[Antennaware] Problems on a 2el Vertical Array

[i4jmy at iol.it]

Toby,

with small antenna spacings, expecially below .25 WL, real pattern is a combination between directly excited elements and parasitic excited elements. In real world, antennas will not behave as pure current sources, phased as you want and with half power on each one. They will radiate a consistent amount of energy, parasitically, as it happens with yagis. 
It’s hard to predict what happens when elements are nearly resonant. Depending if they will look a bit inductive or capacitive, for example, the re-radiated component will have an opposite sign. 
Therefore, it's practically impossible to built a system of resonant antennas that behaves like a model, expecially if pretending it’s symmetrical when reversing direction. 
Actually, best pattern with close and phased elements requires the verticals are not the same lenght and phase lag is set some 33% more than spacing. Exactly like it happens with rotable arrays, i.e. HB9CV. 
Like that is relatively easy to get big F/B in real world, but's elements reactance is there forced and defined using short or long ones, and the array is inherently not symmetrical. 
Using KB8I method you'll have some cut-and-try to do with cables lenght, still with no insurance that reversal of antennas will give the same F/B performances on both directions.
The MFJ 259 is basically unhelpful, except as rough estimate that each vertical has a similar impedance. The measured values don't offer infact the right answers for the phasing components.
You've to use a receiver, far enough from the antenna where the far field is already built and vary the cable lenghts to maximize F/B.
I think You should better go to a lewallen method for phasing.

73,
Mauri I4JMY

---------- Initial Header -----------

>From      : antennaware-bounces at contesting.com
To          : antennaware at contesting.com,w7el at eznec.com
Cc          : "Joerg Puchstein" dl8wpx at attglobal.net,"Wiedmann, Michael" mwiedman at nt.hirschmann.de,"lau" lau at ba-ravensburg.de,"Kasper, Dietmar" Kasper at zmd.de,Rick at dj0ip.de
Date      : Thu, 08 Jan 2004 01:37:50 +0100
Subject : [Antennaware] Problems on a 2el Vertical Array

> Hi guys,
> 
> over Christmas I set up a 2el vertical Array for 40m.
> I used 60 quaterwave long radials for each vertical.
> The spacing is 7m and I wanted to phase them with
> 123° (which seemed for me to be the best trade
> between ground-losses and f/b).
> 
> I calculated the following impedances, using
> EZNEC with Mininec ground.
> (incl 3 Ohms ground losses):
> 
> Ant1: 30,7 +j31,4 Ohm
> Ant2: 19,2 -j16,9 Ohm
> 
> Applying the Christman method, I obtain the following
> cable length (RG-213 - vel. factor 0,66)
> 
> to Ant1: 8,43m
> to Ant2: 13,79m
> 
> At the common point of the cables I calculated the
> following Impedance:
> Z = 34,4 +j3,7
> 
> 
> ----so far the theory-----
> 
> In order to build up the array as precise as possible, I messured the
> Z-parameters with an MFJ-259b:
> 
> Z11 = 37 +j3,7
> Z21 = 25,0 +j13
> Z12 = 25,0 +j13
> Z22 = 37 +j7
> 
> Out of this I calculated the drive impedances (I skip the details):
> 
> Ant 1: 22,3 -j9,9 (1A / 0°)
> Ant 2: 27,5 +27,3 (1  / 123°)
> 
> out of this values I calcuated following cable length (RG-213):
> 
> to Ant1: 10,4m
> to Ant2: 14,92m
> 
> At the common point of the cables I calculated the following impedance:
> Z = 38,8 +j7,36
> 
> 
> I realised the array with the measured values.
> 
> While testing the array on the air, I made several hundert QSOs on 40m
> (as DA0BCC) around X-MAS. I was able to hear / "feel" about 3-4dB
> additional dBs over one single vertical. But I'm unhappy with the f/b ratio.
> The pattern shows, that I should have at least 10dB on EACH station and
> 15-25dB f/b on DX stations.
> I heard only a few DX stations where I really had 25dB f/b... but this was
> the big minority (maybe 5 out of 100).
> Often I didn't notice any f/b at all. (especially on stations closer 
> than 2500km).
> 
> Right now I have two possible answers / questions:
> 
> 1. Is this the typical behavior of a 2el vertical Array ?
> 
> 2. Did anyone set up such an array, using the messured values ?
> 
> 2.5 How MUCH can I trust that MFJ 259b ????
> 
> 
> It would be great to get some feedback from the more experienced
> Array-builders !
> 
> 
> Best greetings, 73
> 
> Toby, DH1TW
> 
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