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

[Bill Tarkington]

Thanks George, read every word and understood most of it.  73, Bill, K3YC
----- Original Message -----
From: "George, W5YR" <w5yr@att.net>
To: <icom@mailman.qth.net>
Sent: Thursday, June 27, 2002 11:46 PM
Subject: Re: [Icom] Icom Pro II and PW-1 (Do I have a problem here!)


| 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?
| ----
| Your Moderator: Dick Flanagan W6OLD, icom-owner@mailman.qth.net
| Icom FAQ: http://www.qsl.net/icom/