Well -- I have found that modelling gets you in the ball park (an
approximation)
A good SWR meter will allow you to tweak it up.

For 1/4 wave vs 5/8 wave 2M antennas -- I have found empirically -- that
when in an area surriunded by big mountain tops -- like Silicon Valley in
the CA Bay area -- the 1/4 wave works better due to its higher angle of
radiation of the 1/4 wave

When on the open road where terrain is flat and the mountains are far
away -- the 5/8 wave works best -- lower angle of radiation.

This is just my empirical observation --- your radiation angle may vary --
hi hi.

--
Caveat Lector Ya All

"Amateur Radio is the best back-up
communications system in the world,
and that's the way it is." -- Walter Cronkite





"Vito" wrote in message
...
I agree completely. Question is, if modeling and prediction is so
unreliable
why do we bother?

"Dan Richardson @mendolink.com" ChangeThisToCallSign wrote in message
...
On 18 May 2004 06:19:50 -0700, (Brian Kelly) wrote:

Then howcum my 5/8 wave mag mount 2m mobile antenna very significantly
outperforms the 1/4 wave mag mount antenna I used to use? ....

My previous response was for a ground plan antenna mounted above
ground and you are addressing a mobile installation. They are different.

... I found that the vehicle's size, shape and whip location
plays a major part in performance.

.... it was possible to find azimuth directions that a 5/8-wave would
produced almost 3 db gain over itself ....

I don't feel you can accurately predict how a the whips will perform
on a vehicle based upon operation on another vehicle....

On Tue, 18 May 2004 12:29:46 -0400, "Vito" wrote:
I agree completely. Question is, if modeling and prediction is so unreliable
why do we bother?

Hi OM,

There is modeling, and then there are modelers. 99.9% of errors are
found with the second. A simple example that explains the illusion of
disparity may be tested with the free version of EZNEC (as is
generally the case).

Model a ground plane antenna at ground level (or simply an inch or cm
above it); and then raise the same antenna a quarter wave (not so
difficult to manage at these breathless reports from VHF-land).

Difference approaches 6dB for this trivial exercise alone. Models
answer the differences quite well, modelers can be found in
commercials wearing lab coats saying "I'm not a doctor, but...."

73's
Richard Clark, KB7QHC

On Tue, 18 May 2004 12:29:46 -0400, "Vito" wrote:

Question is, if modeling and prediction is so unreliable
why do we bother?

Why do you feel modeling is "so unreliable"?

Danny, K6MHE

Dan Richardson wrote in message . ..
On 18 May 2004 06:19:50 -0700, (Brian Kelly) wrote:

Then howcum my 5/8 wave mag mount 2m mobile antenna very significantly
outperforms the 1/4 wave mag mount antenna I used to use? Same ground
condx, same power, same feeder length, same vehicle, etc. I suspect
it's in the differences in the TO angles.


My previous response was for a ground plane antenna mounted above
ground and you are addressing a mobile installation. They are
different.

Agreed.

I've done modeling of 1/4,1/2 and 5/8-wave whips using several wire
grid models for vehicles (small and mid-sized car, small pickup truck
and a SUV). I found that the vehicle's size, shape and whip location
plays a major part in performance.

No surprise there although my instincts tell me that once some number
of "critical" square feet of vehicle sheet metal is "achieved" the
size of the vehicle has a less pronounced effect particulary at VHF
and UHF freqs. But modeling has the annoying ability to deflate
instincts.

I also noted, on the average, the 5/8 produce a slight gain over all,
but it was possible to find azimuth directions that a 5/8-wave would
produced almost 3 db gain over itself - depending what vehicle it was
mounted on.

That would easily explain my offhand experiences. In fact I did it
again last weekend. An buddy of mine is temporarily laid up in a
skilled care facility so I got his home 2M FM station running in his
room. I installed his 1/4 wave magmount whip on one of those typical
steel-shrouded HVAC units often found under the windows of patient
rooms. He couldn't hit the repeater. I went back later and installed
my 5/8 wave whip and yup, now he can hit the repeater.

The most outstanding feature I saw was a 5/8-wave whip azimuth pattern
was less influenced by the vehicle geometry.

The operation is more like a lop-sided dipole with the vehicle body
being on leg.

I don't feel you can accurately predict how a the whips will perform
on a vehicle based upon operation on another vehicle - unless both
vehicles and the antenna locations are the same.

I should add for all models the whips where placed top-dead-center of
the vehicle's roof.

100% agreed and it all fits. Tnx.

73,
Danny, K6MHE

Brian w3rv

Why do you feel modeling is "so unreliable"?

=========================

Because 99.99 percent of models are never properly tested in practice and in
an anateur's back yard, if they WERE tested, the radiation patterns would
only bear a vague resemblance to predictions.

Predictions are mostly wishful thinking. Nevertheless, they can be a
satisfying intellectual activity.
----
Reg.

On Tue, 18 May 2004 20:05:37 +0000 (UTC), "Reg Edwards"
wrote:

Why do you feel modeling is "so unreliable"?

=========================

Because 99.99 percent of models are never properly tested in practice and in
an anateur's back yard, if they WERE tested, the radiation patterns would
only bear a vague resemblance to predictions.

Predictions are mostly wishful thinking. Nevertheless, they can be a
satisfying intellectual activity.
----
Reg.


I see........ Then I assume then that your diagnosis of computer
modeling applies to your antenna computer programs as well?

Danny

"Dan Richardson wrote
Why do you feel modeling is "so unreliable"?

=========================

Because 99.99 percent of models are never properly tested in practice and
in
an anateur's back yard, if they WERE tested, the radiation patterns would
only bear a vague resemblance to predictions.

Predictions are mostly wishful thinking. Nevertheless, they can be a
satisfying intellectual activity.
----
Reg.


I see........ Then I assume then that your diagnosis of computer
modeling applies to your antenna computer programs as well?

Danny

====================================

Danny, there's nothing wrong with antenna pattern-prediction progams. There
are good nunber-crunched side effects. It's only their practical application
by people in their own back yards which is unreliable. It cannot be helped.
It's a fact of life. The environment and performance of an antenna cannot
be accurately predicted unless it is at a height of several wavelengths
above and away from obstructions.

Not a single one of my programs predicts a radiation pattern. Only an
exceedingly few professional antenna design engineers would ever make real
use of such facilities even if I could write them. I don't like wasting
time. I havn't all that amount of time anyway. Nobody has produced a new
type of antenna for many decades. (Not even Fractal). It's all been done
before.

But, as I say, radiation prediction programs are entertaining, educational
and satisfying. Enjoy them while you can.

Nevertheless, unreliable is a fair practical description.
----
Reg, G4FGQ

On Tue, 18 May 2004 23:50:17 +0000 (UTC), "Reg Edwards"
wrote:

But, as I say, radiation prediction programs are entertaining, educational
and satisfying. Enjoy them while you can.

If you will review my posting of the results i reported you will see
that I was comparing 1/4, 1/2 and 5/8-wave whips under the same
conditions for each antenna. I was not and did not report the reported
gain figures, rather, I posted the difference or lack there of for the
comparisons. Under those conditions I feel comfortable stating that
difference between using a 1/4, 1/2 or 5/8-wave whips on a vehicle
(mounted top-dead-center on the roof) the overall gain differences
would be one dB or less.

I have observed, operating mobile using the above antennas that I
really couldn't notice any real difference. Bad areas remained bad
areas and I would loose the repeater in about the same location
regardless which antenna I used.

In other words I was comparing apples to apples and not looking for an
absolute gain value, rather, the difference and I am very comfortable
with the findings - both via computer modeling and actual use.

very 73
Danny, K6MHE

You are only sort of correct.

At HF where the local world is as large as your antenna, that statement
is true, at 6m it starts to become less of an issue as the outside world
becomes big and somewhat ignorable. At 2m it is quite ignorable, and at
70cm it pretty much isn't there. Yagis I have designed for 432 perform
exactly as predicted by the software, with the exception of the ones too
long for the antenna range, where the only discrepency was the gain
tests a bit lower than the model predicts. And then it was .3 dB on an
18.4 dBd prediction.

It is possible to predict antenna gain and pattern very accurately if
you do it at a frequency that does not interact with the local
environment to a any large degree. It is also possible to build those
antennas and have them perform the same time and time again. I've built
dozens of long boom yagis for 2, 222, and 432, and after the first one,
you don't even have to measure the SWR before setting the T match shorts
unless you are anal or an EME're. Oops, same thing, I guess. ;) And
every antenna I've ever range tested (thank you Central States VHF) has
been within .1 dB of predicted except one.

The designs I have built over the last 15 years didn't need to be
adjusted at all from the model plus predicted boom corrections. The
only variable was the matching system on the driven element, which none
of the modeling programs handles very well.

And for you picky folks, yes, the losses from foliage, etc. aren't
ignorable, but the antenna system still performs as predicted unless you
mount it in the center of a tree.

And I do realize I'm preaching to the non-choir here, since this group
is (apparently) not very interested in anything above 10m. :)

tom
K0TAR

Reg Edwards wrote:




====================================

Danny, there's nothing wrong with antenna pattern-prediction progams. There
are good nunber-crunched side effects. It's only their practical application
by people in their own back yards which is unreliable. It cannot be helped.
It's a fact of life. The environment and performance of an antenna cannot
be accurately predicted unless it is at a height of several wavelengths
above and away from obstructions.

Not a single one of my programs predicts a radiation pattern. Only an
exceedingly few professional antenna design engineers would ever make real
use of such facilities even if I could write them. I don't like wasting
time. I havn't all that amount of time anyway. Nobody has produced a new
type of antenna for many decades. (Not even Fractal). It's all been done
before.

But, as I say, radiation prediction programs are entertaining, educational
and satisfying. Enjoy them while you can.

Nevertheless, unreliable is a fair practical description.
----
Reg, G4FGQ

Oh, I forgot the important bit - I did it in my back yard.

tom
K0TAR

Tom Ring wrote:

You are only sort of correct.

At HF where the local world is as large as your antenna, that statement
is true, at 6m it starts to become less of an issue as the outside world
becomes big and somewhat ignorable. At 2m it is quite ignorable, and at
70cm it pretty much isn't there. Yagis I have designed for 432 perform
exactly as predicted by the software, with the exception of the ones too
long for the antenna range, where the only discrepency was the gain
tests a bit lower than the model predicts. And then it was .3 dB on an
18.4 dBd prediction.

It is possible to predict antenna gain and pattern very accurately if
you do it at a frequency that does not interact with the local
environment to a any large degree. It is also possible to build those
antennas and have them perform the same time and time again. I've built
dozens of long boom yagis for 2, 222, and 432, and after the first one,
you don't even have to measure the SWR before setting the T match shorts
unless you are anal or an EME're. Oops, same thing, I guess. ;) And
every antenna I've ever range tested (thank you Central States VHF) has
been within .1 dB of predicted except one.

The designs I have built over the last 15 years didn't need to be
adjusted at all from the model plus predicted boom corrections. The
only variable was the matching system on the driven element, which none
of the modeling programs handles very well.

And for you picky folks, yes, the losses from foliage, etc. aren't
ignorable, but the antenna system still performs as predicted unless you
mount it in the center of a tree.

And I do realize I'm preaching to the non-choir here, since this group
is (apparently) not very interested in anything above 10m. :)

tom
K0TAR

Reg Edwards wrote:




====================================

Danny, there's nothing wrong with antenna pattern-prediction progams.
There
are good nunber-crunched side effects. It's only their practical
application
by people in their own back yards which is unreliable. It cannot be
helped.
It's a fact of life. The environment and performance of an antenna
cannot
be accurately predicted unless it is at a height of several wavelengths
above and away from obstructions.

Not a single one of my programs predicts a radiation pattern. Only an
exceedingly few professional antenna design engineers would ever make
real
use of such facilities even if I could write them. I don't like wasting
time. I havn't all that amount of time anyway. Nobody has produced a
new
type of antenna for many decades. (Not even Fractal). It's all been done
before.

But, as I say, radiation prediction programs are entertaining,
educational
and satisfying. Enjoy them while you can.

Nevertheless, unreliable is a fair practical description.
----
Reg, G4FGQ