Hey, Michael,
Josh suggested adding additional feedline to change the Z at the tuner end, so that it falls within the range of the tuner. An old ham trick and probably the easiest to implement. But I question your varying the radiating element length. Why do you want to change the antenna length, as an 80 meter dipole on 40 meters is “two half waves in phase”. Not a bad gain antenna for 40. [However, in addition to Josh’s suggestion, you could put 40 meter traps in reach leg. Unadilla sells traps. QST has lots of articles about building traps out of coils and caps, but, better yet, just coax.] Another suggestion for matching on 40 is to make the feedline length ¼ wl on 40 (3/4 works too)…..this would provide a Series Section transformer (SST) and transform the high Z at the feed point to a lower Z at the end of the feedline. Check out SST’s and the calculations. These are typically used on loops using 75 ohm coax to transpose the 110 ohm feed Z to about 50 ohms. To answer your question about rolling up wire……I guarantee you it will become a pain in short order to go out and roll up, then roll out, then roll up, then….etc. BTDT. So, better to do it electrically. However, if you would roll up half the wire, say about 20 feet on each side you’d have a very long 40 meter (very reactive antenna) still not being able to match, pending the range of the tuner. Folding it back on itself would work too, as long as it is uninsulated wire and it becomes “one”. So, IMHO, best to handle it electronically. You concern over tuner loss is valid, but loss is happening even now. The MFJ is probably a Pi or T and they can be tuned for minimum loss. QST has a few articles about “Getting the most out of your T-Network Tuner” and Tuner losses/evaluation. They tell you the best way to set up the matching network. Keep up the good antenna experimentation. Great stuff. 73….W5RH From: BVARC [mailto:[email protected]] On Behalf Of Joshua Rose via BVARC Sent: Sunday, December 20, 2015 10:16 PM To: BRAZOS VALLEY AMATEUR RADIO CLUB <[email protected]> Cc: Joshua Rose <[email protected]> Subject: Re: [BVARC] Coiling ends of a dipole? 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] <mailto:[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] <mailto:[email protected]> http://mail.bvarc.org/mailman/listinfo/bvarc_bvarc.org
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