[TowerTalk] Re: Ref. Length 500/F= Ft
K7GCO@aol.com
K7GCO@aol.com
Fri, 1 Sep 2000 08:20:42 EDT
In a message dated 9/1/00 4:29:01 AM Pacific Daylight Time, K7GCO writes:
<< In a message dated 8/26/00 8:32:20 PM Pacific Daylight Time,
wb6psy@earthlink.net writes:<<
Hi ya'all - as I have the Antenna Handbook dated 1976, there must've
been some changes since. I'm looking for the velocity for RG213U.
I'm also looking for the formula for a 5 el parasitic array length and
element spacing if they still use, e.g.
Ref: Length (ft) = 500 ÷ ƒ (mhz).
Last question (for now) is what do people use today to coat (or not) the
elements that slip into each other and boom connections? WD40,
Silicone, etc., or nothing? Comments please!
73 de Rene >>
Rene: Try the greases with aluminum particles.
Rene: The length 500/fMHz=Ref. is not a fixed length for any number of
elements, spacing or element size and taper. Expecting to find a formula
that fits all beams and variations is "Totally Unrealistic"--most of the
time. You would clearly see this after tuning up some beams in Eznec and
it's a great way to enhance Ham Radio interest in the long winter months or
low sun spot cycle. Inaddition the beam Reflector will also be further
massaged depending if the beam is tuned for Gain, F/B or in between. I have
a variety of Reflector lengths which resulted from what ever all the factors
determined after Eznec did it's thing or the test range suggested. The final
lengths for all designs all may be close with some exceptions. Some times
it's a critical setting and sometimes not and varying just the Reflector
spacing may require a reasonable readjustment. After you have done the very
slow and boring "cut&try" on the range or the very quick "cut&compute" in
Eznec for the first couple hundred beams, you see all the variances you
didn't even know existed. Not knowing all this you may just look for a "ball
park figure" on TT which can at times be real close or right on.
A Tale Of An Unknown Reflector Length: In 1956 I observed in Seattle a
great 20M beam that was tuned up in the Boeing Antenna Labs on a week end in
the late 40's and 7 of them were made from Boeing Surplus with 12' X 1 5/8"
center and 1 1/2" tips. It was tuned for gain using .2R and .15D (.35 W/L
total) using a 100 ohm FD DE of proper diameter ratios for a 100 ohm balanced
feedline. It had 2 ears on the back where the 6's came in the back door when
pointed on Europe. They went back and retuned for F/B and lost about a dB
gain--8dBi now. That's just 1 dB more gain than a 2 element quad on a .12 WL
boom. The pattern all around the back is one of the best I have ever seen
and it filled the high angles up front very effectively due to the wide
vertical pattern--an asset (-3DB point @48 degrees). No more problem from
the 6's. This feed system had virtually no vertical noise pick up and with
balanced feed into the receivers of the day, it heard a couple of layers
down. I had to have this design but they all guarded it with their lives. I
finally got the spacing and director length and only needed the DE FD 100 Ohm
Configuration, Reflector length and I knew the tubing sizes.
One day I dropped in to visit W7AJS who is still alive, he had it down for
service, wasn't home and I just happened to have my tape measure. The
Reflector was a Standard Length of 496/F and the DE I could or would have
adjusted to resonance. Had I used the "Standard Reflector Length" of the day
I'd had the use of this design a few years earilier. However even then I
knew all the variations that determined the final lengths and I have to admit
I didn't shot gun the Reflector length 496/14.2 sooner which I'd have used
the first try. This design is one of my F/B favorites. I have that original
beam now. It would cost well over $1000 for the tubing and square boom now
just to purchase the aluminum in small quantity rip-off prcces.
After a couple of hundred beams you tune up in Eznec, you have the strong
desire to get on your knees, point your body toward Roy Lewallen and do
various bowing ceremonies before and after the computer session. I recommend
all that can get Eznec for a "New and Unlimited Horizons of Interest in HR."
Many beams in the past were "shot gun tuned" and occasionally were very
close depending on their simplicity. A 2 element quad perhaps had the least
bad tunings and always had a high feedpoint Z which is always a +.
Inaddition, whatever pattern and DE Z you ended up with, it remained due to
one soldered and permanent joint.
Now instead of playing the massaging game for each and every element
even as fast as it is in Eznec, I still look for short cuts and design
improvements and have a few. I would like to state that I think Chuck Smith
of RaiBeam came up with what is the "first design simplicity break through in
yagi design" since 1922. He found that with his version of a simple 2
element "Critically Coupled" 2 element "gain cell" as I call it, when he
added directors--it increased the gain and didn't up set the great and
original F/B. (NOTE! I've never seen this in any other yagi. You can have
great F/B and add 1 director and it goes to hell and often hard to correct.
Sometimes 2 directors need to be added to regain F/B.). One reason is
attributed to his using a fairly wide spaced 1st director and he was able to
ELIMINATE the normal closer spaced director frequently used now and
essentually get the same gain with one less element for that boom length.
Standard spacing was then normal for additional directors and still the F/B
held. The 1st wide spaced director essentually acted as a "F/B Change
Buffer". Note! I just coined a new term for a new design concept. Therefore
he didn't need another parasitic Reflector to clean up the F/B when adding
directors. That being the case the typical .15 W/L reflector spacing used
for 78 years can now be used for another director which serves a "far more
very useful design and gain purpose." I will later be doing some comparison
testing of similar boom lengths but for the time being I wouldn't be too
concerned if there is anything for the size of better performance--all things
considered. Simplicity of design without sacrificing performance is always
desirable and occasionally done. The Basic Raibeam 2 element has a great
overall pattern on a short boom and the "proper SWR curves" that doesn't rely
on an Antenna Tuner (as some antennas do) for maximum transfer of power. I
think the 6M 2 element weighs 3 lbs if I remember right. I could stack a
couple of these on a "Wet Noodle." I want to see a 2 element 40M RaiBeam.
The Raibeam concept makes the most efficient use of elements I've ever seen.
"Simplicity with a Bonus" is something you don't really see very often these
days--in particular on a 78 year old design.
I'll be happy to recognize any further improvements of "Innovative
Antenna Design" such the GAP Verticals when I can find them and also give
them "K7GCO Advanced Design Seal of Approval Gold Star." This guy "Chuck
Raibeam" is a real clever fellow with his design that obtains maximum gain
and F/B for the boom length with less hardware and a strong virtually
maintenance free design. A 'Magnetic Poll Shift" might put a bend or 2 in
it. Send me your nominations. I'd like to start an "K7GCO Antenna Hall of
Fame" and give credit where due. Right now I have a long list that qualify
for the "K7GCO Antenna Hall of Shame."
I like the Tri-Ex Sky Needle Towers also as they solve major problems if
you can get them. Open wire line still works great with a Johnson Match Box
also and I doubt there will be any replacements for this combo for long
lengths. There is a lower loss tuner than the JMB I use with "1 Adjustment
Knob" for a fast tune up. 100 ohm balanced. k7gco
--
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