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Very confused over this manual tuner

TruckerKevin

KD9NTD
Nov 9, 2017
234
142
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Buckner, Illinois
www.qrz.com
Very confused over this manual tuner the MFJ 989D

The literature boasts of a built in 1:1 balun consisting of two large toroids. And on the back it accepts a balanced feed line . And the documentation says to hook up your balanced feed line.

OK, call me dimwitted, but I thought that the purpose of a balan is to balance an unbalanced feedline. What would be the sense in having a built-in balun if I have to feed a balanced line? So this means that I must already have a balun already in line of my 300 ohm landline before it hits the tuner?

Or am I just completely missing the mark here?
 

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I have the same tuner. It has a 4:1 balun. I have 3 antennas on it, mostly for the antenna switch. One is a 75 meter dipole fed with 450 right to the back of the tuner. I can tune 160-10 on that antenna.
Rich
N8FGB
 
While I ultimately got the right answer I was looking for, the technical aspect of this still pesters me. I’m a new ham, and am slowly learning this stuff. But I’m like a pig on skates here.

The built in balun on this tuner balances the feed to 50 ohms. Great. No need to use a balan because this tuner features a built in one. Is it a misnomer that it says “balanced” or do they refer to it being “balanced” meaning that that’s what it will be after the feed is ran through?

If I’m wrong, and the latter part of my statement above holds true,,,, then someone is in serious er from MFJ by saying in the documentation that it must be fed with a “balanced” line.

That makes no sense to me,,,,,, still. lol
 
Kevin: OK, let me see if I can get this right.
The Balun "transforms" the impedance of the antenna, so the tuner circuit can "match" it to 50 ohms on the transmitter side.
The term "Balance Line" refers to the fact which during transmission the current on each line is equal.
Coax is not a balanced feedline, thus some use a Balun at the feed point to rebalance the current flow in the two lines.
 
The tuner has a balun built in to convert from the 50 ohm unbalanced coax cable that connects the tuner to your radio. This allows direct connection of a balanced feedline such as your 300 ohm feedline to the tuner without the need for an external balun.
 
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Remember RF is AC...Just like the wiring in your house. Current in one conductor travels thru one line and returns in the other and then reverses at the frequency of the cycle.
House wiring is 60 cycle AC, so this transformation happens 60 times a second in the US!:whistle::ROFLMAO:
 
A balun is used to make the transition from balanced to unbalanced. BAL(anced) UN(balanced).

Coax is unbalanced feedline and the 300 ohm twin lead, 450 ohm window line, ladder line or whatever the case may be is balanced feedline.

Your tuner is an unbalanced tuner. The built in balun allows you to connect balanced feedline to your unbalanced tuner. Each leg of the balanced feedline must have equal current and be 180 degrees out of phase or the feedline will radiate. Similar problems as common mode current and coax.
 

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