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G5RV to Remote Auto Tuner

Apr 14th, 18:47

KD5DRY

Joined: May 15th 1998, 00:00
Total Topics: 0
Total Posts: 0
I want to run the ladder line from a G5RV into a remote auto tuner. I am confused as what auto tuner would work for that and whether the auto tuner must be a balanced auto tuner. Also, some G5RVs come with a 1-1 current balun already installed. Should I get a G5RV that does not have a balun to run the ladder line into the auto tuner or is it okay just to connect the balun to the balanced auto tuner? Thanks very much. Warren
Apr 14th, 19:29

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

http://www.eham.net/articles/14338
This eham article describes a another possible method, putting a balun at the feedpoint of the tuner.

Ideally, a balanced autotuner would be best, but these are less common and more expensive.

It is not a good idea to connect a balanced feedline directly to a single ended autotuner--RF current from the feedline can be expected to disturb the ground reference unless the tuner is well grounded--in which case the feedline won't work properly because of the ground.

In theory, you could just use a balun between the autotuner and the feedline, but in practice you run into several significant issues. The balun may create an undesirable impedance match that moves the antenna out of the matching range of the tuner--perhaps making a high impedance load even higher. Alternately, the balun may make a low impedance load too low, reducing system efficiency. Finally, the balun may overheat and be permanently damaged--this is usually evidenced by SWR drift as the balun heats up. Theoretically, balun heating is most likely with high impedance loads and low/modest choking impedances offered by the balun.

An air core balun can avoid the heating issue, but air core baluns typically work well on just one ham band, with modest performance on adjacent bands. This is good enough for triband beams with low impedances on all bands, but not for loading up a center fed dipole on all bands.

Perhaps a better all band antenna for the typical autotuner is the Inverted-L.

Zack Lau W1VT
ARRL Senior Lab Engineer


Apr 14th, 19:38

KD5DRY

Joined: May 15th 1998, 00:00
Total Topics: 0
Total Posts: 0
Thank you VERY much. MFJ makes this claim on their auto tuner: "The MFJ-993BRT handles 300 Watts SSB/CWpower level and matches 6-1600 Ohm antennas. The MFJ-993BRT lets you tune any antenna automatically balanced or unbalanced -- extra fast. It’s a comprehensive antenna tuning center."

What do you think? I may give that a try. The G5RV antenna I am looking at has a 1-1 current balun at the end of the ladder line that eventually connects into the 50 ohm coax. Or do you think I should get a G5RV without a balun? Really appreciate your assistance. Warren
Apr 14th, 20:27

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

As pages 7 and 8 of the instruction manual for that tuner explain, you can and do run into matching issues--you can't really expect this tuner to handle any load you throw at it.

A G5RV can be a good antenna on several bands if put up high, but I do run into a lot of hams that are frustrated if they can't get good performance on every band--a G5RV falls short of this expectation. In contrast, while the Inverted-L requires more work to set up properly, it is much more likely to work as an all band antenna. More importantly for a lof of hams, the Inverted-L needs less space.

One also has to realize that efficient wire antenna interact with nearby antennas, wires and metal structures--antennas with "problem" bands will vary unpredictably depending on location.

In theory, one could swap baluns to optimize performance--using a 4:1 balun for high impedance loads, a 1:4 balun for low impedance loads, and a 1:1 balun for moderate impedance loads. In practice, folks want to use one balun for everything.

Zack Lau W1VT
ARRL Senior Lab Engineer

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