[TowerTalk] Steve Best on Mismatch loss, transmitters and tuners
Jim Reid
jreid@aloha.net
Wed, 22 Jul 1998 10:41:02 -1000
Steve wrote:
> I will use this one message to respond to many.
Me to:
I have to join into this discussion, miss-concepts must be refuted:
> Ward's (N0AX) statement:
>
> "I think there is some confusion on nomenclature. I disagree that
> VSWR affects antenna gain, but agree that VSWR affects *system
> efficiency*. Antenna gain is only a function of antenna geometry.
> System efficiency includes antenna gain and line loss, but the two are
> independent."
Steve wrote:
> I will agree with this statement. The IEEE standard definition for
> antenna gain does not include the effect of VSWR. VSWR does impact
> system gain. However, as an antenna manufacturer, we directly compare
> antenna performance to a reference antenna with a good match. Since
> the difference in the two antenna's VSWR's results in a "gain"
> difference between the two, we include it in our antenna gain figures.
> The real issue is working with system engineers - they always need to
> know how we define "antenna gain". Most end users (maybe commercial
> NOT amateur) are not aware of these subtle definitions.
Above OK.
Steve goes on:
> Ron's (KK1L) statement:
>
> "Now wouldn't this also mean that you are creating a nice reflection
> back up the line to the antenna? This should provide more power at the
> antenna for it to radiate."
KH7M comment: No, the power is re-reflected back from the antenna,
no more power exits the antenna than left the transmitter terminals
Steve goes on:
> "Most tuners will totally dissipate the reflection such that it will
> not be delivered to either the transmitter or the antenna."
KH7M:
This statement IS NOT TRUE!!!! All tuners will again re-reflect the
power; they certainly will not dissipate it, save for a VERY tiny
amount of I-squared-R loss in the tuner inductor.
Steve, I have a Bird 43 watt meter that says I am sending more power
out my transmission line than is coming out of the transmitter!! From
where do you suppose that power came. The Bird is at the OUTPUT
of my antenna tuner which I must use as without it I have a pretty high
VSWR on the transmission line connected to the transmitter. By the
way, the Bird shows a couple hundred more watts up the line after
the antenna tuner than I can measure out from the linear when it is
reading 1500 watts on SSB peaks, or on CW, key down. Funny,
nothing is getting hot either. I suppose even 50 watts dissipated
within my tuner would feel pretty hot pretty quick (or at least
the tuner inductor coil turns involved should); ever touch a
50 watt light bulb, or even a 35 watt soldering iron??
By the way Steve, have you read the book "Reflections" by
Walt Maxwell, W2DU?
Now let me bring in another statement by Steve from an earlier post
by him:
" Consider the reflected power. It too is travelling down the cable but
towards the transmitter. The transmitter is connected to the cable
so, to the reflected wave, it is a load at the end of the cable. This
reflected wave will only be sent back towards the antenna if the
transmitter impedance has a VSWR much much greater than 1:1. Most
transmitters are quite good and should have a low VSWR, therefore,
most of the reflected wave from the antenna will be delivered to the
transmitter where it will be dissipated. Transmitters get hot! "
This is ridiculous!! Transmitters get hot, but not because they
are dissipating reflected antenna power!! If true, the output
inductor of the transmitter would soon melt, as the reflected
power keeps on a'coming!
And Steve goes on:
" On a different subject: Tuners.
" Tuners do nothing more than fool
[ !!!? KH7M, I thought the tuner behaved as any other tuned circuit
would, matches impedances. ]
the transmitter into thinking it is
looking at a matched cable. If you connect a tuner at the input to
your cable and "tune out the reflections" you will have a low VSWR
between the transmitter and tuner and the SAME high VSWR between the
tuner and the antenna. Tuners dissipate or cancel out the reflection
from the antenna so that it does not reach the transmitter. Tuners
get hot too."
Really? Funny, mine never does. Ever reach over, with the
cover off, and touch the inductor within the tuner, when not
transmitting, but immediately after, of course. "Cancel out"?,
how, explain, please.
Steve continues:
" You do not radiate more power
[KH7M comment: Of course not.
Steve, what happens to those couple hundred extra watts read
on my Bird, that did not come out of the transmitter, were they
generated in my tuner?? ]
unless you allow the
transmitter to output more power because it does not see the
reflections. Sometime when you are up and running, put your power
meter between the transmitter and tuner and then between the tuner and
antenna - see what you read. [ What do you think would be read,
Steve??] Make sure the antenna has a high enough
VSWR so that you see reflected power.
73
Steve Best"
Again, more fiction!! I have written above about what my watt meter reads
in both places, that is, before and after the tuner, and I await Steve's
explanation given his beliefs.
More by Steve in an earlier post:
"If you feed an antenna with a 20:1 VSWR you will
give up 7.41 dB of radiated power. VSWR results in a mismatch loss
regardless of cable attenuation. "
Steve, then why, when using the Smith chart to design such a system
as you go on to describe, it is true that the greater the reflection loss,
the better the system match is to the antenna system???
Steve goes on:
" Antenna impedance matching, VSWR and mismatch loss have been one of
the principal aspects of my antenna design career. When I worked
under contract with the US Navy designing and developing HF shipboard
antennas, impedance matching and reducing VSWR was the principal
concern due to power loss. In some cases, we even used 3.5 inch
diameter air heliax coax (very low loss). "
KH7M [No wonder DOD systems are so expensive!]
"At of the companies I worked at, we actually constructed very detailed
1/48th scale models of Navy ships out of brass. We would then
construct 1/48th scale model antennas and perform impedance and
pattern measurements at scale frequencies. The results of these
efforts were translated into the design of full scale shipboard
antenna systems. Once the antennas were installed aboard ship, we
would conduct full scale measurements and matching to ensure the
antenna worked as designed. Again, minimizing VSWR was always the
most important issue. We were the only private company in the U.S.
that conducted 1/48th testing and design of this type.
I wrote a short article on VSWR, reflections and mismatch loss. I
sent it to Ed (K4SB) to look at. If anyone else would like to get it,
please let me know. I would also be very happy to share some of my HF
antenna design experience regarding shipboard HF antennas.
73
Steve Best"
Well, no wonder the Navy spends so much money! Yes, I have seen these
scale models and model test ranges at the Navy electronics facilities
at San Diego, out on the point. Beautiful, and very expensive works
of art they were and are. Do wish you had known Walt Maxwell when
he was designing antenna and antenna feed system for NASA for
satellite applications., many of which are still in orbit, and working
just fine with pretty high vswr in the feedline systems.
Steve, I hope you understand what Tom, W8JI, has posted in the last few
hours, also commenting on your posts. Tom has the 20:1 vswr set up
you quickly judged would have over 7 dB of "mismatch" loss.
I have written enough.
73, Jim, KH7M
On the Garden Island of Kauai
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