Antenna Tuner Questions
Cecil Moore
I am teaching a ham class and have been asked questions that I can't
answer (not unusual) but I can't find the answer in the Handbook or
Antenna Book... Here they are:
1. The Handbook says that an antenna tuner's loss is normally less than
0.5 dB. Assuming a 10/1 SWR, is the loss greater when one is matching
a load of 5 + j0, 500 + j0, 10 + j50,...etc.?
2. Which is the best antenna tuner, a T, Pi, Z,...etc.?
3. What is the transfer function of a transmission-line transformer type
Balun? i.e. Does a 4:1 Balun transform 1000 +j1000 to approximately
250 + j250 or is it like the coax series-section transformer equations?
thanks, Cecil, kg...@indirect.com
Rajiv Dewan
>
>I am teaching a ham class and have been asked questions that I can't
>answer (not unusual) but I can't find the answer in the Handbook or
>Antenna Book... Here they are:
>
>1. The Handbook says that an antenna tuner's loss is normally less than
>0.5 dB. Assuming a 10/1 SWR, is the loss greater when one is matching
>a load of 5 + j0, 500 + j0, 10 + j50,...etc.?
>
The loss in an antenna tuner, or any other tuned circuit for
that matter, is
Quality Factor * Energy stored.
So what affects these two terms?
In general, larger mismatches will require larger energy storage.
This is a loose handwaving argument that can be made concrete
with the circuit and a Smith chart in hand. For instance,
when using a L matcher, both 100+0j and 200+0j can be matched with
an inductor in series and capacitor in parallel to the load. For
a given frequency the 200+j0 will require larger reactances.
The currents and energy stored will be higher and so for the same Q
the losses will be higher.
The quality factor depends on the quality of components and the
the construction. Silver plated large coils with taps instead of
roller contacts, silver plated capacitors of split stator design,
large silver plated straps within the tuner to deal with large
currents and careful placement of components in a huge case (keeping
strong field away from case helps in reducing losses) will result in
a high Q tuner. The only commercial one that filts all of these
criteria is the AEA AT3000 (not produced any more - alas).
I know that roller inductors with turn counter dials evoke the
feeling of quality and dollars - I will take the tapped coil
with Vectronics of AEA style switching any time.
>2. Which is the best antenna tuner, a T, Pi, Z,...etc.?
At the bare minimum, you need two elements - a capacitor and
an inductor to match. You may have to reverse it to match
different kinds of loads. As simple circuits with few components
are desirable from a low loss standpoint, the reversible-L is
favored. It also has the benefit of having a single unique
match point. Down side - the Q varies depending on the load
and can get quite bad.
The T or a Pi is an improvement as the match and the Q can be controlled.
The complicating factor is that there may be many dips and you
have to pick the deepest of the dips to get the best match. Tuning
is more complex. The down side is that more components are needed.
So they may have higher losses and tuning is more complex.
Further, some circuits are better at harmonic supression than
others and this could be a factor.
ARRL book on Antenna Matching is a valuable reference.
>
>3. What is the transfer function of a transmission-line transformer type
>Balun? i.e. Does a 4:1 Balun transform 1000 +j1000 to approximately
>250 + j250 or is it like the coax series-section transformer equations?
>
The former. ARRL book "Transmission Line Transformers" by Jerry
Sevick, W2FMI is the authoritative reference.
Rajiv
aa9ch
r-d...@nwu.edu
Alan Bloom
: 1. The Handbook says that an antenna tuner's loss is normally less than
: 0.5 dB. Assuming a 10/1 SWR, is the loss greater when one is matching
: a load of 5 + j0, 500 + j0, 10 + j50,...etc.?
I don't know what "normally" means, but I can tell you that there are
plenty of poorly-designed antenna tuners being manufactured with more than
5 dB insertion loss. With most designs, the loss is a strong function
of the antenna load impedance. If you measure the loss when "matching"
a 50-ohm load, you probably don't get worst-case results.
: 2. Which is the best antenna tuner, a T, Pi, Z,...etc.?
An L-network is the simplest. However, since any antenna tuner requires
at least two adjustments (to match the real and imaginary part of the load
impedance), an L-network requires that both the capacitor and inductor be
adjustable. Since adjustable inductors are harder to make, a Pi or T
match is usually preferred.
When I was at Drake, I designed the MN-2700 antenna tuner. Unlike most
amateur tuners, it actually has an insertion loss spec (.5 dB) which
applies to any load impedance within the tuner's matching range. Matching
range is specified at up to 5:1 SWR at any phase angle, although it will
typically do much better than that with high (>250 ohm) loads. It uses
a type of Pi/T network:
Fixed Variable
50 ohm -------*---Inductor---*-----Capacitor----- Antenna
source | |
Fixed Variable
Capacitor Capacitor
| |
Gnd Gnd
The fixed capacitor and inductor are band-switched. Since there are no
variable components between the 50-ohm source and the inductor, the current
through it (and thus the loss) are independent of the load impedance when
the tuner is adjusted for perfect match. Since the capacitors are nearly
lossless, the tuner loss is independent of load impedance. All the Drake
tuners (MN-4, MN-2000, MN-7, MN-2700) use this same circuit topology.
: 3. What is the transfer function of a transmission-line transformer type
: Balun? i.e. Does a 4:1 Balun transform 1000 +j1000 to approximately
: 250 + j250 or is it like the coax series-section transformer equations?
Theoretically, both the real and imaginary parts are scaled by 4:1.
So yes, 1000 + j1000 becomes 250 +j250. However, be aware that most baluns
have stray reactances that become more important with high SWR. Also,
high SWR causes increased current or voltage in the balun for a given
power level: With a 10:1 SWR, for example, current can be as high as
3.16 times the 50-ohm value, so your 1 kw balun is now a 100 watt balun!
AL N1AL
Alan Bloom
: ... there are
: plenty of poorly-designed antenna tuners being manufactured with more than
: 5 dB insertion loss.
I don't know how the above line got trashed, but I typed ".5 dB".
(Does vi not like lines that begin with a dot?)
AL N1AL
Andrew Thomason
|> a type of Pi/T network:
|>
|> Fixed Variable
|> 50 ohm -------*---Inductor---*-----Capacitor----- Antenna
|> source | |
|> Fixed Variable
|> Capacitor Capacitor
|> | |
|> Gnd Gnd
|>
|> The fixed capacitor and inductor are band-switched. Since there are no
Forgive me for a stupid question. How do you obtain a variable
capacitor with neither side grounded? Do you use an ordinary
variable capacitor with its body (and shaft) physically isolated
from ground, or do there exist special types with neither side
of the capacitor connected to chassis?
Andrew Thomason
Andrew Thomason
not requiring capacitors designed for high power purposes.
Tom Bruhns
: How do you obtain a variable
: capacitor with neither side grounded? Do you use an ordinary
: variable capacitor with its body (and shaft) physically isolated
: from ground, or do there exist special types with neither side
: of the capacitor connected to chassis?
(It wasn't a stupid question, so I deleted that part ;-)
You can put an insulated coupling on the shaft. I've also
seen ones whose shaft has built-in insulation. Using this
technique, it's even possible to build multi-section variables
with moderately well isolated sections.
I have one variable which must be nearly the ultimate in caps
for an application like Alan described: it has two sets of
circular plates. Each set is attached to a shaft at the edge
of the stack. The shafts are parallel but separated by a
bit less than the diameter of the plates. The thing is
gear-driven, so the sets fully mesh at one extreme, and are
completely separate at the other. The gears and the mounting
plate for the shafts are insulated, and there is a wiping contact
on each shaft. The result is a cap isolated pretty well from
ground, balanced to ground, and capable of a very wide range
from min to max (a bit over 100:1 in this particular case).
73, K7ITM
Alan Bloom
: How do you obtain a variable
: capacitor with neither side grounded? Do you use an ordinary
: variable capacitor with its body (and shaft) physically isolated
: from ground, or do there exist special types with neither side
: of the capacitor connected to chassis?
The most common way is to mount the capacitor on insulated posts and
use an insulated coupling between the shaft and front-panel knob.
AL N1AL
R3...@vm1.cc.uakron.edu
go. If somebody would help me out I would appreciate the help
Thanks
Ron Black
sohl,william h
<R3...@VM1.CC.UAKRON.EDU> wrote:
>I am interested in getting a radio license and don't what to do or where to
>go. If somebody would help me out I would appreciate the help
Call the ARRL (Amateur Radio Relay League) at 1-800-326-3942 and they'll
send you a "New Ham" info package with contact information, etc.
You can also immediately order the license manual for Novice and Technician
class licenses by calling the ARRL order line at 1-203-666-1541.
The license manual/study guide is: "Now You're Talking", item #4173, $19.
Hope that helps.
Standard Disclaimer- Any opinions, etc. are mine and NOT my employer's.
-----------------------------------------------------------------------
Bill Sohl (K2UNK) BELLCORE (Bell Communications Research, Inc.)
Morristown, NJ email via UUCP bcr!cc!whs70
201-829-2879 Weekdays email via Internet wh...@cc.bellcore.com
John Seboldt
>I am teaching a ham class and have been asked questions that I can't
>answer (not unusual) but I can't find the answer in the Handbook or
>Antenna Book... Here they are:
[items axed]
>3. What is the transfer function of a transmission-line transformer type
>Balun? i.e. Does a 4:1 Balun transform 1000 +j1000 to approximately
>250 + j250 or is it like the coax series-section transformer equations?
>thanks, Cecil, kg...@indirect.com
A nice theoretical question. Just a thought on the practical level: from
what I've read, the use of transmission line "baluns" on the output of
antenna tuners is absurd when matching reactive loads, like using a dipole
fed with balanced line on several bands. Feb. 1990 QST deals with a tuner
that puts the balun (1:1, current-type) on the INPUT so you don't have to
deal with this. There are some other articles I remember somewhere about
the behavior of transmission-line transformers with reactiv3e loads, but
can't place them.
What your students should really hear is that your typical commercial
antenna tuner with the balun on the output is taking the cheap way out.
It may work after a fashion, but if you do it right ( a true balanced
tuner, one example of which is the item referred to above), you will have
a non-radiating balanced feedline, and a good multiband antenna system (if
you don't mind twiddling with knobs).
John K0JD
Cecil Moore
: What your students should really hear is that your typical commercial
: antenna tuner with the balun on the output is taking the cheap way out.
: John K0JD
Thanks John, I agree and my homebrew antenna tuner has a balanced output
with no sign of a balun. But these students want to buy an antenna tuner
to use with a non-resonant antenna. I need the equations governing the
transfer function of a voltage and/or current balun that is not arcing
and/or saturating. I can't find them in W2FMI's book, Maxwell's book, or
any of the ARRL publications that I own. Where the heck are they?
73, Cecil, kg...@indirect.com
Alan Bloom
: I need the equations governing the
: transfer function of a voltage and/or current balun that is not arcing
: and/or saturating. I can't find them in W2FMI's book, Maxwell's book, or
: any of the ARRL publications that I own. Where the heck are they?
Ideally, the transfer function for a 4:1 balun is R(load) = 4 * R(source)
and X(load) = 4 * X(source), assuming the load is connected to the
"4" side of the balun.
Or perhaps you meant you want to calculate the power-handling capability
with high SWR. The easy answer to that question is to assume that a balun
designed for the amateur limit can handle 1.5 kW (plus some margin) into
a 50 ohm load. The worst-case voltage or current will be multiplied by
the square root of the SWR. (High-impedance loads increase the voltage,
low-Z loads increase the current.)
The upshot of this is that, under worst-case conditions, the power
rating is degrated by a factor equal to SWR. For example, if the
SWR is 3:1, you can safely use your 1.5 kW balun at 500 watts
without arcing the windings or saturating the core.
AL N1AL
Gary Tennyson
Is the transfer function really this simple ? As a first-order
estimate, I disagree. I think the balun has distributed R, L, G, & C and
therefore looks like both a transmission line and a transformer.
A good place to start (if you really want to chaaracterize a balun) would
be the Fall 1992 Communications Quarterly. An article by Jerry Sevick
discusses two different models for baluns. I think, though, that if you
are wanting to derive a transfer function, you will have to go review
some of the references cited in the article.
I think that a transfer function will ultimately depend on the physical
makeup of the balun of interest, as well as the frequency at which it is
used.
Good Luck.
--
Gary Tennyson BellSouth Telecommunications, Inc.
Internet: ga...@vulcan.com