Antonio Vernucci wrote:
I presume so. It would seem to me that in EZNEC there is no way of
simulating a lossy transmission line.

EZNEC v. 5.0 has that ability. Earlier versions do not.

Sorry Roy,

I still have the old 3.0 version and I shall certainly get the newer
version.

On this occasion, may I ask a question privately?

Sure.

Roy Lewallen, W7EL

Howdy gents,

I thought that the problem of the hairpin or Beta Match was kicked around
way back, please see http://www.k3bu.us/beta_match.htm

Hairpin is shortening the (most important) radiating part of the driven
element where the current is the highest.
The best way to match the long antennas with low feedpoint at the driven
element is to use the folded dipole or Quad element, as I used in my Razor
Beams http://www.k3bu.us/razor_beams.htm

First you shorten the driven element, reduce the effective length, then you
apply lossy matching and then you see less gain and narrower bandwidth.
Folded dipole or quad elements fix that.

Yuri, K3BU.us



"Antonio Vernucci" wrote in message
...
What you're probably seeing is a numerical problem in the NEC calculating
engine. It's very fussy about the region near a source, and doesn't like
small loops which include a source. You should run an Average Gain check
(see "Average Gain" in the EZNEC manual index), which will reveal whether
this is the problem.

The double precision calculating engine in EZNEC+ is considerably more
tolerant of small loops, but can still have problems with average gain
for other reasons.

Roy Lewallen, W7EL

Hi Roy,

thanks for the tip.

As a matter of fact without the hairpin the average gain is almost zero,
whilst with the hairpin is about -0.8 dB, that correponds to the gain drop
I notice. So the problem you had anticipated actually occurs.

I then tried to simulate the hairpin with a proper-length shorted
trasmission line and, doing so, the average gain is almost 0. Evidently
the program does not like the short hairpin loop.

73

Tony I0JX

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

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
please remove abc from the mail address when replying directly

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

Yes, I had overestimated the effect of the matching system, and, as you say, the
antenna Q is far higher then the Q of the matching network. The fact that the
capacitance (corresponding to the capacitive reactance of the shortened D.E)
varies with frequency does not change things.





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.

I agree with the conclusion





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

Yes, after precisely tuning the D.E., it shows a good SWR all over the band of
my interest. Nevertheless, before mounting the antenna, I would not have
suspected such a narrow SWR response.

73

Tony I0JX