swiss short wave radio station
HB9AWJ
Home
German
Luftbalun
Optimization for low loss antenna matching using a symmetric antenna coupler without balun and with remote tuning
Instead of using a power amplifier it would often be worthwhile to take a look at possible RF energy losses on the way from the transmitter to the antenna. Usually there are several elements in an antenna system where - unknowingly or through negligence - sometimes a considerable amount of power is lost. Thanks to the patient support of my HAM radio friend Walter, DL3LH, who is a great RF engineer, I have optimized my antenna system, here adapted to the local conditions, for possible losses. I would like to briefly outline our concept of creating a low-loss antenna system (Figure 1):
Figure 1:

Schematic of the realized antenna system  (top view)
Skizze Antennensituation
The spatial as well as the structural restrictions for the desired antenna sketched above are:

     -  The antenna is designed for the 80m, 60m and 40m band and as an extension for the 160m band
     -  The space conditions allow a dipole of maximum 2 x 17 m using a light inverted-V shape. The maximum antenna height is at 8 m above the ground (feeding point; same
        height as the flat roof of the house), the ends of the dipole are going to poles 3 m above the ground.
    -  The horizontal distance between feed point of the dipole and the metal structures of roof is only 1.5 m, the dipole ends are 6 m away from the house facade.
    -  For structural reasons only a coaxial cable can be installed through the house wall.
    -  The shack is located on the top floor and has direct access to the terrace. Here the coupler can be installed and also be operated.

The above conditions result in a relatively low mounted multiband dipole, which has NVIS characteristics due to its low installation height. The low installation height and the proximity to the house with its many metal elements (metal roof and parapet surrounds and metal railings) affect the base impedance of the dipole. The dipole can be operated on 160m asymmetrically as a T-antenna against ground by short circuiting the two parallel line ends and using one of the two symmetric coupler branches.

Due to the requirement for minimum losses in the antenna feed line system, coaxial feeding is almost automatically prohibited in multiband operation. If nevertheless a coaxial cable has to be used it is only tolerable with impedances near 50 Ω for minimization of reflexion losses. Therefore only a symmetrical feed line can be used for the mentioned dipole.

Because of the impossibility to lead a ladder line into the house the only solution is to operate the antenna coupler outdoors. A H155 coaxial cable leads from the coupler into the house to the transceiver. Thereby the coaxial cable works in a "matched line mode" and no additional losses due to reflection (mismatch) are occuring.

I decided to build a home brew balanced manually remote tuneable coupler. I selected a 600 Ω ladder line, because the 450 Ω Wireman ladder line can cause big losses in case of rain (up to 3 or even to 4 dB) !

The coupler must guarantee a balanced operation. In principle there is the possibility to use a symmetrical tuner or an asymmetrical tuner with a balancing. The latter variant is easier to build and works just as well. Therefore however a balun is necessary.

I would like to make a comment on the balun and on the common mode chokes in general. A balun (or a common mode choke) often causes much higher losses than commonly assumed. Unfortunately calculations on this subject have rarely been published. Walter, DL3LH, has done correlating calculations and found, depending on the conditions of use (frequency, impedance transformation, balancing), partly very high losses (DL3LH, various publications).

Another important preliminary remark concerns the type of the LC matching network. For various and certainly good reasons the T- or Pi-network are particularly popular. There are many publications about the advantages and disadvantages of each of these types of couplers. I have decided to use a simple LC network. Such a network is simple, safe and has normally the lowest losses. Its tuning does not allow multiple and especially no kamikaze settings but only one correct L-C combination. The losses compared to the Pi- or T-network are in almost all cases smaller.


To decide how the antenna coupler must be planned, measurements on the antenna system and the corresponding calculations have to be done. First we have to measure the impedance at the ladder line. Then we have to precalculate the losses of each possible antenna coupler concept.

In the present case, calculated on the base of the ladder line input impedances (see Table 1 below), a balanced coupler in the high-pass variant (with C serial and L parallel) shows the lowest losses. We also calculated the losses for a asymmetrical tuner with balun at the output, but this seems to be less favourable regarding the losses. The losses in an air wounded balun are unfortunately partly high (>1 dB, see below). Therefore the situation here requires a balanced coupler without balun.
When building a coupler the balancing itself is not the problem . In principle, a balanced coupler can be constructed by using three blind elements, whereby these blind elements can be either capacitors or inductors (Figure 2). If you build a balanced coupler with these three elements, you achieve in a first step a completely balanced energy supply to the ladder line. The problem however are possible common-mode currents returning due to a certain asymmetry in the feeder - antenna system (in the real world no antenna system is completely symmetrical, so common-mode components always occur to a certain extent). These common mode currents must be equalized or eliminated by  discharging them.
Figure 2:   basic circuit of a balancing system
Prinzipschema Symmetrierglied
Since in our case mathematically the best version is a symmetrical CL network (high-pass configuration) with a parallel inductance at the output, the galvanic coupling of the two parallel wires of the ladder line through the coil allows a compensation of the common mode currents (Figure 3a). Thus, if the asymmetry in the feed line antenna system is not too big, a balun is not necessary. (If instead - due to the impedances at the ladder line input - a balancing configuration with a parallel capacitance at the output is necessary, i.e. L serial and C parallel, the common mode currents must be discharged via centrally grounded 2.7 kΩ resistors (or two 2.5 mH chokes) at each wire of the balanced line; Figure 3b).

Figure 3 a:

Cs - Lp - configuration:

The inductivity is parallel. Commen mode currents are equalized through the galvanic coupling of the coil
Gleichtaktausgleich

Figure 3 b:

Ls - Cp - configuration:

Equalization of the common mode currents through the capacitor in parallel position is not possible. The discharge is done over the centrally grounded 2.7 kΩ resistors (or over 2.5 mH chokes)
Gleichtaktableitung
According to the antenna condition I built a manually remote tuned, balanced Cs-Lp coupler in high-pass configuration, ommitting a balun (Figure 4; Picture 1). This coupler is directly connected to ground (i.e. the metal on the house, corresponds here to the lightning protection ground, Picture 2) on the terrace.  My realization of the coupler is a modified "Christiankoppler" (by DL3LAC) in highpass constellation with only one L-bank (in parallel configuration) and with an extension of the two C-banks (in serial configuration) with an additional 800 pF capacitor (which results in a total capacity per C-bank of 1'597 pF). This results in a total switchable capacity of 798 pF (the two C-banks in serie) what I need considering the impedance situation here. The coupler is built without balun.

For the remote tuning and the relais steering I use a simplified system with sliding switches (Picture 4).

For a detailed description of the in the German-speaking area so called "Christian-Koppler" see: https://www.darc.de/der-club/distrikte/e/ortsverbaende/39/bauprojekte/christian-koppler/

The practical testing of these configuration shows a completely problem-free behaviour of the coupler with no detectable common mode currents or vagabonding RF in the shack. The symmetry of the so built coupler is shown in Picture 3. The results of the calculations for this system designed with the intention of minimized losses are shown in Table 1.



Figure 4:

Since C and C' are in serial configuration, a maximal capacitance of 798 pF is achieved
with steps of 1.55 pF each.

The inductance can be set by steps of 0.25 µH. It ranges from minimal 0.25 µH to maximal 31.75 µH .
symmetric coupler schematic


Table 1:
Dipole 2 x 17 m  (slight inverted-V shape),
3 m to 8 m (feeding point) over ground,
2.7 m 600 Ω parallel wire feed line,
Quality factor for capacitor and coil:
QC = 500, QL = 100,
10 m coaxial cabel (H 155) Transmitter to coupler

 

 

MHz

Eingangsimpedanz

Paralleldraht-

leitung 600 Ω

Verlust

HL 600 Ω

CL-Netzwerk (Hochpass)

 

C           L

Verlust

CL-Netzwerk

Verlust Koaxial-kabel 10 m H155

(matched line loss)

Total

Verluste

 

dB

pF         µH

dB

dB

dB

3.500

18 - j 168

0.100

256  ;   3

0.66

0.20

0.960

3.650

20 - j 90

0.094

306   ;   2

0.33

0.20

0.624

3.800

47 - j 59

0.041

798 ;   1.25

0.12

0.20

0.361

5.358

290 + j 1280

0.015

  56  ;   27

0.35

0.25

0.615

7.100

630 - j 2230

0.002

   31  ;  10

0.74

0.28

1.022

As table 1 shows, when transmitting with 100 watts output power on the 80m-band, 80 to 92 watts are reaching the antenna. On the 40m-band 80 watts will reach the antenna. If you transmit on the 60m-band with 17.25 watts output, 15 watts will reach the antenna. These values are tolerable, considering the low foot point impedance of the dipole, the low suspension of it and the proximity to the metal elements of the house (Table 2). The symmetrical CL matching network has to transform up a low real resistance on the 80m band and has to compensate larger complex components on the two higher bands.

Imagine the additional loss of around 1 dB inserting a balun for the elimination of common mode currents ! We have experimentally inserted a 1:1 air balun of 3.3 µH (quality factor Q = 130, k = 0.92) and calculated the additional losses: 0.11 dB on 80m, 2.5 dB on 60m and 0.7 dB on 40m. Such considerable additional losses can now be avoided by omitting a balun due to the concept of the balanced matching circuit. As you can also see from the table, the coaxial cable causes a considerable part of the losses, even if it is only 10m long and if it is only operated in "matched line mode", so that no additional losses are occuring by reflection (mismatch).

The efficiency of this multiband dipole, that means of the antenna itself, has not yet been calculated and has to be taken into account additionally concerning the amount of RF actually radiated into space. Especially on 40m the antenna works as a full wave dipole and hence has a higher antenna gain which compensates for certain losses.

In our case due to the existing space restrictions we could not change the length of the symmetric line to create more favourable impedance conditions at the input of the 600 Ω double line.

The conclusion from the above is:

1. first carry out measurements and calculations for an antenna system and based on these search for the best concept of an antenna coupler.
2. use as little coaxial cable as possible and if needed, use it only for energy transfer through a wall into the house.










Picture 1:

The symmetric coupler with a C-Bank on each side (serial configuration) and a L-Bank parallel in the middle and with no balun.
This coupler is a modified so called  "Christian-Koppler" (by DL3LAC).

The relais control for the capacitors und inductors is achieved by sliding switches in the shack (see also Figure 4 and Picture 4).
modified symmetric Christiankoppler





Picture 2:

The symmetric coupler outdoors in position.
The shields of the coaxial cable and of the steering cable are directly connected (grounded) to the metal of the railing.
Symmetrischer Koppler im Aussenbereich




Picture 3:

Control of the symmetry in the system by inserting bicycle light bulbs into each wire of the symmetric feed line (this foto shows a prebuild test coupler with identical electrical configuration) .
control of symmetrie








Picture 4:

The relais control is realized by sliding switches. For the used bands only a few configurations are necessary. Therefore it is easy to read the values in the table and configure the settings manually.
Relais steering unit
Just for interest the impedances at the feeding point of the dipole have been calculated. Note the rather low real components which are due to the low height of the dipole and the proximity to the metal structures of the house and note as well the negative reactive components on the 80m band showing the capacitive load of the dipole which is shortened on this band to a length of 34m  (Table 2).


Table 2:

calculated impedances at the feeding point of the described dipole with 2 x 17 m length and a 2.7m  600 Ω feed line

MHz

Eingangsimpedanz an der 600 Ω Doppeldrahtleitung

Impedanz am Fusspunkt des Dipols (berechnet)

3.5

18 - j 168

21 - j 319

3.65

20 - j 90

22 - j 235

3.8

47 - j 59

51 - j 206

5.358

290 + j 1280

106 + j 631

7.1

630 - j 2230

983 + j 2760

This presentation is dedicated to Walter, DL3LH, to whom I owe a very competent in-depth introduction into the hugh field of RF technology and a lot of the calculations for my antenna system.

A very good program to calculate losses is offered by Walter, DL1JWD: "Antenna-Matching-Analyzer" see : https://dl1jwd.darc.de


HB9AWJ, ( contact: hb9awj [at] kurzwellenstation.ch )

Literature: DL3LH, different publications; on request directly from DL3LH: (https://gutachten-emvu.jimdo.com/kontakt).