I will probably regret this but let me jump in while the water is still
fine.
I looked over Steve's article while on the Stairmaster for an hour a few
days ago.
And basically without checking all the math I found no problems with his
basic line of reasoning and his summary. With all due respect to Mr.
Reid I also find this thread somewhat interesting since I deal with it
on a day to day basis in my high power RF plasma work. So thank you Jim
for bringing up these issues once again.
What I need to do is now go through the math and double check the
article, but I bet Mr. Best did his homework before he ever wrote the
article, which is a very difficult task to acheive.
I would encourage everyone reading so far to try to duplicate Steve's or
for that matter Jim's effort in his analysis below in following the
article and write your own rebuttle.
Now to some thoughts of mine, which I hope to be at least somewhat
interesting to those who care.
Antennas or plasma loads do have very low impedances, especially gas
plasma loads, which act simply put as zener diodes. But the coupling
device (antenna if I may be so bold) is usually either a capacitive
electrode or in the case of some semiconductor processes it is an
inductively coupled plasma or ICP load. These have typical R +/- J
values of 1 ohm or more to hundreds of ohms of +/- J. They are
definetly weird loads to the RF and they present very high reflected
powers on the transmission line feeding them. Even at 13.56 mHz we try
to put the matching network as close as to the load as possible. Inches
away. Why you ask would we do so if in the case that Jim may be making
the line loss would not make much difference if we put the matching
network say 5 feet away. At 13.56 mhz this is not that far away
considering the wavelength. Right? ................Wrong.
The losses in the cable are huge. We try to minimize them by using
RG393 teflon cables and in some cases double runs of this cable. But we
are talking of hundreds of RF amps which are trying to be sent to the
load and are being sent into heat by R losses in the cable and also in
dielectric loss. And it does not have to be at 5,10, or 20 kW either.
Most plasma processes require 500 watts to 2 kW of power. On top of
that we don't use wimpy matching networks like you find in a typical ham
consumer network. These guys are $6000.00 matching networks, they will
do 1,2, and 5 kW forever. And they use very high quality Vacuum
Variable capacitors and silver plated water cooled coils at the higher
power levels. If you compared one of these to the stuff we buy for
$500.00 you imediately see the difference. But they are no magic pill
for the transmission line loss. They just can accept more reflected
power.
Even with all this, the cables get very very hot. At higher powers we
use Hardline cables, or water cooled rods, but thats another story. I am
still talking at levels of 1 kW to 2 kW more common to our world of
radio.
The point of all this in real practicle terms, no decimal points or
integral calculus needed, is that power loss in cables with a mismatch
is very real, very difficult to deal with, and must be well understood
to really know what your power loss is. Jim, when you traded your coax
in for twinline a few months ago (yes I remember) you did a very
intelligent move. You understood that you had alot of loss in the
coaxial cable and you changed. By doing so you basically agreed with
Mr. Best, and also Mr. Maxwell. Although I do not represent myself as
speaking for them. They can do so for themselves.
I also made choices in my antenna systems, I am willing to live with
some loss, but try to minimize it where possible but also enjoy some of
the benifits of having coax in the system.
My intent here is not to embarrass anyone(especially myself), I enjoy
both Jim's and Steve's input to this reflector, and I hope Jim and Steve
will continue to contribute as well as others. This subject is not well
understood. Even though I deal with it daily, I still learn from my
customers, and also am sometimes amazed at what some of my customers get
away with.
Did I ever tell you gus about the guys who were using 20 kW to excite a
Co2 lazer at 80 mHz........man did they ever burn up some cable. But
thats for a couple of beers later at Dayton this year.
73 and GN to all.
Jay, WX0Bannana
Jim Reid wrote:
>
> Aloha,
>
> Well, Dr. Steve Best, VE9SRB, of Cushcraft has tried again,
> and, in my view, erred again in his 16 page(!) article in the
> Winter, 1999 edition of CQ's Communications Quarterly. I
> feel justified in this, my reply, as he names me as one of the
> commenters about his work earlier on (see the last sentence
> of his text material, just before the 5 pages of tortuously
> complex math, including complex algebra calculus, but that
> is what Ph.d's do, isn't it.) I was not alone in trying to help
> Steve understand Walt Maxwell's work which he published
> nearly 25 years ago, particularly in the Dec. 1974 issue
> of QST.
>
> In his 16 page piece he attempts to prove that conjugately
> matching the antenna system at the shack end of the
> transmission line results in great signal losses.
>
> He uses a rather absurd mathematical and experimental
> antenna model; one with a feed point complex impedance
> of 6.2+j14.4 ohms, or an antenna which will present a
> VSWR of 8.743 to a 50 ohm characteristic impedance
> transmission line! (Note: three 3 decimal place calculation
> is used consistently through out his complex analysis!).
>
> I cannot imagine any amateur setting out to use such an antenna
> on the HF bands, but who knows. Anyway, after verbiage, he
> concludes that this antenna, when conjugately matched to
> the "output impedance" of the tuner/matching network will
> have a total loss of 4.448 dB of the power launched onto the
> line at the output of the tuner. And of course, he is absolutely
> correct, because in his mathematical analysis he does not
> bother to set up a 1:1 VSWR match between the transmitter
> and the input terminals of the tuner network!! In fact he leaves
> that VSWR as being a value of 3.393!!(Note again, all this
> 3 decimal place exactitude is Steve's work).
>
> Yes, it is a chore to attempt to read and follow his work, as he
> scatters the parameters with which he is working all through
> the piece; you really have to dig around to find out how
> he gets places at times. And I am not sure he did his
> conversion of the line alpha attenuation for dB to nepers
> per meter correctly, se the 12th page of his piece. Here
>
> he divides the alpha dB value by 868.59; per what one
> learns in engineering school, you convert alpha in dB to
> nepers by dividing by 20 times the log of the base of the
> natural logarithm, or divide by 8.6859. Seems to be a
> factor of 100 here that I did not follow in Steve's math, but
> perhaps he is correct.
>
> If you dig deep enough into the article, you find, in addition
> to the impedance of the antenna Steve is using, that his
> work is being done at 21.2 MHz, and the line he is using
> is 156.19 feet of RG213 coax.
>
> Of course his value for the total loss of 4.44 dB is absolutely
> correct. And you could learn this for yourself from Maxwell's
> published info of long ago. Walt updated all in the 1990
> publication of this early material in his ARRL published
> book, "Reflections." In fact Maxwell's figure 6.1 in his book,
> to which Steve was long ago referred ( even Ed Sleight
> loaned Steve a copy of Walt's book, as Steve was not
> aware of his work!) shows precisely this same value
> of total loss when the antenna system is NOT correctly
> and conjugately matched. If you have access to Walt's
> book, have a look at the lower right hand corner of
> his Fig. 6.1, page 6-8 of the book. There it is, in
> very black ink: 4.44 dB loss when there is a VSWR on
> the line of 8.7, and when NO conjugate match exists.
>
> If I am reading Maxwell's book correctly, and using his
> Fig. 6.1 as he intends, then with reasonably low loss
> line, and even a VSWR of 8 or 9, the total loss, when
> the line is conjugately matched would include, say for 175
> feet of RG213 with about 1.3 dB loss per 100 feet at
> 21 MHz, added loss of only about 0.5 dB caused by
> the operating VSWR of 8 or 9 because of the odd
> antenna impedance of 6.2 + j14.4 ohms.
>
> I cannot imagine any HF amateur putting up with that
> high a VSWR on any antenna line, and attempting to
> match it away using a tuner in the shack. Such an
> antenna impedance might be presented by a short
> whip on a mobile set up, but then base loading at
> the antenna is used to result in a much lower line
> VSWR. However, in a short vertical whip,
> the proper loading is to use an inductor coil,
> as the antenna appears to be a series RC circuit,
> so the complex reactance is capacitive, or a
> negative value, rather than positive as in Steve's
> example. Guess Steve was modeling a long wire
> antenna, though he never describes it beyond
> giving the impedance numbers.
>
> I believe it is wise to try to get within around 3 or 4
> to 1 VSWR's, then use the tuner to get all the power
> from the transmitter up to, and radiated by the antenna.
>
> As I wrote at the beginning, I have written this because
> Steve mentioned me in his piece. I argued with Steve
> long ago, and as he seems to acknowledge in his
> reference to those who had commented earlier on his
> work, I do not support nor endorse his work. Perhpas
> I am completely wrong. I have given my views above. I
> firmly believe that Walt Maxwell made the issues very
> clear, and very succinctly in his writings over the past
> 25 years, and it is not clear to me, in any case, what
> Steve is hoping to achieve in his complex work.
>
> He clearly points out the problem of great transmitter
> power loss, and a very hot transmission line, if
> an antenna of great difference from the transmitter's
> output impedance is not somehow compensated
> by a proper impedance matching network. Of course,
> with tube output rigs/amplifiers using either the old
> swinging link coupling, or the modern Pi-L circuits
> in use in most of today's linear amps, you can
> dispense with the separate tuner unit, and just
> use the output plate circuit to do the impedance
> matching to whatever impedance the transmitter
> "sees" at the input to the transmission line running
> on out and up to the antenna.
>
> Now, I am sure if I am wrong in criticizing Steve's
> work, it will be brought to my attention by many,
> and I will indeed be chagrined and humbled!!
>
> Copies of this are sent to all parties interested in
> this discussion in the past,
>
> 73, Jim, KH7M
> On the Garden Island of Kauai
>
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