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Author Topic: Question on tuners:  (Read 358 times)
ROBFINDLAY
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Posts: 76




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« on: March 25, 2007, 10:28:29 PM »

I'm looking at this http://www.mfjenterprises.com/products.php?prodid=MFJ-941E

For connecting to a random wire antenna, I'm still reading the docs and ALOT of it doesn't make sense to me yet, but here's a question: Once the antenna is tuned to the band you're going to use it on, do you leave the MJF on?  
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NZ5N
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« Reply #1 on: March 25, 2007, 10:37:03 PM »

The On-Off switch is only for the meter light, so you can turn it off if you don't need the light on.
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ROBFINDLAY
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« Reply #2 on: March 25, 2007, 11:27:16 PM »

"
The On-Off switch is only for the meter light, so you can turn it off if you don't need the light on.
"

But it does indeed stay on correct?
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K7PEH
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« Reply #3 on: March 25, 2007, 11:41:50 PM »

A tuner is merely a variable capacitor (maybe more than one) and a variable inductor that are adjusted (in this case) manually to create a match between the input and the output.  The input being your transmitter and the output being the antenna.  After obtaining the match by deftly adjusting the manual controls, you must leave the tuner in circuit.  The tuned inductance and capacitance is what produces the match and therefore the low SWR you need.

In the case of this particular tuner, the power is not required.  It is merely used for light as mentioned previously.  But, there are other tuners, auto-tuners, that use active devices, sometimes relays, and other types of circuits to help find the desired match.  Auto-tuners do need power and I think they may require power even after you have found the match.  Now, it is possible to build an auto-tuner that does not require power to be applied after the match is found but this is not important, keep the power on in that case anyway.
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W7ETA
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« Reply #4 on: March 25, 2007, 11:58:28 PM »

Every tuner I remember has a coil(s) and adjustable capacitor(s).

Nothing in the circuit has an "open" (off) or "closed" (on).

This page has the schematics for a couple of tuners:

http://en.wikipedia.org/wiki/Antenna_tuner

By looking at the schematics, you can see there isn't an on or off, just adjustments.

If you can, find an old HandBook from the 40-60s--cheap on eBay.  They have tons of information, and are easier to follow than current HandBooks--circuits were simpler then?
73
Bob
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KX8N
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Posts: 542




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« Reply #5 on: March 26, 2007, 04:40:18 AM »

"But it does indeed stay on correct? "

Does what stay on?

The light turns on and off.  It has absolutely nothing to do with the functioning of the tuner.

The tuner does not turn on and off.  It works by you adjusting the knobs.
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KB9CRY
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« Reply #6 on: March 26, 2007, 05:28:06 AM »

Yes you leave it On; the tuner matches or adjusts the mismatch between your rig and an antenna that does not, on it's own, exhibit a 50 ohm feedpoint impedance.

So, if you're using nonresonant antennas, then yes the tuner is required to be properly adjusted for the antenna and frequency and stays in that position throughout your use of the antenna

You change antenna or frequency, then you have to readjust the tuner to compensate.
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AA4PB
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« Reply #7 on: March 26, 2007, 03:37:12 PM »

Your tuner is a manually operated tuner. It is ALWAYS in the circuit whether you have the on/off switch on or off.

Most of the "automatic" tuners on the market have coils and fixed capacitors. Relays select the appropriate coil and capacitor combination under microprocessor control. Most of these require power to be applied in order to keep the necessary relays closed. Remove power from one of these and the tuner is bypassed, connecting the antenna directly to the radio.
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K0ZN
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Posts: 1548




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« Reply #8 on: March 26, 2007, 10:19:20 PM »


Hi.

Respectfully, pick up a copy of the ARRL Antenna Book and/or the Handbook. A few hours of basic book time would explain a lot more than a few quick comments here on eHam....especially regarding antennas. The knowledge you pick up will pay itself back many times over in reduced frustrations and more fun with the hobby. Fundamentally, this is a technical hobby and the more you know about how stuff works the better you can make it work!

73,  K0ZN
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KB3KCJ
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« Reply #9 on: March 27, 2007, 12:34:35 PM »

Turn off the light.
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W0FM
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Posts: 2055




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« Reply #10 on: March 27, 2007, 01:26:35 PM »

Rob,

The better way to "visualize" the tuner you are looking at is that it is either "in-line" or "off-line", there is no on and off for the tuner circuit.  That said, the tuner you specify has a "BYPASS" switch position, which removes the tuner from the transmission line circuit, effectively letting the transmitter see the antenna directly, without benefit of matching.

As stated, the ON/OFF switch is merely for the meter lighting.  For the tuner itself, think "bypass" rather than "off" and you'll be OK.

73,

Terry, WØFM
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