Beverage design 160 mtrs Bi Directional

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Two times Bi-directional beverage’s design and construction

For our 160mtrs contest operation we had the opportunity to set up two 1 to 2 wavelength beverages. This articles describes the design criteria, components used and the final electrical and mechanical construction.

 

Design considerations

For the beverages and control following design considerations were given:

  • Both beverages bi-directional;
  • Feed points could be close together or further apart;
  • Possibility to use one or two feedpoint coax connections;
  • No need to use separate control cable assembly;

 

Design implementation

Bi-directional beverage

With reference to [on4un], w0btu beverage antennas and  [zl3] the beverages consists of 2 parallel wires (1.6 mm diameter aluminum wire spaced 11 cm as discussed later) terminated at the far end by the differential/common mode transformer and at the near end by the excitation circuitry (transformers and dual pole signal relay. Key for getting good directivity and front- back ratio is to have all the impedances correct. Impedances are given by the mechanical construction (a.o. height and wire diameter) and the transformer constructions (turns ratio).

If all impedances are correct signals entering from the far end will only be heard/measured at the receiver connected at R(near-far)=Rnf and signals entering from the near end only at R(far-near) = Rfn. Important is also to take into account the finite ground impedance. For an estimate of this impedance see  w8ji ground resistance.

 

Common mode behavior:

Common mode (i.e. voltage between one of the 2 parallel wires and ground, both wires carrying the same voltage) behavior is given by the characteristic impedance of 2 parallel wires above ground.

This impedance is:

[eq 1]

With:
h = height over ground
s = spacing between wires
d = diameter of wires
(h, s and d all in the same unit)

For good performance the beverage should be terminated at both ends with this impedance. This is accomplished via transformers T3 and T2:

  1. The near end is accomplished via transformer T3. This will need to transform the receiver impedance Rnf as such that the secondary impedance added with Rground equals the common mode impedance Zcom

    T3 impedance ratio:

     [eq2]

    T3 turn ratio:

     [eq 3]

  2. The Far end is terminated (common mode) by the series connection of the ground impedance and the transformed differential mode impedance (via transformer T2).

    [eq 4]

    T3 turn ratio:

    [eq 5]

 

Differential mode behavior:

Differential mode (i.e. voltage between both parallel spaced wires) behavior is given by the characteristic impedance between two parallel wires.

This impedance is:

 [eq 6]

With:
s = spacing between wires 
d = diameter of wires 
(s and d both in the same unit)

For good performance the beverage should be terminated at the near end with this impedance. This is accomplished via transformers T1.

  1. The near end is accomplished via transformer T1.This will need to transform the receiver impedance Rfn as such that the secondary impedance equals the differential mode impedance Zdif.

    T1 impedance ratio:

    [eq 7]

    T1 turn ratio:

     [eq 8]

 

Transformer design

Transformers T1, T2 and T3 will need to

  • Provide the turn ratio as given by equations 3, 5 and 8 as close as possible (5 % error) using only integer number of turns (e.g. no half turns).
  • Provide enough reactance ( impedance of the inductor) compared to the impedances involved by the transformation (impedances as used in the equations 2, 4 and 7). It has been proven that a reactance of 5 times the to be transformed resistance is adequate. (2 * pi * 2 MHz * coil inductance > 5 * Rtransf).

Coil inductance is given by the number of turns and the specific transformer core material : Al [nH/turn].

 [eq 9]

 [eq 10]

 

Beverage/Transformer implementation

Taken into account above mentioned design rules and equation a particular implementation is:

  • Wire height 2,3 mtrs above ground
  • Wire spacing 11 cm
  • Wire diameter 1.6mm
  • Rground taken as 60 Ohm
  • Receiver/cable impedance 50 Ohm
  • Transform material used BN73-202 (Al =12000 nH/turn)

Above mentioned formulae’s and design considerations are consolidated in this spreadsheet.

From this spreadsheet the following figures are applicable:

[eq1] Zcom = 371 Ohm
[eq6] Zdif = 591 Ohm

 

T3 (near end, common mode to 50 Ohm transformer):

[eq2] T3 impedance ratio = 6,2

[eq3] T3 turn ratio = 2,5

With the help of below given table the actual turn count can be determined. Taken into account that we only can have integer number of turns (error smaller than 5%) and that the ratio of Reactance and Resistance should be greater than 5.

 

T3-s T3-r (calc)   T3-r (turns) Error (<5%)  Ratio XL/Zcom (>5)
2 0,8 1 20 2
3 1,2 1 20 3
4 1,6 2 20 6
5 2,0 2 0 10
6 2,4 2 20 14
7 2,8 2 6 19

So near end, common mode to 50 Ohm transformer T3 will consists of 5 turns secondary (T3-s) and 2 turns primary (T3-r).

 

T2 (far end, differential to common mode transformer):

[eq4] T2 impedance ratio = 1,9

[eq5] T2 turn ratio = 1,4

With the help of below given table the actual turn count can be determined. Taken into account that we only can have integer number of turns (error smaller than 5%) and that the ratio of Reactance and Resistance should be greater than 5. Also note that transformer T2 primary consists of two coils with a middle connection. Both primary coils should have integer number of turn count.

 

T2-n(2 times) T2-m (calc)   T2-m (turns) Error (<5%)  Ratio XL/Zdiff (>5)
1 1,5 1 45 1
2 2,9 3 3 4
3 4,4 4 9 9
4 5,8 6 3 16
5 7,3 7 4 24
6 8,7 9 3 35

 

So far end, differential to common mode transformer T2 will consists of 2 times 4 turns primary (T2-n) and 6 turns secondary (T2-m).

T1 (near end, differential to 50 Ohm transformer):

[eq7]T1 impedance ratio = 11,8

[eq8] T1 turn ratio = 3,4

With the help of below given table the actual turn count can be determined. Taken into account that we only can have integer number of turns (error smaller than 5%) and that the ratio of Reactance and Resistance should be greater than 5. Also note that transformer T1 primary consists of two coils with a middle connection. Both primary coils should have integer number of turn count.

T1-q (2 times) T1-q (calc)   T1-p (turns) Error (<5%)  Ratio XL/Zdif (>5)
1 0,6 1 42 1
2 1,2 1 16 4
3 1,7 2 13 9
4 2,3 2 16 16
5 2,9 3 3 24
6 3,5 3 16 35
7  4,1 48 
8  4,7 62 

So near end, differential to 50 Ohm transformer T1 will consists of 2 times 5 turns primary (T1-q) and 3 turns secondary (T1-p).

 

Direction control and switching

With the above described setup we get two RF signals, respectively for the HF signals originating from the far end and one for the signals from the near end of our beverage. In case we have two bi-directional beverages a total of 4 RF signals are available. As one of the design consideration was to use only one (long) coax feed and no separate control cabling, below figure provides a solution for that.

Inside box will put on the coax (besides the RF signal) control voltages of +12VDC, -12VDC, 12VAC, or nothing. The different (outside located) switch boxes will only react (switch) when a specific control signal is active. This can be easily arranged by a simple diode circuitry. With this construction the 4 directions are:

  • Not connected = Far end of beverage 1
  • +12V = Near end of beverage 2
  • -12V = Near end of beverage 1
  • AC 12V = Far end of beverage 2

 

Although the near end of both beverages could be at the same physical location it is advisable to use different ground connections, this to eliminate any crosstalk due to the ground resistance. Therefore SW2 and SW3 will be some distance apart en connected with a short run of coax to the box containing SW1. SW1 can be connected to the inside located control box by a long run of coax. Of course it is also possible to use two separate long run of coax to SW2 and SW3 and have SW1 connected directly at the inside placed control box.

Direction control and switching implementation

A possible implementation is given here.

dual bidirectional  beverage v1.2 800 600

(click schematic for high resolution .pdf version)

 

Hardware implementation

Figuur 1, switch box

Figuur 2, near end

box Figuur 3, far end box

Copyright © 2011 Carel Mobach, PC5M

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