On Mon, 17 May 2004 10:42:49 -0500, (Richard
Harrison) wrote:
[snip]
Power varies as the square of the voltage. One can see the difference in
the field strength is hardly worth the effort for an amateur to try to
increase the length of his antenna. It`s about a 3 dB gain from 1/4-wave
to 5/8 wave.
[snip]

The 3 dB gain figure is valid when mounted on theoretical perfect
ground. For a ground-plane elevated above real ground you'll find the
gain to be rarely greater than 1 dB.

Modeling 1/4 & 5/8-wave ground planes at 30 feet above ground (@ 24
MHz) as was described earlier NEC reports the following maximum gain:

1/4-wave 2.35 dB

5/8-wave 3.06 dB

73
Danny, K6MHE

Dan Richardson wrote in message . ..
On Mon, 17 May 2004 10:42:49 -0500, (Richard
Harrison) wrote:
[snip]
Power varies as the square of the voltage. One can see the difference in
the field strength is hardly worth the effort for an amateur to try to
increase the length of his antenna. It`s about a 3 dB gain from 1/4-wave
to 5/8 wave.
[snip]

The 3 dB gain figure is valid when mounted on theoretical perfect
ground. For a ground-plane elevated above real ground you'll find the
gain to be rarely greater than 1 dB.


Dunno. My real world tests don't quite agree. In using 30 mile ground
wave tests across town, I tested 1/4 GP's, 1/2 waves including
decoupling sections, and a 5/8 GP with 3/4 wave radials. All at 36 ft.
The 5/8 ate the 1/4 GP for lunch. Probably 2 plus S units better than
the 1/4 GP. The 5/8 beat the 1/2 wave by 1.5 S units. And this was
tested and repeated over a period of months. Never varied. Ground wave
testing is quite stable, and accurate for those low angles involved.
Much more accurate than trying to compare using constantly varying
skywaves. In real world gains, thats more than 1 db. 5/8 antennas are
weird animals. On 2m, they suck. On HF, they can do fairly well, cuz
the angles involved are not as critical. I used a 5/8 GP on 17m for
2-3 years. "also at 36 ft at the base".
It mangled every other antenna I had on that band. On 10m, the 5/8
beat any other length radiator quite handily. Again, on the critical
2m band, peeeyooooo.....they stink. BTW, on skywave, using a quick A/B
test, all preferred the 5/8, over the other antennas. So it wasn't a
low angle ground wave fluke. MK

With a typical ground plane antenna, the feedline can radiate
significantly, distorting the pattern. This effect could easily be
different for the different antennas. Modeling indicates that two baluns
are often needed to suppress the current on the outside of the feedline.
A model which includes the feedline might give some insights as to why
the antennas behave so differently.

Roy Lewallen, W7EL

Mark Keith wrote:

Dan Richardson wrote in message . ..

On Mon, 17 May 2004 10:42:49 -0500, (Richard
Harrison) wrote:
[snip]

Power varies as the square of the voltage. One can see the difference in
the field strength is hardly worth the effort for an amateur to try to
increase the length of his antenna. It`s about a 3 dB gain from 1/4-wave
to 5/8 wave.

[snip]

The 3 dB gain figure is valid when mounted on theoretical perfect
ground. For a ground-plane elevated above real ground you'll find the
gain to be rarely greater than 1 dB.



Dunno. My real world tests don't quite agree. In using 30 mile ground
wave tests across town, I tested 1/4 GP's, 1/2 waves including
decoupling sections, and a 5/8 GP with 3/4 wave radials. All at 36 ft.
The 5/8 ate the 1/4 GP for lunch. Probably 2 plus S units better than
the 1/4 GP. The 5/8 beat the 1/2 wave by 1.5 S units. And this was
tested and repeated over a period of months. Never varied. Ground wave
testing is quite stable, and accurate for those low angles involved.
Much more accurate than trying to compare using constantly varying
skywaves. In real world gains, thats more than 1 db. 5/8 antennas are
weird animals. On 2m, they suck. On HF, they can do fairly well, cuz
the angles involved are not as critical. I used a 5/8 GP on 17m for
2-3 years. "also at 36 ft at the base".
It mangled every other antenna I had on that band. On 10m, the 5/8
beat any other length radiator quite handily. Again, on the critical
2m band, peeeyooooo.....they stink. BTW, on skywave, using a quick A/B
test, all preferred the 5/8, over the other antennas. So it wasn't a
low angle ground wave fluke. MK

Roy Lewallen wrote in message ...
With a typical ground plane antenna, the feedline can radiate
significantly, distorting the pattern. This effect could easily be
different for the different antennas. Modeling indicates that two baluns
are often needed to suppress the current on the outside of the feedline.
A model which includes the feedline might give some insights as to why
the antennas behave so differently.

Roy Lewallen, W7EL

It's quite possible. The 1/2 was the only one I actually added a
decoupling section to. It was a 1/4 wave length of coax below the
feed, and a grounded set of 1/4 wave radials at the base of that
section. About the same scheme as cushcraft uses on their ringo ranger
2 verticals. It did improve the antenna. I never added extra
decoupling to the 1/4 or 5/8 antennas. But I did try both 1/4 and 3/4
wave radials on the 5/8 antenna. Ended up prefering the 3/4
radials..The 1/2 wave was a very good antenna, but in my case, not
once did it ever beat the 5/8. Actually, thinking about it, and even
included many CB antenna setups going back years and years, I've never
seen a 5/8 GP that didn't handily beat a 1/4 GP by 2 S units to a
local ground wave station. Even using the 1/4 wave radials, which I'm
not crazy about for a 5/8 antenna. These were all using various
length feedlines I'm sure. MK

Dan Richardson wrote in message . ..
On Mon, 17 May 2004 10:42:49 -0500, (Richard
Harrison) wrote:
[snip]
Power varies as the square of the voltage. One can see the difference in
the field strength is hardly worth the effort for an amateur to try to
increase the length of his antenna. It`s about a 3 dB gain from 1/4-wave
to 5/8 wave.
[snip]

The 3 dB gain figure is valid when mounted on theoretical perfect
ground. For a ground-plane elevated above real ground you'll find the
gain to be rarely greater than 1 dB.

Modeling 1/4 & 5/8-wave ground planes at 30 feet above ground (@ 24
MHz) as was described earlier NEC reports the following maximum gain:

1/4-wave 2.35 dB

5/8-wave 3.06 dB

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.

73
Danny, K6MHE

Brian w3rv

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 plan antenna mounted above
ground and you are addressing a mobile installation. They are
different.

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.

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.

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.


73,
Danny, K6MHE

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....

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