<< In a message dated 5/17/01 9:39:16 AM Pacific Daylight Time,
tmartin@chartermi.net writes:
<< I have read so many posts about baluns and SWR that I am thoroughly
confused! I am going to be erecting a 96 foot tower with a 5 element
Telex 20 meter monobander and a 2 element S-402 by Mosley and a 3
element Telex 10 meter monobander. I want to avoid the ferrite core
balun and use a coax balun. I can get no consensus about how many
windings, how many inches for the form, etc. The manuals, antenna
books, and TowerTalk members suggest 12 turns, 6 turns, 8 turns, 4-6
turns, all on either a 4 inch form, or a 6 inch form, or air wound and
taped. The list of suggested turns and methods of windings goes on and
on, not to mention what type of coax to use: 213, 58, 8U, 8X, teflon,
etc. The Mosley instructions don't mention a balun at all which I didn't
use at a previous QTH. I didn't notice any problems with front to back,
gain, or TVI/RFI without a balun on the 402.
So, what's a fellow to do???
Also, I agree that trying to get the perfect match 1:1 SWR is an
obsession with a lot of hams. Of course, the use of solid state rigs
has fostered this mania. My 830 will get out fine with a 5-1 SWR.
However, it is a bit disconcerting to see the reflected meter pegging
the post on voice peaks when using the amp. 73 to all. Tom
>>
Tom: The so called "coax choke" does very little in the high current
section near the feedpoint. You really need a better feed system instead of
another "Band Aid." There isn't enough capacity between the too few turns to
create resonance and an effective Hi-Z choke contrary to what many have been
led to believe. If you check the RF Spill Over beyond the coax coil choke
with a "Palomar RF Meter," there is still a lot of RF below the choke in this
area contrary to what you have been lead to believe.
The area in the coax where the coax coil choke really works is in the
Hi-Voltage Area 1/4 wave lower but that is too far down. This is right at
the Hi-Voltage area where the capacity is higher between the turns and an
Effective Hi-Z Choke is formed with the right number of turns. When you have
the right number of turns you can run your fingers down the coil and the SWR
is upset progressively less and less to near the bottom where there is "no
affect" on the last 2 turns. I call it the "Truman Choke-- the RF stops
here."
I use it in making 1/2 wave center fed verticals. The 1/2 wave L
configuration works great. The coax shield is at the bottom 1/4 wave up to
the coax choke.
All these "Band Aids" on TT on beams, rotators and towers usually don't
solve the major problems. The major problem is a feed system that has RF
Spill Over. So far I seem to be other only one warning everyone of this
"Evasive Problem" that is so prevalent with coax. Most Mfgs don't even know
how prevent it and in some cases--couldn't care less. A simple test is to
place your hand on the coax shield near the feedpoint and see if the SWR
changes on the MFJ SWR Analyzer. If it does you have a bad feed system. The
gamma is the worst of all unless it's unbalance is compensated for and there
is a way to do it. Shift the DE to one side and use the "K7GCO Hand Balance
Test."
On uncompensated gamma match systems you can also place your hand on the
center of the DE and the SWR will change. When feed systems are "properly
balanced" you can touch the center of the DE or the coax shield and there is
no change in SWR on the MFJ SWR Analyzer. I can also touch the ground of the
MFJ back at the center of the DE wiht a length of coax and there is no change
of SWR "when things are balanced". The Palomar RF Meter on the coax reads
"Zero."
Would you believe that when you get the DE and feed system properly balanced,
the center of the DE where "Zero Voltage" now exists perhaps for the first
time, there is "Zero Current." It is so cold there a "RF Frost" is created.
If you put your hand there for over 5 seconds it will be frozen there. You
will have to unbalance the DE to get some RF current there again to thaw out
your hand to remove it.
Now that's an "Absolutely true story--I just made it up."
When I make the balancing adjustments I use just a 1/2 wave of coax to the
MFJ. I also find less SWR change as the beam is rotated. On some beams and
towers I've had 1:1 SWR for 360 degrees of rotation which is not often seen
in particular with a 75M inverted vee 10' below used as guy wires. A lot of
so-called surrounding objects affect is not the object unless real close but
the RF Spill Over from bad feed systems on to it that varies as the beam is
rotated. In other words the beams feed system is often the problem--often
not the nearby object. Try a good feed system and you will see this.
The 200 ohm T match like Telrex used was a good feed system and a factor of
their good performance. Telrex lengthened the DE to compensate for the T
Match reactance. I like balanced 100 ohm coax which is ideal for quads. I
have lengthened Yagi DE's and used a series Xc's to obtain 100 ohms or to
even 50 ohms and 1/4 wave 75 ohm balanced coaxial stubs to get the 100 ohms
for the feedline. A 25 ohm beam Z can be matched to 100 ohms with a 50 ohm
balanced coaxial 1/4 stub. A 36 ohm beam Z can be inverted to 100 ohms with
a balanced 60 ohm coaxial feedline (there is a way to make them)--No RF Spill
Over. Baluns or ferrite cores or anything else needed just as you desire.
I've used FD DE's of 100 ohms also for the 100 ohm balanced coax--No RF Spill
Over.
I have a 10M 3 element beam on a .35 WL boom tuned for absolute max gain of
10.2 dBi. It has 8 dB F/B, a 6 ohm feedpoint Z and narrow bandwidth. 6 ohms
is too low to step up so I use another technique to get 50 ohms and it's
patentable. No RF Still Over either. I now have 3 ways to broadband it
again. I have always liked great F/B but 3 years ago the band was dead on
10M and if there was someone on the front of the beam there was never anyone
on the back. I needed max gain and not F/B. I wanted to see just how this
design would work since no mfg has ever marketed this design and I certainly
found out. It turns out that this beam has been a "Real Killer" on 10M and
with the band open, poor F/B hasn't been a problem. The poor F/B keeps the
band open on the rear also. It often has "Infinite F/B." I will build this
on 12,15,17,20&40M also. I will also tune 3 element quads this way also.
W7WJP had one and it "Really Talked." It had a 25 ohm feedpoint Z matched
with a 36 ohm 1/4 WL stub.
With link coupling in the final tank circuit the 100 ohm balanced coax
connects nicely. Or use a Johnson Match Box. There is another simple way to
convert 100 ohm balanced coax into a single 50 ohm coax without a tuner in
the shack.
I've mention the need for "Zero RF Spill Over" many times, the advantages
balanced 100 ohm coax and other better ways to get max RF into the antenna.
Yet all the "Band Aids" for poor design dominate on TT. This is the year
2001 and bad techniques still predominate with the mfgs and hams. The open
wire line from the 20's still works absolutely great into a T Match and beam.
Open wire line still works great into LF dipoles using 1/4&1/2 wave
multiples when you know how to use tuners. Some do not and state so
regularly without realizing it..
There is a saying "Progress comes one (Opinionated Old Timers) death at a
time." Many mfgs thrive on "John Cheap Skate Retiree Hams Budget". I've
made Special Arrangements in Hell for Special Ovens for them. Others like
M2, Antenna Mart and Raibeam are doing a great job in giving better designs.
I've been talking to them and who knows--there may be some more better ideas
showing up--with No RF Spill Over. If you can get to Sea Side, Or Ham Fest
June 1,2&3 you can learn some new techniques presented at the antenna talks
for beams and a great Mobile Antenna idea that makes Mobiles "Speak Up." I
just got a FT-100D.
I have just installed 2 Raibeam 2 element beams vertically polarized on a 10'
boom and 10' higher the 6M Raibeam 5 element "Butt Kicker" on a 22' boom
horizontally polarized 10' higher. I can instantly select either 2 element
individually or both tegether, or the 5 element on top or both--In, 90 or 180
out of phase.
I had a similar set up on 10M around 1960 and I owned 28.503 MHz. I never
had to move. I used 2 of the old Workshop Beams from the late 40's of 5.5 dB
gain each on a .29 WL boom detuned to 50 ohms vertically polarized and one
20' above horizontally polarized. I still have a great 4X5 negative picture
of that. What really surprised me was that the 2 vertically polarized 10M
beams of only 8.5 dB gain 35' high over Seattle's low ground
conductivity--did so well.
Would you beleive the angle of radiation was so low it kicks up a dust--"RF
Dust That Is." I have a lake in the back yard and it kicks up an "RF Mist
Fog Cloud" in the shape of the reflected RF beam pattern. You have to wear
"Special RF 3D Glasses" to see it. It's been there all this time but no one
ever used the "RF 3D Glasses". When the ducks on the lake or boats sail into
it, they dissappear like in the Bermuda Triangle.
That's an absolutely true story also--I just made it up.
One Ham meter reader heard me on his Hearing Aid 3 blocks on away on 10M and
honed in on me. That was "HAI". I will recreate that configuration on 10M
and other bands again as I just have on 6M. Now if 6M doesn't open this
year I'll really be "RF Pissed".
There is a spray for beams that prevents frost and ice from forming I'm told.
It's Maine product I will test it in SD this coming winter where Ice
loading can occur. An ice laod is bad enough and if the wind comes up--you
are in trouble.
K7GCO
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