Elevated vs buried radials
 

Owen wrote:
On 01/10/10 07:44, Jim Lux wrote:

The other thing is whether the difference is big enough to "make a
difference" in observed system performance. For a lot of operators, a 1
dB change in performance might not be noticeable. If you're in a "either
propagation is there, or it isn't" situation the difference between good
and bad is 10s of dB. There are relatively few people who work at 0dB
SNR (where tenths count) on a regular and continuing basis, and they're
not necessarily the ones who are interested in doing experiments on
antennas on the scale needed.

Yes, there will be differing view on what is significant difference. I
am not in the school of declaring less than one or two S points is
insignificant in general.

yeah, but there's a big difference between 6-12 dB and 1dB.. I think
most users would care about 6 dB. Many fewer about 1 dB. And even
fewer care about 1 dB AND have the desire and means to perform the
experiment in a controlled way. (well, this latter category probably has
less than 10 people in it, and only 1 has published in the last 50 years)



In the case of a four square in the DX segment, users are looking for
performance... and it seems to me that the elevated three radials, eight
wave vertical with capacity had is very close to quarter wave over
buried radials... depending of course on the soil type.

Hmm.. and there the real question is what kind of performance are we
talking about: the power radiated in a desired direction (Tx) or the
ability to null unwanted signals (Rx). Given the generally high noise
levels on low bands for Rx, a 1 dB change in efficiency of the antenna
might not make any difference for the latter.

A bigger effect on a phased array is the relative phasing. For a 4
element array, you can have pretty big errors in phase on transmit
without changing the forward gain much (30 degree phase error on one
element might give you a 1dB change). But a 30 degree phase error on
receive could turn a -30dB null into a -7dB one..

And for that, the lower loss of your elevated radials might make things
"pickier".. that is, as frequency or surroundings change, the reactive
term for each element changes, which could change the power distribution
and phasing among the elements (depending on the feed system used).
(obviously, one of the "current forcing" drive schemes would be less
sensitive to this)



You mention the modelling tools, I am not so much concerned as to
whether the elevated radials model is good, but whether the NEC4 buried
radials model is good, and likewise for radials on and just above the
ground because those models are setting the benchmark for the
performance of the alternative.

The modeling performance of NEC4 for buried wires and wires just above
the surface is quite good. Where I would be suspicious is for a wire ON
the surface or partly embedded in the surface.

Look for that paper by Burke and Poggio on validating NEC3 and NEC4 (it
was published at some conference in Ankara Turkey)

Elevated vs buried radials
 

Owen Duffy wrote:
LA4RT Jon wrote in :

I know my interpretation of Rudy's measurements and my NEC4 model don't
fall in line with some traditional thinking, and will not appeal to many.

I think you can take the fact that Rudy's measurements match the model
pretty well as experimental validation of the model. indeed, NEC4 works.

Traditional thinking (or more accurately, mindless repetition of
tradition) could well be wrong, eh?


I think it is time to build a prototype.

Thanks for the links.

Owen

Elevated vs buried radials
 

On Oct 1, 4:00*pm, Jim Lux wrote:
Richard Clark wrote:
On Thu, 30 Sep 2010 14:44:53 -0700, Jim Lux
wrote:

It was written by BL&E that
120 radials work,

2, 15, 30, 60, and 113.

No discussion whatever of 120.

I stand corrected.. thanks..
So they extrapolated to 120 as a "nice round number" for the future
purposes of the FCC.

The only reason the FCC used 120, is overkill for the
stations to be able to avoid a costly survey.
If they didn't use 120, they had to do tests to prove
that the system was efficient enough. So most used
120 to avoid all that.
For most cases, 120 is almost twice overkill..
For ham use 60 is usually plenty to get well into
the near optimum range. Any more than that is
a small increase, and usually not worth the cost
of the wire.
Since Owen posted this question, I did a lot more
checking around, and I had already seen the MW BC
examples.
Seems I'm not the only one that doubts that a small
number of barely elevated radials will give a large
increase over ones on the ground.
One that is in my camp is... Yuri will love this..
W8JI.. Tom seems to agree with my stance from
what I can tell. He has done tests in this regard
and his results did not show much of an increase
over the ground installed radials.
In fact, he gave one example where they changed a
MW station from four elevated radials to the usual
buried radials.. I assume 120 of them..
They then had to explain to the FCC why the buried
radials suddenly gave 5 db+ gain over the supposedly
"near perfect" elevated set.. :/
He also did tests on 80m comparing this same thing.
The results did not pan out and pretty much were in
the same ballpark as the results I saw when I tried it.
IE: the elevated radials are slightly better than the same
number on the ground, but only by a small amount.
His tests showed that the usual buried radials using
60 or more greatly outperformed the three or four
elevated radials. By 5 db+..
Myself, I think for four elevated radials to equal even
sixteen on the ground would require them to be almost
1/8 wave off the ground.
So it seems I'm not alone in my doubt of this
supposed free lunch program. W8JI seems to be
in my doubtful camp. A few others too actually.

Elevated vs buried radials
 

Jim Lux wrote in news:i85i4p$enq$1
@news.jpl.nasa.gov:

Richard Clark wrote:
On Thu, 30 Sep 2010 14:44:53 -0700, Jim Lux
wrote:

It was written by BL&E that
120 radials work,

2, 15, 30, 60, and 113.

No discussion whatever of 120.

There might not have been much "discussion" though it is mentioned, but
the summary does contain the following:

"It is also found that a ground system consisting of 120 buried radial
wires, each on half wave long, is desirable".



I stand corrected.. thanks..
So they extrapolated to 120 as a "nice round number" for the future
purposes of the FCC.

It does appear that if someone blessed the number 120, it was probably
BL&E who did it, even if they were talking about half wave radials.

Owen

Elevated vs buried radials
 

Thanks Jim, all fair comment and noted.

The end application is a four square phased array for the 80m DX window.

The location is at another ham's property, a rural location with ground
ranging from dryish clay to rock.

I do expect noise to be lowish compared to residential precincts.

The excercise is really about a design for a monopole that gives
reasonably good performance if extended to the four square configuration.

Yes, I note your points about the phase sensitivity to components. That
would be a challenge even with buried radials as although we have been in
drought for a long time with 'controlled' low moisture content of the
soil, rain changes that and the soil is no longer as homogenous.

Nothing is as perfect as a modeller's world, but the discussion and some
of the links offered give confidence that a shortened vertical with
capacity hat and three radials, and shunt coil matched should give
similar performance to full quarter wave verticals with 32 buried
radials.

I have just reread Cebik's article on buried radials, and my own models
seem fairly consistent.

As you say, Rudy's work is further confirmation allowing for the
difference in configuration and the |S21| use.

Owen

Elevated vs buried radials
 

On Oct 1, 3:02*pm, Owen Duffy wrote:
Richard Fry wrote:
Using NEC-4 to incorporate buried (or elevated) radials into the model
should show groundwave fields within 1 km of the monopole that are
very close to the theoretical maximum for the applied power when
radiated along a perfect ground plane, if the model is optimal, and
accurate.

It may do, I can not comment. My interest is for an antenna for sky wave
path, and I have not explored ground wave performance.

Just to point out that for vertical monopole heights of 5/8-lambda and
less, the peak elevation plane relative field (E / E max) _always_
occurs in the horizontal plane, regardless of the r-f losses in the
buried radial system or counterpoise wires they are driven against,
and the conductivity of the earth in which those radial wires are
buried, or above which they are elevated.

IOW, the relative field actually "launched" at all angles above the
horizontal plane from such antenna systems _always_ is LESS than that
in the horizontal plane. The reason for this is related to the r-f
current distribution, and its relative phase along the lengths of
those monopoles.

NEC analyses showing low to zero relative field in the horizontal
plane being launched by a monopole of 5/8-lambda height and less and
regardless of the r-f ground they are driven against need to be
understood in due context. The link next below leads to further
development of this ...

http://i62.photobucket.com/albums/h8...at_Compare.gif

The longest, great-circle, single-hop, skywave paths are related to
the relative fields launched by a monopole system at elevation angles
of less than ten degrees (see Figure 55 in the link below) -- where a
NEC analysis may show very low relative field.

But if such low relative fields really were true for the fields
actually launched by such monopoles, then the nighttime skywave
coverage of MW AM broadcast stations would be much different than is
shown by real-world experience (and applicable theory).

http://i62.photobucket.com/albums/h8...Comparison.gif

RF

Elevated vs buried radials
 

On 10/01/2010 06:13 PM, Jim Lux wrote:
Owen wrote:
On 01/10/10 07:44, Jim Lux wrote:

A bigger effect on a phased array is the relative phasing. For a 4
element array, you can have pretty big errors in phase on transmit
without changing the forward gain much (30 degree phase error on one
element might give you a 1dB change). But a 30 degree phase error on
receive could turn a -30dB null into a -7dB one..


How come ?
Can you elaborate how can these differences happen ?

Thanks
--
Ing. Alejandro Lieber LU1FCR
Rosario Argentina

Real-Time F2-Layer Critical Frequency Map foF2:
http://1fcr.com.ar

Elevated vs buried radials
 

On Oct 2, 4:28*am, Alejandro Lieber alejan...@Use-Author-Supplied-
Address.invalid wrote:
On 10/01/2010 06:13 PM, Jim Lux wrote:

Owen wrote:
On 01/10/10 07:44, Jim Lux wrote:

A bigger effect on a phased array is the relative phasing. For a 4
element array, you can have pretty big errors in phase on transmit
without changing the forward gain much (30 degree phase error on one
element might give you a 1dB change). But a 30 degree phase error on
receive could turn a -30dB null into a -7dB one..

How come ?
Can you elaborate how can these differences happen ?


it's the difference between the effect on a peak vs effect on a null.

consider a simple 2 element array.. for sake of argument, say it's 1/4
wavelength apart and phased 90 degrees, so it has a cardioid
pattern.... a gain of 2 in one direction (where the signals from the
two antennas align), and a gain of zero in the opposite direction.
The gain is 1+cos(phi - spacing*cos(theta)) where phi is the feed
phasing, and theta is the direction.. in the preferred direction
1+cos(90 - 90*cos(0)) = 1+cos(0) = 2
in the 45 degree direction: 1+cos(90-90*cos(45)) = 1+cos(90-90*.707) =
1.895
in the 90 degree direction: 1+cos(90-90*cos(90)) = 1+cos(90) = 1
in the 180 degree direction: 1+cos(90-90*cos(180)) = 1+cos(90-90*-1) =
1+cos(180) = 0

Now spoil the feed phase (phi) by 10 degrees... (80
on boresight: 1+cos(80-90*cos(0)) = 1+cos(-10) = 1.984
on 45: 1+cos(80-90*cos(45)) = 1.959
on 90: 1+cos(80-90*cos(90)) = 1.174
at 180: 1+cos(80-90*cos(180)) = 1+cos(80+90) = 1.52E-2

The gain on boresight didn't change much... from 2 to 1.984 (0.03dB)
But the null in the back came up from zero to 1.5E-2.. (instead of -
infinity, it's now -18dB)

Change the phase error to 45 degrees...)
@theta=0: 1+cos(45-90*cos(0)) = 1.707
@theta=180: 1+cos(45-90*cos(180)) = .292

So, from the 10 degree error case, the forward gain went from 1.984 to
1.707, about 0.6dB...
but the null went from 1.52E-2 to .292 (from -17dB to -5 dB)..


The thing to remember on any gain antenna is that it takes very little
power to disrupt a null (after all, a -30dB null means that if you're
radiating 1kW in the forward direction, you're radiating 1 W in the
null.. so just another watt will double the energy in the null,
turning it from -30dB to -27dB...)

(And, you can see why making antennas with sidelobes -60dB is VERY
challenging... )


Now, change the phasing to, say, 80 degrees.. in the preferred
direction, the gain is now 1+cos(10degrees)

Elevated vs buried radials
 

Mark wrote:

I always think in terms of wavelength when calculating
the approximate efficiency of an elevated radial set.
For instance, three radials at 1/2 wave up will be pretty
much equal to about 120 on the ground.
Three at 1/4 wave will be equal to about 50-60 on the ground.
Three at 1/8 wave might be equal to 15-20 on the ground.
Three at cigarette pack height will be equal to about twice
as many as actually used at best. "slightly guessing
on that one, but my real world tests seem to pretty much
agree".
So being as the increase is fairly small at such low heights
in wavelength, it is probably practical to just bury them so
people won't trip over them.. :/

I have explored what you have said in an NEC4 model of a quarter wave
monopole with three quarter wave radials at varying heights over 'average
ground'. The results are summarised at
http://www.vk1od.net/lost/Clip053a.png . The reference for the graph is
the efficiency of the same antenna with 120 buried radials in the same
soil type.

If the models are correct, laying just a few radials on or very close to
the ground (eg the popular method of pinned into the turf) would appear
to be a very poor option. The model indicates efficiency improves with a
very small increase in height above the dirt, just 30mm is a 6dB
improvement of lying on the dirt, just half a metre achieves 90% of the
available efficiency.

Owen

Elevated vs buried radials
 

On Sun, 03 Oct 2010 22:31:07 +0000, Owen Duffy wrote:

I have explored what you have said in an NEC4 model of a quarter wave
monopole with three quarter wave radials at varying heights over
'average ground'. The results are summarised at
http://www.vk1od.net/lost/Clip053a.png . The reference for the graph is
the efficiency of the same antenna with 120 buried radials in the same
soil type.

If the models are correct, laying just a few radials on or very close to
the ground (eg the popular method of pinned into the turf) would appear
to be a very poor option. The model indicates efficiency improves with a
very small increase in height above the dirt, just 30mm is a 6dB
improvement of lying on the dirt, just half a metre achieves 90% of the
available efficiency.

Owen

Owen,

Based upon your findings above, have you thought of increasing the height
of your model to determine at what height would be necessary to equal the
same efficiency as your 120 radial reference?

Danny, K6MHE