Two new ham questions about antennas
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| Dec 9th 2012, 16:56 | |
KO0YTotal Topics: 0 Total Posts: 0 |
Aced my General test yesterday, thanks to the excellent study guide published by ARRL. I am setting up a FT-857D, and have two questions about antennas, one HF and the other VHF. HF- I will use a center-fed V-dipole at least 100' long and put as high in the trees as I can. The transmission line to the house must be about 70' and I have 300 ohm twin lead for this. At the house, the twin lead will feed an LDG balun, then 25' of LMR-400 to an antenna tuner and the radio. The dipole diagrams I've seen show twin-lead feeding each half of the dipole, but the LDG manual shows one wire to the antenna, and one to ground. Which is correct, and if the latter, should I short the twin-leads at the balun, and run a separate wire to ground? The balun is right next to the ground rod, and there is a transi-trap between the balun and the coax. VHF- We live in a park (Western term for a place completely surrounded by mountains, also known as a hole). The entrance is not far, and offers a view of the flatlands of eastern Colorado stretching to Kansas. I could put an antenna there among the trees, but the transmission line would be 150-200' long. Is this feasible, and if so, what line and antenna should I use? The view from there is almost 180 degrees. Thanks for any help. |
| Dec 9th 2012, 20:33 | |
W1VTSuper Moderator Total Topics: 0 Total Posts: 0 |
A balun goes between the twin lead and the open wire. The tuner works best if it is near a grounding point. The popular multiband baluns use ferrite loading to achieve a wide bandwith--this works well, but only if the impedance is kept low. If the impedance gets too high, the SWR will drift as the balun heats up and eventually fails. In the old days, hams would use a half wave on their favorite band--but this often results in balun failure. A more modern approach is to compromise, and chose a length that isn't a half wave on any amateur band--due to the harmonic relationships of the amateur bands--choosing a half wave on a popular ham band results in a full wave high impedance dipole on its harmonic. I'd suggest using either the popular G5RV or ZS6BKW antenna designs for the length of the dipole and length of the twin lead feeder. http://www.nonstopsystems.com/radio/ZS6BKW-2.pdf Design details on the ZS6BKW antenna You can get more range on 2M by using SSB/CW--but you need to use a horizontally polarized antenna--such as a turnstile or horizontally polarized Yagi, as opposed to the vertical commonly used for FM. 200 ft of coax isn't excessive if it has a much better view of the horizon. Most hams use a low loss 1/2" coax such as LMR-400 or 9913F7. Surplus Hardline or large diameter CATV coax may also be an option--as the stuff isn't recyclable, hams have been able to get the stuff for cheaply--though not everyone is physically up to working with large coax-. Zack Lau W1VT ARRL Senior Lab Engineer |
| Dec 9th 2012, 22:07 | |
W0BTUTotal Topics: 0 Total Posts: 0 |
I think Zack was tired when he typed that. :-) "Balun" means BALanced to UNbalanced. A balun generally goes between a balanced line (or balanced antenna) and unbalanced coax. Depending on (1) the length of the antenna, (2) the length of the balanced line and (3) the impedance of that line, the impedance at the tuner and/or balun may be too high or too low to match on some bands. I agree with Zack about the G5RV and ZS6BKW. Or, you can build my version ("The W0BTU Antenna") which uses inexpensive 75 ohm coax and covers more bands than either one of those. See http://www.w0btu.com/g5rv_antenna.html#w0btu_antenna . It has a reasonable SWR on all HF bands between 80 and 10 meters except 30 and 60. There is a complete set of SWR curves for each band there. I used a ferrite-core broadband 1:1 current balun (choke) right at the junction of the window line and coax to keep RF off the outside of the coax shield. We used one of K9YC's designs using about 6 turns of the 75 ohm coax feedline (RG-6 or F-6 CATV coax) wound through 4 or 5 stacked Fair-Rite 2.4" diameter #31 ferrite cores. The 75 ohm coax can be any length needed from there to the tuner. If you build my version using 300 ohm twinlead instead of what we used, its length will have to be different. 73, Mike www.w0btu.com |
| Dec 18th 2012, 21:12 | |
KO0YTotal Topics: 0 Total Posts: 0 |
Focusing on the HF- I put up a dipole, but it doesn't work very well. It is in a V configuration, with the center feed up about 30 feet and the ends about 8 feet off the ground. The 12 AWG solid copper wire is about 100 feet long, fed with 68 feet of 300 ohm twinline, That connects to a 4:1 balun and thence via 25 feet of LMR 400 to an LDG YT-100 tuner and 857D transceiver. It works in reception, but the tuner shows SWRs far too high (5:1 or more) when tested in AM mode. Is it that my feeder is too long? I could hang the center from the house (stucco over concrete) and that would reduce the feed length up to the balun to 12 to 16 feet. The angle between the legs of the dipole is 60 degrees. Is that too sharp? Thanks again. |
| Dec 19th 2012, 12:12 | |
W1VTSuper Moderator Total Topics: 0 Total Posts: 0 |
I don't know whether your feeder is too long--you might experiment with making it a little longer as well as shorter--and see what works best for you. Inventing new designs by trial and error is a popular amateur activity. That is one of the great things about amateur radio--nobody is forcing you to copy a proven design if you want to try something different. 60 degrees is on the sharp side--you get current cancellation and higher losses. But, a compromise antenna is always better than no antenna. The Antenna Book comes with Transmission Lines for Windows, a program that can take impedances measured with an antenna analyzer and figure out the SWR and impedance at either end of a transmission line--this can be very useful for figuring out the optimum transmission line lengths for you station. Zack Lau W1VT ARRL Senior Lab Engineer |
| Dec 25th 2012, 18:21 | |
KO0YTotal Topics: 0 Total Posts: 0 |
I will try trimming the line length slightly to match Z66BKW and I can probably open up the angle to 90-100 degrees. The 300 ohm feedline is actually 75 feet long and connects to a 4:1 balun. My understanding is that the transmission line is a loss factor but not a resonant factor since the impedance is known. Am I wrong here? Regarding the VHF antenna- I measured out to my view site and it is 350 feet from the radio. Is this practical even with a low-loss coax? Could an amplifier be used here? Thanks again and Merry Christmas to all! |
| Dec 26th 2012, 18:21 | |
aa6eTotal Topics: 0 Total Posts: 0 |
For VHF 350 ft is too much for normal coax: 14.5 dB loss for RG-8X at 144 MHz for example. (See Times Microwave Coax Calculator.) That means that 97% of your transmit power is lost, and (just as crucial) your receive sensitivity is much diminished. Open wire lines are a lot better, maybe under 3.5 dB for 350 ft, see this info (pdf). However, maintaining the line dry and well supported (away from absorbing or reflecting obstacles) is not always easy. A bi-directional amplifier close to the antenna should help considerably, but it is still going to be tough if you're thinking of reasonably priced coax. You're on the borderline, but for very long separations people start using remote base technologies. I.e. put your transceiver at the antenna site and use wires (or wifi or whatever) to communicate audio and control signals. 73 Martin AA6E |
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