Efficiency of 200-ohm hairpin matching
 

On Apr 10, 2:24 pm, "Antonio Vernucci" wrote:
I am also surprised by the high resistance you are seeing with the
hairpin wires removed. From what you posted before, and from the 3-
element NBS and the 6-element designs I ran in EZNEC, I would expect a
lower feedpoint resistance than that. If I put a pure inductance in
parallel with your 42.4-j39 ohms, I get the same answer as you did,
that it moves to 78 ohms resistive.

My offer to look at your EZNEC file is still open, of course. Maybe I
could see something in it--I know that I can look and look at
something and not see an obvious problem, even when I know perfectly
well what to look for.

If your personal mail on the newsgroup is correct, I could send you the EZNEC
file of my antenna.

Also--in the model before you split the D.E. into three wires, did you
get a more reasonable feedpoint impedance (with no hairpin, just
looking at the D.E. feedpoint)? I think we were expecting something
like 19-j58 ohms. If you get that, then I would look for the reason
things change (so very much!) when you split the D.E. into three
separate (but connected, end-to-end) wires. I can understand a small
change but not so large a change. Is the total number of segments for
the three wires making up the D.E. still about the same as it was in
the original version of the model, with just one wire for the D.E.?

I am also surprised as I would have expected a much lower resistance (as you
suggested)

I did the test you have suggested and I noted some change, but not much. With a
"solid" D.E., i.e. not broken in three pieces (and without the hairpin of
course), impedance is 43.27 - j 40.97 ohm, not terribly away from 42.4 - j39
ohm. In that test I configured EZNEC for 17 segments, whilst when the D.E. was
broken in three parts I had used 17 segments for the two outer arms and only 5
segments for the 10-cm center section, due to its much shorter length (I see
that result depend somewhat on the number of segments).

73

Tony I0JX

Yes, the email should be right. I do have Eznec5+, so I can check
against Roy's suggestion. I'm sure he'll be happy to help with things
that may be related to how NEC/EZNEC behaves, if we can identify them.

Cheers,
Tom

Efficiency of 200-ohm hairpin matching
 

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

Efficiency of 200-ohm hairpin matching
 

Yes, the email should be right. I do have Eznec5+, so I can check
against Roy's suggestion. I'm sure he'll be happy to help with things
that may be related to how NEC/EZNEC behaves, if we can identify them.

Cheers,
Tom

Tom,

the mistery seems to be solved:

- with the "real" hairpin the program does not yield correct answers, neither in
terms of gain nor in terms of impedance. Roy W7EL anticipated that the problem
is due to the feedpoint being in a short loop. I did the test he suggested (i.e.
calculating the Average Gain) and the result I got clearly showed that there was
a problem.

- instead, with the hairpin simulated by means of a shorted piece of
transmission, the results are fully consistent. Firstly the gain does not vary
with and without the hairpin. Secondly the impedance I read removing the hairpin
is 24.5 +j65.37 ohm, a value fully consintent with the pure 200 ohm I get with
the simulated hairpin.

At least I learned one should not try to model "real" hairpins.

At this point I would not further bother you, by sending you files that would
not tell you much. It was a real pleasure for me to have such an interesting
discussion with you and, if I may, I will write you should some other strange
situation occur in the future.

73

Tony I0JX

Efficiency of 200-ohm hairpin matching
 

Antonio Vernucci wrote:
I then tried to simulate the hairpin with a proper-length shorted
trasmission line and, doing so, the average gain is almost 0.

Is that shorted transmission line lossless?
--
73, Cecil http://www.w5dxp.com

Efficiency of 200-ohm hairpin matching
 

I then tried to simulate the hairpin with a proper-length shorted trasmission
line and, doing so, the average gain is almost 0.

Is that shorted transmission line lossless?
--
73, Cecil http://www.w5dxp.com

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

73

Tony I0JX

Efficiency of 200-ohm hairpin matching
 

On Apr 11, 11:54 am, "Antonio Vernucci" wrote:
Yes, the email should be right. I do have Eznec5+, so I can check
against Roy's suggestion. I'm sure he'll be happy to help with things
that may be related to how NEC/EZNEC behaves, if we can identify them.

Cheers,
Tom

Tom,

the mistery seems to be solved:

- with the "real" hairpin the program does not yield correct answers, neither in
terms of gain nor in terms of impedance. Roy W7EL anticipated that the problem
is due to the feedpoint being in a short loop. I did the test he suggested (i.e.
calculating the Average Gain) and the result I got clearly showed that there was
a problem.

- instead, with the hairpin simulated by means of a shorted piece of
transmission, the results are fully consistent. Firstly the gain does not vary
with and without the hairpin. Secondly the impedance I read removing the hairpin
is 24.5 +j65.37 ohm, a value fully consintent with the pure 200 ohm I get with
the simulated hairpin.

At least I learned one should not try to model "real" hairpins.

At this point I would not further bother you, by sending you files that would
not tell you much. It was a real pleasure for me to have such an interesting
discussion with you and, if I may, I will write you should some other strange
situation occur in the future.

73

Tony I0JX

Hi Tony,

I suppose you are not using EZNEC 5+. If that is the case, I would be
happy to run your model on the 5+ version with double-precision
calculations, and see if that makes a difference. It should be easy
enough to run it. It it no problem to me, just a fun diversion. And
of course I would be happy to have discussions with you in the future.

Cheers,
Tom

Efficiency of 200-ohm hairpin matching
 

Antonio Vernucci wrote:
Is that shorted transmission line lossless?

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

So what would be the results if the real-world
transmission line losses were accounted for?
--
73, Cecil http://www.w5dxp.com

Efficiency of 200-ohm hairpin matching
 

On Apr 11, 12:49 pm, "Antonio Vernucci" wrote:
I then tried to simulate the hairpin with a proper-length shorted trasmission
line and, doing so, the average gain is almost 0.

Is that shorted transmission line lossless?
--
73, Cecil http://www.w5dxp.com

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

73

Tony I0JX

At least in EZNEC 5+, the Transmission Lines window has an entry for
line loss, in dB/100 feet at a frequency you also enter. But the loss
for your 0.8cm diameter aluminum tubing spaced 10cm is only about
0.06dB/100 feet at 50MHz. It is certainly safe to ignore that,
considering that the loaded Q of the matching system is vastly lower
than the Q of the hairpin line as a resonator.

Cheers,
Tom

Efficiency of 200-ohm hairpin matching
 

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.

Roy Lewallen, W7EL

Efficiency of 200-ohm hairpin matching
 

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?

73

Tony I0JX

Efficiency of 200-ohm hairpin matching
 

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

Efficiency of 200-ohm hairpin matching
 

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

Efficiency of 200-ohm hairpin matching
 

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

Efficiency of 200-ohm hairpin matching
 

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

Efficiency of 200-ohm hairpin matching
 

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