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1-7 of 7 messages
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DIPOLE TUNING
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by N5RMQ on August 14, 2003
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In tuning a dipole antenna, do you count the wire that is wrapped back around the antenna as length, or just the wire that is between the end insulators? Thanks.
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RE: DIPOLE TUNING
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by WB2WIK on August 14, 2003
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That has nothing to do with "tuning," but I understand the question.
Unless you're speaking of VHF-UHF dipole antennas which are very short, so that inches will greatly influence resonant frequency, it really doesn't matter.
Technically, if you make a dipole using insulated wire (which I strongly recommend for anything except temporary use), when you "wrap the end" back on itself, the wire that is doubled back is still part of the antenna and contributes to overall length. If you make the dipole from bare (non-insulated) wire, the doubled back portion only adds to antenna length if it doesn't contact the wire between the center feedpoint and the end insulator.
However, again, this isn't very significant unless you fold back an awful lot of wire. Normally, folding back just a few inches (or several inches) and wrapping it around the main element wire is sufficient to hold up an antenna for a lifetime, and if in doubt, you can tie the wire in a knot, solder it or otherwise mechanically secure it so nothing can slip. With a 10 meter dipole, a 1 MHz change in resonance amounts to a dipole length change of seven inches. On 80 meters, a 500 kHz change in resonance (as wide as that whole band is!) amounts to a dipole length change of two hundred inches (!!) -- so obviously, a few inches one way or another is pretty meaningless.
This subject involves "fabricating" the antenna, though, and not "tuning" it. "Tuning" a dipole implies changing its length to adjust for actual resonance (not calculated, but measured) once the antenna is installed. Due to earth proximity and end effects, most dipoles are "too long" if fabricated according to the formula and will have to be adjusted "shorter" to achieve desired resonance. The influence of the earth below and dielectrics close to the tips of an HF dipole influence its length MUCH more than any extra wire wrapped back on itself at each end insulator.
WB2WIK/6
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RE: DIPOLE TUNING
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by K5ENA on August 14, 2003
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I have always measured my dipoles, basically, as the length between the insulators. More specifically, where the wire goes into the insulator hole on one of of the dipole to the insulator hole on the other end of the dipole. You always end up "pruning" the antenna for the lowest swr anyhow, so it is not that critical. An exact measurement is not that critical either for those folks who use antenna tuners.
Good luck.
John, K5ENA
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RE: DIPOLE TUNING
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by W7WIK on August 14, 2003
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Just the wire between the end insulators. The antenna will be affectd by the local environment. Therefore it's a good idea to make it a little long to start with, then trim a little at a time until it comes to resonance. It's a lot easier to cut wire than add wire!
The easiest way to check and adjust the antenna is to use an SWR/antenna analyzer. If you don't have one or can't borrow one, you'll have to use the tried and true method of using an SWR meter and transmitter running at the lowest power setting that will work with your meter. Use a continuous mode such as CW or FM. SSB won't work. Keep transmissions very short and use the lowest power possible.
Check SWR above the frequency you wish to use and below. If the SWR is high above the wanted frequency and low below the wanted frequency, you'll have to trim the antenna - the antenna is too long. Take measurements after trimming just one side, then the other. Sometimes due to local conditions or to get a better match to the coax, the antenna will be slightly longer on one end. Keep trimming and testing until you achieve the lowest possible SWR for the frequency you're likely to use.
Hope this helps, 73,
Marco, W7WIK
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RE: DIPOLE TUNING
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by WB6BYU on August 14, 2003
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Actually I'd suggest that you NOT put the insulator
at the end of the wire, in spite of all the neat
drawings in the antenna books.
Calculate the desired length using the standard formula
and add a bit for good measure (to make sure it is too
long.) Then slip the end of the wire through the hole
in the insulator some distance further than normal and
tie/twist it off at that point. This leaves some of
the antenna wire hanging down from the insulator.
(How much? I'd probably try a foot/30cm on the higher
bands or 3'/ 1m on 40m for starters.)
Now when you go to tune the antenna by adjusting the
length, you don't need to retie the insulators each
time you trim the wire.
And another heresy: I rarely use insulators on the ends
of my antennas anyway. Or at least, not discrete pieces
of plastic with holes drilled in them. If you are using
synthetic rope (nylon, Dacron, or my favorite orange
plastic baling twine) you already have several feet
or metres of insulator attached to the wire. Why would
adding an extra few inches/cm of insulator make any
difference?
Certainly I would consider using them if I were running
a kW, or the antenna was subject to salt spray, or
other such cases. But for a common dipole running 100w
I haven't found them necessary.
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RE: DIPOLE TUNING
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by N2MG on August 15, 2003
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WB6BYU wrote:
<<
And another heresy: I rarely use insulators on the ends of my antennas anyway. Or at least, not discrete pieces of plastic with holes drilled in them. If you are using synthetic rope (nylon, Dacron, or my favorite orange plastic baling twine) you already have several feet or metres of insulator attached to the wire. Why would adding an extra few inches/cm of insulator make any difference?
>>
If the rope gets really wet, it can act like a conductor. A real insulator (plastic or ceramic) has less of this problem.
Most of time you probably wouldn't notice the difference.
Mike N2MG
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RE: DIPOLE TUNING
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by N2MG on August 15, 2003
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In general I would ignore the difference between the actual length of wire (including the length wrapped around) and what the effective length might be. Cut the antenna long and experiment. Proximity to ground and other factors will likely have more effect than an inch or two wrapped around an insulator.
However, you can use the "fold" to your advantage:
I have built several 80 meter inv-vees over the years (all insulated wire) on which I folded (or unfolded)back several feet of the ends so that I could get it to resonate in either the phone band or the CW band. I simply lowered the antenna enough to reach the ends, then pulled some of the wire through the insulator, folded the end of the wire back up the vee toward the feed point, and twisted it around to hold it in place. This shortening of the effective length allowed an antenna originally cut for 3.550MHz to resonate at around 3.800MHz. After some experimenting I marked the wire with tape and I could switch segments in about 15 minutes (25 mins in the dark). Eventually, I attached an extra insulator up each leg of the antenna: then I had not only a better place to which I could attach the folded end, but it was easier to find in the dark.
Mike N2MG
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