>There is a scenario that you are both correct:
>
>
>If the large vertical has quite high ground resistance in the beginning, the
>base impedance can actually be greater than 50 ohms. As radials are added,
>the impedance will drop and vswr will drop as well. That is until you reach
>the
>50 ohm point where the vswr will rise as more radials are added.
>
>Gerald K5GW
Let us say the radiation resistance is 12 ohms, not an unusual value for a
short, top loaded vertical, which basically what an inverted-L is. With a poor
ground system, the ground loss might be on the order of 90 ohms. So, once the
system is tuned to resonance (all the capacitive or inductive reactance tuned
out using the ATU), the load is seen as 102 ohms resistive. That would be
slightly over 2:1 SWR, but the antenna efficiency would be very poor, something
like 10%.
Now we add enough radials to bring the ground loss to 38 ohms, and retune the
system. That brings the load to 50 ohms, purely resistive. The SWR is now a
perfect 1:1, but the efficiency is still only 24%.
We keep on adding radials and retuning, bringing the load down lower as the
ground resistance is reduced. The SWR begins to climb, as the total resistance
descends below 50 ohms, but the antenna efficiency goes up. Finally, we
accumulate a total of 120 quarter wave radials, equivalent to the ground system
used by many AM broadcast stations, and we achieve a ground loss resistance of
only about 2 ohms. Now, once the ATU is retuned to resonance, the load
resistance is 14 ohms, but the antenna efficiency is 86% while the SWR is up to
3.6:1.
The point is, that there is nothing magic about SWR, and a 1:1 SWR does not
necessarily mean better performance. Even considering feedline losses in the
above example, we are better off running the 3.6:1 SWR than we were with the
1:1.
To improve feedline efficiency and power handling capability, we can readjust
the ATU so that it not only tunes out the reactance, but transforms the
impedance. This can be done with a simple L-network at the base of the
vertical radiator. This brings the SWR at the feedline to 1:1, and the antenna
efficiency is still 86%. There may be some loss in the L-network, but a well
designed unit should be at least 90% efficient. That would still give us a
total efficiency of 77%, minus feedline losses.
Rather than run the vertical wire of an inverted L adjacent to the tower, I
would attach the horizontal wire of the inverted L to the top of the tower, and
use the tower itself as the vertical radiator, using shunt feed or unipole
feed. The wire that is now serving as the vertical part of the L would be
converted to the shunt feed wire, with the top end attached to the tower at
some point to allow a good impedance match to the ATU. The radials would be
bonded to the base of the tower and that would serve as the common grounding
point for the antenna, ground plane and ATU.
The problem with the nearby tower, even if everything is made to tune up
perfectly, is that unless the radials are securely bonded to the tower base,
the tower is floating free with a substantial resistance to ground. Because of
the close proximity between vertical radiator wire and tower, there will
inevitably be substantial coupling between the inverted L and the tower, and
the high ground resistance at the tower base will result in significant power
loss in the system, thus reduced efficiency.
Another factor is bandwidth. With a good ground plane and low resistive losses
at the antenna, the tuning may be very sharp and the antenna may have a narrow
bandwidth, necessitating retuning with a relatively small change in frequency.
OTOH, if the ground losses are high, which adds a lot of ohmic resistance into
the equation, the tuning may become very broad and the antenna may appear to
have a very good bandwith allowing a much wider tuning range without
readjustment of the ATU, but this is deceptive because of the poor overall
efficiency of the antenna.
The foremost consideration should be radiation efficiency, even when this is at
the expense of SWR and antenna bandwidth.
Otherwise, we might just as well be satisfied to use a "Maxcomm automatic
antenna matcher". (Do a Google search if you are not familiar with those.)
Don k4kyv
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