|
Mobile antenna question
I'm putting together a homebrew mobile antenna based more or less on
"20 dollar mobile antenna" that appeared in QST some years ago. For those not familiar, the antenna consists of a tapped coil at the bottom, a spiral wound lower section, a tapped loading coil, and an upper stinger. My question involves that spiral wound lower section. What would this be thought of? As antenna below the loading coil, as an inductor in addition to the other two, or some sort of hybrid of antenna/inductor? How would this be modeled in EZNEC? Apologies in advance if this was an incredibly stupid thing to ask! 8^) - 73 de Mike KB3EIA - |
Mobile antenna question
On Jan 24, 12:32*pm, Michael Coslo wrote * * * * For those not familiar, the antenna consists of a tapped coil at the bottom, a spiral wound lower section, a tapped loading coil, and an upper stinger. * * * * My question involves that spiral wound lower section. What would this be thought of? As antenna below the loading coil, as an inductor in addition to the other two, or some sort of hybrid of antenna/inductor? * * * * How would this be modeled in EZNEC? * * * * Apologies in advance if this was an incredibly stupid thing to ask! 8^) * * * * - 73 de Mike KB3EIA - Hi Mike, I built one similar to this myself. I always assumed the spiral wound lower section added some distributed inductance that would reduce the amount needed for the upper loading coil. The inductance added by the spiral section could be modeled by adding a couple of inductive loads on the lower section. The effect of the loads will be to change the current distribution slightly on the lower section of the antenna. The spiral bottom does radiate. The goal on this type of antenna is to adjust the center loading coil such that the antenna looks capacitive, and adjust the bottom loading coil to cancel Xc. Hopefully your impedance now looks like 50 ohms resistive. The spiral and center loading attempt to move the current distribution up the antenna so that more radiation occurs on the upper parts. When I modeled mine, I only had a center inductive load, and the results were satisfactory. My goal in modeling was to design matching networks, I wasn't too concerned about current distribution. Also had a top hat, modeled as crossed wires. Later discarded top hat, not aerodynamic. Homebrew mobile antennas IMO need center loading, no top hat, can accept power (loads up), and will stay attached to your vehicle at 80mph in a driving rainstorm. Gary N4AST |
Mobile antenna question
|
Mobile antenna question
Michael Coslo wrote:
I'm putting together a homebrew mobile antenna based more or less on "20 dollar mobile antenna" that appeared in QST some years ago. For those not familiar, the antenna consists of a tapped coil at the bottom, a spiral wound lower section, a tapped loading coil, and an upper stinger. My question involves that spiral wound lower section. What would this be thought of? As antenna below the loading coil, as an inductor in addition to the other two, or some sort of hybrid of antenna/inductor? How would this be modeled in EZNEC? Apologies in advance if this was an incredibly stupid thing to ask! 8^) - 73 de Mike KB3EIA - Hi Mike, Anything other than a straight conductor would add inductive loading, with less (if any) contribution to radiation. Close spacing of the inductor turns will add distributed capacitance. The closer to the feedpoint, the greater the I^2*R loss and the larger conductor should be, to keep the loss as low as possible. So, electrically, the best place for loading is at/toward the top end. My mobile antenna setup consists of Hustler/Newtronics parts... but not quite out-of-the-box. For 20m, I use their RM15 (15m) loading coil + a 49" whip with about 6" cut off of it. The whip came from Larsen (pn W490). Though this effectively moves the loading coil toward the high-current end, it adds considerable radiating surface area. When the band was in, it was good enough to work the UK (using 100W) from my QTH near Seattle! I haven't (yet) tried an RM12 loading coil with an even longer W640 (64") whip. My setups for the other HF bands are similarly taller than the Hustler setup... the 75m antenna is quite tall! :-)) Bryan WA7PRC |
Mobile antenna question
On Jan 24, 12:32 pm, Michael Coslo wrote: I'm putting together a homebrew mobile antenna based more or less on "20 dollar mobile antenna" that appeared in QST some years ago. For those not familiar, the antenna consists of a tapped coil at the bottom, a spiral wound lower section, a tapped loading coil, and an upper stinger. My question involves that spiral wound lower section. What would this be thought of? As antenna below the loading coil, as an inductor in addition to the other two, or some sort of hybrid of antenna/inductor? How would this be modeled in EZNEC? Apologies in advance if this was an incredibly stupid thing to ask! 8^) - 73 de Mike KB3EIA - Basically the same as my "plastic bugcatchers" I make out of old CB and ham fiberglass antennas. The spiral just adds a bit more inductance. How tight, and how many turns would determine the overall loading. IE: just a few spiral turns won't do much for 80m. Your bigger lumped coil is going to be doing most of the dirty work. The spiral will just add a bit. But on the higher bands like 10m, that spiral could have quite a bit of effect on tuning. Myself, I don't use the lower spiral section. Those smaller coil turns are more lossy than using the bigger lumped coil. It's really best to just use a straight lower section, and have all the loading at the center lumped coil. That coil is higher Q, and more efficient. You don't want to degrade things by using the lower Q lower spiral windings, when you have a big coil above those. My present best antenna is a 6 ft 20 meter hamstick that I butchered up. I did away with all the thin coil windings, but kept enough wire so I could have a fairly straight section at the bottom. I'll wrap maybe a turn or two just to keep it in place better, but I don't consider it part of the coil. I wanted max performance. So..Instead of placing the coil at the center of the fiberglass whip, I mounted it nearly at the top. The top one foot of the six foot whip is where the coil is. Then I have a 5 ft stinger whip attached to the top of the glass whip. That makes it a true center fed, with 5 ft under and above the coil. The tapped coil at the bottom is the matching device. That can vary to the install.. The higher you can mount the loading coil, the better, unless maybe the stinger is too short. I don't have that problem with a 5 ft stinger. The antenna is a stout talker. Even on 80m. And didn't cost me a cent. All was junk I aquired... BTW, for added effect, I often extend the base below the coil with a 3 ft hustler mast. In that case, I have 8 ft below the coil, and 5 above. In that "parked" mode, it really kicks butt. Mine tunes 80-17 with the 5 ft stinger. On 15-12-10, I use shorter fixed length stingers to tune, with the coil bypassed. Naturally, the usual low band 11 ft antenna is a bit tall for the highest bands.. :/. I actually don't work the higher bands much. I'm on 40 in the day, 80 at night most all the time. But I can work all the others if I want to. MK |
| All times are GMT +1. The time now is 10:10 PM. |
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
RadioBanter.com