28 May 2013

EFHW Measurements coupler comparisons

I wanted to compare my T-50-2 coupler with my T-200-2 coupler. The measurements did not line up with what I had measured previously.

Antenna:  66ft 8inch wire in my garden. Vertical approx 20ft. Horizontal approx 38ft. Other end hanging down 8ft.

Resonant frequency: 7020Khz

Coupler 1:  T-50-2. 28:3 turns. Impedance ratio 10:1 (50*100=5000ohms). Resonated with a variable capacitor.
  • Counterpoise 1: 6ft 8inches.  SWR 1:1. 2:1 BW to 7.1Khz. (0.06wavelength)
  • Counterpoise 2: 23ft 1inch.   SWR 1:1  2:1 BW to 7.1Khz  (0.17wavelength)
Coupler 2: T-200-2  18:2 turns. Impedance ratio 9:1 (50*81-4050ohms). Resonated with a length of coax.
  • Counterpoise 1: 6ft 8inches SWR 1:1. 2:1 BW at 7.080Khz (0.06 wavelength)
  • Counterpoise 2: 23ft 1inch   SWR 1.8:1. (could not achieve a good match)
In both cases the most stable system was achieved with the short counterpoise.

Both systems exhibited roughly the same bandwidth. (this is different to what I have seen in the past where the longer counterpoise resulted in a wider bandwidth??) 

NOTE: In the case of the T-50-2 the secondary was tied to the primary at the cold end.

Calculated inductance for Coupler 1. ~ 1.8 uH
Calculated inductance for Coupler 2 ~ 4.0uH

Capacitance to resonate Coupler 1. ~ 285pf
Capacitance to resonate Coupler 1. ~ 128pf

Coupler 1  Xc = 79ohms = Xl  (Q= 5000/79 = 63. not sure if this the correct calc?)

Coupler 2 Xc = 177ohms =Xl (Q= 4050/177 = 22. )

However the problem with the above calc is that the input impedance to the antenna/counterpoise must change with different length counterpoises?? See fig3 of the AA5TB efhw writeup.

More to think about here? No wonder this antenna is not well understood!

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