Dipole or loop?
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| Aug 19th 2014, 03:02 | |
KO0YTotal Topics: 0 Total Posts: 0 |
My QTH is good in that I have a large area with many pine trees in back of the house, to put up wire. It is bad in that none of the trees are very high (30 feet or so), and the earth is poor (granite mountainside). I have a Hustler vertical with many radials for 30 meters and above, and use wire for 40 meters and down. I am active in MARS, so transmit outside the amateur bands as well. At present, I have a 150' dipole, center fed with 300 ohm ladder line to a 4:1 balun in the shack. It works ok, but I often have to resort to the mechanical tuner as the electronic tuner (LDG YT-100) cannot handle the mismatch. The run is not long, about 90' total, so I think I should replace this setup with coax run to a good 1:1 balun at the center of the dipole. (Someone tell me if I'm wrong on this). My other question is whether I should keep the dipole as it is and just replace the feed line setup, or string a horizontal loop. I could probably hang as much as 500-600' feet in 4 to 6 trees. Would that work better than the dipole? The objective is not resonance, since I operate over a wide range, but as much as possible of the HF band under 10:1 SWR so the electronic tuner can handle it. Also, I would trade DX for omni-direction radiation. Thanks for any help. |
| Aug 19th 2014, 14:41 | |
W1VTSuper Moderator Total Topics: 0 Total Posts: 0 |
While a larger loop has more resonances in the HF spectrum, the resonances become more narrow, so I doubt the large loop will meet your SWR objective. YT-100 specification Tunes 4 to 800 ohms. (16 to 150 on 6M) This suggests using a 4:1 balun at the feedpoint of the dipole. This isn't ideal--at low impedances the loss of a tuner tends to be much higher, but it may work well enough for your needs. Beware of SWR drift--this indicates that a lot of power is being lost in the balun--which is actually quite likely given the limitations of balun technology. Zack W1VT |
| Aug 19th 2014, 14:58 | |
N0NBTotal Topics: 0 Total Posts: 0 |
I will guess that your objective is more for local coverage than DX in which case a 30' height is reasonably close to optimum for 80/75m and will still work well enough for NVIS on 40m. When I first tried the all-band doublet style of antenna I used 300 ohm ribbon line as well--the type with the solid poly insulation, not foam. After a year or so I bought a roll of the 450 ohm "window" line from a local dealer and installed that in place of the 300 ohm stuff. I was pleasantly surprised with the improvement in signal strength and the tuning action improved on my small MFJ manual tuner. A month or two later I replaced the small 941C tuner with an MFJ 986 differential T tuner and gained another measure of improvement. I have since replaced the 986 with a Palstar AT1500DT (no longer made) that offered a measure of improvement with ease of tuning and is definitely a higher quality unit. My take away is thus: * Use 450 or 600 ohm line (600 may need to be constructed) * Use the biggest/baddest tuner you can afford to reduce losses as much as possible and have increased tuning range. I've now happened on a third item to that list and that is to make a doublet at least 3/8 wavelength in overall length on the lowest band of interest. I am using a doublet with a 204' total flattop on 160 and 80/75m (it also works well on the higher bands although tuning can be tricky and lobes may be to some direction other than one desires). As to a loop, they work. I have not had the area available to put one up myself but I have seldom worked anyone on the low bands using a loop who did not have an excellent signal. Since you have the trees available, give a loop a try. There will be plenty of sources available on the Web to give you some good ideas. At least for a time you will probably want to keep the dipole although I would suggest upgrading it to 450 ohm line and you'll likely want a beefier tuner. Perhaps even one of the balanced line jobs (I've not used one of them either). You can then build your own dossier of comparison between the doublet and the loop. Have fun! 73, Nate N0NB.us |
| Aug 19th 2014, 15:25 | |
KO0YTotal Topics: 0 Total Posts: 0 |
Thanks, Zack; I'll continue with the dipole and just replace the feed line. I am planning to use one of the Comtek baluns sold by DX Engineering; these have excellent reviews on eham, and seem well-constructed. Their site recommends 1:1 for dipole, but you are saying that may supply out-of-range impedance to the tuner. I also have a mechanical tuner (MFJ-949E) and that will tune a broader range, but it is slower, and I can't use it with ALE. So I guess my question would be: how much power might I give up for the convenience of the auto-tune? By the way, I operate at 100 watts with no intent to use an amp. Thanks again for advice. |
| Aug 19th 2014, 15:33 | |
KO0YTotal Topics: 0 Total Posts: 0 |
Thanks, Nate. My understanding now is that the function of ladder line is to reduce loss on long runs from the shack. Since it is only 90 feet at most, it would be better to hang a balun at the center point (which is supported by a mast), and use 213 coax from there. Please correct me if this is wrong. |
| Aug 19th 2014, 15:38 | |
W1VTSuper Moderator Total Topics: 0 Total Posts: 0 |
You could easily lose half the power in the balun--typical baluns are designed for 200 ohms at the output, not 2000ohms. Ideally, you would use a balun optimized for 1.8 to 8MHz, rather than something that tries to cover the entire HF spectrum. A good all-around compromise, given that you have a radial field already installed, is an inverted-L. The feedpoint impedance is roughly half that of a dipole of twice the length, plus a bit of ground loss. With an autotuner, you typically want to avoid multiples of resonant half wavelenths, to keep the impedance down and within the autotuner matching range. http://www.g8jnj.net/usingautotuners.htm If you are using a longer length of wire, perhaps as a Sloper or inverted ‘L’ good lengths to choose are either 19.4, 22.8 or 34.3m long, as these avoid high impedances on most of the Amateur bands from 160 to 6m. Zack Lau W1VT ARRL Senior Lab Engineer |
| Aug 27th 2014, 15:14 | |
KO0YTotal Topics: 0 Total Posts: 0 |
I bought a 1:1 maxi-core balun from DX, and (for now) left everything else the same(150 foot dipole, 90 feet 300 ohm twin lead to shack). I used a RigExpert to measure SWR and Z with this balun. From 2 to 10 MHz, more than half of the range is 200 ohms or lower, so I guess it's good that I didn't get the DX 4:1 balun. Z peaks at 1200 ohms around 4.2 MHz, which is unfortunately near my primary MARS frequency. I notice that it is difficult to tune there, and cannot get SWR much below 2:1. The antenna is resonant at 3.62 MHz, which is higher than the length of 150 feet would suggest. Is this because it is low (22-24 feet) off the ground? I could hang a longer dipole, but not higher. DX suggests 220 feet for a multi-band non-resonant antenna, and I could go as long as 260 feet. Would that shift the impedance peak, as well as resonance, downward? |
| Aug 27th 2014, 20:46 | |
W1VTSuper Moderator Total Topics: 0 Total Posts: 0 |
You are seeing a resonance shift because of the mismatched feedline. A shorter length of 300 ohm feedline may move the resonance up and make the impedance at 4.2 MHz easier to match. But, accurate prediction are hard, given the way wire antennas interact with the surroundings. You may want to experiment between several of the most practical antenna choices for your station--combinations of wire and feedline lengths. Take good notes. Then, go back to the one that works best. If you have a lot a room, it may make more sense to put up two (or more) antennas to meet your needs. Zack W1VT ARRL Senior Lab Engineer |
| Aug 29th 2014, 13:28 | |
W1VTSuper Moderator Total Topics: 0 Total Posts: 0 |
Another option may be to put up a wideband 80 meter antenna. http://rudys.typepad.com/ant/files/antenna_broadband_dipole.pdf Here is a clever open sleeve design by Rudy, N6LF--that actually covers frequencies above and below the 80M band! |
| Sep 1st 2014, 02:05 | |
KO0YTotal Topics: 0 Total Posts: 0 |
I shortened the feed line by about 14 feet, which did shift the impedance curves upward enough to make the 4.2 MHz region easier to tune. I wish I could attach those graphs here, but doesn't seem possible. Two dipoles would be better, I can see, but that is a project that will have to wait. Thanks again for all comments; I'm beginning to understand this a bit better now. |
| Sep 5th 2014, 03:43 | |
tuulenTotal Topics: 0 Total Posts: 0 |
Hi Zack, The link you posted looks interesting http://rudys.typepad.com/ant/files/antenna_broadband_dipole.pdf I have plenty of room to put one of those up, in regard to the length of the antenna, and can probably figure out how to get it up to the 70 foot height discussed in the article. BTW, there has been a fair amount of discussion about sunspot cycles and why the coming years could present a particular challenge to radio operation. There is a lot of guesswork and speculation involved about that, but it is said that the high bands could take a hit and even 20 meters could become troublesome. It also is said that low sunspot numbers and low bands go together and that now is a good time to get low band antennas set up and ready to go, at 30, 40, 60, 75/80 and 160 meters. And so that link to an 80 m antenna that you posted http://rudys.typepad.com/ant/files/antenna_broadband_dipole.pdf comes along at just the right time, thank you. Doug |
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