Results (with new matching system) pa0nhc 20200607 / 20201029 / 20210817

(C) The use, publication, copy and modification of all info on this site is only permitted for non-commercial purposes, and thereby explicitly mentioning my radio amateur call sign "PA0NHC" as the original writer / designer / photographer / publisher. (C)  
These data are results of investigations by G0CWT and PA0NHC, mental owners.

20210817 :  My 145MHz loop antenna free field measurement experiment, and a directivity calculation, showed that the G0CWT video demonstration about the directivity of such antennas is faulty. His faulty results are generated due to UNbalanced feed, lack of coax common mode suppression, and reflections from surroundings. 
A horizontal polarized 1/4 lambda circumference delta loop in a free location has nearly no directivity, but is nearly omni directional. 

            The results of balanced feed :
On 80m, the frequency of maximal magnetic field strength, and VSWR 1 : 1.0 now coincides exactly.
            Just tune to VSWR = 1.0, and you are sure to be tuned for maximal transmitted field strength too.
-   The loop is now :
        - Better balanced,
        - No common mode problems,
        - Very insensitive for non-resonating metal objects in the near field (See my location photo).
-   Signal reports received are definitively stronger.
-   Selectivity is far better.
  No TVI / BCI in consumer equipment, at a distance of about 3m below the loop.


          Calculated loaded loop Q  (formula according to DL4KCJ) :
           Q = Xc  / Zraam.        B =  F   /  Q. 
 7,1 MHz : Q = 299 / 5,5  = 54.    B = 7100 / 54 = 131 kHz.
3,65 MHz : Q = 208 / 0,61 = 340.   B = 3650 / 340 = 10.7 kHz.
Due to the relative large loop circumference (> 1/20 lambda circumference), the feedpoint-Z is HIGH, resulting in low loop Q, low loop currents, and high efficiency. With optimal matching circuit having tight coupling between the matching circuit and the loop feed point.

          Antenna bandwidth and selectivity (3700 kHz).
Magnetic transmitted field strength bandwidth measured in the antenna near field :.
-3dB @ 3680 kHz and -3 dB
@ 3720 kHz  (40kHz).

Selectivity : Received signal strength :
-20dB @ 3800 kHz and -20dB @ 3600 kHz (200 kHz).

          Cross band selectivity :
Very favorable for co-site operation  (field day / multi antenna club station).

       Antenna tuned to 40 m :
RX signal strength on 80m
-35 dB !! 

      Antenna tuned to 80 m :
RX signal strength on 40m 
-40 dB !! 

REM : my coax feeder is about 19m long. Measurements were not done at the antenna, but at the beginning of the feeder.
The VSWR properties of this loop are better than here measured ! 

antenna bandwidth
for VSWR < 1.5 
Antenna fixed tuned to 7,1 MHz : B(vswr<1.5) = 70 kHz.
Antenna fixed tuned to 3,7 MHz : B(vswr<1.5) =   8 kHz. 
REM : On 80m still enough bandwidth exists for good SSB and AM performance.

Measured tuning range for VSWR < 1.5 : 
(3.0 to 4.4 MHz) and (5.3 to 9.1 MHz).
This is FAR WIDER than of a 1/2 wave dipole antenna.
In these large tunable ranges the feeder VSWR and feeder losses are LOW.

Loop is tunable to resonance between 3.0 MHz and 9.95 MHz.

New matching system. 
Pure resistive transformer =>
Lowest VSWR = max.radiation.
Symmetrical feed into
the loop current center
=> NVIS.

(Old matching system)
Asymmetrical power injection.
=> Skewing radiation.
Transformer not optimized.
=> Max field strength
lowest VSWR.