[Lowfer] re Overall antenna efficiency

[Andy - KU4XR]

--- On Thu, 10/21/10, Alan Melia <alan.melia at btinternet.com> wrote:

> The problem with monitoring the antenna current is that you
> dont really know why it changed ....hopefully the loss resistance 
> dropped but the figure quoted suggest that the improvement if it 
> all went into radiation was about 0.5dB or 10%. But did the tuning 
> change and cause the PA to run more power, has the antenna just 
> got a lower feed impedance? Is the extra current traversing the
> radiation resistance. To get real advantage you need to be
> looking for more than that.....but it might now be attainable 
> within your local contraints.

*** But, did the tuning change ****

Alan, thank you for your comments, as always very helpful.. This is
twice now that I have seen that question about the tuning changing
after an alteration in the antenna setup. I want to tell what I
saw here, even though I can't explain it theoretically... In a 
previous experiment, some may remember that I ran some wires angling
downward from the tip of the top hat to the mast, about 5 feet down.
It was recommended that I move them from the mast to the top of the
antenna and connect them to the 3 foot tensioning pipe. I did, and
it made no difference in anything. No extra OR less current, no
change in the tuning... Now; commenting on the connection of one of
the radials to the service entrance ground connection; That DID !! 
cause a change in tuning. Here's what I saw: I tuned, and peaked the
transmitter before connecting the new ground lead. I use 25 volts DC
on the PA, and set it at 40 Milli Amps of current for 1 Watt Input.
This generated 150 Milli Amps of RF current showing on the ammeter.
AFTER connecting to the power company ground at the pole; I checked
the transmitter, and the PA current had dropped to , if I remember
correctly, around 38.7 Milli Amps maybe, and I honestly don't
remember what the RF current was reading. But after Re-Tuning 
everything. ie: getting the voltage to read 25 volts at 40 Milli Amps
and the antenna current peaked with the transmitter generating the
1 Watt Input; I now had 155 Milli Amps showing on the RF ammeter.

So; before the new ground connection; 25 V @ 40 mA = 150 mA of RF " I "
After the new ground connection; 25 V @ 40 mA = 155 mA of RF " I " 
And the answer to the question; " did the tuning change " is YES, it did.

As Alan mentioned in his post; I don't know what parameter changed to
cause the rise in antenna current, My hope is that the earth resistance
went down, because that is what I was working on was the earth ground.
No alterations were made to the antenna itself, so the change came from
altering something in the earth part of the antenna setup.

And, as has been said; ( if I had the equipment to do so ) Far Field
measurements of the signal would tell the truth about any alterations
in the system, beneficial, or not.

73 Gents :


Andy - KU4XR - EM75xr - Friendsville, TN. USA
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