Absolutely, and, "If it works don't complain!", is my motto.
Still, my mind ponders and responds with questions...



One thing I haven't been forth coming with is the 1/4 wave antenna. Really,
I don't know what specs a "proper" one should be constructed to (working off
very basic ARRL diagram).

The one I built has a very heavy stainless whip--most likely is military
surplus, but years ago I bought a whole slew of them and still have not used
them up, the whip base is about 3/8 and tapers at approx. 12 feet to a 3/16
dia.

I just hacked it to a 1/4 wave with the 1/4 ant length formula provided by
the ARRL book and removed length with a very course file, until resonate.
The radials are 1/2" aluminum rod and heavy. The insulating material for
the whip is cut from 1-1/4" thick nylon sheet. The SWR was originally high,
but "drooping" (ARRL book suggested this) the ends of the radials, down near
a 45 degree angle, provides 1:1.5. The radials are cut 5% wave longer than
the whip (again, suggested by ARRL book), they are grounded to the mast and
the mast has a good earth ground. Antenna is directly fed with 50 ohm coax.

Perhaps my construction of the 1/4 is less than optimum? And, this may make
it appear that the antenna from my original post is performing better than
it really is.... thoughtful frown

The weather here is high wind and rain. I will not be tempted to do any
more experimenting until this passes...



Regards



Hey! If you can't provide and suggestions, how about just spell checking
this for me?



"Roy Lewallen" wrote in message
...
If there's no ground system loss, a half wave vertical antenna has about
1.3 dB gain over a quarter wave vertical due to the sharper pattern shape.
This is the line-of-sight or surface wave gain difference. If reflection
from real ground is included in the analysis, the gain difference is about
1 - 1.5 dB at lower angles, and the quarter wave does much better, by 3 or
4 dB or so, at high angles (very roughly above 50 degrees or so).

Coil loading to achieve half wave resonance does have one potential
advantage, and that's to reduce ground loss when substantial ground
resistance is present. Because of the much higher base impedance, ground
current is much less with a half wave resonant base fed antenna than with
a quarter wave antenna, even when the resonance is achieved with coil
loading. Ground loss is typically pretty low on ten meters in mobile
applications, but in some other setups (such as ground mounting over a
buried radial system, or a hand-held radio), the reduction in ground
current could reduce ground loss more than enough to compensate for coil
loss.

I suspect this is the explanation for at least some of any advantage you
see in the coil loaded antenna over a straight wire of the same length.
However, there's a good sized handful of other possible factors. (Every
last one of them, incidentally, explainable perfectly well by "standard
theory".) If the measurement results don't agree with the model, it nearly
always means that the actual antenna -- that is, the "antenna", the
"ground" system, and everything connected to either one or in the near
field of either one -- differs from the model in some significant way. Or
the measurement system is faulty.

Someone interested in understanding the operation of an antenna will
commonly spend a great deal of time tracking down the factors that differ
between the model and real antenna, and it's just about always a truly
educational experience. When done, the result is usually a model that
really represents the antenna system, and that shows results very close to
careful and competent measurements. And an experimenter who knows more
about antennas, modeling, and the importance of some factors he never
before considered. You go through this exercise a few hundred times, like
professional engineers routinely do, and you come out with a great respect
for "standard theory" and an even greater level of skepticism toward
people who have a weak understanding of it and are convinced they've
witnessed a miracle. Those of us who have spent a career using "standard
theory" to design real, useful items that work as designed haven't become
skeptical because our brains have been petrified by education; it's
because we've seen "standard theory" work, over and over and over, and
every last miracle inevitably fall, one after another, to careful scrutiny
and understanding.

But we also eventually come to realize that astrology, homeopathy, and
feng shui(*) will be with us forever. People will simply believe what
suits them, and won't be bothered by evidence. Especially if the evidence
takes effort, knowledge, patience, and understanding to acquire.

The "S-Unit" is a nice homey amateur unit of measurement, but it has no
meaning. (Some people seem to think it's equal to 6 dB, but vast numbers
of others think it's the the size of the markings on their receivers' "S
Meter". The two can be very different.) dB, on the other hand, is a
universally defined and understood unit. Anyone unable to accurately
measure relative values in dB is unable to make quantitative gain
measurements at all.

(*) My spell checker didn't recognize this, so I did a quick google search
to see if I got it right. It came up with 2,180,000 hits. There's no lack
of believers.

Roy Lewallen, W7EL

John Smith wrote:
Well, I have 3 test antennas on this project:
1) 1/4 wave stainless steel whip w/4 heavy 1/2 inch, aluminum ground
plane radials
2) 1/2 wave without any ground plane/counterpoise, lower 1/4 wave section
is 3/4 copper pipe, upper 1/4 is a stainless whip, total of the
combination is 1/2 wave... ferrite beads are used on the outer shield of
the coax at the point where it connects to the antenna, match is though a
simple l-network (ferrite beads are actually un-needed but used anyway,
no real detectable radiation from coax either way)
3) This is the antenna I described in the first post(s), I tend to refer
to as the "quarter/half", it is constructed on 3/4 ID pvc pipe, and has
an OD of just over an inch, the coil is constructed of #8 copper wire and
the top whip is the 28 inch stainless whip I mentioned. Is a resonate
1/2 wave and physical 1/4 (whip + helical coil length.) Matching network
is a l-network, capactior is constructed of acrylic insulator plates
(acrylic "window pane") with copper sheet plates, coil is a 1.3 OD
torroid. coil is approx. 1+ uh (computed from toroid data)--I have not
measured the capacitance of the capacitor (probably somewhere from 20+ pf
to 30+ pf), no groundplane/counterpoise, ferrite beads as above.

Naturally, as the models predict, the 1/4 is lower in preformance than
the half 1/2, and very noticable in the most distant contacts.
However, the "quarter/half" out preforms the 1/4 but falls lower in
preformance than the 1/2. Contacts out at 30+ miles are where the
electrical 1/2 wave antennas differ most from each other (antennas 2 and
3 above.)
FSM measurements of the two 1/2 waves are very simuliar--however, these
measurements could have been taken at a greater distance to increase
accuracy (meters sensitivity I currently am using limited this.)
Franky, I was surprised by the preformance. As others have predicted--I
expected the preformance of the shortened 1/2 wave to be poorer than the
1/4 (mainly due to the helical coil skewing the radiation pattern and
adding a slight resistance to a physical 1/4 antenna.)
However, it seems to fall in the middle between the 1/4 and the 1/2 wave
(full length) antennas, favoring being closer to the 1/2 full length by
1/2 S-unit+
The biggest difference between the 1/4 and 1/2 wave full length is 2-3
s-units at stations in the 30+ mile distances.
The full 1/2 wave and the shortened 1/2 wave seem close to 1 s-unit on
all apparent distant stations.
I find this hard to believe, and the models I calculated did not reflect
what I had actually seen in s-units.

All three antennas were placed at the same height, on the same mast while
testing (32 ft above real ground.) And all other conditions the same.
The l-network match is the only difference between the two 1/2 wave
antennas, in the future I will correct this and finalize the tests... the
difference in the matches could be responsible for the difference in
expected results.
Right now, it actually looks to me, from the above--that the "1/4
physical, 1/2 wave electrical (quarter/half)" antenna was and is
exhibiting properties of both a 1/4 and a 1/2 wave antenna. This is the
main reason I tossed out this "antenna problem" here, to see others
reactions and draw from their experience.

Warmest regards

"Buck" wrote in message
...

On Sun, 20 Mar 2005 18:26:42 -0800, "John Smith"
wrote:






My question(s):

For an example, on the 10 meter band:
If I take a 28" whip and mount it at the end of helical wound coil
(wound on
1" diameter form), where the "wire length" of the coil, PLUS, the length
of
the whip (28" + coil wire length) is equal to 1/2 wavelength (electrical
length)--BUT, the overall physical length of the antenna (top, tip of
whip
to base of helical wound coil) is 1/4 wavelength, what would the
radiation
pattern of such an antenna be?

Would it favor the pattern of a 1/4 or 1/2 wave antenna--or, would the
pattern be a compromise between the two--or, would the pattern be
totally
unrelated to either?

What could I expect the impedance of such an antenna be? Would the
reactance be capacitive or inductive? What would be the best way to
provide
a match to 50 ohm coax from such an antenna?

What software is available to model such an antenna?

Thanks in advance,
warmest regards



EZNec will model your antenna. As for the antenna, I believe that the
shortened 1/2 wave antenna will radiate with less effect than a
stretched 1/2 wave, but possibly better than a 1/4 wave. How much?
That depends on the coil and the matching network pending losses.

Good luck,
Buck
N4PGW

--
73 for now
Buck
N4PGW