[TowerTalk] Problems on a 2el vertical Array

[Tobias Wellnitz <T.Wellnitz@gmx.de>]

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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