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