QRP Antenna
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I have enjoyed the occasional queries and responses from this august group on the subject of antennas and the answers have added greatly to my understanding. I have yet to see much advice on the closeness of buildings, hence my query today. This is a rather lengthy message, so if you'd like to skip it, I certainly understand.
Given that at QRP antenna considerations are paramount, I installed a commercial multiband inverted V at my QTH and have had poor results for DX. It looks like a Windom design with the feedpoint off-center and comes with a balun to allow for coax feed. According to the specs, it should provide a match at 80, 40, 20, 17, 12, and 10, and the ATU within my K1 matches fine on 40, 20, and 17. (I've also had no problems matching other rigs using their ATU to the other frequenccies). I am using a low loss coax from Davis RF and the rig(s) are grounded to a ground rod just outside the shack. The apex of the antenna is about 40 ft with each leg at the approximate height about ground that the manufacturer's specs require. It is strung in a NE/SW direction -- broadside to southern Africa. The antenna seems to 'hear' fine; it's in the transmitting that it appears to be deficient. I've tried to check into the Elecraft net with no luck. I tried to contact Peter One (using 100W I must admit) with no luck at all times of the day and night. Standing on my deck looking at the antenna and wondering what, if anything could I change, it occured to me that perhaps my problem is that the antenna is signficantly BELOW the houstop. How can that be, you ask? Well, here in Atlanta, I have what we call a 'ravine back yard'. My house is below street level and the back yard has a deep slope. The tree to which the antenna is strung is in the backyard and while the height above ground is 40', it is also at the eye level of my first floor (2 story house). The longest leg of the V is well below the house level and actually facing (for half its length) the concrete basement that holds up the house. Sooooo.....how much impact on the take-off angle would you think this configuration would have? Should I replace the V with a flat-top doublet at the same 40' height which would result in most of the antenna being at 40' but still below the roof level? Thanks for any suggestions you have. 73, Clark Macaulay KE4RQ _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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Which way does the ravine face? What's on the other side. You've described
half the system<g> (The half that won't work! The other broadside direction, and the ends have some possibilities). Dan / WG4S / K2 #2456 / Moving from a mountaintop to an Atlanta ravine lot next week<g>. <snip> My house is below street level and the back yard has a deep slope. The tree to which the antenna is strung is in the backyard and while the height above ground is 40', it is also at the eye level of my first floor (2 story house). The longest leg of the V is well below the house level and actually facing (for half its length) the concrete basement that holds up the house. </snip> _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by Clark Macaulay KE4RQ
Clark, KE4RQ, asked:
I installed a commercial multiband inverted V at my QTH and have had poor results for DX. It looks like a Windom design with the feedpoint off-center and comes with a balun to allow for coax feed. According to the specs, it should provide a match at 80, 40, 20, 17, 12, and 10, and the ATU within my K1 matches fine on 40, 20, and 17. (I've also had no problems matching other rigs using their ATU to the other frequencies). I am using a low loss coax from Davis RF and the rig(s) are grounded to a ground rod just outside the shack. The apex of the antenna is about 40 ft with each leg at the approximate height about ground that the manufacturer's specs require. It is strung in a NE/SW direction -- broadside to southern Africa. The antenna seems to 'hear' fine; it's in the transmitting that it appears to be deficient. I've tried to check into the Elecraft net with no luck. I tried to contact Peter One (using 100W I must admit) with no luck at all times of the day and night. Standing on my deck looking at the antenna and wondering what, if anything could I change, it occurred to me that perhaps my problem is that the antenna is significantly BELOW the housetop. How can that be, you ask? Well, here in Atlanta, I have what we call a 'ravine back yard'. My house is below street level and the back yard has a deep slope. The tree to which the antenna is strung is in the backyard and while the height above ground is 40', it is also at the eye level of my first floor (2 story house). The longest leg of the V is well below the house level and actually facing (for half its length) the concrete basement that holds up the house. Sooooo.....how much impact on the take-off angle would you think this configuration would have? Should I replace the V with a flat-top doublet at the same 40' height which would result in most of the antenna being at 40' but still below the roof level? ---------------------------------------------------------------------------- ---- Expect your strongest signals to be opposite the direction of the slope. That is, the ground sloping up to the house will act as a reflector. Actually, below a wavelength or two above ground, the ground ALWAYS acts like a reflector. That's why horizontal antennas at the right height - about 1/2 wave up - enjoy 4 to 6 dB gain over a vertical for fairly low angle radiation. Closer to the ground horizontal antennas see that gain straight up for short skip propagation. That's why folks currently call as a NVIS or near-vertical incidence system. In your case the ground isn't horizontal, but it'll work just the same. At 40 feet, the major lobe from your antenna will be opposite the ground, especially on 40 and 20 meters. Only in this case it's off at an angle opposite the slope of the ground instead of straight up. That's good for working stations in that direction, particularly DX and lousy for working stations in other directions beyond short-skip range. "Mountain topper" portable operators often try to duplicate your situation on purpose, erecting a low doublet or dipole off the side of a steep cliff to get low angle radiation from a horizontal radiator that is not very high off of the ground. Bottom line: your propagation will be greatly affected by that slope. Hopefully it points your signal in interesting directions even if that doesn't include Oregon for the ECN. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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Ron makes a good point about an antenna backed by an upslope, such that the
antenna is below the highest objects, be it hill or house. You can possibly get more interesting contacts by arranging a rotating guy ring to which to attach the high point of the antenna. Then, get some rebar to act as temporary tie off points for the ends. When the propagation is not good in one direction, "rotate" the whole line of the antenna wire to other directions and tie off to another pair of rebar or other anchors. Wood fence posts work, too, as tie off points. Allow some insulated guy line between the antenna end insulators and the tie off if anchor is metal re bar. Stuart K5KVH _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by Clark Macaulay KE4RQ
On Feb 21, 2006, at 9:57 AM, Clark Macaulay wrote: > Sooooo.....how much impact on the take-off angle would you think > this configuration would have? Should I replace the V with a flat- > top doublet at the same 40' height which would result in most of > the antenna being at 40' but still below the roof level? The key question is -- what band? The antenna will act very differently on 80m than on higher bands. The single most important dimension of a horizontal antenna is the height above ground in wavelengths. At heights less than 1/4 wavelength, the horizontal antenna basically sends all the energy straight up. At heights above 1/2 wavelength, broadside pattern starts to appear. As the height increases above 1 wavelength, the broadside pattern approaches the horizon -- low angles that favor distant DX. Lots of hams fret about the length of their doublets, but fail to account for the height above ground. While the length may affect resonance, the height affects the incidence angle of the antenna. The incidence angle makes all the difference for DX. A dipole may be (near) 0 dBd at one angle, but what is its gain at the angle the DX signals arrive (and depart)? At 40 feet, that's about 12m tall. That's a reasonably effective height for 10 and 12m. At 20m, it's barely 1/2 wavelength, so the broadside pattern starts to disappear. Below that, the antenna becomes an omni-directional cloud-warmer. The inverted-V installation will also tend to round out the pattern, since it reduces the nulls off the ends. Installing effective antennas for DX is tough, especially with limited-height supports. If you want to work DX, and your support is less than 1/2 wavelength high, consider installing a vertical antenna. Ground-mounted verticals with 30 or more 1/2 wave radials are very effective DX antennas. At my station, I have a 15m (almost 50 foot) tower with a small tribander. The tribander has a 40m dipole kit, too. I feed the whole tower as a vertical for 80m and 160m. It's not perfect, no. The 40m dipole is too low to show much of a pattern (I do see a 2 S-unit (12 dB) null off the ends), but it works better than anything else I've put up. The shunt-fed tower is only 1/10 wavelength long on 160m, but I've gotten 43 states confirmed and 10 countries using 100 watts, mostly in the last year. Bill Coleman, AA4LR, PP-ASEL Mail: [hidden email] Quote: "Not within a thousand years will man ever fly!" -- Wilbur Wright, 1901 _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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