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