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Remote tuners, random length wire matter? and what about MFJ-998RT?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by NZ9Y, Dec 18, 2011.

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  1. NZ9Y

    NZ9Y Premium Subscriber QRZ Page

    I'm curious about remote feed point tuners. Does it even matter if an antenna attached to one is resonant? Would not a randon length wire at least 1/4 wave work just as well as a 1/4 wave tuned wire on the frequency of interest? For example, at 7.2 mHz, would a wire 32.5 feet long work any differently than one 35 or 40 feet long?

    Also I'm curious about the MFJ-998RT tuner. Anyone have experience with this? Any reason it shouldn't work as advertized? I'd like to use it at the top of a tuned shortened multi-band sloper to get full bandwidth without feedline loss.

    My concern is that when installed it will be in an location that will cost a bucket lift rental anytime I need to access it, so I want to know it will work pretty well. Another tuner would be considered if it can handle at least as much power (legal limit) and isn't way more expensive.

    Thanks!
     
  2. KL7AJ

    KL7AJ Ham Member QRZ Page

    Your understanding is perfectly correct. There is nothing sacred about a self-resonant antenna. The prime example is the extended double Zepp, which has considerable gain over a dipole, yet is non resonant. Depending on the tuner design, some types are better at tuning out large reactances, but it doesn't matter really where you do the resonating. Read the classic article by Byron Goodman, W1DX, called "My Feedline Tunes My Antenna." It was a classic in 1956, and should be required ready by every ham now.

    Eric
     
  3. KB4QAA

    KB4QAA XML Subscriber QRZ Page

    Some tuners can handle a wider range of impedances some can handle less. Same with power ranges.

    Random length antennas are chosen for the convenience in placement. It can be difficult to predict their characteristics even with some modeling. And modeling is really the only way to estimate if a given tuner can match it.
     
  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I've never used the 998RT and I think it's a pretty new product, so offhand I don't know anybody who has one.

    However, matching and performance aren't well related.

    If it could match a paperclip, that wouldn't make it a good antenna system.

    The thinking that matching at the feedpoint is better has some merit, with respect to overcoming transmission line losses; no doubt about that. But on HF, line losses can be held pretty low even if the line is long, and you know what you're doing.

    In lieu of a nearly $800 remote tuner, I'd probably opt for adjusting the antenna itself, or using a lower loss transmission line and a tuner in the shack.
     
  5. N0AZZ

    N0AZZ Ham Member QRZ Page

    Have a look at DXengineering's remote tuner they sell with there multi-band vertical that whole setup is sold in pieces or the whole kit.
     
  6. NZ9Y

    NZ9Y Premium Subscriber QRZ Page

    If its the one I saw, it doesn't handle enough power. In fact the only remote tuner I can find in production that handles legal limit is the MFJ-998RT.
     
  7. NZ9Y

    NZ9Y Premium Subscriber QRZ Page

    This much I've done as far as I can. There are, however limitations in doing this. Bandwidth is the biggest problem. I know on 80-160 meters my antenna is near resonant on part of these bands, and losses are minimized with good feedline and a low loss L tuner in the shack, but there ARE losses.

    I figured with an autotuner at the feedpoint losses could be reduced furthur, AND convenience would be great. I suspect having an antenna that is "in the ballpark" self resonant is still useful as its more likely an autotuner will match.

    Thanks everyone.
     
  8. W4FO

    W4FO Premium Subscriber QRZ Page

    I just received the 998rt for early Christmas gift from Wife and Son.
    It tunes my 84 foot inverted L on 80 -10 and handles 1300 to 1400 watts no problem.
    I have used the 998 on the same antenna for about 2 years with no problems.
    It is same tuner as 998rt just not in waterproof case.
    PAT
     
  9. N0AZZ

    N0AZZ Ham Member QRZ Page


    What are your early thoughts on this tuner? I have had very bad luck with anything MFJ and have sold off all that I owned and swore off them. But a legal limit remote tuner hum I have one end fed inverted L for 160m that is about 75' away from the shack it would be nice to feed it with LMR-400.
     
  10. W4FO

    W4FO Premium Subscriber QRZ Page

    Fred I just took the case off the 998,after two years of everyday use,it looks like new.
    I have had zero problems with this unit,all components,solder joints,first class.
    As noted the RT is basically the same tuner as the 998.
    Also it will let you no if you try to run 1500 plus.
    I keep the amp at 1300/1400 and no problems.
    pat
     
  11. KA4DPO

    KA4DPO Ham Member QRZ Page

    I have a dandy spread sheet calculator for use with auto couplers like the SGC series and the Icom AH4. Send me an Email and I'll forward it to you. You initial assumption that an antenna does not have to be self resonant is correct but if you are using an end fed antenna of any configuration then 1/2 wavelength must be avoided as the impedance at the feedpoint and hence the RF voltage are extremely high and can damage an auto tuner.
     
  12. KE5MC

    KE5MC Premium Subscriber QRZ Page

    Blind comment as there are no details on the installation.

    Use appropriately sized pulleys and UV resistant line. Then only one trip up to place the hardware. This has worked well with my dipole suspended from three points. Not the same weight you have to deal with, but I don't see why it would not work unless there is something about the mounting support location that makes hoisting impractical.

    Mike

     
  13. NZ9Y

    NZ9Y Premium Subscriber QRZ Page

    This is very good to know, thank you. I wonder if a sloper would qualify as well as the tower acts as the other half, sort of. I have alot to learn about slopers and haven't found much to study.
     
  14. NZ9Y

    NZ9Y Premium Subscriber QRZ Page

    Ahhh, but details were given, at least enough to know this wont work for me. The antenna is a sloper from the tower and that means it grounds at the feedpoint to the tower. I WISH I could use your idea, but its not a dipole.

    There seems to be so much misunderstanding about slopers. Maybe I have it wrong, but I understand a sloper to be half a dipole with the tower and beam acting as the rest of the antenna. Some call it a half sloper and call a sloping dipole a full sloper, but that makes no sense. A "half sloper" is a sloper. A "full sloper" is a sloping dipole, but it is a dipole just like an inverted V is a dipole. Or maybe I have it all wrong. :)

    Now whether slopers work or not is a different discussion. Mine does, at least better than my alternative, which is nothing for now.
     
  15. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    You have it right. In my experience (and I've tried a lot of this stuff many times with different towers at different locations) "half" slopers are quite tricky. "Full" slopers (sloping, center-fed dipoles) really aren't, and usually work just as you figure they would.

    I've had half slopers that worked very well, by coincidence and not necessarily by design. I've had others that didn't work worth a darn and just took too much tinkering for my patience level.

    An interesting spin on a "full sloper" when the tower isn't tall enough to actually do this properly is to make a "vee sloper," where the top is supported by the tower, and the bottom supported by a rope to a ground anchor, and the center is pulled out away from the tower -- by as much as 20-30-40 feet, to make the wire shorter vertically. This requires another rope and another ground anchor, and the resulting antenna looks like a sideways vee >. It's full sized electrically, but shortened mechanically by virtue of pulling the center far away from the tower. It works. As far as I can tell, it works about as well as a regular full sloper does if you had a taller tower.

    We did this with four of them hung off a 140' self supporting tower some years ago; the antennas were all for 160m and center fed slopers, all about 255' long or something. Obviously, a 255' long antenna is too long for a 140' tower unless you can place the anchor points very far away from the tower. So, we made them all > configurations, and fed them through a 4-port remote switch so we could "change directions" N-E-S-W and it worked. Not a lot of directivity, but a little bit, maybe 6 dB or so, and using that system it was the first time we broke 50 DX entities on 160 during one of the major DX contests (I think it was the CQ WW). Die hard 160 guys do better, but we're not among them; just trying to add to the score in a multiop, multiband contest station.

    So, you might consider the "sideways full sloper vee" configuration...it does work and the tower requirement is greatly reduced from what a standard full sloper would require.
     
  16. N4NW

    N4NW Premium Subscriber QRZ Page

    I am consider this to use on 160m Tee. What happens at 1501 watts?
     
  17. K8JD

    K8JD Ham Member QRZ Page

    wear asbestos gloves when you find out.
     
  18. K5RCD

    K5RCD XML Subscriber QRZ Page

    No big deal. Over 1500 Watts it will trip itself off line and must be reset via the power injector control (from in the shack).

     
  19. K4SAV

    K4SAV Ham Member QRZ Page

    Sloper performance can vary all over the place depending on the tower configuration and how much you do to minimize the losses. On a typical 50 ft tower with a tri-bander on top, if you just throw up a shortened loaded sloper for 160 meters you might be in the range of -10 dBi, or less. If you do all the good things including adding radials to the tower you might get it up to about -3 dBi. Not including tuner and feedline losses.

    If you already have a sloper and are just looking to widen the bandwidth, first calculate the feedline losses to see what the improvement will be. Coax loss on 160 meters is pretty low and adding an expensive tuner to reduce that may not be worth the trade. If you know the SWR at the input to the coax and the length and kind of coax, you should be able to calculate the coax loss, or at least very close to the real number. Just plug some numbers into this calculator to get the loss.
    http://www.ac6la.com/tldetails.html

    Jerry, K4SAV
     
  20. NZ9Y

    NZ9Y Premium Subscriber QRZ Page

    I guess this thread isn't too old to dredge up again, and I have a related question.

    I usually at least use a coax choke at most feedpoints. I know its not always needed, but I figure it can't hurt.

    In a remote tuner setup, would it help/hurt to run a coax choke right before the tuner? In other words, rig, needed coax feedline, choke, tuner, antenna.

    Could it help if I was unlucky enough to have my feedline be some multiple of whatever frequency I happened to transmit on to reduce the feedline radiating?

    Please.
     
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