“Zip Cord” Antennas for Portable Applications
Introduction
I have been investigating the use of “zip cord” for dipole antennas and parallel-conductor feedlines. This is a subject that has been discussed previously in the Amateur Radio literature, and I believe that my results are in general agreement with the previous work.
The approach that I will describe here is to use the zip cord as a half-wavelength feedline, so that the dipole’s impedance is repeated at the transmitter end of the feedline. Because of this, the characteristic impedance of the feedline is of secondary importance. Because the feedline is relatively short, losses are minimized, although this means that it is not possible to erect the dipole at “optimum” height. However, since the goal of this project is to create light-weight, compact, “pocket sized” antennas that can be used in the field with temporary supports, great antenna height is not an important consideration.
The specific wire that I have investigated is Radio Shack’s No. 278-1385, #22 Speaker Wire, which is sold in 100 foot rolls. Do not confuse this with #24 speaker wire. I investigated the latter product briefly and found it to have somewhat different characteristics. These differences do not render the #24 wire unusable, although I did find it to be somewhat more lossy than the #22. My measurements on the #24 product were not as extensive as those that I made on the No. 278-1385 wire, and reported only briefly here.
All electrical property measurements were made using an Autek Research Model VA1 Vector RX Antenna Analyst. Formulas and physical property information were taken from the ARRL Antenna Handbook, the ARRL Electronics Data Book, and the VA1 Instruction Manual.
Characteristic Impedance
This is a property that I am not able to measure directly with my instrument, although I can present (and support) some “educated guesses.”
The measured center-to-center distance (S) between the conductors of Radio Shack No. 278-1385 Speaker Wire is approximately 0.082 inch, and the conductor diameter (d) of #22 wire is given as 0.0253 inch. For parallel conductors with air dielectric, the characteristic impedance is given by:
Z = 276 • log(2S/d)
Z = 276 • log(2x0.082/0.0253) = 224 ohms
Again, this is for air dielectric. Note that a similar calculation for “300 ohm” twin lead yields about 400 ohms, meaning that the polyethylene insulation reduces the calculated value by a factor of about 0.75. Next, note that for coaxial cable the characteristic impedance may be calculated by multiplying the “in air” value by the inverse square root of the dielectric constant for the particular insulation being used. For polyethylene the dielectric constant is 2.3, so the adjustment factor is about 0.65. The type of insulation used on the Radio Shack speaker wire is unknown to me, but I have made the assumption that it is polyethylene or a similar material (many plastics appear to have a similar dielectric constant). In addition, since the insulation between the conductors in the speaker wire is thicker (transverse to the wire’s axis) than is the insulation for 300 ohm twin lead, I have assumed that it must have a greater (reducing) influence on the characteristic impedance. From these assumptions I have further assumed that a not unreasonable adjustment factor for the characteristic impedance is about 0.7. Thus, the characteristic impedance for this line might be:
Z = 224 • 0.7 = 157, or about 150 ohms.
This value is not particularly important, but will be used later in the section on Loss.
Velocity Factor
The velocity factor of the line was measured using the VA1. The technique involves shorting the end of the line, then sweeping the instrument over a range of frequencies to find the lowest impedance at several points. The first will be the frequency at which the line is a half wavelength, the second will be two half wavelengths, etc. The velocity factor is then calculated by the ratio of the line length to the value of a half wavelength in vacuum at the particular frequency (given by the formula L = 492/f, where L is in feet and f is in MHz). The results for my roll of No. 278-1385 speaker wire (which measured 102 feet in length) were:
Frequency Velocity Factor
3.31 0.68
6.75 0.69
13.67 0.70
27.77 0.71
Since the frequencies of greatest interest are in the 20 meter band and above, I chose to use 0.70 as the velocity factor of this line. (I could just as well have said that I picked 0.70 because it is a round number. Note the approximate 4% variation if velocity factor over an a frequency range of about an order of magnitude. This is probably an artifact of the instrument and measurement technique. The results are amazingly consistent considering the use of a "cheap" handheld instrument.)
Loss
The line loss was also measured using the VA1. For this calculation, the series of minimum impedance measurements taken during the frequency sweep (see Velocity Factor, above) are applied to the formula:
Loss = 8.69 • Zminimum/Zcharacteristic
which is then adjusted by the factor of (100/102) to correct the result to the standard reporting value of loss in dB per 100 feet of line. (Remember, my roll was 102 feet long.)
The calculated loss is presented in the table below, and is compared with values for RG-174 and RG-58. (The latter were taken from the log-log graph presented in the ARRL Antenna Handbook, which was read as closely as possible.)
Frequency Loss RG-174 RG-58
3.31 0.97 2.7 0.8
6.75 1.48 3.3 1.2
13.67 2.39 4.0 1.6
27.77 3.41 5.3 2.4
As frequency increases so does loss, but the length of a half wavelength feedline decreases. As a result, the feedline loss remains less than 1 dB for the particular application that I have described over the entire range of the Amateur HF bands. (In fact, loss decreases from about 1 dB at 80 meters to about 0.5 dB at 10 meters.) If the assumption of a 150 ohm characteristic impedance is correct, then the SWR on the line will be about 3:1, which introduces at most an additional 0.7 dB of loss. Total feedline loss is then on the order of 1.5 dB. It is significant to note, I believe, that although the line is only slightly more lossy than RG-58, it is closer in size and flexibility to RG-174. This is an important consideration given the intended use. Because the length of a half wavelength of line decreases faster than the loss for a given frequency increases over the frequency range of interest, a reasonable approach might be to make the feedline a full wavelength long on the higher bands in order to allow an increase in antenna height. Whether the increase in height offsets the increase in loss remains to be determined (feel free to experiment).
[Spring 2002 Update] The idea of making the feedline one-wavelength long has been tested by constructing a speaker-wire dipole for 15 meters. Results were satisfactory.
As an aside, I would note that my measurement of the #24 speaker wire indicated a loss of about 6 dB per hundred feet at 30 MHz, which compares well with RG-174. The loss of the Radio Shack No. 278-1385 speaker wire also appears to be lower than that for “lamp cord” previously reported in the literature. ("Zip Cord Antennas -- Do They Work," by Jerry Hall, K1TD, available on the ARRL Members Only Web Site.)
Construction
In order to construct a single-band dipole and feedline using this wire, the antenna length is calculated using the formula La = 234/f, and the feedline length is calculated using the formula Lf = 492/f. Note that the dipole uses the quarter wavelength formula since the wire will be split to form the two halves of the antenna, and is reduced by 5% for “end effects.” The feedline formula is NOT reduced by 5%: 234 = 0.95 • (492/2). A single piece of wire of length L = La + Lf + X is then cut, where “X” is “a couple of feet added to the cut length for margin of error.” The feedline length (Lf) is then measured and marked, and the remainder of the wire (which includes the extra length for margin of error) is “unzipped” to make the dipole section. An electricians’ knot should be tied at what is now the junction of the dipole and the feedline.
At the transmitter end of the feedline, the wire is “unzipped” a couple of inches. A banana plug is attached to one side, and an alligator clip is attached to the other. In operation the banana plug will be inserted in the transmitter’s SO-239 connector, while the alligator clip will be attached to the transmitter’s ground connection at any convenient point. (On the FT-817 this might be the heat sink, for example.) Note that the issue of feeding a balanced load with an unbalanced source has been ignored.
For the initial test of the dipole, measure and mark the calculated antenna length (La). Fold the wire back on itself at this point and tape in place. In addition to shortening the dipole while leaving the option of lengthening it without splicing and soldering, you have created a convenient attachment loop. Tie a piece of light nylon line to the loop on each end, and proceed to deployment.
Deployment
I will describe the configuration that I have found to work for me while testing Speaker Wire Dipoles on 17, 20, and 30 meters. You are encouraged to experiment and find what works best for your particular application. Please report any refinements and improvements on HFpack and/or Live-Wire.
Initial testing was done by installing these antennas in an inverted-vee configuration with the apex at about 20 feet. This was done using either a telescoping fishing pole, or by tossing a line over a tree branch. The ends of the dipole were brought down to within six to eight feet of the ground and tied off with nylon line using a sheet bend, and then tied to tent stakes. The dipole was then pruned to resonance using the VA1 by changing the fold point at the end -- the “extra” wire was left in place and was not trimmed off. The 20 meter and 17 meter antennas were also tested as indoor dipoles, and were easily tuned to resonance.
In practice, I found that once the antenna was tuned for resonance it was possible to adjust and optimize the feedpoint impedance by changing the azimuthal angle between the two legs. In my particular installation the best match was found with the dipole legs arranged at an azimuthal angle of between 90 and 120 degrees. For indoor applications the feedpoint impedance was found to be adjustable by changing the amount of droop in the legs, proximity to walls, floors, etc., and the angle between the legs. As is typical of parallel wire feeders, the feedline should be kept clear of other objects and should be kept equidistant from both legs of the dipole to the maximum extent practicable.
[Spring 2002 Update] I have found that the 15 meter dipole can be used on 17 meters, and the 17 meter dipole can be used on 20 meters by connecting them to the rig through the Fishing Pole Tuner. I was not successful in tuning either antenna to a higher band, and could only get them to work on the next lower band.
Performance
Antenna performance was evaluated subjectively based on on-the-air contacts using SSB and power levels between 2.5 and 5 watts. No comparisons with other antennas were attempted. It is my judgment that these antennas performed acceptably.
KR8L/7
March 29, 2002 (Updated May 19, 2002)