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Fan Dipole?


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  • Ionosphere

Hello Gentlemen,

A question for all the guys out there who are familiar with fan dipoles,

I built my 40 meter dipole myself that I use for 40 meters with my Yaesu FT-890AT.  Awhile back I saw a "headline" for an article titled something about "How to use a fan dipole with an autotuner" or something to that effect somewhere but I didn't click on it and read the article.

I'm sort of wanting to have the option for 20 meters as well without having to invest in another coax run, so I was considering a fan dipole with 20/40 meters.

I thought about it and my question is, how will the FT-890 with the auto-tuner know that band I want to use?  Will it be trying to tune both wires to the frequency or is there some way the radio will "know" to not try to tune the 20 meter wire while I'm on, say 7.185 MHz and to switch over to the other wire when I switch to the other band?  Sorry to say I don't know very much about "antenna science" but the article that I didn't read lead me to believe that there was some trick to trying to tune two antenna off one feedpoint with an autotuner.

Am I "good to go" with the simple 2-band fan dipole and an autotuner or is there something I need to know?  It's a case of I don't know what I don't know.   Copper wire being what it is nowadays price-wise, Id like to avoid the expense of the "try it and see what happens" method of learning if I can.  LOL

Thanks in advance.
Anthony, KD3Y



Edited by KD3Y
spelling erurs
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  • Elmer

The autotuner uses the frequency set by the radio.  When you are transmitting on 20 m, the 40 meter antenna does not resonate well at all.  Some tuners will try to make a 40 m antenna work on 20 m. My Yaesu will not even try. But, once you have a 20 m antenna attached, the tuner sees something it can work with and adjusts to lower the SWR at the radio. A 40 meter dipole is not too shabby at 15 meters, so you can run your tuner and it will likely match ok on 15 meters.

Likewise, the 20 m antenna is a lousy antenna for 40 m. So, your tuner will mostly notice the 40 m antenna when you try to operate on 40 m. 

There is a bit of interaction between the antennas. If you add a 20 m antenna to your 40 m antenna, spend a little time re-adjusting the length of the 40 m to get the lowest SWR at a good place on the 40 m band. 

In the end, you actually don't care which wires the tuner or the radio are using. If the SWR is low (without the tuner), it suggests that wires in the tree are providing a decent antenna. Note, the reading you get at the radio is not exact. To get an exact reading you have to compensate for the signal loss in the coax cable. But, that is another story.


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