Re: Elecraft Digest, Vol 8, Issue 10

[Charles B. Wilber]

--- John W2XS wrote:
The 42-foot wire received slightly lower signal reports than the inverted V,
but most people said that they could still hear me fine on the wire. To my
surprise, though, most people reported no real difference with and without
the counterpoise.  I don't know if this was a circumstance of my particular
installation, but I would have thought that the counterpoise would make more
difference.
--- end of quote ---

John,

I've read in a number of places that the counterpoise should ideally be slightly
longer than the antenna wire. Perhaps that is why little difference was noted.

73,
Charlie
N1AOK
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[Don Wilhelm-3]

Charlie, John and all,

One of the 'funny' properties about RF is that it will always find a
(ground) reference somewhere - that may be from any wire, chassis, PC board
or even your body.  Whatever the RF finds convenient will be used as its
ground reference.

The idea of using a proper counterpoise is that one will assure an RF ground
at the point the counterpoise is connected.  To have that condition, the
counterpoise must be an electrical 1/4 wavelength long and the far end must
be isolated (just like the end of an antenna - actually an elevated resonant
radial).  The physical length will vary with placement of the wire (and a
lot of other things too - just like an antenna will be influenced by nearby
objects).  A random radial wire can be tuned with lumped constant elements
to create this RF ground too, and that is exactly what the 'artificial
ground' boxes do. the really are 'antenna tuners for your ground wire'.
BTW, a half wavelength wire connected to ground at the far end will behave
just like a quarter wave that is open at the far end - think transmission
line (and antenna wire) behavior.

The bottom line message here is that all factors must be considered before
one concludes that a counterpoise is or is not beneficial to a particular
installation.

73,
Don W3FPR

----- Original Message -----

> --- John W2XS wrote:
> The 42-foot wire received slightly lower signal reports than the inverted
> V,
> but most people said that they could still hear me fine on the wire. To my
> surprise, though, most people reported no real difference with and without
> the counterpoise.  I don't know if this was a circumstance of my
> particular
> installation, but I would have thought that the counterpoise would make
> more
> difference.
> --- end of quote ---
>
> John,
>
> I've read in a number of places that the counterpoise should ideally be
> slightly
> longer than the antenna wire. Perhaps that is why little difference was
> noted.
>
> 73,
> Charlie
> N1AOK
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[David A. Belsley]

On Dec 10, 2004, at 9:27 AM, W3FPR - Don Wilhelm wrote:

> One of the 'funny' properties about RF is that it will always find a
> (ground) reference somewhere - that may be from any wire, chassis, PC
> board or even your body.  Whatever the RF finds convenient will be
> used as its ground reference.

> BTW, a half wavelength wire connected to ground at the far end will
> behave just like a quarter wave that is open at the far end - think
> transmission line (and antenna wire) behavior.

Doesn't this cause a bit of an RF-philosophical problem?  If one end of
the half wave is supposed to determine RF ground, how does the RF know
what's at the other end is also ground?  I should think that simply
grounding it to the earth would not do the trick if, as in your
original message, the RF hasn't already "found" it as a convenient
ground.

best wishes,

dave belsley, w1euy

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[Don Wilhelm-3]

David,

Sorry about the confusion.
I was not proposing that the grounded halfwave was a practical solution - my
intent was only to point out that the behavior of RF on a 'counterpoise
wire' is not magic, but follows the same rules as other wires carrying RF,
just like an antenna element, or one wire of a transmission line.  There is
no 'one solution' or one answer to such questions, we have to consider the
behavior of all elements involved.

The REAL answer is: "It all depends ..."

73,
Don W3FPR

----- Original Message -----

>
> On Dec 10, 2004, at 9:27 AM, W3FPR - Don Wilhelm wrote:
>
>> One of the 'funny' properties about RF is that it will always find a
>> (ground) reference somewhere - that may be from any wire, chassis, PC
>> board or even your body.  Whatever the RF finds convenient will be used
>> as its ground reference.
>
>> BTW, a half wavelength wire connected to ground at the far end will
>> behave just like a quarter wave that is open at the far end - think
>> transmission line (and antenna wire) behavior.
>
> Doesn't this cause a bit of an RF-philosophical problem?  If one end of
> the half wave is supposed to determine RF ground, how does the RF know
> what's at the other end is also ground?  I should think that simply
> grounding it to the earth would not do the trick if, as in your original
> message, the RF hasn't already "found" it as a convenient ground.
>
> best wishes,
>
> dave belsley, w1euy
>
>

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[Mychael Morohovich]

A while back, Ron AC7AC designed, built, and kindly sent me a simple yet
elegant
rf current sensor that operates along with my DMM to give an accurate
quantification of
how my rig is delivering power to the radiator. More current equals more
radiation. One thing it allows me to do is to calculate power out into a
known load, and particularly relative to this discussion, it facilitates the
ability to make comparisons between different antenna configurations,
including the relationship between radiator and counterpoise.

Instead of a 1/4 wl wire, I use an MFJ Artificial Ground to tune a wire for
maximum current per given band. While measuring, one thing that
surprised me was to discover that with my 86' EFW, the use of a properly
tuned counterpoise didn't always increase the current to the radiator.
For example, on 40 meters the use of a counterpoise helped
to realize a substantial increase of current into the radiator, while on 80
meters I actually saw more current into the radiator with the counterpoise
unhooked on that band. The effects on 30-10 were really fairly minimal or
neutral in this regard.

73,

Mychael AA3WF
K2#1025


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[Vic K2VCO]

Mychael Morohovich wrote:

> Instead of a 1/4 wl wire, I use an MFJ Artificial Ground to tune a wire for
> maximum current per given band. While measuring, one thing that
> surprised me was to discover that with my 86' EFW, the use of a properly
> tuned counterpoise didn't always increase the current to the radiator.
> For example, on 40 meters the use of a counterpoise helped
> to realize a substantial increase of current into the radiator, while on 80
> meters I actually saw more current into the radiator with the counterpoise
> unhooked on that band. The effects on 30-10 were really fairly minimal or
> neutral in this regard.

Keep in mind that you are measuring the current in the radiator AT A PARTICULAR
POINT.  Due to the standing waves on the radiator, that current varies depending
on where you measure it.  Now, since the 'antenna' you are feeding consists of
the counterpoise PLUS the radiator, tuning the counterpoise will change the
current distribution on the whole system.  So measuring the current at the
original point on the radiator does not tell the whole story.  Perhaps adding
the counterpoise has moved the maximum current point away from the feedpoint (a
desirable situation)?

What you can do is listen to distant stations while connecting and disconnecting
the counterpoise.  That will test the overall effectiveness of the antenna.  Of
course, the counterpoise will also change the horizontal and vertical pattern of
the antenna, so you will need to listen to a large number of distant stations to
  control for this effect!

--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco

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[Ron D'Eau Claire-2]

Vic, K2VCO, wrote:

Keep in mind that you are measuring the current in the radiator AT A
PARTICULAR
POINT.
---------------------
The counterpoise is tuned for maximum current at the *rig*!

The idea is that the more current flowing into the counterpoise for a given
power output the lower the RF voltage at the rig. The objective is to
provide the lowest impedance "RF Ground" at the rig.

A well-tuned 1/4 wave wire does a very good job, of course. A 1/4 wave wire
shows an impedance of about 35 ohms. Parallel several and you can reduce
that proportionately. The problem is that in many situations it's not
possible to carefully arrange even one resonant 1/4 wave wire, much less
several. Many so-called grounds consisting of a random length wire thrown on
the floor or a wire running several feet to a ground rod show an impedance
at the rig in the hundreds of ohms, making the rig "hot" with RF.

In that case a 35 ohm "ground" provided by a single tuned counterpoise is
vastly superior for "grounding the rig".

Will it change the efficiency of the antenna? Perhaps. If the antenna is an
end-fed 1/2 wave wire presenting an impedance of  perhaps 2000 ohms to the
rig, the difference in radiation efficiency will be negligible. One can
consider the two resistances in series: the ground resistance, 35 ohms in
this example, and the radiation resistance of the antenna, 2000 ohms in this
example. If .1 amps is flowing inn that circuit, the rig is putting out
20.35. Of that 20 watts, 0.35 watts is dissipated in the counterpoise and 20
is radiated. The advantage of the counterpoise in this case is to keep the
RF voltage at the rig down. It will tend to "float" to a high RF potential
if there's no ground and cause all sorts of problems.

If the antenna is shortened to 1/4 wavelength, then the efficiency drops to
50% with equal current and power in the counterpoise and antenna. That
sounds awful but that is, in reality a very respectable figure for such an
antenna. Many short vertical antennas have efficiencies of much less than
50%.

As the antenna is made shorter than 1/4 wave long (that's overall physical
length, adding a loading coil doesn't help the efficiency - it only helps
match impedances) the radiation resistance plummets, and with it the
efficiency unless a really, really good ground system is used. That's the
ultimate challenge with anyone using a "loaded" monopole of any sort. There
simply is no "magic" answer to efficiency for physically small antennas -
until we get some really cheap, really good room-temperature superconductors
to make them from...

Ron AC7AC


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[Jim Larsen - AL7FS]

Thanks for the good info....I am slowly learning.

73, Jim, AL7FS

Ron D'Eau Claire wrote:

--
Jim Larsen
http://www.CSTAlaska.com/
http://www.AL7FS.us/
Anchorage, Alaska

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[Vic K2VCO]

Ron D'Eau Claire wrote:

> Vic, K2VCO, wrote:
>
> Keep in mind that you are measuring the current in the radiator AT A
> PARTICULAR
> POINT.
> ---------------------
> The counterpoise is tuned for maximum current at the *rig*!
>
> The idea is that the more current flowing into the counterpoise for a given
> power output the lower the RF voltage at the rig. The objective is to
> provide the lowest impedance "RF Ground" at the rig.

Hi Ron,

I agree.

The post I was responding to included:

> For example, on 40 meters the use of a counterpoise helped
> to realize a substantial increase of current into the radiator, while on 80
> meters I actually saw more current into the radiator with the counterpoise
> unhooked on that band. The effects on 30-10 were really fairly minimal or
> neutral in this regard.

Mychael was using an RF current sensor to measure current into the radiator.  My
point was that tuning the counterpoise may have the effect of moving the high
current point in the antenna system that includes the counterpoise and the
radiator.  I was making the point that his experiment of unhooking the
counterpoise and measuring the current into the radiator did not indicate that
the counterpoise was or was not improving antenna performance.

Since his antenna was an unbalanced end-fed wire working against ground, the
counterpoise was an essential part of his system.  Tuning it would be expected
to have an effect on performance.  Of course, with a balanced or
ground-independent antenna, no counterpoise should be needed.  RF in the shack
is indicative of imperfect balance or isolation.

--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco


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[Mychael Morohovich]

Vic wrote: "I was making the point that his experiment of unhooking the
 counterpoise and measuring the current into the radiator did not indicate
that
 the counterpoise was or was not improving antenna performance."
-------------------------------------------------------------

The method I used was to first tune the antenna with my KAT100 for a flat
SWR, tune the counterpoise for max current at the rig, then tweak both until
the KAT100 reported a flat SWR for the antenna while the Artificial Ground
reported maximum current into the counterpoise. Using the rf sensor, I
measured the current of the antenna [and counterpoise because I was curious]
at a spot close to the back of the rig. I then removed the counterpoise,
retuned the antenna for a flat SWR, and measured the current again for
reasons of comparison.

This showed me the amount of current that was going into a properly tuned
antenna both with and without a tuned counterpoise in place.

Mychael



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[Vic K2VCO]

Mychael Morohovich wrote:

> The method I used was to first tune the antenna with my KAT100 for a
> flat SWR, tune the counterpoise for max current at the rig, then tweak
> both until the KAT100 reported a flat SWR for the antenna while the
> Artificial Ground reported maximum current into the counterpoise. Using
> the rf sensor, I measured the current of the antenna [and counterpoise
> because I was curious] at a spot close to the back of the rig. I then
> removed the counterpoise, retuned the antenna for a flat SWR, and
> measured the current again for reasons of comparison.
>
> This showed me the amount of current that was going into a properly
> tuned antenna both with and without a tuned counterpoise in place.

Think of the antenna and everything connected to the ground terminal of the
KAT100 -- including the counterpoise -- as an off-center fed antenna.  When you
removed the counterpoise, you moved the feedpoint.  This changed the impedance
seen by the KAT100 (which is why you had to retune it) as well as the current
measured at the feedpoint.  If (for example) the current at this point
decreased, how do you know it didn't increase somewhere else along the wire?

--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco

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[Mychael Morohovich]

> Think of the antenna and everything connected to the ground terminal of
> the  KAT100 -- including the counterpoise -- as an off-center fed antenna.
> When you removed the counterpoise, you moved the feedpoint.  This changed
> the impedance seen by the KAT100 ... 73, Vic, K2VCO

Thanks Vic, I get it now: the method that I am using is valid only when the
antenna is 1/4 wl or less. Since my EFW is 86 feet, it would only
be useful trying this comparison on topband where a 1/4 wl is roughly 125
ft. It was suggested off list that my best bet overall is to employ the
use of a field strength meter. Perhaps it is time to invest in one!

73,
Mychael


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[Vic K2VCO]

Mychael Morohovich wrote:

> Thanks Vic, I get it now: the method that I am using is valid only when the
> antenna is 1/4 wl or less. Since my EFW is 86 feet, it would only
> be useful trying this comparison on topband where a 1/4 wl is roughly 125
> ft. It was suggested off list that my best bet overall is to employ the
> use of a field strength meter. Perhaps it is time to invest in one!

The field strength meter is useful, but doesn't tell the whole story.  Since the
counterpoise becomes part of the antenna system, changing the counterpoise can
change the pattern of the antenna.  So you would need to make a bunch of fs
measurements at various points and plot them.  Not to mention the fact that an
antenna has both horizontal and vertical directivity!

--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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