On 19 abr, 19:02, "Antonio Vernucci" wrote:
After running the required simulations it was possible to conclude the
following:

- the antenna impedance can clearly be transformed into 50 ohm or into 200 ohm
by just changing the driven element length and the hairpin length. By selecting
the proper lengths, an identical SWR curve can be obtained for the two cases,
this meaning that the matching system impedance has virtually no influence on
the SWR bandwidth of the antenna under simulation

- however, for a given RF power, in the 200-ohm case the RF current in the
hairpin is about 1.8 times higher than in the 50-ohm case. This means that in
the former case the power lost in the hairpin ohmic resistance would be about
3.2 times that of the latter case.

73

Tony I0JX

Hello Tony,

Probably you convinced yourself for 100% that the antenna limits the
bandwidth.

One can generally say when the bandwidth of the L network is far
greater then the antenna bandwidth (without matching, with respect to
a reference impedance equal to the resonance [real] impedance), the
overall BW is just a little less then the antenna bandwidth.

Or you can say when the antenna Q is far higher then the Q of the
matching network, overall Q factor is determined by the antenna. The Q
of an L network is about sqrt(Zin/Zout –1), when ZinZout, uses
sqrt(Zout/Zin-1). For a step from 20 to 200 Ohms, the VSWR=1.5
Bandwidth is about 11% (5.72 MHz in your case)

A nice exercise can be modeling your antenna's impedance (without
matching) as a LCR series circuit and put this into a lumped circuit
simulator (for example a PSPICE simulator). Some simulators allow
direct entry of S-parameters.

Now you can add every other component (also lossy and lossless
transmission lines) and see the effect on the overall BW.

As other people said, the Q of a hairpin made of 5…10mm tubing is over
100. As the BW of your L match is far below that (also for the 200 Ohm
case), losses in the hairpin are that low, that they can practically
not be measured via field strength measurement. So 3.2 times higher
then in the 50 Ohms case is still negligible.

After all the calculations and simulations, I hope your 6m Yagi is
still useful for you.

Best regards,

Wim
PA3DJS
www.tetech.nl
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