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Elevated Screwdriver And Radials?
I am wondering if anyone has used an elevated screwdriver and elevated
radials? I wish to put this up at 22 ft. Above the garden. With at least 1/10 wave radials at 40 meters. Any suggestions? Wing |
Elevated Screwdriver And Radials?
On Sep 24, 7:38*pm, WING wrote:
I am wondering if anyone has used an elevated screwdriver and elevated radials? I wish to put this up at 22 ft. Above the garden. With at least 1/10 wave radials at 40 meters. Any suggestions? Wing -- WING The radials need to be tuned when elevated. If you used 1/10 wave radials, they would have to be loaded to be 1/4 wave electrically, and I wouldn't expect the performance to be near as good as full length 1/4 wave radials. And.. the radials need to be tuned for each frequency you operate. Which usually means multiple elevated radials for each band to be used. Myself, I would try to use a longer radiator as well, but a screwdriver will work.. A longer radiator would work better. I had a full size 1/4 ground plane up for a good while. 32 ft radiator with four 32 ft radials. The base of the radiator was at 36 ft, and the radials sloped down at about a 45 degree angle. I built the radiator from an old 5/8 CB antenna that I reinforced with inner tubing and lengthened to 32 ft. It was totally self supporting and require no radials. Pretty light too. I don't have it up now, but I still have the antenna on the side of the house. To avoid radials of any real length the best bet is usually to go with some type of loaded half wave design. IE: Cushcraft R5, etc.. |
Elevated Screwdriver And Radials?
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Elevated Screwdriver And Radials?
On Sep 26, 9:25*pm, Richard Clark wrote:
On Sun, 26 Sep 2010 18:24:39 -0700 (PDT), wrote: On Sep 24, 7:38*pm, WING wrote: I am wondering if anyone has used an elevated screwdriver and elevated radials? I wish to put this up at 22 ft. Above the garden. With at least 1/10 wave radials at 40 meters. Any suggestions? Wing -- WING The radials need to be tuned when elevated. If you used 1/10 wave radials, they would have to be loaded to be 1/4 wave electrically, and I wouldn't expect the performance to be near as good as full length 1/4 wave radials. And.. the radials need to be tuned for each frequency you operate. Which usually means multiple elevated radials for each band to be used. Myself, I would try to use a longer radiator as well, but a screwdriver will work.. A longer radiator would work better. I had a full size 1/4 ground plane up for a good while. 32 ft radiator with four 32 ft radials. The base of the radiator was at 36 ft, and the radials sloped down at about a 45 degree angle. I built the radiator from an old 5/8 CB antenna that I reinforced with inner tubing and lengthened to 32 ft. It was totally self supporting and require no radials. Pretty light too. I don't have it up now, but I still have the antenna on the side of the house. To avoid radials of any real length the best bet is usually to go with some type of loaded half wave design. IE: Cushcraft R5, etc.. Hi Mark, Of late it seems most questions are of the "drive-by" category with no apparent interest by the OP for responses. However, I know full well you will engage in dialog - so here goes: There is no particular virtue in having the radials being resonant on any band of operation IF the entire antenna is being tuned. *And such it would be with an elevated screwdriver. Well.. Not quite. But I guess it depends how one is feeding it. Normally it's tuned by varying the number of turns on the loading coil, which is motor run. I assume he will have the motor control at the shack. Normally for the lower bands, you would also use some impedance matching for a better match to coax. But that coil or whatever, would normally be at the base of the radiator. He's basically running a normal everyday ground plane, except that the radiator is short and coil loaded. So I don't see it as tuning the whole antenna per say. I still think the radials would need to be resonant to be effective, and also do a semi decent job of decoupling the feedline. It then becomes something of an academic issue: why not discard the radials and simply think of the elevated Screwdriver operating as a vertical dipole with one end grounded (where the coax rises from below where the shield is attached to suitable ground treatment)? *Just sit on the tune button until the ad-hoc VSWR match is found. *That's how Screwdrivers are tuned anyway, aren't they? Humm.. sorta.. But I'd rather have a lower 1/4 wave section to act as the lower half of the dipole. Or have radials sloping down at a steep angle to require less room. If it's high enough anyway.. But the usual screwdriver is just shorting out turns in the coil to adjust. And the impedance matching is usually fixed and fairly broad banded in operation. So when you tune the coil to resonance, the match will generally be good for pretty much the whole band in most cases. I've never really used a screwdriver, even though I have one sitting in the garage.. I preferred my plastic bugcatchers so I could use a higher coil location for more efficiency and better current distribution. But what he wants to do, I consider just an everyday ground plane, except he can vary the coil loading from the shack. I'd still rather have the full size ground plane myself. I'm a radio bully.. :) The one I used really worked well for long hauls on 40 at night. It would smoke my dipoles to DX. The longer the path, the better it did. The break even point where the vertical and dipole were about equal was generally 700-800 miles or so. Anything farther, and the vertical got better. At 1500 miles, usually 2 S units over the dipole.. To VK land, 4 S units over the dipole. The farther you worked, the better it did vs the dipole. "I know the S meters are not accurate, but good enough for A/B comparisons.." The elevated version at 36 ft and four radials did much better than the same 32 ft radiator ground mounted with 32 radials. The improvement elevated was like day and night. I calculated my four radials at 36 ft were probably about equal to 60 or so on the ground. It was very effective for a simple antenna. The only state side guys that would regularly beat me were either running bob tail curtains, or yagis's.. They had to be running multi elements to win. Course, I usually helped things by running about a KW back in those days.. :) Don't want to strain anyone's ears over there. :/ |
Elevated Screwdriver And Radials?
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Elevated Screwdriver And Radials?
Richard Clark wrote:
That was my point about the screwdriver being run as a elevated vertical dipole. So why not just use two screwdriver antennas with each antenna being used as half of the "dipole"? Since Don Johnson was mentioning this being done in the 90's by apartment dwellers, it's obviously been tried before and may even work. It likely won't be the most efficient at getting the most RF out but likely will work. I note incidentally that homebrew screwdrivers would drastically lower the cost of this experiment. |
Elevated Screwdriver And Radials?
Art Clemons wrote:
Richard Clark wrote: That was my point about the screwdriver being run as a elevated vertical dipole. So why not just use two screwdriver antennas with each antenna being used as half of the "dipole"? Since Don Johnson was mentioning this being done in the 90's by apartment dwellers, it's obviously been tried before and may even work. It likely won't be the most efficient at getting the most RF out but likely will work. I note incidentally that homebrew screwdrivers would drastically lower the cost of this experiment. Sure, you could do this, but there's really not much advantage over a single screwdriver and a fixed element opposite it. Either way, you're basically putting an adjustable inductor in series with the feed. Whether it's symmetric is almost immaterial, especially for a short (relative to wavelength) radiator. Short antennas look capacitive. Putting in a series L compensates for that, but doesn't do anything about the low resistance at the feedpoint. What most mobile screwdrivers do is put a 4:1 transformer at the feed.. that gets a nominal 12.5 ohms up to 50, the inductance takes care of the reactive component, and you're "good enough". In fact, it's typically even more clever, the 4:1 transformer has a fair amount of leakage capacitance, so at higher frequencies, it looks more like a 1:1, which works out nicely, since the feedpoint R is closer to 50 ohms with a longer (in terms of wavelength) radiator. More details and measurements he http://home.earthlink.net/~w6rmk/ant...crewdriver.htm I think there are better ways to do a space limited antenna. |
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