A ham consulting the experts on QRZ asked:
On 30 meters, my SWR reads 3:1 to my antenna (an EndFed 53 feet long wire up about 25 feet). Reading a chart I have, I see that at 80 watts output, my reflected power should be 20 watts. I verified this by looking at my Diamond SX-200 Meter which also indicates the reflected power is 20 watts. My questions are these: does the 20 watts reduce my 80 watts output to 60 watts at the antenna? I have a choke on my feed line in my shack (near my transceiver) & the SX-200 Meter is between the choke & transceiver….
The OP later explained that the transceiver is a IC-7300 and it appears that the internal tuner is in use above… so let’s proceed on that basis.
Analysing the OP’s report, his SX-200 indicates VSWR=3 Pf=80, therefore Pr=20, and P=60W. Note that \(P=P_f-P_r\) is valid because Zref is real, so the answer to his question about power to the antenna is 60W, he is quite correct.
He went on to ask where the 20W reflected goes to… but I will leave that to Walt Maxwell devotees to discuss energy sloshing around and re-re-re reflections… the stuff of ham lore.
Understanding the IC-7300
As an example of what might be expected of the IC-7300 with a mismatched load, I did a series of measurements at 7MHz with a sample variably mismatched load.
Above is a plot of power output vs VSWR for a sample mismatched load. Also plotted is the measured reflected power and the calculated power output based on the ham lore \(P=P_f (1-\rho^2)\).
Firstly, let’s dispatch the nonsense of the last expression. It is valid only for a true linear source with Thevenin equivalent source impedance that is real and is 50Ω (ie 50+j0Ω), neither of which is satisfied by most instances of this type of transmitter. This accounts for the glaring gap between the orange predictions and blue measurement. As mentioned, this is enshrined in ham lore and is uttered by people who just do not understand.
Above is a Simsmith model of a 100W Thevenin 50+j0Ω source with a 150+j0 load which delivers a VSWR=3 load to the source. Simsmith predicts load power is 75W, in line with the \(P=P_f (1-\rho^2)\) calculation… but it does not match the measured 30W because the transmitter is not a 100W Thevenin 50+j0Ω source. Simsmith as many possible sources, but none of them are appropriate to this type of transmitter.
It is naive to apply tools are often used without consideration of whether the models are valid. Invalid models mean GIGO.
Note how the reflected power levels off at about 10W, exposing the VSWR protection mechanism. Pr greater than the 10W threshold creates an ALC voltage to reduce drive to that threshold. This is a common method of implementing VSWR protection, though the threshold may vary (often in the range of 5-10% of nominal power output).
The nominal power rating of the IC-7300 is 100W, and it does have an internal ATU which brings two issues:
- the internal ATU is less than 100% efficient so it degrades available power output;
- the internal ATU is likely to be less than optimal (it uses discrete steps) so it may degrade available power output; and
- the internal ATU is limited in matching range so it may degrade available power output.
These also apply to some extent to an external ATU, but one might expect less so for a good external ATU.
I might caution that depending on the transmitter’s VSWR protection to reduce output to level that might not cause damage depends on the ALC system, and in SSB telephony, the repetitive ALC transients create a spectrally dirty signal.
Analysis of the OP’s report
The OP later reported that the IC-7300 power meter indicates 80W. It is likely that this is derived from the directional coupler and that Pf=80 at the input to the internal ATU (power is less than rated power, hinting that VSWR>1 at that point). We do not know what Pr is at that point, net power is probably in the range 70-80W prior to the internal ATU (depending on the match achieved).
The OP reports measurements that indicate that the net power to the antenna at the transceiver output (ie the output of the transceiver’s internal ATU) is 60W, well short of the rated 100W.
Given the above, it is not surprising that net power after the internal ATU is 60W.
Contributions to this 40% or 4dB) reduction in transmitter power output include failure of the transmitter to deliver 100W into the load as transformed by the internal ATU and loss in the internal ATU.
So, the antenna with intrinsic high VSWR and the internal tuner to present a more acceptable load to the RF Power Amplifier is perhaps convenient, but comes at a performance deficit.
Of course there are more downstream losses in this scenario, it is a EF quite probably with a transformer of unknown loss.
Ham lore comes to your aid here, “less than one S-point or 6dB does not matter” -or the cry for mediocrity “any antenna is better than no antenna at all”.