Rick:
Just to confirm what Guy mentioned regarding the visual impact of FCPs...
They can truly be made virtually invisible from a distance even when a
person knows where to look. I know this for fact because N3ND and I did the
camouflage paint job on his FCP just two weeks ago. The method we used was
similar to how I hide my 43' Rohn 25 among rows of pines at my QTH.
Years and years ago we did the same thing in Vietnam during the war in order
to camouflage aluminum and fiberglass antenna masts so Charlie and NVA
Regulars had a more difficult time putting us off the air. It is easy to do
if the rules of nature are maintained - no straight lines...
A camouflaged FCP truly is a thing of beauty to behold and also to use on
Top Band, I know because I use one (hi hi) Mine is hidden in the woods no
need for spray paint but from a distance it doesn't stand out as even being
there...
Hi Rick,
We need to look at the current distribution and amplitudes on a 5/16 wave
single wire folded counterpoise (FCP) quite carefully. It really is NOT the
same as two loaded 1/16 wave radials.
The deciding factor between the two is definitely not appearances. When an
FCP is either naturally in black or spayed with black, brown and green
against a background of trees or woods, our experience is that it becomes
invisible from the street, even when you know it's there and where it is,
actually up to HOA stealth levels. But significantly more critical than
that, if you really understood the camouflage FCP's "Christine-approved"
seal of approval both in nature and depth of criticism, you would not
propose appearance as a deciding factor for two 1/16 radials. You'd be back
to performance issues.
:>)
Just in general, if two loaded 1/16 wave radials were as good as the
experience we have had with the FCP, two loaded sixteenth waves would have
already been out there in number, tons of folks happy as a pig in slop using
it, it would be in ON4UN's book and we would not be having this discussion.
Even with the single inductor for both radials, a pair of 1/16 radials
requires something like a whopping 55 microhenries to tune it, and with a
lossless coil has a 1.5:1 SWR bandwidth of only 26 kHz. The coil required is
almost all of a $70 ten inch B&W #3026 Miniductor if you want to run QRO.
Myself, the last time I tried to do something that extreme I melted the
plastic rods with overheating.
The smallest successful radial set I have seen is ON4UN's proffer of 4
loaded 1/8 wave elevated radials, which is in use with good results at one
active contest station in my local area. John's radial length and count
choice, to my analysis anyway, seems a carefully chosen balance between the
loading inductor's size and losses, and narrowing of bandwidth, and the loss
multiplier of length over ground. But that requires the 90 foot square for
the radials, vs. the 66 foot line for the FCP.
The FCP is not a shortened counterpoise like a pair of 1/16 radials. The FCP
is a LENGTHENED counterpoise, LONGER than than a quarter wave in order to
pull off a field cancellation trick by folding it. It also has a useful
side effect of moving a typical tuned resonant Z of smallish vertical
radiator plus counterpoise above 50 ohms.
The junction of the FCP and vertical radiator is driven with a 1:1 isolation
transformer (required) between it and coax center conductor, coax shield,
and any connection to ground. A wind, rain and snow static ground is
provided by a 10 watt 5 meg resistor bridging the windings which is the only
connection between the two windings. The length of the radiating wire is
adjusted at the far end to present zero reactance on the shack side of the
isolation transformer. This way the residual inductance of the isolation
transformer and the capacitive reactance of the FCP is soaked up in the
pruning and tuning. Back to the current distribution...
Remember that the current entering the FCP is set by the radiating wire
because the apparent series resistance in the FCP is so low relative to the
radiation resistance of the vertical radiator. The FCP's beginning current
would be the same amplitude as the beginning current on the two radials. Set
our imaginary power drive to get one ampere at the base of the antenna in
both cases.
Counting FCP segments 1 through 5. 33 feet per segment. Directions used
are for illustration only.
1: center to 33 feet east
2: 33 feet east back to center
3: center to 33 feet west
4: 33 feet west back to center
5: center to 33 feet east and end insulator.
Segments 2 through 5 carry the typical cosine current curve of a 0 to 90
degrees quarter wavelength. This is enforced working backward from the end
insulator. Segment 1 has the cosine of -22.5 degrees to 0 degrees. The
current max is at the connection between segments 1 and 2.
Distributed current on segment 2 is the mirror image of distributed current
on segment 1. Segment two is reverse current direction from segment one,
therefore segments one and two completely cancel fields.
Distributed current on segment 3 minus distributed current on segment 4
(distributed effective current on the west side) is almost identical to a
mirror of distributed current on segment 5 which all that is left
uncancelled on the east side.
This means that the UN-cancelled distributed currents in the FCP are pair of
ramps either side of center corresponding to the cosine of 67.5 degrees
through 90 degrees, PROPORTIONAL to the ramp currents on two opposed
elevated 1/16 wave radials, EXCEPT that on the two loaded radials the ramp
current begins at 1.0 relative, and on the FCP begins at 0.38 relative at
the center of the FCP.
Since the cancel at ground due to opposite currents near center is the same
in either system, and the ground loss is the integration of the square of
the field intensity at ground times the ground area, the loss of the FCP is
0.145 relative to the radials' 1.00. So 1/16 wave radials with perfectly
loss-less loading coils inherently have seven times more ground loss than
the FCP fed equal current.
The actual dB dun to antenna system performance from a choice of
counterpoise depends on these effective series losses summed with the feed Z
of the vertical radiator. At my location up 90 and out 105 produces a tuned
feed Z of 120 something. Choice of counterpoise will not make nearly so
much difference to me with that height and length as to someone who can only
get 50 feet vertical.
The rather large loading coil(s) needed to tune two 1/16 wave radials on 160
puts one in a d**ned if you do and d**ned if you don't situation. If the Q
is high and loss low, the coils seriously reduce the bandwidth of the
antenna system, whereas the FCP leaves the bandwidth quite more controlled
by the vertical radiator. If the Q is low and losses high, this adds coil
wire loss to the already higher ground loss of the two radials. Four times
1/8 wave elevated seems to have this balanced.
Preliminary work seems to indicate that the FCP has better performance than
any elevated SPARSE radial system, except a virtual tie with ON4UN's 4 times
1/8 wave elevated radial system with a lossless inductor. BUT, and a big
BUT, there is lot of work left to do to PROVE that, best laid plans of mice
and men, wishful thinking syndrome, and Murphy all apply here. We will
likely be able to do some direct sky wave validation by balloon suspension
of a string of Elecraft XG3's driving impedance matched 1 meter vertical
antennas on frequencies separated by .5 kHz and run the tests in reverse
direction. Those 30's and 40's researchers would have killed to get their
hands on some of the stuff we have.
The real difference between those two is a 90 foot square and a 66 foot line
for the property disadvantaged. If the comparison is between ON4UN's 1/8
waves and a *PAIR* of FCP's at right angles fed in parallel, there is no
contest. 45 foot square beats 90 foot square and should be visible on RBN
if the radiators are low Z. (W0UCE pay
attention.)
Carefully controlled, the RBN comparisons seem to be good to within a couple
dB range. That's a lot less than the ten dB accuracy range needed to
positively identify really sucky radial systems as really sucky.
73, and may your 160 antenna make you happy,
Guy.
On Wed, Nov 16, 2011 at 12:22 PM, Richard (Rick) Karlquist
<richard@karlquist.com> wrote:
> On 11/15/2011 9:10 PM, Guy Olinger K2AV wrote:
>
>> field, either buried or elevated. Therefore, presuming that undense
>> irregular radials that would fit would be excessively lossy, per RBN
>> data previously gathered, the +33, -33 foot linear folded
>> counterpoise
>> (FCP) is used instead, elevated at 8 feet. The folds in the
>> counterpoise are designed to self-cancel fields as much as possible,
>> thereby minimizing ground induction, which is loss to skywave. The 66
>
>> 73, Guy.e
>
> It seems to me that the folded counterpoise is equivalent to a couple
> of loaded short radials, except that "linear loading" is used instead
> of lumping loading coils. Thus the ground induction loss is not
> reduced by the folding. So this is just a non-traditional
> implementation of 2 short loaded elevated radials. Nothing wrong with
> that, if implemented carefully. The decrease in gain is probably
> within the margin of error of RBN.
>
> In the described small backyard situation, I would think that making
> radials out of plain wire and loading them with coils at the feedpoint
> would be more acceptable from the visual clutter viewpoint.
>
> Rick N6RK
>
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
|