hf radio direction finder?
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| Aug 4th 2014, 00:17 | |
tuulenTotal Topics: 0 Total Posts: 0 |
Hi, I have a small portable radio and have been a shortwave listener for some time, but I am thinking about getting a license and setting up a ham radio station and have searched the web for information about that topic, where it seems that a good antenna is a very important piece of equipment. Fortunately I have a small field by my house and enough room for as much as an 80m flat-top dipole antenna in any direction of orientation, and I am aware that a dipole has its strongest performance at an angle broadside to the length of the antenna, with performance which gradually decreases toward the ends of the antenna. But that leads to a question, as to how to find the best or optimal orientation direction for general purpose use, and it seems that a radio direction finder could be the right tool for that job. On the web I found historical information about World War I and World War II RDF equipment, then there are modern manufacturers specializing in military and other such heavy-duty RDF applications, and then for hf radio I found a limited selection of 80m RDF equipment and a lot of 2m "foxhunting" RDF equipment, but nothing in between those two bands and my main interest is in the 80m through 10m bands. I can understand that there probably is not enough demand for such equipment from amateur radio operators to get the attention of commercial manufacturers, but then I doubt that I could be the only person to ever have considered hf RDF equipment and so I am wondering if there could be any decent DIY homebrew hf RDF plans available. Any suggestions? |
| Aug 5th 2014, 14:11 | |
W1VTSuper Moderator Total Topics: 0 Total Posts: 0 |
Most of the time, the beam heading is along the great circle path. http://www.njdxa.org/dx-tools/beam-headings.php Here is a calculator that will create a list of custom headings for your station. For someone just starting out in the eastern part of the country, a good compromise is to point the antenna toward Europe--where there is a very high level of activity on the amateur bands. However, for many hams just starting out, it is more important to get the antenna high, rather than in a particular direction. A low antenna will have high ground losses, and a more or less omnidirectional pattern. You should try to get the dipole up at least half wavelength on the band you want--this is 35 ft on 20M and 17 ft on 10M. A second choice may be to orient the antenna to cover as much of the continental USA as possible. Zack Lau W1VT ARRL Senior Lab Engineer |
| Aug 5th 2014, 22:35 | |
N0NBTotal Topics: 0 Total Posts: 0 |
You're likely overthinking this a bit. :-) Until a dipole antenna is about 0.4 wavelengths above ground it doesn't resemble the classic donut pattern in the text books. For 80m this wroks out to roughly 100 feet above ground. Keep in mind that this means the entire antenna must be at least that high not just the apex. Zack gave you some good figures that are the minimum for a dipole to be a good DX antenna. However, a lower antenna on the low bands (160-40m) can work to our advantage for local coverage. The technique is now called NVIS (Near Vertical Incident Skywave (a military term as I recall)) or, informally, a cloud warmer. By having the antenna for these bands about 0.1 to 0.25 wavelengths above ground a lot of the energy will go straight up. That sounds useless, and it largely is for working DX, but it really shines when using these bands for "local" contacts. i.e. out to a few hundred miles or so pretty much omnidirectionaly. I will tell you that DX is certainly possible with such a low wire as I used a center fed doublet with a flat top about 200 feet long to work Amsterdam Island earlier this year on 80m. Amsterdam Island is near the antipode from this location (within a few hundred miles) and my flatop was no more than 22 feet above ground at its highest. Condititions were right and I happened to catch the opening at the right time, but that "cloud warmer" worked nearly half way around the globe that morning. No, that is not an everyday occurance! Now, you didn't give a hint as to your location. I've lived in the middle of the USA all of my life and I typically orient my antennas to favor the continental US and that often works well for Europe and Oceana as well. For really working DX on the low bands most ops will choose a vertically polarized array such as a four-square. Even a single vertical element will do far more for DX efforts than most of us can do with a dipole. One thing about HF antennas is that none will offer a pattern as sharply defined as a flashlight beam that the text books show. Even the best HF yagis will have some response to the sides and rear. Just about anything you can put up, especially if you have decent sized trees, will work stations. It's actually rather difficult to put up an antenna that doesn't work anyone! 73, Nate N0NB.us |
| Aug 8th 2014, 19:49 | |
tuulenTotal Topics: 0 Total Posts: 0 |
Zack and Nate, Yes, thank you! My now learning that a Great Circle antenna orientation is a good direction to begin with and then having a calculator available for custom headings gets me off to a great start. I was thinking it would be easier to monitor RDF equipment and to make a map of station locations than it would be to determine optimal antenna orientation by trail and error, by erecting an antenna, then taking it down and then re-erecting it in a slightly different direction, possibly again and again until an optimum direction is found. Stations generally always identify themselves and I know where some of those stations are, but apparently an east coast operator might have a west coast call sign or some such switcheroo and so many of the stations are known to me only by radio frequency. Anyway, I am going to go with your suggestion, thank you. After all, RDF equipment looks to be complicated and expensive, and then after its being successfully used it gets put on a shelf to collect dust forever. Yes, apparently antenna height does make a difference, and yes, apparently 40% of band wavelength is about the minimum mast height needed for good all-around performance. I earlier mentioned that I have enough space available for as much as an 80m dipole, but the mast for a 40m dipole is about as much as I can handle. For simplicity I prefer going with a wood mast supported by guy-lines, one which could be tipped over and lowered to ground height for any repair/service, than dealing with the legal and technical complications of a proper metal tower, and a wood mast of about 50 feet should be just fine for 40m use. And yeah, for quite a few reasons a dipole seems to be an antenna that I could get along with very nicely, not having ultimate performance such as a yagi could have but good enough for all-around use. I am in eastern Connecticut, about 40 miles east of Newington and where I can receive W1AW Lima Charlie, loud and clear. My main interest is in CW radio and while an 80m antenna would be great to have I can still get along just fine with 40m and the higher bands. Actually I am just getting back into radio, after many years, where in the early 1970s I was a US Army 05B Morse code radio operator. I have been listening to W1AW code practice to get my ears back into shape by listening to clear and clean high quality code and have been supplementing that with daily time using the Koch method on the Just Learn Morse Code website http://www.justlearnmorsecode.com Thanks and good to meet ya! 73, Doug |
| Aug 13th 2014, 13:02 | |
N0NBTotal Topics: 0 Total Posts: 0 |
You're welcome, Doug. Good to have you coming back into radio. I'm guessing that where you're located that you'll have little problem working the whole of the USA and Europe. Low band antennas need a lot of real estate before they begin to have the directional characteristics that are easier to obtain on the higher bands. 73, Nate N0NB.us |
| Aug 22nd 2014, 02:58 | |
tuulenTotal Topics: 0 Total Posts: 0 |
Hi Nate, Speaking of antennas, which lend themselves to so much homebrew innovation, etc., I see there is a forum here for "Antennas, feed lines, baluns and antenna tuners" and so I am going to redirect some antenna questions to that forum. 73, Doug |
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