Topband: tuning Inverted L

[cris blak]

Hello Mike and others,

The "procedure" for tuning/optimizing vertical antennas and such (ie: INV-L) is 
as following:

1. Doing impedance measurement at the base of the antenna (at the feed point 
but, without feeder!) with (* appropriate equipment, should install the radials, 
one by one, one after the other; The aim is to obtain as low impedance (at 
resonance! - this means, X=0 or the complex/reactive part of the impedance 
should be zero) as possible (close to the theoretically one) and as much current 
on each radial as possible; current should be measured on each radial, one after 
the other as installed
2. After all the above optimization were done, the antenna feed complex 
impedance should be derived (real and imaginary parts, active and reactive part 
of that impedance); having this impedance and knowing the feed line impedance 
(ie: coaxial 50ohm), one should calculate the tuning circuitry (ie: L network 
should be fine); for this task, the freeware program RFSim99 is all that you 
need
3. Build the tuning circuitry, install it than measure again the impedance at 
its input (where the feed line will be connected); if that impedance is not 
reactive at the resonant frequency and equal with feed lines one than connect 
the feed line
4. Last, a final VSWR check with full power should be done "in the shack"; if 
everything is OK than, enjoy! If you want the joy to be "full" than install a 
un-un transformer for common mode current suppression. 

 
Congratulation, you just optimized your vertical antenna for maximum available 
efficiency on your location! 

One should have in mind that not all the time, the more radial the better is 
true!!! And this is validated by experiment and hard to simulate due to local 
condition which is far, far from the theoretical situation.

For example, I use also an INV-L, 15m vertical, about 27m horizontal sloping 
down to about 10m over the ground and lots of objects in its near field. I have 
one rod ground "system". First, I used 36 elevated radials about 1/8 lambda and 
the feeding impedance were about 25ohm. Checking the current on each radials, 3 
of theme carry about 1/2 from the overall current, another 5 the rest till about 
70% of the antenna current. After applying the above optimization method, the 
impedance were down to 15ohm. This was the best I can do. The theoretical 
impedance for such antenna is about 11ohm which means, there will be about 26% 
more place to improve its efficiency...or, looking form the other point of view, 
now, 26W are lost (transformed in heat) from 100W injected in the antenna. Far 
better than with 36 radials when 64W were heat and only 44W radiated! Remember, 
I'm talking from the real life (measurements), not from the books. 


Apart from direct impedance measurement, field strength measurement are best to 
do in order to be sure about the overall result. 

In my case, after each antenna modification, field strength measurement were 
done on 4 direction and three points, 1, 2, 2.5Km away having line of site with 
the antenna validating the impedance measurements within 0.3dB tolerance.

 Remarks:
- (* "Appropriate measurement equipment": for HF impedance measurement I'm using 
U/I method with an dual channel oscilloscope as an vector voltmeter/ammeter and 
the transceiver with about 10W as a RF signal source; for current measurement I 
use current transformer with an 100uA ammeter embedded (ie. W8JI web page); 
miniVNA, MFJxxx and such equipments are not suited for such precise 
measurements; of course a proffesional VNA is far better to have but is very 
expensive...
- such optimization should be done having the local weather as a constant; this 
means that the soil and air humidity/conductivity should be constant along the 
optimization process!!! This is very important. Otherwise, the optimization 
process will be corrupted. Doesn't matter the absolute value of the 
humidity/conductivity but to be almost constant during the process.

Saying all this "story", I hope you can understand how you (and maybe others) 
should optimize your (there) vertical antenna for better efficiency. 

Speaking of effectiveness is a different matter and was not discussed here. 

Good luck and hope to hear from your experiences.

73 de YO3FFF
Cristi



----- Original Message ----
From: Michael Kincaid <w7fkf at fast.net>
Sorry for the BW but after weeks of
tinkering I'm perplexed.

Mike - W7FKF