DXman,
while the figures resulting from your measurements are indicative of what is happening when the shaft is lengthened, the test for distance comparison would have been much more meaningful if it had been conducted entirely on either am or ssb.
10K- SWR 1.3:1, R=44 X=4
M.Coily - SWR 1.4:1, R=42 X=5
Replacing the 17" with a 22" shaft for both antennas:
10K - SWR 1.1:1, R=49 X=1
M. Coily - SWR 1.2:1, R=47 X=2
(if i misinterpreted those numbers let me know)
in any event, there is much more going on here than just lower swr and reactance figures.
several things are readily apparent. as the height of the inductor is raised higher into the air the input impedance is moving closer to 50 ohms and more of the reactance is being cancelled. as the coil is raised the destructive interference created by surrounding objects (including the close proximity of the counterpoise underneath) and the attendant capacitive coupling is lessened, distorted capacitive distribution across the inductor corrects itself as well. the overall effect is to move the higher levels of voltage being created towards the top of the antenna, where they belong. this is exactly the reason why top-loaded antennas excel in terrain that is level and wide open for miles in all directions. if you spend a lot of time in urban/city areas you might want to opt for an antenna design providing the pattern characteristics of a unity antenna. the antennas we're discussing here are great examples of unity or less-than-unity antennas in that the majority of them are simply physically shortened, electrically lengthened 1/4 wave antennas.
the full-size 102" whip with spring and ball at about 107.5" inches mounted in the top center roof area of a typical vehicle yields 0.0dbd, or unity gain when compared to the reference dipole.
raise the inductor as high as necessary so that:
R= 50
X= 0
or as close to both these values as is possible.
and once you get there don't let the absence of a flat swr bother you. i believe most of these shafts can be connected to one another providing for a wide-ranging assortment of shaft lengths to experiment with.
be something to do if someone wants to play.
</p>Edited by: <A HREF=http://p067.ezboard.com/bworldwidecbradioclub.showUserPublicProfile?gid=freecell>freecell</A> at: 6/5/04 2:27 pm
