[TowerTalk] 1/4-1/2 wave nonsense
K7GCO@aol.com
K7GCO@aol.com
Tue, 26 Sep 2000 11:53:03 EDT
This Post has info not found anywhere it seems and "clarifications" for those
who have trouble with routine feedline practices.
In a message dated 9/19/00 2:19:13 AM Pacific Daylight Time,
w8ji@contesting.com writes:
<<
<<Adding to Wes's well-made points about mismatched lines
narrowing bandwidth, and losses preventing a perfect repeat of the
impedance at the other end, it's also important to consider what
the tuner or matching device sees.
*******After that deep and profound consideration is made of the feedline
length change with frequency example given I have to say "Enough Theory" &
"Too Much Math"--lets get down to basic and good engineering practices that
reduce these impedance variables and math. If a long 75 ohm hardline is used
on 10M with Phone/CW wide excursions of frequency connected to a 50 ohm load,
expecting a 75 ohm feedline of 1/2 wave multiples to bail you out and reduce
the over all SWR, resistive and reactance variances in particular when
connected to a 50 ohm output final--is just not good thinking. You can get a
good match at one frequency where it's a 1/2 wave multiple and 50 ohms at the
antenna. Short lengths of line can be added as suggested by N4KG to
compensate for the electrical shortening of it operating lower in frequency
and unfortunately is another "Band Aid." You will be on a SWR Roller
Coaster" on each side. You can do this on 30,17&12M with 100 KHz bandwidth.
50 ohm finals reduce power out with loads other than 50 ohms and then at
certain levels of SWR, most of them start to reduce power output even more to
protect the finals from excess heat dissipation--it's the price of
convenience first developed for the CB'ers. It's a build in safety feature
simplifying tune up of mismatched tuners. When the desired 50 ohms is
obtained or close you have "RF Lift Off." But this system example was
convoluted with 75 ohm coax hardline used on a 50 ohm Ioad and 50 ohm RF
Source. I have 2 rolls of it also but I'll use it as suggested next.
PLAN A is a simple corrective measure of changing the antenna matching device
to 75 ohms (which any creative ham should be able to do) and a much lower SWR
is obtained over the same frequency range on the coax for lower losses. The
Zload values center around 75 ohms now, however. The built in tuners in most
rigs can handle this fairly well although it's kind of an "up hill matching
process" requiring a tuner which at high power is a chunk of money and a
nuisance to keep tuning up--or burning up. In the transceivers they are
automatic. The ultimate goal should be to use an antenna system that
"doesn't need a tuner"--when possible.
PLAN B is to use a 50 ohm pi-network final as it will still allow it to be
loaded to full power input over a wide SWR variation without a tuner although
the output is slightly lower due to the wrong Zload reflected back the final
tube for loads other than 50 ohms.
PLAN C is to alter the inductance in the tank pi-network so it's optimum for
75 ohm loads. This can be done in many finals. No additional tuner is
needed which is a big savings and nuisance eliminator with legal power. That
will increase the average output over the whole band--for those who "split
dB's". I split a few myself.
PLAN D is to use a "Link Coupling Final Tank Circuit". By selecting the
number of turns in the link circuit and the use of a series Xc for balanced
or unbalanced lines, any Z up to 1000 ohms can be matched with the optimum
Zload reflected to the final tubes. Translation: Full power output with the
least heat dissipated over the band and No External Tuner is needed. Good
tube life was common. I'm here to tell you Art Collins hasn't buried link
coupling and open wire line yet. As Truman said after reading of Dewey's
Presidential Victory--something to the affect the "election results were a
bit premature--I'm the winner".
If feedline length isn't "planned", it's quite possible to have an
impedance that is nearly impossible to match on some bands.
Lengths to especially "avoid" are odd-quarter waves on bands
where the antenna is 1/2 wl long.
****** Yes and No. End fed or center fed?? A very poor "unplanned"
example--use specifics! A 1/4 wave of open wire line will give a Hi-Z load
at the end of the feedline from the center Lo-Z of a 1/2 wave. Without
resonant lengths of either or both you can indeed have reactive loads that
can be difficult to match. An intelligent selection of length avoids that
totally. It's a "resistive load" that is easy to match when both the antenna
and the feedline are resonant even reasonably close and that is why either
1/4 or 1/2 wavelengths are suggested by all authorities--to reduce or
eliminate reactance. One must learn how to "Follow Instructions". NOTE! I
should like to point out that the Johnson Match Box will handle higher
reactive loads than most tuners due to the use of a dual differential
variable capacitor--it allows balanced ir unbalanced operation also.. Learn
how to use it. That's specifically why it was developed.
1/2 wave feedlines connected to Hi-Z feedpoints like 2 half waves in phase
will present a similar Hi-Z value to the tuner as the previous example. It
would therefore based on your general condemnation, be another no-no that
would be hard to or impossible for the MB or other tuners to match and
virtually make these antenna impossible to use. Gimmee a Break! ABSOLUTELY
NOTHING COULD BE FURTHER FROM THE TRUTH AND SUGGESTS YOU HAVE PROBLEMS WITH
THESE TECHNIQUES OF TRANSFERRING RF FROM THE FINAL TO THE ANTENNA. THIS TYPE
OF ANTENNA SYSTEM WAS THE "BREAD & BUTTER ANTENNA" PRIOR TO COAX AND THERE IS
NO SYSTEM YET TO THIS DAY THAT EXCEEDS ITS FLEXIBILITY, EFFICIENCY, INITIAL
AND LONGEVITY LOW COST. IF YOU CAN'T MAKE IT WORK--THOU SHALT NOT SPEAK
BADLY OF IT.
If you had a problem I'd review your procedures and use the very lengths that
the books and charts suggest. The authors have been there and done that.
I've even modeled open wire lines in dipoles in Eznec and it gave the
expected Z's--even from the "Dreaded 1/4 Wave" and is was "full steam ahead"
with my projects as I "saw absolutely nothing or any red flags" that would
prevent the desired goal and what MB Tuners couldn't match. There are many
hams (I'm not alone) that can make theory work with proper planning.
For example:
With a 450 ohm line and a 50 ohm (dipole) load, using a 1/4 wl line
(lossless case) input impedance will be over 4000 ohms.
*****The match above is close but shy by 50 ohms--no big deal.
On the second harmonic input impedance of the system feeding the above
dipole,
impedance could be as high as several thousand ohms.
*******In order for the 450 ohm line to invert the antenna impedance to say
7000 ohms, the antenna R has to be 28.929 ohms to be exact using the 1/4 wave
stub formula. Do you have a different one? The charts and Eznec for LW show
the R at the Lo-R points get "progressively higher" (not lower) as the LW
increases in length ir the frequency increases. So where is this Antenna
Lo-R originating from at the higher frequencies? It just doesn't not exist.
In over 60 years of using LW's I have yet to observe this? Is there anything
left that doesn't work? Where do these problems all come from?
Very few tuners, including the old Matchbox, will work with such a load.
******* "Absolutely Nothing" could be further from the truth. The MB was
designed especially for these antennas and--Johnson didn't screw up. I
haven't found a Hi-Z load I didn't like or couldn't match in LW's, Bobtails,
top loaded verticals or at the end of feedlines with L-networks and other
tuners. 1/2 wave verticals are self resonant and don't require a radial
system for that reason and any ground losses that exist in any radial system
if used are insignificantly small compared to the Hi-Z feedpoint of the
antenna at the end and that's a big advantage of 1/2 waves or any "voltage
fed antenna". Forget about feedpoint losses compared to 1/4 waves. The
"lossless case" mentioned above is totally irrelevant as when the Z's are
high the currents are low in feedlines also. The affect of typical or even
higher wire resistance is virtually unseen ratio wise. Hey, open wire line
is "air cooled" and with porcelain spacers will take a lot of heat--if you
have that much power. Enamel and or other insulation doesn't turn black
either like that that modern and expensive coax. In fact stainless steel
wire is used as a very inexpensive lossy 600 ohm feedline and termination
load. It's air cooled also out of necessity.
Every Match Box you unjustly create non existant limitaitons for that I have
including the ones I made with plug in coils and variable inductance's
instead of taps, the "Parallel and Series Configurations" designed
"specifically for Hi-Z and Lo-Z loads" shown in ARRL Handbooks starting back
in the 20's & 30's and others tuners I have, match these Hi-Z's just fine to
50 or 75 ohms for the feedline to the rig. I do not have one single
exception. I wouldn't have recommended the MB many times if it couldn't
match the Hi-Z's of the antennas I have suggested. No one else in ham radio
has ever reported to me it was deficient in any way. If there was a major
deficiency which I've been able to find, I'd have fixed it as I did on some
minor ones. I'd like to meet the MB designer. Does anyone know who designed
it? Please let me know. I'd like to give him the "K7GCO Best Designed Tuner
Award" even though he has critics.
Most of the ARRL tuners didn't have the series Xc in the link to ground and
that limited some loads it would match. They were told about that starting
back in the 30's, repeatedly after and never ever changed it. QST has also
consistently shown balanced feedpoints fed with coax directly, obviously
don't know what RF Spill Over is, ruined F/B's or RF in one's audio does,
have been told about it repeatedly but it still occurs in article after
article and year after year. In ON4UN's Antenna Book printed by ARRL he has
donut toroids on the feed end of all his coaxes. To me this gave "Instant
Technical Credibility" to his book. Somehow he got it past ARRL and I
purchased the book. Why do circuit improvements continually take 20-40 years
to finally get accepted? Thre is an old Axiom of "the present policy setters
have to die off."
The reason I point this out is some handbooks suggest making the
feedline a 1/4 wl long or multiple of 1/4 wl so the feedline is a
"tuned feeder". That's generally a bad suggestion!!
******That is "specifically the feature" of open wire line that reduces the
reactance created with other lengths. The term "tuned feeder" is a bad term.
It's a "resonant feeder." A "Classic Example" is: 1/4 wave matching stubs
match one resistive load to another--sometimes fairly wide--WITHOUT the
introduction of reactance's. Note! No tuner needed! 1/2 waves repeat what
it sees on the other end and also WITHOUT any additional reactance. Why is
this a "bad suggestion?" Explain in detail. (No reply yet). What part of
resonance and lack of reactance don't you understand?
When the Johnson Match Boxes first came out I tested the 275 and 1 KW models
for Johnson. The 275W model had a problem. The input link around the tank
coil has 5 turns with a 5 turn tap for a 300 ohm and a 2 turn tap for a 50
ohm input. They had them reversed. They did listen contrary to ARRL. All
275W MB's should be examined for any with the reversed link connections.
Either model should be totally taken apart and examined for burnt switches
and other problems as some have been abused and or cooked with feedlines
other than resonant lengths.
QST frequently suggests "to use any length of open or ladder line and a 4:1
balun or tuner." That is very very bad advice and a "Technical Spanking" is
in order here. That will seldom work. They have poorly advised many which
has been clearly evident. A switch added to select 2,3,4 & 5 turns of the
link for a 50 or 75 ohm coax input is a great idea to enhance it's Z matching
capabilities. The MB doesn't normally need the series Xc in the link either
although it can prove useful with some odd loads. It's basically a "2 Knob
Tuner" that is easy to adjust. I and many others never needed the series Xc
or even other than the standard 2 turn link in the MB for resonant antennas
and feedlines of 1/4 wave multiples as the Handbooks have shown for over 70
years written by those who practice Basic Transmission Line and Tuner Theory
101 and know how to handle RF reasonably well. Frank Jones who wrote the
"Radio Handbook" in the 30's had a total grasp of all this circuitry, was a
great technical friend and Elmer.
Review http://www.karlquist.com for a great 80M vertical installation.
It's fed wtth 1000' of open wire line that has a measured ".3 dB loss" with
transformers. Eat your heart out coax lovers. Open wire line made very
practical a vertical 1/5 mile from the shack. Rick Karlquist N6RK gets the
"K7GCO 80M Vertical, Mechanical, Economical and Low Feedline Loss Award."
For the many who has asked, I will be publishing tuner info (or lack of it's
use) using open wire line information in the future with antennas you can
duplicate and embarrass many of the coax users. Open Wire Tranquilizers are
available for frustrated "call'em again coax users" and the equivalent of
"Coax Food Stamps." As you can see there are many areas of confusion that
still exist perpetuated by many in Ham Radio and signal improvement areas
where you can improve your signal for the least one time cost. I deal with
"Skin Flint dB's." Honest to gosh I wouldn't mislead you as it wouldn't
serve any useful purpose. Some have other goals which continue to evade me?
I have to ask "how long is all this bad mouthing of open wire line and tuners
going to continue when there is nothing to justify it whatsoever? Their
statements totally discredit their arguments to any one who has properly used
tuners and open wire line. Enough is Enough! This information hasn't
appeared anywhere for many years and you got it FREE courtesy of TT and
K7GCO. In you paid for it and it was wrong in any way you have a right to
complain. To any possible critics, we would rather hear "new, useful and
productive ideas" for a change. The goal on TT should be to "Advance the
State of the Art" even with old techniques if there isn't anything any
better. Put up or shut up! K7GCO
73, Tom W8JI
W8JI@contesting.com
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