Yagi plans
 

Does anyone know where I could get plans for a 440 yagi that uses a
conductive boom and is designed for 50 ohms?

I have seen plans but all of them use an insulated boom or insulated
elements.

If i use a conductive boom and ground it, is there no need for a
coaxial lightening arrestor?

wrote:


If i use a conductive boom and ground it, is there no need for a
coaxial lightening arrestor?

I wouldnt plan on avoiding lightning protection, just by useing a
conductive boom, nor for that matter, even loop (electrically short
circuit to DC) feed for ANY antenna! When lightning strikes, it is
going where ever it wants to, in spite of the best grounding intentions
Remember, that if lightning can travel a mile, in open air, it will
go where it wants when it hits an antenna! tho the bulk of the energy
will disperse to ground, there is always SOME thjat will go down the
coax , even in a close strike! And whan that happens, dammage will
occur, especially without protection. I remember working on a base
station during a storm, haveing removed an antenna- the coax was some
1 inch from the edge of the base station, and produced a spark (Ill
let you figure the voltage at that point) , and that wasnt even a
direct hit-- the strike occured some 3-4 miles away! Look at it like
an insurance policy- 50 to 100 bux is cheap when talking about
replaceing a FT-847 (or other expensive rig). "Course the best
protection would be disconnect the radio, and ground the coax. But
then there is still no garentee. Jim NN7K

Jim - NN7K wrote:
wrote:


If i use a conductive boom and ground it, is there no need for a
coaxial lightening arrestor?

I wouldnt plan on avoiding lightning protection, just by useing a
conductive boom, nor for that matter, even loop (electrically short
circuit to DC) feed for ANY antenna! When lightning strikes, it is
going where ever it wants to, in spite of the best grounding
intentions
Remember, that if lightning can travel a mile, in open air, it will
go where it wants when it hits an antenna! tho the bulk of the
energy
will disperse to ground, there is always SOME thjat will go down the
coax , even in a close strike! And whan that happens, dammage will
occur, especially without protection. I remember working on a base
station during a storm, haveing removed an antenna- the coax was some
1 inch from the edge of the base station, and produced a spark (Ill
let you figure the voltage at that point) , and that wasnt even a
direct hit-- the strike occured some 3-4 miles away! Look at it like
an insurance policy- 50 to 100 bux is cheap when talking about
replaceing a FT-847 (or other expensive rig). "Course the best
protection would be disconnect the radio, and ground the coax. But
then there is still no garentee. Jim NN7K

Is it possible to make a coaxial surge supressor? Are there still plans
that use a metal boom? What about static electricity?

Stryped wrote:
"If I use a conductive boom and ground it, is there no need for a
coaxial lightning arrester?"

That is my experience, provided that the driven element is a
short-circuit for lower frequencies, such as a folded dipole or a
short-circuit 1/4-wave stub.

My company had countless examples in high places over the world with no
damage to radios, including many solid-state models, operating 24-7.

W6SAI, Bill Orr gives plenty of examples of "plumber`s delight" Yagi-Uda
antennas in the eighteenth edition of "Radio Handbook" (1970). He also
warns on page 548: "Short large-diameter elements have low Q and are not
practical in parasitic arrays. Orr also says: "The Yagi antenna,
however, remains "the antenna to beat" for the 50-,144-, and 220-MHz
amateur bands."

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
Stryped wrote:
"If I use a conductive boom and ground it, is there no need for a
coaxial lightning arrester?"

That is my experience, provided that the driven element is a
short-circuit for lower frequencies, such as a folded dipole or a
short-circuit 1/4-wave stub.

My company had countless examples in high places over the world with
no
damage to radios, including many solid-state models, operating 24-7.

W6SAI, Bill Orr gives plenty of examples of "plumber`s delight"
Yagi-Uda
antennas in the eighteenth edition of "Radio Handbook" (1970). He
also
warns on page 548: "Short large-diameter elements have low Q and are
not
practical in parasitic arrays. Orr also says: "The Yagi antenna,
however, remains "the antenna to beat" for the 50-,144-, and 220-MHz
amateur bands."

Best regards, Richard Harrison, KB5WZI


What do you mean it is "short circuited for low frequencies?"

I am trying to decide whther to go with two antennas or one.

I would like to build at least one of them, but can not find plans for
one with a conductive boom.

Jim, NN7K wrote:
"When lightning strikes, it is going where ever it wants to, in spite of
the best grounding intentions."

Yes, the trick is to send it to where you would rather it goes. You do
this with your grounding techniques.

The coax itself is important protection against lightning. Opposition
within the coax to lightning is severe. Lightning is a common-mode
phenomenon. Lightning takes the easy path on the outside of the coax.
All coax has a flashover potential which depends on its spacing and the
dielectric of the coax. Once it flashes (not far within the coax) the
voltage is clamped to the sustaining potential of the arc. This is a
steady low potential with a very high conductance in the arc.

Lightning on the outside of the coax must be properly diverted to ground
to avoid damage.
A few twists and turns in the coax increase its opposition to the
lightning (a transient) on the outside of the coax. Such a coil is
advisable between the antenna support and your operating position. Let
the antenna tower carry the lightning to the earth.

Best regards, Richard Harrison, KB5WZI

Stryped wrote:
"What do you mean it is a "short-circuit for low frequencies?"

A folded dipole, you might agree, is essentially a short-circuit for
d-c, also for 50-60- Hz a-c. At some high frequency, the folded dipole
is no longer a short-circuit, but offers considerable opposition to
current at its input terminals.. Its impedance is very high at its first
resonance where its overall length is 1/4-wave, and its impedance is
often about 300 ohms near its second resonance where its overall length
is about 1/2-wave.

At frequencies much lower than the resonant frequencies of the dipole
(most of the energy contained in a lightning stroke) the opposition of
the folded dipole is very low, or almost a short-circuit. This puts the
voltage at both terminals of the dipole to the same value and makes the
lightning applied to the transmission line a common-mode phenomenon.

Best regards, Richard Harrison, KB5WZI

Richard- the point I make-and the company I worked for, until I retired
on disability- has Thousands of base stations- and yet still sustains
major damage every year-- they have a manual on grounding, and a 2 week
program on grounding/insulateing and , in fact have so many grounds that
the wonder is they dont have major problems with ground loops! That
company is Union Pacific Railroad. If you can find a employee of the
communications dept, you might ask to borrow a copy of it. It is
interesting to read, just for information's sake. One thing that I
DIDN'T point out (as it has no bearing to grounding antennas an a ham
(or normal business) environment, is that because of Federal Railroad
Administration rules, that in many applications (primararily involveing
switching equipment), that any GROUNDING is strictly prohibited! As
most signals are now either radio controlled, or soon will be (the old
open wire "Code Line" is being rapidly retired), this brings new meaning
to ISOLATION for the radios/antenna , via the RS-232 bus to the Coder
logic-- a forest, if you will, of Opto-Couplers at each site. This also
applies to "Talking hot box,dragging equiptment, weight in motion, and
Automatic Car Identification eqpt! Even where phone cables cross
under/over the tracks brings new meaning to arcing in a lightning storm.
Had one here in the yard, that was a dead cable (cut off at both ends)
when lightning struck (not even anywhere near the cable/tracks) you
could see arcing between the pairs (talk about induction!) The length of
that cable was about 500 feet! The protectors that U.P. recommends (and
buys by the hundreds is poly-phasor , tho for my own use like the Alpha
Deltas. The Alpha-Deltas use standard gas protectors as are in
telephone circuits, and break down at 200, and 300 volt levels- while the
polyphasor measure a wide open at the center conductors of your 2 pieces
of coax The difference is the Alpha Delta is a relatively broad band
device, while there are several different models (depending on freq. for
the Poly-phasors. The point that I was trying to make (perhaps poorly),
is :1) WHEN you sustain a DIRECT HIT, something is going to happen - and
that wont be good news, except to the parts jobbers/manufacturers and 2)
Even a nearby strike can leave great damage if no preventive measurs
have been taking to isolate the antenna from the radio. If you open the
preamp of a 150 MHz motrac, you see a neon lamp-- that was for
protection of a nearby strike, but for a direct hit-- doesnt do a lot of
good! as info, Jim NN7K




Richard Harrison wrote:

Yes, the trick is to send it to where you would rather it goes. You do
this with your grounding techniques.

The coax itself is important protection against lightning. Opposition
within the coax to lightning is severe. Lightning is a common-mode
phenomenon. Lightning takes the easy path on the outside of the coax.
All coax has a flashover potential which depends on its spacing and the
dielectric of the coax. Once it flashes (not far within the coax) the
voltage is clamped to the sustaining potential of the arc. This is a
steady low potential with a very high conductance in the arc.

Lightning on the outside of the coax must be properly diverted to ground
to avoid damage.
A few twists and turns in the coax increase its opposition to the
lightning (a transient) on the outside of the coax. Such a coil is
advisable between the antenna support and your operating position. Let
the antenna tower carry the lightning to the earth.

Best regards, Richard Harrison, KB5WZI