Updated 26 September 2001 URL is http://our.tentativetimes.net/opine/antenna.html
ow that you have upgraded to Tech Plus, you want to do some operating on the HF bands. You need an antenna and don’t know what to buy or build. Since you have operating privileges on four HF bands, you probably want a multiband antenna of some sort.
s we are at the point of minimum solar activity in the 11 year sunspot cycle, propagation is poor during the hours of darkness on the 10 and 15 meter bands. The segment of 40 meters which you may operate is very active during daylight hours but full of foreign broadcast stations at night, making it nearly unusable after dark. The 80 meter band is pretty much the only band usable for you at night. During daylight you will usually find some activity on all four of these bands but your effective communication distance will be different from band to band.
ome amateurs favor coax fed half-wave dipole antennas but to operate multiple bands, this requires multiple antennas with two exceptions. A forty meter dipole will usually work fairly well on the 15 meter band. The other exception is the multi-band trap dipole antenna. While these can be home-brew, the traps are rather difficult to build and adjust so most amateurs use commercial versions. These are basically one-half wave coax fed dipoles for the lowest frequency band used with one or more traps (tuned circuits) in each half of the antenna wire. These traps electronically isolate sections of the antenna at higher frequencies thus making the antenna resonant on these other bands.
here are a variety of multiband beam antennas available that cover the 20, 15, and 10 meter bands. They are directional antennas thus most of your radiated power can be concentrated in a single direction. They also will help significantly on reception. They do have the drawback that when you consider that you must also invest in a tower or mast and an antenna rotor, you can have quite a lot of money invested in a beam antenna system, and you still have no antenna for 80 and 40 meters.
here are several multiband vertical antennas commercially available. The R-7000 by Cushcraft and a similar antenna manufactured by Hy-Gain cover all the bands between 40 and 10 meters including the three WARC bands. Both have an optional 80 meter add on. While expensive, this system would cost significantly less that a beam antenna system.
he antenna I use is a non-resonant multiband dipole and works reasonably well on all the HF amateur bands from 160 meters to 10 meters. A misconception is that an antenna must be of a resonant length to function and this is simply not so. This stems from the fact that most all ham radios have a 50 ohm output impedance. It so happens that a one-half wave resonant dipole has a feedpoint impedance of something in the neighborhood of 50 to 70 ohms at or near the antenna resonant frequency, thus a one-half wave dipole fed with 50 ohm coaxial cable usually presents a pretty good match to common amateur radios.
ny piece of wire will radiate RF provided that the power can somehow be fed into it but the impedance will probably be something grossly different than 50 ohms. My antenna happens to be 102 feet long fed at the center feed point with 450 ohm ladder line. We have used similar antennas for a number of field day setups with the difference being that 300 ohm television twinlead was used as the feedline.
ome might ask how this antenna can work. If you figure out the math of it, this would be one-half wave dipole on 4.6 MHz which is not even a ham band. On none of the ham bands will this antenna present a 50 ohm impedance and it probably won’t even match the 450 ohm or 300 ohm feedline either. As a consequence of all of this, the SWR (standing wave ratio) on the feedline is going to be quite high most of the time.
e have all been indoctrinated that this is a sin but is it really? 450 ohm open wire balanced feedline and 300 ohm TV twinlead as well are practically lossless feedlines on the HF bands. Coaxial cable is not and the higher the SWR, the greater the power lost and dissipated as heat in the feedline. As a matter of interest, RG-58 coax has an attenuation (loss) of 2 db per 100 feet at 21 MHz and a loss of about 2.6 db per 100 feet at 30 MHz while the larger RG8 has a loss of 1 db per 100 feet at 30 MHz. Common TV 300 ohm twinlead has a loss of about .4 db per 100 feet at 30 MHz and 450 ohm ladder line has a loss of about .16 db per 100 feet at 30 MHz.
one db loss may not seem significant but what it means is that if you put 100 watts into a system with a 1 db loss, about 80 watts will come out. A 2 db loss with 100 watts in means that about 63 watts are coming out. Bear also in mind that this line loss is only true in a matched system (the antenna feedpoint impedance, the feedline impedance and the transmitter output impedance are all the same). When such is not the case, there is an additional loss due to the mismatch as a function of the SWR and line attenuation.
or example, say we are using a one-half wave 40 meter dipole fed with RG 58 on the 10 meter band. At 30 MHz the line loss is 2.6 db. Let us say we have a SWR of 4. At an SWR of 4 with a matched loss of 2.6 db there will be an additional loss due to the mismatch of 1.5 db for a total of 4.1 db loss. If we had 100 watts going into this system we would have only 39 watts arriving at the antenna. Sixty-one watts have been lost in heating the coax cable. On the other hand using 450 ohm ladder line with a matched loss of .16 db, the additional mismatch loss if we had an SWR of 4 would be only about .15 db for a total of .31 db loss. If we had 100 watts going into such a system, we would have 93 watts getting to the antenna. We have lost only 7 watts in the line.
he point is that with balanced feedline, if the power can somehow be put into it, nearly all of it will get to the antenna and be radiated. The way we manage to do this is with an antenna tuner or a transmatch that changes the 50 ohm unbalanced output of the radio to whatever the balanced impedance happens to be at whatever frequency we happen to decide to operate. Consequently, the radio is happy as it is seeing a 50 ohm load and the output power is getting into the feedline and from there most of it gets to the antenna.
oes this kind of an antenna have any drawbacks? Well, yes and no. An antenna tuner or transmatch must be used with this antenna and some hams would consider this to be a major disadvantage. Myself, I use a tuner with any HF antenna I operate. Back before we moved to town, I used one with my tri-band beam. I had assembled the beam give me the best match in the CW portion of the band so when I went to the phone portions, I used the tuner to adjust so that the rig would see a 50 ohm impedance.
ith modern solid state rigs, as the SWR begins to deviate from a matched condition, the power output is reduced to prevent damage to the output transistors. To make matters easier, I keep a listing of tuner settings for a frequency I operate in each band or band segment so when I change bands I can preset the tuner and then only make a minor adjustment for the best match.
urrently, I am using a tuner manufactured by MFJ. In the past, I have used several home brew tuners with this type of antenna and they worked quite well. Perhaps construction of a tuner would be suitable for a future article.
hile not the ultimate in antenna systems, the type described is a very versatile multiband antenna and inexpensive to build. For the field day antennas we used pieces of scrap ½" or ¾" PVC water pipe to make the center and end insulators. The antenna wire itself was scrap insulated #12 or #14 insulated copper building wire that had been pulled from conduit at the factory where I was working at time. Yes, insulated wire will work fine for an antenna. The only real cost was the TV twinlead which now costs about $10.00 per 100 feet. Ladder line would cost a little more but still not an outrageous expense.
f anyone would like to build and use such an antenna and has questions about it, you may call me or you can e-mail me firstname.lastname@example.org
article ©1996 Wm. J. Weinhardt
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