True Ladder Line 10-80 Antenna with Balun/tuner?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KB1QYH, Dec 31, 2018.

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  1. KB1QYH

    KB1QYH XML Subscriber QRZ Page

    Hello-

    I have a "True ladder line" brand 10-80 doublet antenna. It is fed with 600-ohm ladder line. I have not installed it yet, and I'm contemplating putting it up today. I have a balun designs 4116 3kw 4:1 hybrid balun. I plan on using about 20 feet of coax to get it into the shack. (Maybe 15) I know the coax losses will be very high, and I'm wondering what kind of performance I'll get compared to the "Myantennas 10-80 end-fed antenna I have right now. Losses are similar to 100ft of lmr-400 to a dipole as far as I can see. One benefit of the doublet will be that I will be able to get it much higher in the air- 80+ feet. What I would really like to do is buy a link-coupled manual tuner to use this antenna with, but I don't want to go the johnson matchbox route. Why aren't these made anymore? It seems to me like this is the most efficient wire antenna available. Why are link-coupled tuners not made by anyone anymore? I've read there is a lot of loss in the balun of the current Matching-T Tuners.

    Thoughts appreciated.


    '73,

    Jay
     
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    If you're talking about a relatively low loss coax like LMR-400 the losses shouldn't be very high at all. For instance if you run 20 feet of LMR-400 to your balun and the SWR50 looking into the ladder line is 20:1 the total loss in the coax jumper would be less than 1dB on 20 meters with roughly 3/4 of a dB related to the high SWR. Even if your 20' coax jumper was a piece of RG-8x you'd only expect around 1.5dB of loss related to the high 20:1 SWR on 20 meters. Sure, it's a good idea to keep that coax jumper to the balun as short and low loss as feasible but even a 20' jumper can be fairly low loss.

    That's the big thing here. Get any horizontal antenna up high and you'll see improved performance. I expect you'll be very happy with your open wire fed Doublet up at that height and wouldn't worry about the coax jumper especially if you run low loss coax.
     
    N2EY likes this.
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    There are balanced line tuners available today, like this one, which has more matching range than the old Johnson Matchbox: http://www.palstar.com/en/bt1500a/

    The MFJ-974 products are quite good also (balanced line antenna tuners).
     
    N2EY likes this.
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    How long will the ladder line be?
     
    W2VW likes this.
  5. KB1QYH

    KB1QYH XML Subscriber QRZ Page

    Such quick responses! Thank you all-

    K7TRF- Thank you for the comment. I read the article in the June 2015 QST regarding baluns, and it had quite a bit of info on loss. I'd like to get the least loss possible. And yes- Height is Might!

    WB2WIK- I didn't realize that! I was looking at the AT-5k, and the reviews are terrible! This balanced line tuner may be the way to go!

    WA7ARK- the 600-ohm ladder line is 100 feet long.
     
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    So I modeled a 125ft #16 wire 80ft agl fed with 100ft of 0.98vf 600Ohm OWL, with a balun between 20ft of 0.87vf 50 Ohm coax.

    Here is a plot of the SWR on the 50 Ohm coax as a function of the impedance ratio in the Balun. The 1:1 balun is the worst, I would say that a 1:9 or 1:16 balun is best overall (actually least bad):

    doub.png

    Here is the reason that I think that 600 Ohm OWL fed 80m doublets are not very good:

    doub1.png

    Note that the resistance seen by the tuner is quite low (~8 Ohms) on all bands except 10m. This is much worse if you select a 1:1 or 1:4 balun. This is the worst possible way to run a T-match tuner (where it has the highest losses); isn't a good place to operate the coax or the balun, either.

    The doublet-OWL-tuner crowd is shooting themselves in the foot most of the time, but this has become a religious argument. This why I play with coax fed antennas...
     
    K2XT likes this.
  7. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Not sure what's going on Mike, but I modeled the same system with the same parameters and ended up with a very different look at SWR50 across the HF bands when using a 4:1 balun at the coax to 600 ohm balanced feed line junction. Did you actually model this with a 1:4 balun like you'd use in a Cobweb antenna, IOW 50 ohm coax to 12.5 ohm load?:
    125 foot Doublet 80 feet 100 foot feedline 600 ohms plus 20 feet Coax and 4-1 Balun Custom.gif

    I agree the real portion of the load impedance is pretty low on 80m but overall this isn't a difficult load for a decent tuner to match on any of the HF bands.

    Here's the model I used based on generic 600 ohm open wire line and LMR-400:
    upload_2018-12-31_14-25-18.png
     
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    I used a 1:3 turns ratio, so (50 Ohms in) : (450 Ohms out), i.e., the tuner is looking into the 50 Ohm port through 20ft of 50 Ohm coax, and the 450 Ohm port of the transformer is connected to the 600 Ohm OWL.

    doub2.png
     
    Last edited: Dec 31, 2018
  9. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Fair enough, we modeled different systems. The SWR the tuner sees isn't half bad when using a conventional 4:1 balun (4:1 impedance ratio as baluns are typically described or 2:1 turns ratio).
     
  10. N2EY

    N2EY Premium Subscriber QRZ Page

    Well, there's the problem. You're modeling what most of us would call a 1:9 balun (impedance ratio). A 1:4 (impedance ratio) or 1:1 balun is a better choice on most bands.

    The real trick is to find a length of OWL that provides relatively low SWR on the bands of most interest.
     

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