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Modelling, building and measuring a 4 element Dual Band Cubical Quad antenna (6m-4m).

Methodology adopted

1) Make your antenna choice according to your exeptations and your attempted utilization.
2) When done, search for what already exists.
3) Model the antennas found, then optimize them. I use for this the MMANA-GAL software (free) (download MMANA-GAL here). Chose the modelized antenna who gives your best expectations.
4) Build this antenna.
5) Install this antenna into a environnement witch is the most favourable to take your measurements.
5) Make your measurements with a correctly calibrated Vectorial Network Analyser  - Look at the official site of NanoVNA (In addition, where possible, measurements will be made in the same conditons  as those chosen for your modelling).
6) Compare your measurements with the results obtained by software modelling.
7) If needed, make physical correction to the antenna , re-modeli, re-measure until you're satisfied.
 

1st Step  :


Model with MMANA-GAL software the existing descriptions of the 4 elements cubical quad antennas given into the publications of L.B. Cebik (W4RNL), William I.Orr (W6SAI), Carl O. Jelinek N6VNG.
The modelling-based antennas gived very similar results. Each model have been optmized several times.

The here given dimensions below are derived from the several opizations (gain, F/B/ SWR) made with MMANA-GAL software.

Perimeter of the loops (in mm)

  REFLECT. RAD. D1 D2
50.2MHz 6288 6128 5740 5753
70.2 MHz 4616 4436 4356 4292

 

 

 
Spacing REFL.-RAD. = 1228 mm
Spacing REFL.-D1. = 1798 mm

Spacing REFL.-D2 = 2752 mm

=> : Download the MMANA-GAL file here (right mouse button: save as)

(*) MMANA-GAL users : for 50.2 MHz source w11c  - for 70.2 MHz source w17c

Settings MMANA-GAL :
  • Calculation above real ground (not on freespace); soil conductivity chosen with world atlas of conductivities (World atlas of ground conductivities (hamwaves.com)). I make a modelling above real ground because I do my measurements above a real ground. (Now,... if the NASA wants to take to the space my antennas for free and make my measurements, I have no objection!)
  • I set a soil conductivity of 30 mS/m (just of curiosity, I made modelling with other conductivity values,there was little difference between  the results, excepted for sea water)
  • Antenna height : 7 mètres
  • Wire jauge for the loops : fence aluminium wire diameter 2 mm
  • Modelling doesn't include the boom, cross arms structure,loop roots, mast and roof. Many modelling have been made with/without those neighbouring objects with little difference into the results, so I didn't included them.

 

Step 2 : building the antenna

The hardware is coming from a hobby store.
Cross arms and structure are made with aluminium corners and "U" profiles,In order to ensure that the cross arms are square and reinforced, I use flat iron strip 1 cm wide, they will be painted and coated with epoxy resin.
Screews and bolts are M5 satinless steel.
Aluminium fence wire 2 mm diameter

Roots are made with hardwood dowel dia.12mm, painted and coated with epoxy resin.

cubical qaud on8im 2

cubical qaud on8im 3

cubical qaud on8im 5cubical quad on8im

 

Step 3 : MMANA-GAL modelling, NanoVNA measurements & compare the results

 

Loop 50.2 MHz


Modelling MMANA-GAL
 
mmanagal50 1
 
mmanagal50 2
 
mmanagal50 3
 
 
NanoVNA measuring conditions fully respecting the conditions set for the modelling

• Device Under Test (DUT) height : 7 m
• Neighbouring objects at least 10m from the DUT
• VNA connected directly to the loop , connection the computer via a 10 meter USB cable.
• Software nano-VNA Saver v 0.2.2
• VNA calibration :
• STIMULUS START : 49.5 MHz
• STIMULUS STOP :51.0 MHz
• CALIBRATION : RESET – CH0 OPEN – CH0 SHORT – CH0 LOAD – CH0 ISOLN (NO LOAD) + CH1 I(LOAD)
• SAVE 0
• Connectors SMA + adapter -> N (is included into the calibration) 
 
Measurements are made with a unbalanced loop feed (coax connector directly connected to the loop).

 

MEASUREMENTS @50.2 MHz

• Sweep start : 49.5 MHz
• Sweep stop : 51 MHz
 
nanovna 502MHz
 
 
Impedance : Software v/s Measurement @50.2 MHz

• MMANAGAL : 50.2-j0.0253 Ω
• Mesure NanoVNA : 50.69+j.0759 Ω
 
Plots MMANA-GAL and impedances from 49.95MHz to 50.45MHz
 
mmana plots 500
 
 
Software results v/s measured values
Freq (MHz) MMANA—GAL NanoVNA
49.95 45.3 - j12.2 Ω 45.57 - j10 Ω
50.075 47.8 - j6 Ω 48.11 - j4.79 Ω
50.2 50.2 - j0.0253 Ω 50.69 + j0.759 Ω
50.325 52.6 + j5.5 Ω 53.41 + j6.41 Ω
50.45 54.9 + j10.9 Ω 56.51 + j12.3 Ω
 
MMANA-GAL : resonance
MMANAreso
 
NanoVNA : resonance measurement
 
 
Capture resonance50RjX
 
 
Marker @ 50.186 MHz

Resonance : software v/s measurement @50.2 MHz 

MMANA-GAL

F.RES 50.203 MHZ Z = 50.2 - j253 10-4
NANOVNA F.RES 50.186 MHZ Z = 50.41 – j876 10-4
 
Smith Chart  (SWR circles included)
 
Smithchart50MHz
 
 
MMANAGAL : SWR curve

 

MMANAGAL SWR 50

 

NanoVNA : SWR measurement

CaptureSWR50

 

 

Software: Forward Gain (DBi) & F/B (dB) 

 

MMANA Gain 50 

 

LOOP 70.2 MHz 

Modelling MMANA-GAL

mmanagal70 1

mmanagal70 2

mmanagal70 3

 

 

NanoVNA measuring conditions fully respecting the conditions set for the modelling

• Device Under Test (DUT) height : 7 m
• Neighbouring objects at least 10m from the DUT
• VNA connected directly to the loop , connection the computer via a 10 meter USB cable.
• Software nano-VNA Saver v 0.2.2
• VNA calibration :
• STIMULUS START : 69 MHz
• STIMULUS STOP :72.0 MHz
• CALIBRATION : RESET – CH0 OPEN – CH0 SHORT – CH0 LOAD – CH0 ISOLN (NO LOAD) + CH1 I(LOAD)
• SAVE 0
• Connectors SMA + adapter -> N (is included into the calibration) 
 
Measurements are made with a unbalanced loop feed (coax connector directly connected to the loop).
MEASUREMENTS @70.2 MHz

• Sweep start : 69.0 MHz
• Sweep stop : 72.0 MHz

 

nanovna 702MHz 

 

Impedance : sotfware v/s measurement @70.2MHz 
·         MMANA-GAL : 50.2-j0.0253 Ω
·         Mesure NanoVNA : 12.57+j.2.84 Ω

 

 
Plots MMANA-GAL and impedances from 69 MHZ to 72 MHz

mmana plots 702

 

Software results v/s measured values
Freq (MHz) MMANAG—GAL
NanoVNA
69.8 63.3 – j13.0 11.48 - j16
70.0 56.8 – j8.5 12.22 – j7.15
70.2 48.7 – j2.2 12.57 + j2.4
70.4 39.6 + j6.8 13.31 + j13.8
70.6 30.6 + j18.6 14.31 + j25.8

 

MMANA-GAL : resonance

MMANAresonance702 

 

NanoVNA : resonance measurement

Capture resonance70RjX

 

Marker @70.167 MHz

Résonance : software v/s measurement

 

MMANA-GAL F.RES  70.274 MHZ Z = 45.37 – j0.8206  
NanoVNA F.RES  70.1476 MHZ Z = 12.45 – j34 10-4  

 

 Abaque de Smith :

Smithchart70MHz

 

Marker @70.167 MHz

 

Software : SWR

MMANAGAL SWR 70

 

NanoVNA : SWR measurement

CaptureSWR70

 

Software : Forward Gain (DBi) & F/B (dB)

MMANAGAL FB70 

 

 REMARKS :

 

calculation 1 @70.2 MHz with 50 MHz loop open 
calculation 2 @70.2 MHz with 50 MHz loop closed
compare cadre ouvert ferme
 

calculation 3 @50.2 MHz with 70 MHz loop open
calculation 4 @50.2 MHz with 70 MHz loop closed

compare cadre ouvert ferme50

 

I have no explanation (not yet) but 

she would like to know a re-modelling of this antenna with other softwares (NEC, 4NEC2) and see if whe have the same differnces.

In the present study case, this antenna is principaly purposed to be used on the begining of the 6 meter band, the use of the 4 meter band constitutes a balanced and realistic compromise about forward gain and F/B ratio (this applies for all the multiband cubal quad using common loops).

 

STEP 4 : feeding the antenna

 

FEEDING THE 50.2 MHz LOOP

The measured SWR for the 6m band is 1.02:1 thus a balancing of the antenna feed system is only needed to avoid a disorsion of the Far field pattern due to the unbalance of the coaxial cable.

Easy UNBAL to BAL solutions

I used coaxial stubs.

Illustration : (from ARRL Antenna Book)

balun2

How it works : 

The purpose is to obtain 2 currents in opposed phase to feed the loop. We send simultany a current trough 2 coaxial lines, the second coax line is 1/2 wave (180°) longer, so the current is 1/2wave (180°) later then the current into the first coax line. The 2 currents are phased 180° each of other.

After installation of those stubs on the antenna, I re-measured all on 50.2 MHz : resonance & SWR remains unchanged.

 

A alterned method.

balun

 

FEEDING THE 70.2 MHz LOOP

 

The gamma match method adapt the impedance and balance the feed of the loop.

To calculate tour gamma match see link : 

Microsoft Word - N6MWGamma4.doc (nonstopsystems.com)

Figure and dimensions

gamma match

 

Note :

 

Diameter of both wires are identical.(aluminium fence wire diameter 2mm).
The variable condensator can be replaced by a open piece of coaxial cable .

 capacite

The value of C (pF/m) is given by the cable manufacturer or can be measured with a capacimeter.

 

STEP 5 : Measurement of the antenna patern & antenna Gain 

"Le diable n'avait pas encore de terrain, et il y plantait déjà l'arbre"
 
 

Measuring the antenna patern

 

I use for this a software called Polarplot created by G4HFQ , it can be dowloaded here G4HFQ Radio Programming Software .

Al the explanations are given in the help file of the program.

How I used Polarplot.

Polarplot

 

I have taken h = 1λ  & D = 10λ

There are other many methods. 

Example: Relevé du diagramme de rayonnement d'une antenne yagi 144 (pagesperso-orange.fr)

You can use your DIY field meter, thsi will give you already a good evaluation of your antenna caracteristics.

 

Best 73's  ON8IM