seen out of the corner of my eye: Power delivered is not very stable after switch on, I think it started in the upper 40 mWs somewhere.

Increasing to -26 dB in WSJT-X increases the ALC to about the level that’s below the S2 mark of the S scale. Power delivered is now 67 mW.

Increasing to -20 dB in WSJT-X: ALC about below S 4.5, output power of rig 86 mW.

Increasing to -15 dB in WSJT-X: ALC below S 7, output power of rig 103 mW.

At 5 W max tx power

Now decreasing max tx pwr to 5 W in IC-705.

ALC engages at -29.3 dB in WSJT-X. Output power is again 53 W.

At -20 dB pwr in WSJT-X: 85 mW, ALC as before at that WSJT-X audio level.

At 0.5 w max output

ALC engages at -29.4 dB, output power 52 mW.

Other percentages

At max tx power 500 mW

RF Power at full level.

ALC engages at -24.6 dB. At that level, the power bar is not at 100% yet.

For full power level, -20.2 dB from WSJT-X is needed. At this level, the ALC bar is below about S 2.7. Output is now 399 mW.

Now, -25.0 dB in WSJT-X is the power that just engages the ALC, so at 0.4 dB lower than just a few minutes ago. Power delivered is 347 mW.

Reducing rf power from full (100%) in the IC-705 does not change anything until 4% is displayed.

Without changing the audio drive from WSJT-X:

RF power, % power, mW quotient, mW / %
≥5 347 ≤69
4 266 66.5
3 182 61
2 124 62
1 74 74
0 74 NaN

Re-adjusting WSJT-X to just engage the ALC:

RF power WSJT-X, dB out, mW quotient, mW / %
0 % -30.3 52 NaN
1 % -30.3 52 52
2 % -27.4 105 52
3 % -26.5 165 55
4 % -25.6 256 64
5 % -25.3 344 69

At max tx power 5 W

ALC engages at WSJT-X -22.8 dB. Output power is 4185 mW.

With that drive, output remains unchanged when IC-705 rf power is anywhere between 50 % and 100 %.

Now start to stop transceiving while typing, as I’m worry about overheating of the CPU and the batteries.

Next, I try several of the IC-705’s rf power settings and determine which wsjt-x audio pwr setting causes the ALC to just engage, and what RF power is delivered.

rf pwr, % pwr, dB pwr, mW
40 -22.5 3175
30 -22.8 2380
20 -23.2 1540
10 -24.2 767
5 -25.3 349

Tx max power 5 W with battery

Now the external power supply is switched off. The battery is fully charged.

RF power to 100%, ALC engages at -22.2, power delivered is 4190 mW.

At 10 W with external battery

TX audio is changed from 1572 Hz (all previous measurements) to 1500 Hz.

ALC engages at -21.5 dB, power delivered is 8.83 W.

Now with a different frequency.

I move to 28.1246 Mhz. ALC engages at WSJT-X -19.1 dB, power delivered is 9.03 W.

I move to 144.1215 MHz. ALC engages at -18.7 dB, power delivered is 8.60 W.

I move to 432.3015 MHz. ALC engages at -20.2 dB, power delivered is 7.48 W.

How to get 5 W output

In the 40 m band (7.040161 MHz), with external power, setting the trx limit to 10 W.

At 100 % power adjustment in the TRX, this resulted in 8.3 W.

At 64 % power and much audio input (driving the ALC), 5.26 W output was achieved. At 61 %, 5.04 W. At 60 %, 4.9 W.

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