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best HF antenna system next to a trailer?
Hi,
I'd like some advice for determining the best antenna to put up in my situation. I am getting set to move to a trailer park in northeast arizona, about 50 miles west of Gallup, NM. Locals have no objection to antennas so long as people don't have to run into them via normal passage ways. The trailer is 12 feet high by 15 feet wide by 50 feet long, facing N-S. Nearest certain ground is electrical panel. The water pipe is metal but could be interrupted throughout system by PVC. There may also be significant noise from AC systems in park. Good news is that my wife is giving me an entire walk-in closet for my station, and that I can lay aluminum foil against all surfaces to create a Faraday cage. I have approx 25 to 30 feet between my trailer and neighbors, and a 35 foot altitude streetlamp 6 feet away curb. Soil conductivity is red clay, extremely poor. Significant rainfall (monsoons) summer to fall. Winds gusting to 50 or 60 mph during winter. Soil frosts between October to March. I would like to work CW DX on 40, 30, 20, and 17 meters. 80 and 160 would be a bonus. Conventional options such as tower or surplus telephone pole are out of the question due to cost and lack of available area. Radials must be buried as children are playing nearby. Probable options: 1) Load up the streetlamp with an antenna matcher, work against 180 degrees of buried radials out to 1/8 lambda. 2) Solder a series of tin/steel cans (cantenna) using pocket torch and copper tape to 1/4 lambda with added capacitance hat(s), brace the cans against the ground and the trailer, work against 270 to 360 degrees of buried radials out to 3/8 lambda. 3) Create a mast from 40 feet of metal pipe and 15 feet of wood rod, brace against trailer burying pipe end 10 feet, mount an inverted vee trap dipole in N-S direction for E-W DX. 4) Pair of masts on either side of the trailer, mount a delta loop from each mast, feed one loop and use the other as a reflector. 5) Pair of cantennas on each side of the trailer, operating as out of phase pair of 1/4 lambda verticals. Which would be best? And where do I place the lightning arrestor(s)? Thanks, The Eternal Squire |
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Eternal Squire wrote:
"I`d like some advice for determining the best antenna to put up in my situation. I am getting set to move to a trailer park in northeast Arizona." Phil Rand, W1DBM distilled 35 years of trailering experience in QST and it was reprinted in the 1978 ARRL Antenna Anthology. As Richard Clark wrote, there is no miracle antenna. Phil found a simple dipole only a few feet above ground would outperform a mobile whip on 40 or 75 meters. Here is Phil`s Table 3: Hustler 75-meter Mobile whip mounted vertically on top rear corner of trailer-------S7 Same as above with 60-foot counterpoise connected to trailer-----------------------------S9 Two Hustler mobile whips back to nack as a horizontal loaded dipole-------------S9+5dB 60-foot horizontal wire 8 feet high using trailer (30-ft. Airstream) as ground------S9+10dB Hustler 4BTV trap vertical with 75 meter resonator-------------------------S9+10dB 120 foot dipole, 15 feet high at center------------------------------------------S9+20dB Airstream Loop antenna------------------S9+20dB Home station dipole 50 feet high------S9+30dB Feet = 0.3048 m There is a lot more in the article which may interest operators from trailers, but I`m not a typist. Check the Airstream Loop antenna. Nothing extends laterally from the trailer to trip anyone up. Best regards, Richard Harrison, KB5WZI |
Richard Harrison wrote:
Phil found a simple dipole only a few feet above ground would outperform a mobile whip on 40 or 75 meters. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Without more information, this comparison is flawed. A mobile whip has a lower angle of radiation than a horizontal dipole. On 40 or 75, the vertical component of radiation can be quite significant for close-in stations (100 miles or so). At night, working long distances, the whip may outperform the dipole. During the day, the dipole will probably outperform the whip. It all depends. 73, Bill W6WRT |
Phil found a simple dipole only a few feet above ground would
outperform a mobile whip on 40 or 75 meters. NVIS sure...Wouldn't be so sure about long haul... My 40 meter mobile beats my home dipole at 40 ft on a 1000 mile path. Here is Phil`s Table 3: Hustler 75-meter Mobile whip mounted vertically on top rear corner of trailer-------S7 Part of the problem...His mobile is stunted... :( Same as above with 60-foot counterpoise connected to trailer-----------------------------S9 Two Hustler mobile whips back to nack as a horizontal loaded dipole-------------S9+5dB Again kinda stunted due to the lousy hustler coils... :( Could be better than that if better coils were used. 60-foot horizontal wire 8 feet high using trailer (30-ft. Airstream) as ground------S9+10dB Pretty mediocre if NVIS... Hustler 4BTV trap vertical with 75 meter resonator-------------------------S9+10dB 120 foot dipole, 15 feet high at center------------------------------------------S9+20dB Airstream Loop antenna------------------S9+20dB Home station dipole 50 feet high------S9+30dB Sounds like these are all NVIS paths... For those, I agree, a dipole/loop is usually best. One problem though... Often when mounting a low dipole next to a large metal trailer, etc, the coupling often will make tuning quite difficult. I'd try to get the dipole as far away from the trailer as possible *if* it acts squirrely... But a *good* mobile antenna could often be quite good to longer hauls. On the higher bands, a good mobile antenna should be just fine. If it were me, I'd #1 run the best mobile antenna I could rig up as a vertical. Then I'd run a dipole for low band NVIS stuff. In my case, I prefer paralleled multiband dipoles, at right angles, but if I can only run one wire, I'll make a multiband dipole split up with clipable insulators. If thats not workable, I suppose a trap dipole could be used, but thats always my last choice for a multiband dipole setup, being I like every drop of efficiency I can muster. But the losses with those is not that bad. With my mobile antenna, I could easily use *just it* if I wanted, on any band. But my mobile ain't no stunted hustler antenna. When I'm parked, my usual coil position is higher than the total height of the average hustler whip. My mobile eats hustlers for lunch and spits out the seeds... :) It's ugly. I did some tests using hustler coils vs my usual homebrew...Wasn't pretty... Adding the hustler coil is like turning the antenna into a dummy load, *even* considering that in most mobile setups, ground loss overshadows coil loss. So if you see a *drastic* decrease in perfomance when changing coils, Houston, we have a problem. I've seen many claim the "small" hustler coils are actually more efficient than the "super" coils, which was the type I tried. Luckily , I didn't pay for it, and I gladly gave it back after testing... I think he stuck it on a hustler vertical... Poor thing.... :( I'd forget the "can" antennas, etc...A good mobile whip would likely do about as well. I'd use wire, or regular masts to make a tall vertical. To me, cans sound like a soldering nightmare... :/ MK |
Bill Turner wrote:
"Without more information, this comparison is flawed." I agree the information was incomplete. I dfid not reproduce the whole article. The fault was mine, not Phil`s. A low dipole has a high radiation angle. For comparison, Phil was working Airstream net stations in New York, New Jersey, Maryland, Pennsylvania, Ohio, and New England. Phil was located in Ontario near Buffalo, New York when he collected his data. His in-laws lived there. Phil`s home QTH at the time was the highest spot in Western Connecticut, with a line-of-sight path to New York City. Phil had surrounded his mountain top with rhombic antennas pointed toward his likely targets. Amateurs answered when he called. In the Airstream net, most of the contacts were made Sundays on 3963 kHz at 8 am local time. Sky wave was mostly near vertical incidence. The low dipole was good for the job. Not too directional and a lot of radiation nearly straight up. Phil noted that several times when he switched to to the mobile whip, he could not be heard through the QRM. The numbers Phil put in Table 3 are only true under the conditions prevailing when he made the checks. Best regards, Richard Harrison, KB5WZI |
On 13 Sep 2005 16:15:08 GMT, "Bill Turner" wrote:
Richard Harrison wrote: Phil found a simple dipole only a few feet above ground would outperform a mobile whip on 40 or 75 meters. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~ Without more information, this comparison is flawed. A mobile whip has a lower angle of radiation than a horizontal dipole. On 40 or 75, the vertical component of radiation can be quite significant for close-in stations (100 miles or so). At night, working long distances, the whip may outperform the dipole. During the day, the dipole will probably outperform the whip. It all depends. 73, Bill W6WRT Correct on radiation angle, however the average mobile whip at 3.8mhz is around 10% efficient. Even at 7Mhz it doesn't improve much efficientcy wise. The low dipole (low being less than .25WL) is close or better than 95% efficient but has a rotten radiation angle for DX however close in it will be very good. Myself in that situation.. I'd put a poles at either end of the trailer (thats 50ft length) and if possible get it up 30ft or better and hang a dipole. If the antenna is 66' (40m) the excess length can hang. The support poles can be anything that will stay up. At 20m 30ft is 1/2WL up and will be decent. Even if you can't do two support poles and only one make that one high as possible and mount a dipole as a sloper. It will be somewhat directional but performace will be far better than any ground mounted vertical that has no ground plane. If money wasn't a limiting factor. put down a base and put up a freestanding tower. The rules remain. More metal, higher the better. Allison KB1GMX |
An antenna doesn't have a single "radiation angle". It radiates at all
angles. The relevant question is how much does it radiate at the particular angle of interest, not at which angle does it radiate the most. An antenna which radiates its maximum at a high angle might well radiate more at a low angle than an antenna with a lower angle of maximum radiation. Roy Lewallen, W7EL wrote: On 13 Sep 2005 16:15:08 GMT, "Bill Turner" wrote: Without more information, this comparison is flawed. A mobile whip has a lower angle of radiation than a horizontal dipole. . . Correct on radiation angle . . . |
Money and space are actually limiting factors, but at least CC&R's
arent! My wife and I have been discussing this, and she really doesn't like the idea of something 30 to 40 feet high on a small lot. Her interest actually is gardening and we had been discussing containing an area of decent topsoil within a square formed by railroad ties. She has no objection to 20 to 25 foot high metal poles on the corners of a square 20 to 25 foot on the side... which turns out to be the core geometry of a 20 meter 4-square broadside phase array. I would lay the poles first in concrete reinforced holes, connect the feed network and radials on the dirt, and lay feed line underground from the array to just near the trailer. Then I would lay the railroad ties along the square, and then fill the square with topsoil. Additional radials would need to be buried under a couple inches of red clay fanning out from the square. Variations: 1) could I create trap verticals from the poles for 20, 17, 15, and 10 meters, or do I need inscribed squares of seperate verticals because seperate phased feeds might be needed for these other bands? 2) could I simply operate the square outside of 20 meters with a tuner for local operation? I think because space, money, and aesthetics are limiting factors, I need to use precision to my advantage rather than size or height, and a 4 square may help there. Comments? The Eternal Squire |
On Tue, 13 Sep 2005 20:43:23 -0700, Roy Lewallen
wrote: An antenna doesn't have a single "radiation angle". It radiates at all angles. The relevant question is how much does it radiate at the particular angle of interest, not at which angle does it radiate the most. An antenna which radiates its maximum at a high angle might well radiate more at a low angle than an antenna with a lower angle of maximum radiation. Roy Lewallen, W7EL Hello Roy, I do understand that. I also understand when you say radiation angle your talking about the primary or dominent lobe(s). There may be many other lobes at useful or less than useful angles present as well. However, how does that relate to using a shortend antenna with maybe 10% radiation efficientcy to a dipole at a reasonably attainable height? Allison KB1GMX wrote: On 13 Sep 2005 16:15:08 GMT, "Bill Turner" wrote: Without more information, this comparison is flawed. A mobile whip has a lower angle of radiation than a horizontal dipole. . . Correct on radiation angle . . . |
Roy Lewallen wrote:
An antenna doesn't have a single "radiation angle". It radiates at all angles. The relevant question is how much does it radiate at the particular angle of interest, not at which angle does it radiate the most. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ That's true, except few if any hams have a specific "angle of interest", since different angles are used at different times. For most of us, the angle of maximum radiation gives a general indication of how the antenna will perform. A better indication would be a graphical representation. It's always a problem when one tries to reduce a complex situation like this down to a single number. 73, Bill W6WRT |
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However, how does that relate to using a shortend antenna with maybe
10% radiation efficientcy to a dipole at a reasonably attainable height? Well....Exactly as he described. An antenna which radiates its maximum at a high angle might well radiate more at a low angle than an antenna with a lower angle of maximum radiation. The thing is "might".... My mobile antenna on 40m is *much* less efficient than my dipole at 40 ft. But...It still is the best to longer hauls over about 800 miles, and to dx. In it's case, it does radiate more at the lower angles I'm using at that time, vs the dipole. With some lesser mobiles, "mine is fairly stout", this might not be the case. The best antenna should always be decided to fit the usual paths to be used. In the case of my mobile vs the dipole, it's possible that if the dipole were raised another 1/4 wave higher, it could match the mobile at those lower angles. At home, I often ran a dipole at 40 ft vs a full size ground plane at the same height. Both were pretty efficient. Efficiency comparisons were fairly useless as to actual performance. What really decides which is best at a given time, is the path, and angles to be used. Now, if you compare two same length low dipoles, both to NVIS, both shooting straight up, and one is less lossy than the other as far as feeding method, etc, then yes, efficiency will decide which one is best. MK |
A better indication would be a
graphical representation. The EZNEC demo does that well. There is a little green ball that you can grab with your mouse, and place it at any angle you wanna check. Makes it quite easy to see, or compare various angles. MK |
I think because space, money, and aesthetics are limiting factors, I
need to use precision to my advantage rather than size or height, and a 4 square may help there. Sounds like too much work for my lazy a$$... :/ If you did that for 20m, it's kinda stuck , as far as spacing, etc, for good performance on the other bands. Me, on 20m...I'd be using my mobile antenna. And I'd probably be hanging pretty close with any "fancy" antenna. When I'm mobile, 20m up, there is *no* disadvantage vs being at the house on dipoles, etc.. At least as far as talking. On 20m, my mobile is LOUD. I can work dx at the drop of a hat, and still do well stateside. Even on 40m, there is little real disadvantage to being mobile vs the dipole at home. Even in the daytime. Say if I'm 20-30 over 9 at home, and drop to 10-15 over 9 in the mobile. Doesn't amount to a hill of beans... Same for 75m. But in the summer on 75m, I do recommend a dipole/loop, as it will be worth quite a bit in getting over the noise running NVIS. I could talk with the mobile just fine, but the dipole will make me "full quieting" to use an expression... It's like throwing on an extra 500w when you are shooting upwards... I wouldn't underestimate how well a *good* mobile antenna could work on the higher bands. If it's mounted up high on a trailer, etc, it should work well. It's like being on a huge car. The only problem is changing bands...You need to be able to reach it to change taps, etc.. But that can be easy with a quick disconnect. That's what I use on mine. I can't reach my coil when the antenna is on the trucks. With the disconnect, a twist, and it's off. Also good for theft prevention.. A good screwdriver could be an option too. Not saying you can't try other antennas, but just saying , a good mobile will get the job done, all by itself. And it's pretty simple and easy to look at visually. It will often outplay ground mounted trap verticals with mediocre radial systems on the higher bands. MK |
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Bill Turner wrote:
Roy Lewallen wrote: An antenna doesn't have a single "radiation angle". It radiates at all angles. The relevant question is how much does it radiate at the particular angle of interest, not at which angle does it radiate the most. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ That's true, except few if any hams have a specific "angle of interest", since different angles are used at different times. For most of us, the angle of maximum radiation gives a general indication of how the antenna will perform. A better indication would be a graphical representation. It's always a problem when one tries to reduce a complex situation like this down to a single number. 73, Bill W6WRT I agree, which is why EZNEC produces a graphical output. I encourage people to look at it rather than reducing the pattern to a single number. And I have to emphasize once again that what really counts is the field strength, not the pattern shape. An antenna can have a wonderful looking pattern with nearly all its radiation at low angles, and still be a poor antenna for DX. Or with nearly all its radiation at high angles and be a poor antenna for short range communications. One familiar example is a Beverage antenna, which has a lovely pattern shape but makes a poor transmitting antenna. A quarter wave vertical will nearly always do much better for transmitting, even at the angles favored by the Beverage. Roy Lewallen, W7EL |
wrote in message ups.com... Hi, I'd like some advice for determining the best antenna to put up in my situation. I am getting set to move to a trailer park in northeast arizona, about 50 miles west of Gallup, NM. Locals have no objection to antennas so long as people don't have to run into them via normal passage ways. The trailer is 12 feet high by 15 feet wide by 50 feet long, facing N-S. Nearest certain ground is electrical panel. The water pipe is metal but could be interrupted throughout system by PVC. There may also be significant noise from AC systems in park. Good news is that my wife is giving me an entire walk-in closet for my station, and that I can lay aluminum foil against all surfaces to create a Faraday cage. I have approx 25 to 30 feet between my trailer and neighbors, and a 35 foot altitude streetlamp 6 feet away curb. Soil conductivity is red clay, extremely poor. Significant rainfall (monsoons) summer to fall. Winds gusting to 50 or 60 mph during winter. Soil frosts between October to March. I would like to work CW DX on 40, 30, 20, and 17 meters. 80 and 160 would be a bonus. Conventional options such as tower or surplus telephone pole are out of the question due to cost and lack of available area. Radials must be buried as children are playing nearby. Probable options: 1) Load up the streetlamp with an antenna matcher, work against 180 degrees of buried radials out to 1/8 lambda. 2) Solder a series of tin/steel cans (cantenna) using pocket torch and copper tape to 1/4 lambda with added capacitance hat(s), brace the cans against the ground and the trailer, work against 270 to 360 degrees of buried radials out to 3/8 lambda. 3) Create a mast from 40 feet of metal pipe and 15 feet of wood rod, brace against trailer burying pipe end 10 feet, mount an inverted vee trap dipole in N-S direction for E-W DX. 4) Pair of masts on either side of the trailer, mount a delta loop from each mast, feed one loop and use the other as a reflector. 5) Pair of cantennas on each side of the trailer, operating as out of phase pair of 1/4 lambda verticals. Which would be best? And where do I place the lightning arrestor(s)? Thanks, The Eternal Squire How about the screwdriver antenna. This is compact, can even be "disguised" inside a PVC pipe, can be positioned vertically OR horizontally and can be used to tune a random wire strung into a nearby tree. I used this system on a motor home where we positioned the antenna on the rear ladder with a swivel mount. When traveling, the antenna was positioned at a 45 degree angle and rested in a wooden cradle on top of the bus. At rest, the antenna could be raised to the vertical in a minute or two and tightened with thumbscrews. Because the motor home was mostly fiberglass, we quickly discovered that the antenna could be used in the semi-horizontal position while going down the road with good results! In your case, your trailer would be a "mobile", but you could set it up in several ways to take advantage of various situations, and, also tune a random wire with it. I've done this while on field exercizes with CAP. Now I know I am partial since I make screwdrivers, but you can find a variety of makers on the web. Just try to avoid the ones made of PVC and, of course, there are those that are SUPER expensive. Go for the ones with larger diameter coils and wire sizes and avoid the ones that promise 160 thru 10. Those that promise 160 thru 10 are the ones that are wound on small diameter forms and are wound with 20 gauge wire CLOSELY together (in effect, transformer-wound) which destroys your "Q". I doubt seriously such an antenna would do much "punkin" on 160! Every body runs into compromises at times, so you have to be do the best you can do with what you have to work with. If you have 75 feet of room over your lot, then you might visit www.k1jek.com for an all-band dipole 80-10 Meters. I'm told this is a good choice for short lots. Hope this helps 73 Jerry K4KWH www.qsl.net/k4kwh |
Smack her in the head and put up a tower, It has worked on all 7 of my
wives. :) wrote in message oups.com... Money and space are actually limiting factors, but at least CC&R's arent! My wife and I have been discussing this, and she really doesn't like the idea of something 30 to 40 feet high on a small lot. Her interest actually is gardening and we had been discussing containing an area of decent topsoil within a square formed by railroad ties. She has no objection to 20 to 25 foot high metal poles on the corners of a square 20 to 25 foot on the side... which turns out to be the core geometry of a 20 meter 4-square broadside phase array. I would lay the poles first in concrete reinforced holes, connect the feed network and radials on the dirt, and lay feed line underground from the array to just near the trailer. Then I would lay the railroad ties along the square, and then fill the square with topsoil. Additional radials would need to be buried under a couple inches of red clay fanning out from the square. Variations: 1) could I create trap verticals from the poles for 20, 17, 15, and 10 meters, or do I need inscribed squares of seperate verticals because seperate phased feeds might be needed for these other bands? 2) could I simply operate the square outside of 20 meters with a tuner for local operation? I think because space, money, and aesthetics are limiting factors, I need to use precision to my advantage rather than size or height, and a 4 square may help there. Comments? The Eternal Squire |
I'm deciding to go with a 20 meter 4-square because it can hopefully
provide me with similar directivity and DX takeoff angle as a 20 meter beam but without the hassle or height of a tower. A dipole only has directivity in 2 directions, a 4 square can give me directivity in 4 directions with basic phasing and 8 directions high tech using ARRL suggestions. The EZNEC plot was pretty awesome. The Eternal Squire |
Roy,
I decided to go with a 20-meter 4 square. I wonder if any people have experience with 4-squares that they can share with me. I have considered some construction details give available materials, and I have some questions. 1) Can I shorten each element by using an inverted L rather than straight vertical, with a pipe as vertical part and a wire as horizontal part? I have heard that matching is far less of a problem this way also. 2) Where can I find or build a reasonably inexpensive phase box? 3) For the vertical part, I am wanting to a dig a hole 2 foot across by 3 foot down, and fill with concrete. Into this I would insert a 5 foot length of 1 1/2 inch steel support pipe about midway, so that 2 1/2 feet are above ground. Into this I would mount a 10 foot length of 3/4 inch steel pipe with a 2 1/2 foot insulated overlap of PVC pipe. The 3/4 inch steel pipe would be the bottom of the actual driven element. Into this I would mount a 10 foot length of 1/2 inch aluminum pipe with a 2 foot metallic contact overlap, and then I would finish with rod for vertical or wire for inverted L. Question: how would the 2 1/2 foot overlap of a non-grounded metal support pipe interfere with radiation of the vertical element? Thanks in advance, The Eternal Squire |
There's that "takeoff angle" again. Having a good "takeoff angle" is no
guarantee of good DX performance, and isn't a valid way to compare the performance of two antennas. You should model both the beam and the 4-square. Make sure you include a realistic amount of ground loss resistance for whatever ground system you think you can put down for the 4-square. Superimpose their elevation patterns on the same plot, and see which really does best at low angles. If you don't want to go to the trouble of modeling a beam, you can model a simple dipole which has almost the same elevation pattern as a beam of a few elements (in the forward direction) at the same height. Mentally add the beam's gain relative to a dipole to the dipole's pattern. See if the 4-square really is as good. It might change your mind. Roy Lewallen, W7EL wrote: I'm deciding to go with a 20 meter 4-square because it can hopefully provide me with similar directivity and DX takeoff angle as a 20 meter beam but without the hassle or height of a tower. A dipole only has directivity in 2 directions, a 4 square can give me directivity in 4 directions with basic phasing and 8 directions high tech using ARRL suggestions. The EZNEC plot was pretty awesome. The Eternal Squire |
wrote:
Roy, I decided to go with a 20-meter 4 square. I wonder if any people have experience with 4-squares that they can share with me. I have considered some construction details give available materials, and I have some questions. I've built and used a few, for 40 meters. 1) Can I shorten each element by using an inverted L rather than straight vertical, with a pipe as vertical part and a wire as horizontal part? I have heard that matching is far less of a problem this way also. You can make a 4 square from any kind of element. EZNEC can tell you what effect the element shape will have. I strongly recommend against designing the antenna to get the best or easiest match. Design the antenna for the best performance, then design whatever matching arrangement you need in order to match it. An exception to this general rule is that antennas with an exceptionally low resistance or high reactance might not be practical because of the problem of matching system loss, so such an antenna might need redesign in order to be practical. 2) Where can I find or build a reasonably inexpensive phase box? Chapter 8 of the ARRL Antenna Book describes how to design one. See also "The Simplest Phased Array Feed System - That Works" and accompanying program Simpfeed, available from http://eznec.com/Amateur/Articles/. 3) For the vertical part, I am wanting to a dig a hole 2 foot across by 3 foot down, and fill with concrete. Into this I would insert a 5 foot length of 1 1/2 inch steel support pipe about midway, so that 2 1/2 feet are above ground. Into this I would mount a 10 foot length of 3/4 inch steel pipe with a 2 1/2 foot insulated overlap of PVC pipe. The 3/4 inch steel pipe would be the bottom of the actual driven element. Into this I would mount a 10 foot length of 1/2 inch aluminum pipe with a 2 foot metallic contact overlap, and then I would finish with rod for vertical or wire for inverted L. Wow, for a 20 meter 4-square? For each element on 40, I drove a 1-1/4" 8 foot galvanized chain link fence line pole 4 feet into the ground. (Our soil is clay.) I cut a piece of heavy wall PVC pipe lengthwise into quarters for insulators, and clamped the element to the line pole with muffler clamps with a couple of pieces of the split PVC pipe in between. The elements are three pieces of telescoping 6061-T6 tubing, beginning with, as I recall, 1-1/8" at the bottom. They've been up for around 20 years now and survived a couple of pretty strong wind storms. Question: how would the 2 1/2 foot overlap of a non-grounded metal support pipe interfere with radiation of the vertical element? Any shunt impedance will reduce the null depth if the array is adjusted for the correct base current ratio. This is because a different fraction of the current will be diverted from each element because of their differing base impedances. However, I've found that the 4 foot overlap I have doesn't reduce it noticeably. But my overlapping pipes are parallel and, if I understand your description, yours will be coaxial. That'll result in a lot more shunt capacitance, and a correspondingly greater effect on the null. The main lobe won't be affected much. Roy Lewallen, W7EL |
Dear Group:
Long, long experience with angles above the horizon that are used by DX signals (at HF) indicates that the most useful angles are between 2 and 12 degrees. Comparing the expected gain of antennas at 6 degrees provides a good figure-of-merit. That said, if one has a low, horizontally polarized antenna with very little gain at 6 degrees, you might still work DX using more than an optimum number of hops (angle of more than ten degrees). However, you will work DX fewer days per month than someone who gets significant gain at angles smaller than ten degrees. I emphasize what Roy has said: the so-called take-off-angle (equal to the smallest angle at which peak gain occurs) of an antenna is not necessarily an indicator of DX performance. Another example is the case of a horizontally polarized antenna that is over 3 WL high: it has a small TOA but is likely to have a null at an important angle smaller than 12 degrees. In other words: the too-high-antenna works very well some of the time, but a lower antenna works better at other times. A useful goal for the (single) optimum (for DX) antenna is an antenna that has its second null (first null is at zero degrees) at an angle greater than 12 degrees and a first maximum (what is called by many the TOA) between 2 and 12 degrees. The actual angle used at the transmitter end of a DX circuit is sometimes quite different from that used at the receiver end. 73 Mac N8TT -- J. Mc Laughlin; Michigan U.S.A. Home: "Bill Turner" wrote in message ... Roy Lewallen wrote: An antenna doesn't have a single "radiation angle". It radiates at all angles. The relevant question is how much does it radiate at the particular angle of interest, not at which angle does it radiate the most. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ That's true, except few if any hams have a specific "angle of interest", since different angles are used at different times. For most of us, the angle of maximum radiation gives a general indication of how the antenna will perform. A better indication would be a graphical representation. It's always a problem when one tries to reduce a complex situation like this down to a single number. 73, Bill W6WRT |
To Novices -
It is use of the term "take off angle" which causes all the confusion surrounding DX and the "best" take off angles. It is a misnomer. The elevation angle of a radio path between two stations is purely a geometric function of their locations on the Earth's surface and the heights of ionospheric reflecting layers. It has nothing whatever to do with either of the antennas or ground conditions - except that it is the best elevation angle at which an antenna beam should be pointing. If, purely by coincidence, the "take off angle" indicated by Eznec happens to be the same as the exceedingly changeable "path elevation angle" then all is well and good. The true "take off angle" having maximum gain (another misnomer) for any vertical antenna is always zero degrees, ie., it corresponds to the always existent very strong groundwave. Whereas Eznec always reports the groundwave strength as being zero. It is of no use in the prediction of often-used ground waves between stations. Whenever a resistive ground is involved, programs like Eznec do not produce the true radiation pattern of an antenna. Not that there is anything incorrect with Eznec. It is just the confusing description of what it displays. ---- Reg. |
Reg Edwards wrote:
. . . Whereas Eznec always reports the groundwave strength as being zero. . . If you're using the strict definition of "groundwave" as being the field at an elevation angle of zero, only EZNEC's far field analysis reports it as zero, because (as the manual explains, and as I've explained here several times before) the far field results are valid at a distance beyond the point where the surface wave has decayed to essentially zero -- a few miles at HF. And at that distance, the field at zero elevation angle is zero if the ground conductivity is finite. If the surface wave strength is wanted, it can easily be found using EZNEC's near field calculation, which calculates the total field at any point in space -- including just above the ground surface. It is of no use in the prediction of often-used ground waves between stations. Unless you use the near field results, which do give an accurate indication of the field at any point in space. I assume you've just forgotten the several times I've explained that to you. Maybe this will be the magic time it'll sink in. Whenever a resistive ground is involved, programs like Eznec do not produce the true radiation pattern of an antenna. . . For sure, the modeling of ground is the weakest point of all antenna modeling programs including EZNEC. But the pattern is generally a good representation of reality. Remembering, of course, that the far field pattern is just that -- the pattern at a distant point at which the surface wave has decayed to zero. A graphical pattern which includes the surface wave component would be different at every distance from the antenna up to the distance where the surface wave has decayed to essentially zero (a few miles at HF). The field strength at angles greater than zero would be of little interest to amateurs doing local communication by surface wave. Those who want to know the field strength at ground level at any distance can easily get this information from EZNEC's near field analysis (which reports the total field, not just the near field). Most amateurs who are interested in local communication over a few miles using surface waves don't need to see the overall elevation pattern, and they can get numerical results of the surface field strength from the near field analysis. Amateurs communicating by sky wave, by far the more common situation, can benefit from the graphical results afforded by EZNEC's far field elevation pattern. Not that there is anything incorrect with Eznec. It is just the confusing description of what it displays. It's interesting that in the 15 years EZNEC and its predecessor ELNEC have been available, and the thousands of users, no more than a half dozen people have expressed any confusion regarding its far and near field analysis. And none of the others has required repeated explanations. But some people are sure to have more trouble with the concept than others. It's explained in the EZNEC manual, and I always welcome questions and suggestions which would help me make it more clear. I am, however, resigned to the fact that some small number of people aren't capable of, and some simply aren't interested in, understanding. Because of your deep interest in surface wave propagation and field strength prediction, and your characterization of it as "often-used", you must do a lot of communication by this mode. What bands do you use, and what sort of range do you reliably communicate over? How many hams are within this radius whom you talk to? Roy Lewallen, W7EL |
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