Michael, Try adding another 5 to 10 feet of ladder line. I had a similar issue with my 80m doublet. After adding just a bit more, it was able to find a match across all bands.
Just one of the drawbacks of not having dedicated resonant antennas for each band! Josh KB3VQQ On Dec 20, 2015 10:10 PM, "Michael Rapp via BVARC" <[email protected]> wrote: > Hi all, > > My last antenna experiment was a center-fed zepp cut for 80 meters. > (Dipole fed in the center with 450 ohm feed line, requiring a tuner.) > > I found this antenna to be a decent multi-band antenna, with one > exception: I never could get my MFJ manual tuner to find a match on 40 > meters. For the other bands, it seemed to find a match fairly easily. As > I had another antenna -- an inverted L vertical -- for 40 meters, I > shrugged this off as something to look into later. > > Well, my inverted L has succumbed to the elements and I need to rebuild > it, so I turned my attention to trying to figure out my center-fed zepp 40 > meter problem. > > After much research (remember, I'm a beginner at this!), the problem has > become somewhat obvious. One drawback to a center-fed zepp, I have > learned, is that the impedance at the second harmonic of the lowest > frequency will be very, very high. The second harmonic of an 80 meter > antenna is 40 meters. Oops. Apparently the impedance is so high the MFJ > tuner can't match it. > > I thought about purchasing a fancy tuner, but I remembered that tuners > aren't miracle devices. They don't change the impedance of the antenna and > I am concerned about power loss at the tuner. It seems the only thing to > do is to find a way to temporarily electrically shorten the dipole when I > want to operate on 40 meters. > > The 'zepp is an inverted V, so the ends of each leg of the dipole are > fairly low to the ground and easily accessible. (They actually run along > the top of my backyard fence for about 15 feet on each side!) What if I > coiled enough of the wire such that the second harmonic of the new length > was off the 40 meter band? Is coiling the excess wire enough to > electrically shorten the antenna, or do I really need to run the excess > wire back along the elements (parallel to them) for this to work? > > I like the coiling idea as I could just uncoil the wire on each end, > stretch it back to its original length when I want to operate on 80 (or 60) > meters. > > -- > /*/-=[Michael / KT5MR]-=/*/ > > _______________________________________________ > BVARC mailing list > [email protected] > http://mail.bvarc.org/mailman/listinfo/bvarc_bvarc.org > >
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