What influence has a metallic boom on a Yagi antenna?
Normally the design is made without taking the boom into
consideration.

http://yfrog.com/janextgen14p

The voltage curves on the parasitic elements have their maximum
in the center. When using a non-conductive beam the situation is not
affected.

With a metallic boom and with the parasitic elements fastened
to the metal would the oscillation pattern be changed?

http://img28.imageshack.us/img28/3096/boommounted.jpg



Do I have to insulate the parasitic elements from the metallic boom
or is it advisable and allowed to fix them with metal fasteners?
Does the connection have to be made electrically good,
or does it not matter at all?


In fact there are both designs used, but probably without further
consideration. Some manufacturers use plastic holding clamps
for mounting the elements, for no other reason than saving time
during assembly. Other manufacturers just drive a self cutting
thread screw through everything.

The YAGICAD software does talk about mounting elements
on the boom, and NEC is too difficult for me to understand.

Thank you in advance for your answers, and a sixpack
will be kept ready to compensate your efforts at your next visit.

w.

On 21 jun, 10:20, Helmut Wabnig [email protected] --- -.dotat wrote:
What influence has a metallic boom on a Yagi antenna?
Normally the design is made without taking the boom into
consideration.

http://yfrog.com/janextgen14p

The voltage curves on the parasitic elements have their maximum
in the center. When using a non-conductive beam the situation is not
affected.

With a metallic boom and with the parasitic elements fastened
to the metal would the oscillation pattern be changed?

http://img28.imageshack.us/img28/3096/boommounted.jpg

Do I have to insulate the parasitic elements from the metallic boom
or is it advisable and allowed to fix them with metal fasteners?
Does the connection have to be made electrically good,
or does it not matter at all?

In fact there are both designs used, but probably without further
consideration. Some manufacturers use plastic holding clamps
for mounting the elements, for no other reason than saving time
during assembly. Other manufacturers just drive a self cutting
thread screw through everything.

The YAGICAD software does talk about mounting elements
on the boom, and NEC is too difficult for me to understand.

Thank you in advance for your answers, and a sixpack
will be kept ready to compensate your efforts at your next visit.

w.

Hello Helmut,

There is influence, whether or not the elements are electrically
connected. When (thickness of the boom) (element length), the
influence is negligible.

When the above isn't met, you may experience a noticeable shift in
design frequency (it will go up). when connecting the elements to the
boom, the influence will be more. So to compensate for this, the
elements should be somewhat longer. Note that it also depends on where
the metallic contacts are. An element through a boom will experience
more shift then an element having just a single-point electrical
connection to a boom.

When you have bad connections (for example due to corrosion or a loose
nut), movement (induced by wind) will change the electrical
characteristics in a time varying way. This may result in cracking
noise, even on strong signals. Clean strong signals may introduce
noise at other frequencies due to the time varying change in antenna
properties.

This may be a reason for manufactures to use non-contact mounting of
beam elements. I saw several UHF yagis (with relative thick square
booms w.r.t. element length) with loose elements making (bad or no)
contact with the square boom. So if you decide to make electrical
contact, make sure it is long lasting.

Here you may find useful info http://www.qsl.net/yu1aw/Misc/vhf_ant.htm

With kind regards,


Wim
PA3DJS
www.tetech.nl

On Tue, 21 Jun 2011 02:44:16 -0700 (PDT), Wimpie
wrote:

On 21 jun, 10:20, Helmut Wabnig [email protected] --- -.dotat wrote:
What influence has a metallic boom on a Yagi antenna?
Normally the design is made without taking the boom into
consideration.

http://yfrog.com/janextgen14p

The voltage curves on the parasitic elements have their maximum
in the center. When using a non-conductive beam the situation is not
affected.

With a metallic boom and with the parasitic elements fastened
to the metal would the oscillation pattern be changed?

http://img28.imageshack.us/img28/3096/boommounted.jpg

Do I have to insulate the parasitic elements from the metallic boom
or is it advisable and allowed to fix them with metal fasteners?
Does the connection have to be made electrically good,
or does it not matter at all?

In fact there are both designs used, but probably without further
consideration. Some manufacturers use plastic holding clamps
for mounting the elements, for no other reason than saving time
during assembly. Other manufacturers just drive a self cutting
thread screw through everything.

The YAGICAD software does talk about mounting elements
on the boom, and NEC is too difficult for me to understand.

Thank you in advance for your answers, and a sixpack
will be kept ready to compensate your efforts at your next visit.

w.

Hello Helmut,

There is influence, whether or not the elements are electrically
connected. When (thickness of the boom) (element length), the
influence is negligible.

When the above isn't met, you may experience a noticeable shift in
design frequency (it will go up). when connecting the elements to the
boom, the influence will be more. So to compensate for this, the
elements should be somewhat longer. Note that it also depends on where
the metallic contacts are. An element through a boom will experience
more shift then an element having just a single-point electrical
connection to a boom.

When you have bad connections (for example due to corrosion or a loose
nut), movement (induced by wind) will change the electrical
characteristics in a time varying way. This may result in cracking
noise, even on strong signals. Clean strong signals may introduce
noise at other frequencies due to the time varying change in antenna
properties.

This may be a reason for manufactures to use non-contact mounting of
beam elements. I saw several UHF yagis (with relative thick square
booms w.r.t. element length) with loose elements making (bad or no)
contact with the square boom. So if you decide to make electrical
contact, make sure it is long lasting.

Here you may find useful info http://www.qsl.net/yu1aw/Misc/vhf_ant.htm

With kind regards,


Wim
PA3DJS
www.tetech.nl


Thanks, and also thanks to Owen,

I made an error in the description
http://yfrog.com/janextgen14p
because those are current curves, not voltage.

What I learned is, a metallic boom requires a small +length correction
of the dipole elements. YAGICAD does it automatically with a cryptic
value called L+BCF (boom correction factor?)
An electrical connection with the metallic boom will need more
correction than an isolated element mount, if I am correct.

A metallic boom should also affect dipole spacing,
but to what extent, I do not know.
YAGICAD does not care for that, or does it?
Am not sure.


w.

On 6/21/2011 1:20 AM, Helmut Wabnig wrote:
What influence has a metallic boom on a Yagi antenna?
Normally the design is made without taking the boom into
consideration.

http://yfrog.com/janextgen14p

The voltage curves on the parasitic elements have their maximum
in the center. When using a non-conductive beam the situation is not
affected.

With a metallic boom and with the parasitic elements fastened
to the metal would the oscillation pattern be changed?

Yes.. not a lot, but important on a high gain antenna (one that is
"super directive".. i.e. has gain N, where N is number of elements)
because the amplitude may not change much, but the phase might, and
small phase errors ruin the nulls in the pattern, which degrades the
overall directivity.

A modeling program like NEC can easily show the difference.


http://img28.imageshack.us/img28/3096/boommounted.jpg


That's the distribution of the voltage, but your picture doesn't show
phase and magnitude, which is what's important.

There are a whole variety of techniques to account for the boom in a
model, ranging from actually modeling the boom (tricky with some
modeling codes) to empirically determined correction factors.




Do I have to insulate the parasitic elements from the metallic boom
or is it advisable and allowed to fix them with metal fasteners?
Does the connection have to be made electrically good,
or does it not matter at all?

It does matter. What you really don't want is something that changes.
Either always connected or always insulated is what you want.





In fact there are both designs used, but probably without further
consideration. Some manufacturers use plastic holding clamps
for mounting the elements, for no other reason than saving time
during assembly. Other manufacturers just drive a self cutting
thread screw through everything.

The YAGICAD software does talk about mounting elements
on the boom, and NEC is too difficult for me to understand.


basically, a directive antenna gets its directivity from having
different phase and magnitude of current in each of the elements. In a
Yagi-Uda, only one element is driven, so the current in the other
elements comes from mutual coupling among the elements.

That coupling is affected by the spacing, length, and diameter(s) of the
elements, so having the boom connected is sort of like having an element
that is somewhat "fat" in the middle. It's slightly different.

On a 3 element wideband Yagi, the "boom correction" is going to be
pretty small. But on a 432 MHz moonbounce array with 20 elements, and
where you want very low side/back lobes to keep the noise temperature
down, it gets more important.

back before computers got fast and cheap, there were a lot of analytical
(or semi-analytical) approaches to designing Yagi antennas. You'd have
an approximation of mutual coupling (from, say, King) and form a matrix
equation. You'd figure out what element currents you wanted for your
pattern, and iterate your element lengths and spacings until it all
worked out. (or, you use the matrix equation to determine the element
currents, given the excitation, and then calculate the pattern from
that, using simple phased array techniques).

The problem is that simple approximations like King's typically assume
constant diameter elements. So, by dint of a bunch of empirical work,
people found "correction factors" that you could apply to an idealized
model (no boom) that would make it work ok on a boom.








Thank you in advance for your answers, and a sixpack
will be kept ready to compensate your efforts at your next visit.

w.

On 6/21/2011 11:13 AM, Jim Lux wrote:

It does matter. What you really don't want is something that changes.
Either always connected or always insulated is what you want.

Dead right.

This seems to be one reason why yagis at HF tend to be grounded elements
and at VHF and up tend to be insulated. At HF it's fairly easy to get a
low resistance with fairly low impedance change over a few years.
That's harder to do at VHF especially with through the boom uninsulated.
Hence through-the-boom or over-the-boom insulated at 2m and up.

I've built many 6m beams with no correction added and using simple
homebrew U clamps. I have seen little discernible difference in their
performance from predicted. Of course I shouldn't except perhaps in the
side nulls. And they seemed about right. One small confirmation was
the SWR curve also matched predicted.

Anyway, the problem with connected is corrosion. Can only slow it down.

tom
K0TAR

On 6/22/2011 7:31 PM, tom wrote:

I've built many 6m beams with no correction added and using simple
homebrew U clamps. I have seen little discernible difference in their
performance from predicted. Of course I shouldn't except perhaps in the
side nulls. And they seemed about right. One small confirmation was the
SWR curve also matched predicted.



Based on a fair number of simulations over the years, I'd say that SWR
curves are not very sensitive to pattern degradation (except in an
egregious case)..

Consider a 5 element medium gain Yagi where the currents are all roughly
equal (which is what you'd have with moderate superdirectivity) and it
has a F/B ratio of, say, 20dB.

Screw up the current in one of the elements by, say, 25% (so the overall
excitation is now off by 5%).. That's enough to turn your -20dB null
into a -10dB null (in round numbers..)

But, would you even see that in a VSWR.. say it was 1.2:1 before
(reflection coefficient = 0.09).. and now the reflection coefficient
changes by 5%.. so it's 0.0945.. That's a VSWR of 1.208.. I think you'd
have a hard time measuring that..

On 6/23/2011 11:34 AM, Jim Lux wrote:
On 6/22/2011 7:31 PM, tom wrote:

I've built many 6m beams with no correction added and using simple
homebrew U clamps. I have seen little discernible difference in their
performance from predicted. Of course I shouldn't except perhaps in the
side nulls. And they seemed about right. One small confirmation was the
SWR curve also matched predicted.



Based on a fair number of simulations over the years, I'd say that SWR
curves are not very sensitive to pattern degradation (except in an
egregious case)..

Consider a 5 element medium gain Yagi where the currents are all roughly
equal (which is what you'd have with moderate superdirectivity) and it
has a F/B ratio of, say, 20dB.

Screw up the current in one of the elements by, say, 25% (so the overall
excitation is now off by 5%).. That's enough to turn your -20dB null
into a -10dB null (in round numbers..)

But, would you even see that in a VSWR.. say it was 1.2:1 before
(reflection coefficient = 0.09).. and now the reflection coefficient
changes by 5%.. so it's 0.0945.. That's a VSWR of 1.208.. I think you'd
have a hard time measuring that..


You are correct. I have also measured gain and F/B and side patterns.
And they match predicted quite well.

One design was a 7.5 wavelength 432 EME antenna which had nice ground
noise performance in real world use. Which is very pattern sensitive.

tom
K0TAR

On 6/23/2011 8:54 PM, tom wrote:
On 6/23/2011 11:34 AM, Jim Lux wrote:
On 6/22/2011 7:31 PM, tom wrote:

I've built many 6m beams with no correction added and using simple
homebrew U clamps. I have seen little discernible difference in their
performance from predicted. Of course I shouldn't except perhaps in the
side nulls. And they seemed about right. One small confirmation was the
SWR curve also matched predicted.



Based on a fair number of simulations over the years, I'd say that SWR
curves are not very sensitive to pattern degradation (except in an
egregious case)..

Consider a 5 element medium gain Yagi where the currents are all roughly
equal (which is what you'd have with moderate superdirectivity) and it
has a F/B ratio of, say, 20dB.

Screw up the current in one of the elements by, say, 25% (so the overall
excitation is now off by 5%).. That's enough to turn your -20dB null
into a -10dB null (in round numbers..)

But, would you even see that in a VSWR.. say it was 1.2:1 before
(reflection coefficient = 0.09).. and now the reflection coefficient
changes by 5%.. so it's 0.0945.. That's a VSWR of 1.208.. I think you'd
have a hard time measuring that..


You are correct. I have also measured gain and F/B and side patterns.
And they match predicted quite well.

One design was a 7.5 wavelength 432 EME antenna which had nice ground
noise performance in real world use. Which is very pattern sensitive.

tom
K0TAR


In fact, I would assert that today, VSWR measurements are almost useless
for *antenna adjustment and construction*.. and even for adjusting
elements. With modern modeling codes which don't require all sorts of
little fiddly "correction factors" (e.g. taper or boom factors) to
allow simulating a complex structure with a simpler model, a tape
measure would be a better tool.

There's a lot of literature from the 50s-70s and even 80s where antennas
are constructed and you adjust the element length by using the trusty
GDO. (Yep, I remember helping my grandfather assemble a multiband Yagi
of some sort in the early 70s doing it that way..)

But today.. mechanical dimensions and a high fidelity model are the way
to go.


Where the VSWR measurement would be useful is in two places:
1) Adjusting a wire antenna length... they're usually interacting with
the surroundings, so you have to cut and try, and a VSWR sweep is a good
way to get there quickly. Putting up that triband multiwire dipole for
Field day tomorrow? The Antenna analyzer is your friend when trying to
get all those mutually interacting lengths dialed in.


2) A quick check to see if an antenna system that worked before has
"broken" in a big way.

On 6/21/2011 3:20 AM, Helmut Wabnig wrote:
What influence has a metallic boom on a Yagi antenna?
Normally the design is made without taking the boom into
consideration.

http://yfrog.com/janextgen14p

The voltage curves on the parasitic elements have their maximum
in the center. When using a non-conductive beam the situation is not
affected.

The current is maximum at the center, not the voltage.

With a metallic boom and with the parasitic elements fastened
to the metal would the oscillation pattern be changed?

At HF the effect is normally minimal depending on the mounting method
used. Using insulated through the boom mounting it starts to be
noticeable at 2m and by 70cm it definitely needs to be compensated for.
1 and 1/4 inch booms using Delrin through the boom mounts at 432 need
about 7mm added.

The previous statements assume you don't use insanely large booms and
mounting pieces.

tom
K0TAR