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HF antenna for boat portable?
hi,
I am taking a trip on a dive boat to Cocos Island off of Costa Rica. I'm thinking about trying to bring an HF rig. What suggestions have you for an antenna? It would have to fit into a reasonably "standard" suitcase, or go as carry-on luggage, since packing space is limited. -- Dave * N3WTK (DM04xf) * http://isi.mtwilson.edu * VE |
I wanted to add that I will bring either my little homebrew QRP CW
(based on a Norcal 40), or use this as an excuse to buy an FT857. 73, -- Dave * N3WTK (DM04xf) * http://isi.mtwilson.edu * VE |
On Wed, 17 Nov 2004 04:35:36 +0000 (UTC), David Snyder Hale
wrote: I am taking a trip on a dive boat to Cocos Island off of Costa Rica. I'm thinking about trying to bring an HF rig. What suggestions have you for an antenna? It would have to fit into a reasonably "standard" suitcase, or go as carry-on luggage, since packing space is limited. Take a look at the www.njqrp.org/pac-12 website. I built one of these and have been very happy with it. The wire ground radials may prove to be a problem, but the idea might be worth considering. I've used it while camping. ....Edwin ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~ Edwin Johnson ....... ~ ~ http://www.shreve.net/~elj ~ ~ ~ ~ "Once you have flown, you will walk the ~ ~ earth with your eyes turned skyward, ~ ~ for there you have been, there you long ~ ~ to return." -- da Vinci ~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
Dave, N3WTK wrote:
"What suggestions have you for an antenna?" Avoid the sharks off Cocos Island. From a boat at sea, you have a perfect earth connection available. Dunk an electrode and use it for HF. If the boat is metal, just make a ground connection outside to the hull. HF won`t penetrate the metal. Then use a vertical wire as an antenna. The sea short-circuits horizontal polarization. L-antennas are often used on boats. The horizontal portion is just capacitive loading for the real antenna which is the vertical portion. Loaded vertical whips are another type of common boat antenna. Best regards, Richard Harrison, KB5WZI |
On Wed, 17 Nov 2004 04:35:36 +0000 (UTC), David Snyder Hale
wrote: hi, I am taking a trip on a dive boat to Cocos Island off of Costa Rica. I'm thinking about trying to bring an HF rig. What suggestions have you for an antenna? It would have to fit into a reasonably "standard" suitcase, or go as carry-on luggage, since packing space is limited. dunno how big your boat is, but possibly a "tape-measure" dipole: http://www.gotenna.com/ bob k5qwg |
Bob, K5QWG wrote:
"dunno know how big your boat is, but possibly a "tape-measure" dipole:" A Go-Tenna is collapsible for easy packing, but twice as long as an equivalent monopole, used against the sea, a near perfect ground. A Go-Tenna can be deployed as a vertical and so could be effective at sea but requires twice the altitude of an equivalent monopole. Horizontal deployment requires an elevation of a couple of wavelengths at sea to be very effective. That`s usually excessive. Recall that the ground wave is vertically polarized. There is no horizontally polarized wave propagation over the sea. Terman says on page 808 of his 1955 edition: "Examination of these vector diagrams show that with s perfect reflectoe the horizontal components of the electric field will exactly cancel each other at the surface of the perfect reflector. In contrast the vertical components of the electric field of the incident and reflected waves do not cancel, but rather add at the reflector surface with small values of Psi 2 (the vertical takeoff angle from the surface)." Best regards, Richard Harrison, KB5WZI |
David Snyder Hale wrote in message ...
I wanted to add that I will bring either my little homebrew QRP CW (based on a Norcal 40), or use this as an excuse to buy an FT857. http://www.buddipole.com/ Pricey but worth it. 73, w3rv |
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Gary, K4FMX wrote:
"Are you saying that my low (less than 1/2 wavelength high) horizontal antenna will be next to useless if I live on the sea shore?" No. Your antenna will do whatever it does. I said that sea water reflects so well that the reflected ray from the sea is almost as strong as the incident ray. At low angles they cancel when horizontally polarized, being equal and of opposite polarity, and this eliminates low-angle radiation. This is demonstrated in Figs. 13 & 14 on page 3-12 of the 19th edition of the "ARRL Antenna Book". Low horizontal wires tend to send most energy straight up. This can provide near vertical incidence contacts. For distance, when the reflecting surface is good (sea water) and the antenna is low, the antenna had better be placed vertically. Results are shown in Fig 16 on page 3-13 of the same book. Best regards, Richard Harrison, KB5WZI |
Chuck wrote:
"Or that only vertically polarized signals can be intercepted by ships at sea?" I served on a navy ship in WW-2. Our antenna was a low-L. It could intercept either polarity but responded only to line of sight and high-angle signals. This was a deliberate design. The Navy did not want our emissions QRM-ing the world. Our range was limited to about 500 miles. We could contact our shore destinations at about 2 days travel from them (our ship was slow). Best regards, Richard Harrison, KB5WZI |
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A local here used a 20M Hamstick in a canoe -- worked Japan from Arizona
Hamsticks at URL: http://www.hamstick.com/ -- The Anon Keyboard I doubt, therefore I might be |
Hello Richard,
I think Gary and I were taking issue with your statement: "Recall that the ground wave is vertically polarized. There is no horizontally polarized wave propagation over the sea." While the first sentence is correct, the second would be a bit of surprise if it were true. Indeed, your experience on the naval vessel utilized horizontal propagation over the sea. But it is not correct to equate horizontal polarization with low-angle polarization. I think I understand what you meant to say. 73, Chuck Richard Harrison wrote: Gary, K4FMX wrote: "Are you saying that my low (less than 1/2 wavelength high) horizontal antenna will be next to useless if I live on the sea shore?" No. Your antenna will do whatever it does. I said that sea water reflects so well that the reflected ray from the sea is almost as strong as the incident ray. At low angles they cancel when horizontally polarized, being equal and of opposite polarity, and this eliminates low-angle radiation. This is demonstrated in Figs. 13 & 14 on page 3-12 of the 19th edition of the "ARRL Antenna Book". Low horizontal wires tend to send most energy straight up. This can provide near vertical incidence contacts. For distance, when the reflecting surface is good (sea water) and the antenna is low, the antenna had better be placed vertically. Results are shown in Fig 16 on page 3-13 of the same book. Best regards, Richard Harrison, KB5WZI |
Bob, K5QWG wrote:
"The description for the radiation patterns say they are over the ground. Would the patterns be similar over water?" The legend says: "The solid-line curves are the flat, perfect-earth (read sea water) patterns, and the shaded curves represent the effects of average flat earth---." Best regards, Richard Harrison, KB5WZI |
Chuck wrote:
"While the first sentence is correct, the second would be a bit of a surprise if true." Terman writes on page 803 of his 1955 edition: "The ground wave is vertically polarized, because any horizontal component of electric field in contact with the earth is short-circuited by the earth." On page 808 of the same book: "Examination of the vector diagrams shows that with a perfect reflector (read sea water) the horizontal components of electric field will exactly cancel each other at the surface of the perfect reflector. In contrast, the vertical components of the electric field of the incident and reflected waves do not cancel, but rather add at the reflector surface with (small values of elevation angle from the surface) Psi. It`s true. Terman has been tested for 7 decades. Best regards, Richard Harrison, KB5WZI |
Gary, K4FMX wrote:
"Are you saying that my low (less than 1/2 wavelength high) horizontal antenna will be next to useless if I live on the sea shore?" No, but at some distance from the water, its benefits fade away. I`ve read, and I don`t remember where, that the benefit of high conductivity only extends 1 or 2 blocks back from the water`s edge and then it is gone. More money is spent on Viagra and breast implants than on Alzheimer`s research. With aging baby boomers, we may soon have crowds of salient people who don`t know why. Best regards, Richard Harrison, KB5WZI |
This could be a little misleading.
Horizontal antennas perform just about the same over salt water as they do over regular ground, with one exception: If the horizontal antenna is low, very high angle waves will be attenuated when over real ground, but won't be attenuated nearly as much if over sea water. The radiation at zero elevation angle is zero for any antenna height and ground conductivity. So there's nothing about sea water that would make a horizontal antenna work worse than over ground. However, a vertically polarized antenna has much stronger low angle radiation when over sea water (*) than when over plain ground. And this low angle radiation can be much stronger than anything but a very high horizontal antenna. So a vertically polarized antenna is usually the best choice for a boat. I also don't think that an L antenna is the best idea, especially if the horizontal part is low. If it is, the horizontal portion radiates mostly straight up, wasting part of your power. It's a better idea to make a T shaped antenna. Then the top will radiate very little, leaving most of the radiation to the vertical section. (*) The important factor for good low angle radiation is the ground conductivity at the point where reflection occurs, rather than the conductivity just under the antenna. For lower and lower angles, the reflection occurs at farther and farther distances from the antenna. Also, the reflection occurs farther away as the antenna gets higher. For typical HF antennas and low angle propagation, the important region is on the order of up to a few hundred feet from the antenna. Efficiency is a separate issue, and for that, the important thing is the conductivity just under and close to the antenna. Having the antenna directly over salt water makes getting a good low loss ground connection easy, as another poster pointed out. Roy Lewallen, W7EL Richard Harrison wrote: Gary, K4FMX wrote: "Are you saying that my low (less than 1/2 wavelength high) horizontal antenna will be next to useless if I live on the sea shore?" No. Your antenna will do whatever it does. I said that sea water reflects so well that the reflected ray from the sea is almost as strong as the incident ray. At low angles they cancel when horizontally polarized, being equal and of opposite polarity, and this eliminates low-angle radiation. This is demonstrated in Figs. 13 & 14 on page 3-12 of the 19th edition of the "ARRL Antenna Book". Low horizontal wires tend to send most energy straight up. This can provide near vertical incidence contacts. For distance, when the reflecting surface is good (sea water) and the antenna is low, the antenna had better be placed vertically. Results are shown in Fig 16 on page 3-13 of the same book. Best regards, Richard Harrison, KB5WZI |
Roy, W7EL wrote:
"The radiation at zero elevation angle is zero for any antenna height and ground conductivity." It is the equal and opposite reflection which cancels the incident wave at an equal distance from the reflection point, i.e. toward the horizon. Extremely low conductivity would produce radiation as in free space; no cancellation in the horizontal plane. Best regards, Richard Harrison, KB5WZI |
Richard Harrison wrote:
Roy, W7EL wrote: "The radiation at zero elevation angle is zero for any antenna height and ground conductivity." It is the equal and opposite reflection which cancels the incident wave at an equal distance from the reflection point, i.e. toward the horizon. Extremely low conductivity would produce radiation as in free space; no cancellation in the horizontal plane. No, that's not true. The radiation at zero elevation angle is zero for any antenna height and ground conductivity, even "extremely low". Sometimes it's necessary to look beyond simplified quotes read in books, and read and understand the underlying math. In this case, the math can be found as equation 1(*) on p. 717 of Kraus' _Antennas_, 2nd Ed. (I thought this was also in the 1st Ed., but can't find it there.) This is the equation for reflection coefficient from ground for horizontal polarization. It's easily seen that it equals -1 at an elevation angle (alpha) = 0 for any value of ground conductivity or permittivity. (Note that epsilon-sub-r is the complex permittivity, as shown in eq. 4(**), which contains the conductivity.) The total field is given in equation 3(***), which shows that its value is zero when the elevation angle is zero and the reflection coefficient is -1. This is, however, entirely theoretical. As the elevation angle gets lower and lower, the reflection point becomes farther and farther from the antenna. So a reflection at zero elevation angle takes place at an infinite distance from the antenna. Among other problems, this requires an observation point an infinite distance away and a perfectly flat ground plane that's infinite in extent. While this is the standard for a lot of theoretical analysis and in programs like NEC, MININEC, and EZNEC, it of course can't be constructed in reality. In real life, the Earth curves away, so if the terrain is perfectly flat, extremely low angle radiation never strikes the ground. (Without working through the numbers, I'd guess this to be below a small fraction of a degree for a moderate antenna height. But it would be easy to calculate.) Again theorectically, still using a simple reflection model but now with an idealized model of a spherical ground, you'd get a free space pattern at zero elevation angle and up to a fraction of a degree *regardless of the ground conductivity*. But when slicing things this thin, you probably need to use a better reflection model, which takes into account dispersion and refraction. I'm not sure how that would modify the result. (*) rho(horiz) = (er*sin(a) - sqrt(er - cos^2(a))) / (er*sin(a) + sqrt(er - cos^2(a))) where er = epsilon-sub-r = (complex) relative permittivity of the ground a = alpha = elevation angle (**) er = er' - j * sigma/(omega * e0) where er' = epsilon-sub-r prime = (DC) dielectric constant of the ground sigma = ground conductivity omega = radian frequency = 2 * pi * f e0 = permittivity of free space (***) E(horiz) = 1 + rho(horiz) * [cos(2*Bh*sin(a) + j*sin(2*Bh*sin(a)] where B = beta = 2 * pi / wavelength h = height of horizontal antenna above ground a = alpha = elevation angle Roy Lewallen, W7EL |
Roy, W7EL wrote:
"No, that`s not true. The radiation at zero elevation angle is zero for any antenna height and ground conductivity, even "extremely low"." We need no simplified quotes nor formulas. Reflection requires a "discontinuity". The resistance of free space is 377 ohms. As long as a traveling wave in space encounters 377 ohms, its velocity and trajectory are not altered. "Extremely low" could include a soil that for practical purposes appears to the wave as 377 ohms. Surely the earth could be absorbent and not highly reflective. In that case, the reflected wave reaching a distant observer or antenna would come practically from the incident wave alone. 377-ohm earth may be hard to come by. Sheets impregnated with carbon were invnted by Winfield Salisbury at the Harvard Radio Research Laboratory during WW-2. These cloth sheets are patented and are called Salisbury sheets. A similar technique was contemporaneously developed in Germany by J. Jaumann. So it`s not likely to just happen upon completely non-reflective soil. But, surely most soil absorbs some energy from the wave striking it. So, the reflection from the soil will be weakened and will be unequal to the task of completely canceling the incident wave. Cancellation at zero degrees will be incomplete. Best regards, Richard Harrison, KB5WZI |
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