[TowerTalk] Inverted L Tuning

[Donald Chester]

>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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