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Author Topic: The MFJ analyzers look nice. They really do. I just don't have that money....  (Read 5796 times)
WB6BYU
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Posts: 12977




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« Reply #30 on: June 02, 2011, 09:01:52 AM »

Quote from: KB1TXK
So my dummy load resonates? Wink


It does if the reactance is zero - that is the definition of resonance.

If you are measuring one end of a coax cable, and the SWR is 1.0 : 1, you don't know whether there is
an antenna, a dummy load, or the input circuit to an amplifier (or an infinite length of coax) at the
other end by a single measurement of impedance.  The load still looks like 50+j0 to the transmitter -
are you saying that in some cases that is a resonant impedance and sometimes it isn't?


And, like W6RMK, returning to the original post, if the question is whether you NEED an SWR analyzer
to tune up antennas, the answer clearly is no.  You can do it with an SWR meter:  tuning the antenna
for minimum SWR is at least as good as anything else for this purpose.

Beyond that there are two arguments for the SWR analyzer: it is portable, and it gives some indication
of resistance and/or reactance beyond just SWR.  These need to be considered separately.

Portability is good:  I remember lugging my HW-12 and 45 pound power supply (pair of 866 rectifiers)
up onto the roof as a kid to tune my 80m antenna.  Years later the Ten-Tec Argonaut with a built-in
SWR meter made life much simpler.  I've also used the remote key lead technique when I needed to
adjust a variable capacitor at the external tuner:  I left the radio in the shack but put the SWR meter
at the antenna and used a long wire to key the transmitter as needed.  (The conventional approach
was to have a friend come over and key the transmitter while you adjusted the antenna.)  You can
even build your own simple crystal-controlled oscillators with SWR bridges if you are interested in
a limited number of frequencies.  While there are ways around it, portability is the biggest argument
for some sort of SWR analyzer in my mind.  That's what I use my MFJ-209 for (though sometimes
I still take a radio out into the back yard so I can make QSOs on the antenna.)


Measuring resistance and reactance can be useful, but it certainly isn't essential for building antennas.
I've built and experimenting with LOTS of antennas over the years, and can think of one time that I
needed to measure the R and X (to make a matching stub to use a 155 MHz on 2m.)  In that case
I built a simple circuit that reads out impedance on the face of an oscilloscope.  Otherwise I've
managed fine with SWR (which is all I get from my MFJ-209).  Is the antenna too short or too long?
measure the SWR at several points and find out - this is much more accurate than checking the
reactance though an unknown length of coax.  Is the resistance above or below 50 ohms?  Put a
200 ohm resistor across the feedpoint and see if the SWR gets better or worse.  Need to measure
a complex impedance?  Build an experimental "L" network and adjust it to get a match, then measure
the values of the components.  Need to cut a quarter wave stub?   Connect a resistor of the desired
impedance across the far end, or put it on a T connector with a dummy load and short the far end,
and adjust the length for minimum SWR.

Not that having R and X might not make things easier sometimes.  If I had such an analyzer I'd certainly
use it, but the point is that you can do a lot without it.  In fact, sometimes it provides TOO MUCH
information, and the operator loses track of what really matters.  The crowd that insists on X=0 (though
a random length of coax, when that doesn't necessarily coincide with minimum SWR) are one example.
I've also seen folks totally mess up tuning a gamma match by basing their adjustments on the frequency
of minimum SWR rather than the SWR at the desired frequency.


I guess what it boils down to is that the knowledge of what you are doing is more important than how
fancy your test equipment is.  If you can't afford a fancy analyzer, don't let that stop you from
experimenting with antennas.  You may need to learn a bit more about impedances and SWR to know
how to interpret the readings, but that is a useful thing to do anyway.  As you get more experience
with antennas, you'll find some places where an SWR analyzer would be more convenient, and you may
decide that is worthwhile for you to acquire one (or find a friend you can borrow from.)  If I didn't
have the MFJ-209 I probably would have bought a fancier one by now, but so far I don't seen enough
added benefit to upgrade.
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KB1TXK
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« Reply #31 on: June 02, 2011, 12:59:47 PM »

the technical replies are making a lot more sense now that I'm studying for the general test haha.

thanks everyone!
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W5DXP
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« Reply #32 on: June 02, 2011, 01:26:41 PM »

Quote from: KB1TXK
So my dummy load resonates? Wink

It does if the reactance is zero - that is the definition of resonance.

From The IEEE Dictionary: "resonance - (5)(radio-wave propagation)(A) The rapid increase or decrease of the oscillation amplitude as the excitation frequency approaches one of the natural frequencies of the system. (B) The change in amplitude as the frequency of the wave approaches or coincides with a natural frequency of the medium."

Since a dummy load is not associated with a natural frequency, it doesn't seem to meet the definition for "resonance" which does seem to require a natural frequency.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
N3OX
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« Reply #33 on: June 02, 2011, 01:39:18 PM »

I see Cecil beat me to this, but I agree with his comment above.

It does if the reactance is zero - that is the definition of resonance.

I don't want to go off on too much of a hair-splitting tangent on this, but a dummy load is (ideally) entirely NONresonant.  It does not and can not exhibit resonant behavior.  It stores no energy, just immediately dissipates it as heat.  "Zero reactance" is not enough to define resonance.  The system has to be capable of exhibiting resonance AND the reactance has to be zero.

A resonant system needs to have stored energy sloshing back and forth between multiple forms.  In a clock pendulum or a kid on a swingset, energy sloshes between kinetic and potential energy, between moving fast and going high.  In an antenna, it sloshes back and forth between electric and magnetic field.  When you force these systems at a frequency away from the resonant frequency, they push back significantly out of phase with your forcing and you have to adjust what you're doing to accept energy back from the load.  

A swing or a pendulum has a resonance and it becomes really easy to keep it going at that frequency.  The mechanical version of a dummy load more like trying to shove a basketball back and forth in a big vat of cold molasses.  At ANY frequency, it accepts a bunch of oscillatory energy from you.  There's no stored energy to speak of.  If you stop pushing the basketball, it, to good approximation, just stops dead.

That's a dummy load.

When you're talking about a driven, damped system like a swing or an antenna, forcing at resonance is forcing at the frequency where the stored energy is precisely balanced between the multiple forms, so you only have to supply what's being dissipated.  But if you move off that frequency, the load is going to be spitting energy back at you (with a phase depending on which form of energy is stored in greater proportion at that frequency.... reactance in electrical systems).  A dummy load doesn't do this cannot qualify.

There are a few antennas that in their ideal form are nonresonant... horn antennas are one, and real ones come close... it's tougher to avoid weak resonances in systems where the most important loss mechanism is radiation of EM fields.  It's very easy to swamp resonances in an ultra lossy dummy load Grin
« Last Edit: June 02, 2011, 01:48:03 PM by N3OX » Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
N8CMQ
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Posts: 353




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« Reply #34 on: June 02, 2011, 09:43:06 PM »

SWR does not indicate resonance, SWR indicates impedance match.
Resonance is when the wave length of the antenna and the frequency are equal.
A resonant antenna impedance is affected by many external factors, height, ground resistance, and murphy...
Cut an antenna to resonance, and then match the impedance of that antenna to the feed line, and you will have a good antenna.
Improve the ground resistance, elevate the antenna to the proper level (horizontal) and you will have a better antenna system!
Relying on only one tool can lead you astray. I have two MFJ-249, one for work and one for home. I also have a palomar noise bridge, and Heath GDO. The 249 can also be used as a GDO, as well as a frequency counter.
Keep your eyes open, I bought my second 249 for $10.00, all it needed was battery holders.
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K2QPN
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Posts: 70




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« Reply #35 on: June 07, 2011, 06:04:32 AM »

Also, check out the Deluxe Tenna Dipper from www.qrpkits.com. It is inexpensive, comes in kit form and fits in the palm of your hand.

73 Bob K2QPN
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LNXAUTHOR
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Posts: 43




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« Reply #36 on: September 04, 2012, 03:59:16 PM »

You can check Hendricks out (qrpkits.com), and they have a deluxe antenna dipper, which will allow you to change the frequency and get an LED indicator (bright means no match, off means you are good). It's cheaper than the MFJ units. The only reason I don't have one is that it does have one SMT part, and while they claim it's not difficult to install, I can still see myself burning my fingers with it.

just built one - took me four hours - the SMT part was a *heck* of a lot easier than many i've run across... (only 4 tabs)

went out today and set up my MP-1 w/a custom extender and extra radials

then compared my Palstar ZM-30 readings to the Deluxe Antenna dipper... matching results!

btw, you can run the dipper on a single 9V (won't last as long, but enough for a couple hours, according to the designer)

72
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KB5UBI
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Posts: 97




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« Reply #37 on: September 05, 2012, 06:53:34 PM »

The cheap MFJ 204B antenna bridge is quick to use. It will dip at resonance and is useful with different impedances. It's basically a 207 with adjustable impedance.
 
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