[Icom] Icom Pro II and PW-1 (Do I have a problem here!)

[George, W5YR]

Well, now you put me on the spot! But thanks for the kind words.

Best answer I can give you is that "it depends!" On your dummy load and
everything else in the system.

First of all, there is no perfect dummy load. Even the best will have some
residual reactance in addition to the desired resistance component. Plus,
the resistive component may not be exactly equal to the line Zo causing
some small residual SWR. 

Second, no real transmission line has a perfectly real Zo. Consult the
tables and you will find that most coax has a fraction of an ohm of
reactance at HF. Ordinarily we can neglect this with little penalty. But
not when picking nits . . .   <:}

So, the combination of an almost-but-not-quite-perfect load and a line with
an-almost-but-not-quite real Zo means that you will indeed have a tiny
amount of reflected power - probably too small to measure unless you are
socking 1500 watts into that dummy!

So, theoretically and perhaps stretching the point, even with your dummy
load,the Forward Power indication of your wattmeter still represents the
sum of the power in the forward-traveling wave and that in the miniscule
re-reflected traveling wave. The Net Power actually being dissipated by the
load is still the difference between the indicated Forward Power and the
Reflected Power. 

If the system is lossless except for the load itself in this case, then the
power delivered from the transmitter will equal the power dissipated in the
load. But the Forward Power shown on the meter may in fact be slightly more
than that due to the presence of the tiny amount of reflected and then
re-reflected power. The delivered power is always greater than the power
actually absorbed by the load due to losses in the system which have to be
supplied by the only real source of power: the transmitter.

So the transmitter output power budget looks like this: <delivered power> -
<antenna system loss> = <power available to the load>. On the line,
however, the forward power component will comprise some of the loss power
since it carries part of the power supplied to the losses plus the
re-reflected power, if any, plus the portion of the delivered power that
the load actually absorbs. The reflected power component carries the
remainder of the power supplying the loss plus whatever is rejected by the
load. Any re-reflected power combines as a component of the forward wave.
This situation is how you can send 100 watts into an antenna system from
the transmitter, have 95 watts actually radiated, have 5 watts lost in
heating things up, and yet see a forward power of 150 watts and a reflected
power of 50 watts on the line. No wonder folks get confused!

As a real-world practical matter, under the conditions you describe, the
reflected power is almost always completely negligible and the "forward"
power indicated by the wattmeter will be arbitrarily close to the power
being delivered by the transmitter and absorbed by the load. If our
components such as tuners and connectors and coax, etc. weren't as low-loss
as they are, it would be easier to see (and feel and smell!) what is going
on in an antenna system.

Thanks for asking an interesting question - you never know what a little
nit-picking is going to drag up!   <:}

73/72/oo, George W5YR - the Yellow Rose of Texas         
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe   
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771 I-LINK 11735
Icom IC-756PRO #02121  Kachina 505 DSP  #91900556  Icom IC-765 #02437


Bill Tarkington wrote:
> 
> George, thanks for the nitpicking cause I have followed you long enough to
> know you know your stuff!  Anyway, let me either demonstrate my ignorance
> some more or make a point.  Understand the business about determining net
> power.  But, in my case, am using a 1500W dummy load rather than an antenna.
> So, am I correct in believing that there is no reflected power, and thus no
> net, just forward power?