To: "qrp-l@mailman.qth.net" Subject: [QRP-L] Reel-Type Portable Antennas From: Jim Stephens Date: Fri, 17 Jan 2014 12:34:17 -0500 Portable antennas that feature reels from which wire is paid out until the desired length is reached are discussed from time to time. Does wire remaining on the reels have an effect - good or bad - on antenna performance (assuming half-wave dipole with some wire length unused)? Does unused wire remaining on the reels affect the calculations for resonant length (i.e. - shorter or longer than formula would otherwise indicate)? Can the reels be metal? (One design uses fly fishing spools at the center. I am thinking at outboard ends.) TU es 72! Jim Stephens, NX8Z Jim.NX8Z@gmail.com Hurricane, WV --- Subject: Re: [QRP-L] Reel-Type Portable Antennas From: Tom M Date: Fri, 17 Jan 2014 12:52:54 -0500 On 1/17/2014 12:34 PM, Jim Stephens wrote: Back in about 2000, I did an article for NJQRP club re same... ( http://www.njqrp.org/data/qrp_homebrewer.html ) Listed in issue 7 of the qrp homebrewer . As far as my testing went, I found no problems with the spools and the wire left on same... these were plastic camping clothesline spools. I pull out the wire to aprox lengths, put them on the analyzer for swr/etc and then marked off the lengths. The measurements were +/- the norm for diff elevations of the antenna... so rez length was fairly close to expected. Tom aa2vk --- Subject: Re: [QRP-L] Reel-Type Portable Antennas From: Les Gasser W9XC Date: Fri, 17 Jan 2014 12:05:24 -0600 Jim, I use reeled wire all the time as either elements of a dipole/inverted vee, or as the counterpoise element on a vertical Buddipole. The wire end attaches to the feedpoint and the reel end extends out to the field. The reel makes it very easy to tune a Vee or counterpoise to resonance. After adjustment, I just hang the reel end over a light plastic poke-in-the-ground electric fence pole as support. With the excess wire spooled up in the reel at the END of the antenna element (not at the feedpoint) the excess coil has no effect. I doesn't act as a loading coil for example. I've used the Coleman plastic reels (Google "Coleman clothesline reel"). I've also used metal reels made for chalk line (Google "chalk line reel" and look at the Irwin (metal) or Stanley (plastic)). Each of these comes apart to install your wire. Or you can let out most of the clothesline/chalk line, cut off all but a stub, affix your wire to the the stub and roll it up into the reel. All work great. With Wireman 534 26ga 'stealth' wire, these reels will easily fit enough for a workable 80-10M vee/doublet (two reels) or a counterpoise. - Les --- Subject: Re: [QRP-L] Reel-Type Portable Antennas From: David Ryeburn Date: Fri, 17 Jan 2014 20:58:35 -0800 At 12:05 PM -0600 1/17/14, Les Gasser W9XC wrote: > With the excess wire spooled up in the reel at the END of the > antenna element (not at the feedpoint) the excess coil has no > effect. I doesn't act as a loading coil for example. I disagree about "no effect", but only slightly. It certainly does not act as a loading coil. At the end of an antenna (high voltage, low current) the excess wire will not act as an inductor because no current is flowing through it (to speak of). But that hunk of metal will act as capacitive end-loading to a slight extent. The hunk of metal is small, so that's why it's only to a slight extent, and as those who have played with short antennas know, end-loading can be quite effective. If you adjusted the length of the antenna for resonance with the spool in place and then cut the spool off, I'd expect the resonant frequency to increase slightly. If you cut the end loading hat off of a short mobile whip with such a device attached up there, the resonant frequency increases, and more than slightly, but the hat is spread out over more space than your tiny wound up wire on the reel. Let's suppose you put the reels at the centre of a half wave dipole instead, connecting the feedline via alligator clips or something to the wires as they leave the reels (not at their inner ends on the reels). Current wouldn't be flowing through the wires on the two reels so there would be no inductance effect. There would be a bit of extra capacitance involved, just as with the reels at the ends of the antenna, but since the reels would now be located at a high current, low voltage point the effect of that capacitance would be even less than the slight effect it would have with the reels at the antenna ends. Actual measurements, anyone? I'm a mathematician, not a physicist or engineer, but when I suspect something is small, it's nice to measure it and find out for sure. If I had two of those reels, I'd do it myself. David Ryeburn, VE7EZM and AF7BZ --- Subject: Re: [QRP-L] Reel-Type Portable Antennas From: "Lee, KX4TT" Date: Fri, 17 Jan 2014 23:19:27 -0000 My understanding is that it acts like a lumped inductance and plays little part - I use plastic camping reels with both insulated and uninsulated wire -- it seems to make little difference. Lee KX4TT --- Subject: Re: [QRP-L] Reel-Type Portable Antennas From: Don Wilhelm Date: Fri, 17 Jan 2014 18:35:03 -0500 Let's see if the moderator will let me reply -- An inductor at the end of a resonant 1/2 wave dipole will not have a great effect (yes it will have a little), but move that inductor closer to the center of a dipole, and it will have a greater effect the closer it is toward the center - I am talking about a center fed dipole, not OCF. Consider a loading coil for a mobile antenna (or a vertical) - if a loading coil is placed at the base, the inductance to produce resonance will be lower than that required of a center loaded antenna. If an inductor is placed at the top of the whip, it must be huge by comparison to tune the whip to resonance (without a top hat considered). The effective capacitance presented by the whip beyond the inductor makes the difference. The effective capacitance is reduced as you move further away from the feedpoint. Once you move beyond the 1/4 wavelength point, the effective capacitance again begins to increase. 73, Don W3FPR --- Subject: Re: [QRP-L] Reel-Type Portable Antennas From: Albert Woodhull Date: Sat, 18 Jan 2014 12:59:31 -0800 (PST) I have an RC-432/G military surplus antenna reel. It is part of the AN-GRA-50 antenna system which includes two of these, end and center insulators, coax, halyards, and a storage bag. A 56 page manual can be found online if you Google either of the designations. The complete set includes a tape measure which is calibrated in frequency as well as feet and inches, but the manual also gives the standard 468/f formula for the length of a dipole. I take this to mean that no correction is needed for the extra capacitance of a fairly big hunk of metal at the outer ends. This hunk of metal is composed of the reel hardware and the wire in excess of the amount needed for each half of the dipole. The antenna is intended for use at any frequency from 1.6 to 20 Mhz. At the high frequency end there would be 148 feet of the total 160 foot length of wire still wound on the reel. 73, Al, N1AW Leyden, Massachusetts