On Apr 9, 2:08 am, Owen Duffy wrote:
K7ITM wrote :

...

Yes, EZNEC has some nice features. I exercised a couple of them
tonight, doing frequency sweeps with inductive and capacitive
matching. I did cheat: since I do not know the dimensions of the
hairpin match, I elected to just use a pure lumped inductance. I
suppose the error compared with a transmission line stub (hairpin)
won't be great. I also used for the first time ASCII file import for

It probably isn't.

However, you could get an idea from my Two Wire Line Loss Calculator
(http://www.vk1od.net/tl/twllc.htm). It looks like a hairpin made of 4mm
dia aluminium 50mm spacing and 150mm in length give an impedance of 0.02
+j61... so the Q is probably mainly determined by the end connections.

a o/c stub made of 4mm dia aluminium 50mm spacing and 1300mm in length
give an impedance of 0.07-j80... so, it is not quite as good
electrically, it is unweildly and end connection resistance will probably
still be significant. This is no doubt why people use a hairpin in
preference to an o/c stub!

Owen

Thanks, Owen. Actually, the error I was thinking of was not the Q,
since both a good hairpin and a good helical coil will have Qu's very
much higher than the loaded Q of the matching network -- but rather of
the slope of reactance versus frequency, since the hairpin is a
transmission line and will presumably show a sharp resonance at a
different frequency from the coil's self resonance. But that error
should also be small--negligible as Wim notes.

Cheers,
Tom

K7ITM wrote in
:

On Apr 9, 2:08 am, Owen Duffy wrote:
K7ITM wrote

om:

...

Yes, EZNEC has some nice features. I exercised a couple of them
tonight, doing frequency sweeps with inductive and capacitive
matching. I did cheat: since I do not know the dimensions of the
hairpin match, I elected to just use a pure lumped inductance. I
suppose the error compared with a transmission line stub (hairpin)
won't be great. I also used for the first time ASCII file import
for

It probably isn't.

However, you could get an idea from my Two Wire Line Loss Calculator
(http://www.vk1od.net/tl/twllc.htm). It looks like a hairpin made of
4mm dia aluminium 50mm spacing and 150mm in length give an impedance
of 0.02 +j61... so the Q is probably mainly determined by the end
connections.

a o/c stub made of 4mm dia aluminium 50mm spacing and 1300mm in
length give an impedance of 0.07-j80... so, it is not quite as good
electrically, it is unweildly and end connection resistance will
probably still be significant. This is no doubt why people use a
hairpin in preference to an o/c stub!

Owen

Thanks, Owen. Actually, the error I was thinking of was not the Q,
since both a good hairpin and a good helical coil will have Qu's very
much higher than the loaded Q of the matching network -- but rather of
the slope of reactance versus frequency, since the hairpin is a
transmission line and will presumably show a sharp resonance at a
different frequency from the coil's self resonance. But that error
should also be small--negligible as Wim notes.

Hi Tom,

Warning bells sound to me when applications call for reactors fabricated
from TL sections. Not to say that are always bad, but they aren't always
good, and they bear examination.

I think that may have been in Tony's mind over the matching network.

In the case of the hairpin, fabricated from substantial material, it
looks good in this application. Because the line section is so short,
inductive reactance is almost linearly proportional to length, and self
resonance of the line section itself is nearly a decade higher.

My experience with one of the HyGain (now MFJ) 2m Yagis was that VSWR was
stable with weather and over time. I had taken some care to exclude air +
water from key connections, in fact I replaced all the fastners used for
electrical connection with SS when the beam was less than a year old
because of corrosion. Tweny years later the remaining fastners used only
for mechanical retention required replacement due to corrosion. But,
after the first rework, the matching system seemed robust (bird proof)
and reliable.

I have used them since on home made antennas with good success.

Owen

On Apr 9, 2:59 pm, Owen Duffy wrote:
...
Hi Tom,

Warning bells sound to me when applications call for reactors fabricated
from TL sections. Not to say that are always bad, but they aren't always
good, and they bear examination.

....

:-) Yes, indeed. I just this afternoon fabricated a 500MHz LPF that
I wanted to give about 40dB attenuation up to several GHz. My design
was a 5th order elliptical, so I had two parallel tanks for the series
paths and three shunt capacitors. Because the coils (10nH and 15nH)
are so small, I though about using a shorted stub instead, but then
realized that my shorted stub would look like a short when it was 1/2
wave long, not a good thing to have as a series element at a frequency
you want to block. Probably would have worked OK since the two stubs
were different and wouldn't have the same resonance frequency; and by
the time you reached that freq., the shunt caps would be pretty
effective on their own. Anyway, it works fine with tiny coils. Thank
goodness for good microscopes.

Cheers,
Tom