Yagi boom question
Helmut Wabnig
Normally the design is made without taking the boom into
consideration.
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.
Owen Duffy
> What influence has a metallic boom on a Yagi antenna?
> Normally the design is made without taking the boom into
> consideration.
No necessarily, but it is common to design for a non-conductive boom and then make a correction to element lengths for the type of boom in use.
>
> 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.
That is not true.
> With a metallic boom and with the parasitic elements fastened
> to the metal would the oscillation pattern be changed?
Depending on how the boom is or is not bonded to the element, and the geometry (sizes, position etc), the conductive boom may reduce the magnetic flux nearest to the element, effectively reducing its inductance in the middle area, so tuning it higher.
> 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?
If the 'connection' has infinite R (ie no connection) or zero R, there is no power converted to heat, otherwise you invite conversion of RF energy to heat.
>
>
> In fact there are both designs used, but probably without further
> consideration. Some manufacturers use plastic holding clamps
Not by competent designers, they consider the effects of the boom.
> 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.
>
The foregoing may challenge your thinking on the subject.
Owen
Wimpie
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
Helmut Wabnig
wrote:
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.
Owen Duffy
> 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?)
If you Google for "boom correction factor", you should find a wealth of discussion on the subject. It is not all cut and dried, it remains an area of active research.
Owen
John Smith
An interesting experiment might be to cast the upper elements above a
metal slab of 1/4 wave depth which extends a full wave out in all
direction of the compass, in eznec/nec/mmana, and serves the purpose of
the lower elements connected to a huge boom.
A comparison between a standard yagi and such a design should show a
stark comparison of the importance/function of the lower/ground
elements, between the two. Or, perhaps just the extension of the boom
in the frontal and trailing directions would provide usable data.
In any case, I suspect the influence of the boom, in such a greatly
exaggerated example, would be exposed in more defining light ...
What could be done to the upper elements, reflector(s) and director(s),
to provide such a comparison(s) is a matter which provokes thought ...
Frankly, I don't even know if any antenna modeling software would allow
such an "abuse."
Regards,
JS
Jim Lux
> 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.
Jim Lux
basically, it's all about coming up with a simple algorithm to allow the
use of a simpler modeling code (i.e. model without boom, apply BCF, done).
It all depends on what is "good enough". On an HF beam, the elements
typically sag a fair amount, and wave around in the wind. Furthermore,
the boom is a tiny fraction of the element length so it has a "small"
effect (e.g. 10cm boom on a 10m long element) . So the uncertainties in
performance due to other mechanical factors are bigger than the effect
of a boom.
On a 432 or 1296 yagi using a 2cm square tube as the boom, though, this
is less the case. The elements are rigid, 2cm is a big fraction of a 35
cm long element. And, on VHF/UHF, one typically has a LOT more elements
and a LOT more gain, so the tolerances on everything are much tighter to
get the desired performance. If one were build a 5 wavelength long 20m
Yagi (5 lambda isn't unusual for 2m or 70cm), you'd be worrying about
those sorts of effects too, at least if you wanted to get the 17dBi gain
that might be possible.
Jim Lux
I'm having a tough time visualizing what you're describing. Half
elements sticking up from a "perfectly conductive plane"?
>
> A comparison between a standard yagi and such a design should show a
> stark comparison of the importance/function of the lower/ground
> elements, between the two. Or, perhaps just the extension of the boom in
> the frontal and trailing directions would provide usable data.
IN general, the boom extending front or back doesn't make a big
difference. If I had to "rule of thumb" it, I'd say that the
"influence" of the boom is several element radii.
>
> In any case, I suspect the influence of the boom, in such a greatly
> exaggerated example, would be exposed in more defining light ...
>
> What could be done to the upper elements, reflector(s) and director(s),
> to provide such a comparison(s) is a matter which provokes thought ...
>
> Frankly, I don't even know if any antenna modeling software would allow
> such an "abuse."
>
Step right up and fire up that copy of HFSS or ADS
in most modern codes you can model *anything* given sufficient time to
build the model and run it. If you have lots of big chunks of "stuff" a
method of moments code like NEC might not be the best choice. One of
the FDTD or similar codes might be better.
John Smith
A slab of aluminum 1/4 wave deep and extending a full wavelength is all
directions is about as perfect a conductor/elevated-pseudo-ground as I
could ever imagine ... yep, you pictured it correctly ...
>> A comparison between a standard yagi and such a design should show a
>> stark comparison of the importance/function of the lower/ground
>> elements, between the two. Or, perhaps just the extension of the boom in
>> the frontal and trailing directions would provide usable data.
>
> IN general, the boom extending front or back doesn't make a big
> difference. If I had to "rule of thumb" it, I'd say that the "influence"
> of the boom is several element radii.
>
Certainly for the front director, and back reflector, it would look as
two legs of the ground plane from a vertical ground plane antenna --
with the bottom dipole element still attached ...
>
>>
>> In any case, I suspect the influence of the boom, in such a greatly
>> exaggerated example, would be exposed in more defining light ...
>>
>> What could be done to the upper elements, reflector(s) and director(s),
>> to provide such a comparison(s) is a matter which provokes thought ...
>>
>> Frankly, I don't even know if any antenna modeling software would allow
>> such an "abuse."
>>
>
>
> Step right up and fire up that copy of HFSS or ADS
>
> in most modern codes you can model *anything* given sufficient time to
> build the model and run it. If you have lots of big chunks of "stuff" a
> method of moments code like NEC might not be the best choice. One of the
> FDTD or similar codes might be better.
Having all the antenna I want, at this time, I am just too lazy to load
up mmana and see the results ... most run-of-the-mill and
been-around-forever antennas have pretty much been optimized for cost of
materials/work to benefit ratio ... I don't think I have seen anything
new, or even claimed to be new, in decades ... or longer!
Regards,
JS
Jim Lux
> Having all the antenna I want, at this time, I am just too lazy to load
> up mmana and see the results ... most run-of-the-mill and
> been-around-forever antennas have pretty much been optimized for cost of
> materials/work to benefit ratio ... I don't think I have seen anything
> new, or even claimed to be new, in decades ... or longer!
>
I think you're generally right. What the tools give you is the ability
to make a "semi-custom" design that's optimized for a purpose slightly
different than everyone else.
For instance, you might want a 20m Yagi that's optimized for 14.300 as
opposed to 14.050 or something.
Various and sundry multiband schemes also come to mind. Lots of
alternatives for element layout, lots of potential interactions. And
these designs are not well accommodated by analytical techniques or cut
and try on the physical article (because of the interactions at
frequencies well away from the resonant frequencies of some elements).
Doing something like a Force-12 C3 tribander by cut and try would be
tedious at best.
In the VHF/UHF world, where very high directivity is the rule, and
people worry a lot about side/back lobes (for antenna noise temperature
reasons), and because the structure is significant in size compared to
the elements, I think modeling codes are almost essential. And, there's
lots of alternatives to look at (element through the boom, on top of the
boom, connected or not, spacer or not, etc.)
tom
> 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
tom
>
> 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
Jim Lux
> 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..
tom
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
Jim Lux
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.