EMRFD Message Archive 3286
Message Date From Subject 3286 2009-07-01 11:38:27 Ashhar Farhan The importance Q in a matching network The normalized pi-network is a standard in our toolbox. Often, at the
beginning of a design, we have to specify the Q for the network. what
is the significance of this choice? why are most of our power
amplifier matching networks designed for an unusually low Q?
For instance, in a 7 MHz amplifier, our interest is primarily in the
region of 7.0 to 7.2 MHz, a bandwidth of 200 KHz, this point to a
required Q of 35. But the networks are often designed with Q-Loaded of
less than 5.
I am raising this issue because I am struggling to understand what
happens when a standard pi network is used to tune across two bands
(like 7 and 14) by varying the capacitors (keeping the same inductor).
I can visualize that the two capacitors form a divider across the
inductor thereby achieving the impedance transformation. Though the
amateur literature is full of admonitions to keep the Q low, there is
a lack of clarity on why this is required. Can someone guide me?
3287 2009-07-01 19:30:33 Russell Shaw Re: The importance Q in a matching network Ashhar Farhan wrote:
> The normalized pi-network is a standard in our toolbox. Often, at theThe source and load deliver and consume a certain level of current.
> beginning of a design, we have to specify the Q for the network. what
> is the significance of this choice? why are most of our power
> amplifier matching networks designed for an unusually low Q?
> For instance, in a 7 MHz amplifier, our interest is primarily in the
> region of 7.0 to 7.2 MHz, a bandwidth of 200 KHz, this point to a
> required Q of 35. But the networks are often designed with Q-Loaded of
> less than 5.
> I am raising this issue because I am struggling to understand what
> happens when a standard pi network is used to tune across two bands
> (like 7 and 14) by varying the capacitors (keeping the same inductor).
> I can visualize that the two capacitors form a divider across the
> inductor thereby achieving the impedance transformation. Though the
> amateur literature is full of admonitions to keep the Q low, there is
> a lack of clarity on why this is required. Can someone guide me?
The current circulating in the pi network increases with Q, so Q=35
means 35x more current will be circulating (assuming source and load
are resistive, but in practice, they're not 100%). To achieve that,
the pi components need to have a reactance around 1/35th of the source
and load (approx, because Zsource and Zload can be quite different).
This means that for the pi to have low loss, the pi components need
to have *extremely* low loss, and very low impedances, making it more
Similar applies to Tee networks, except that you'll get ~35x higher
voltages and 35x higher reactances than normal.
3288 2009-07-01 22:22:55 Ashhar Farhan Re: The importance Q in a matching network russell,
thanks for the explanation. it is a little counter intuitive to shoot
for lower Q for lesser losses. but i understand that we are talking
about loaded Q and not the unloaded Q. the less the energy in the
resonator for lesser time, the less the losses. i guess this is what
it means at a physical level.
my investigation started with using a pi network (instead of the
venerable L match) to tune the antenna. the currently fashionable
'balanced' balance line tuner uses two L networks to match line. i was
considering using a two pi networks instead. this allows me to skip
the variable inductors or alligator clips on an breadboard AMU (i have
kids tearing around the shack at all times).
3289 2009-07-06 06:47:02 Stan Re: The importance Q in a matching network Farhan, I suspect the reason we do not see the Pi network as much today has more to do with the changes in the design from vaccum tube to solid state transmitters.
The Pi network first came on the scene in the 1930s and often called the Collins tuner in the older radio books. It the 50's it became the major component in the transmitter since it was easier to build a tapped coil than the plug-in ones of the earlier era.
The Pi network works best when the input impedance is a Hi-Z. As the input impedance is lowered the value of the input capacitor has to be increased to achieve a match. When you get to the 50 ohm input level the size and cost of the input capacitor becomes the major cost of building with a pi-network.
Lowering the Q of the inductors allows lower values of input capacitor, it also has the effect one does not have to retune the finals when moving around the band. Retuning is not much fun with running a vaccum tube transmitter.
Do a simple calculation of a Pi network for a 1000 ohm input impedance with 50 ohm output and using the same value of Q do a network for 50 ohm input and 50 ohm output. Watch the effect on the capacitor. Increase the Q and I believe the input capacitor will increase even more, however, it has been a few years since I designed a Pi network stage. They use to be a Pi network design on the web and you could put in freq and vary the Q, etc. think it might have been DX zone that had it. It was fun to play with; the current Pi tuner designer on the web will not run on my Vista computer so did not get a change to check it.
So other factors in addition to a slight increase in loss. Application is different today.
3290 2009-07-06 09:02:20 Ashhar Farhan Re: The importance Q in a matching network thanks stan and everybody else,
the last few days have been of incredible education. the amazing thing
is that everybody is right! there are a number of explanations and
each one of them is so chestful of interesting insights.
my immediate need was to build a balanced antenna tuner for my feed
line without resorting to variable inductors. it occured to me that a
pi network could do the trick. two paralleled pi networks would
essentially become a lumped constants equivalent of the ladder line
itself (two variable capacitors shunted across the input and output,
fixed inductors as series element between the input output posts).
as is usual, i first refered to EMRFD for the cookbook solutions and
then I started digging deeper and deeper into it. right now i am
lugging around my school days' 'hayt and kimberly' (Engineering
Circuit Analysis) trying to understand complex impedances and complex
frequencies with bob's 'caron' impedance bridge in hand.
btw, here is an online calculator:
The Q of a pi network with fixed inductor will vary with frequency. i
found it best to bisect the pi network into two L networks whence the
analysis becomes simpler.
i wish there were a text that goes deeper than EMRFD, explaining the
mathematical concepts like poles, zeros, complex planes, et al for the
hams (not engineers). it would help us be understanding how our
filters and close loop systems work. there probably is something like
that already (IRFD is too theoretical and needs you to be familiar
with fair amount of mathematics).
3293 2009-07-07 11:57:24 davidpnewkirk Re: The importance Q in a matching network 3298 2009-07-08 11:57:40 Kevin Wheatley Re: The importance Q in a matching network 3299 2009-07-08 21:34:25 Ashhar Farhan Re: The importance Q in a matching network wow, it is an amazing tutorial. i am reading it right now ...
3300 2009-07-09 14:44:24 gt25psi2002 Re: The importance Q in a matching network Dear Sir,
Thanks very much for the website. It is very informative (but scared to
see the formulae!) I'll try to absorb it even though only 1% or 2% is my
>and have found a REALLY great site that explains things in
> I've been doing some research on this subject over the last few days,
understandable english (well most of it made sence).
> Here's the link:
> Kevin - M0KHZ