Doublet - Coax Length

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by G5TM, May 21, 2018.

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  1. G5TM

    G5TM Premium Subscriber QRZ Page

    Hi all

    I am planning to install a 44 foot doublet fed by 300 ohm ladder line into a 1:1 or 4:1 balun and a remote tuner. The antenna is at the back of the property with the shack at the front. I have a 30x30 foot garden.

    A remote tuner is needed as I cannot bring the ladderline anywhere near the shack (wife wont have the unsightly run going over the roof of the house).

    The ladderline will be about 36 feet down to the balun with the balun being connected via a right angled PL259 adaptor to the tuner with a run of approx. 120 feet of coax from the tuner to the shack as it needs to snake around the house) I will be using low loss coax.

    Will the fact that the long coax run is after the tuner matches the antenna mean that losses should be fairly low?

    Thanks

    Tim
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    If the tuner can find a good match, the coax losses should be low.

    44' doublet fed with 36 feet of 300 Ohm balanced line will have native impedance at the "bottom" end of the balanced line that wanders all over the place, so hope for the best with the tuner.
     
    NH7RO likes this.
  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    The Z=R+-jX feedpoint impedance looking into 36ft of 300 Ohm ladder line (Vf=0.97), center feeding a 44ft horizontal dipole, 37 ft agl, over average ground.

    (clik to see bigger)
    DOUBL.png

    The Z is all over the place. Ladder-line fed doublets impose a terrible burden on a tuner!

    There is no one Balun that is going to help with impedance transformation, maybe only if band-switched.

    If the tuner is single-ended (unbalanced), then you are stuck trying to build a common-mode choke (to convert unbalanced to balanced) that can contend with a wide range of conditions. Some have placed said choke between the coax and tuner input. Others take issue with this approach.
     
    NH7RO likes this.
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Yes.

    If you plan to work 40m and higher frequency bands I'd suggest a bit longer doublet if you can manage it. I ran a 44' doublet for a while with a good manual tuner but when I switched to a remote auto tuner I had trouble achieving a good matching solution on 40m, the tuner would take a long time to tune and the final SWR once tuned wasn't that great. Extending the doublet legs to ~55' end to end fed with open wire feed line made matching with the auto tuner easy on 40m-10m. A good rule of thumb is to make sure a doublet is at least 3/8 wavelength long on the lowest frequency of operation and 44' is a bit shorter than that which can make matching a challenge.

    Depending on things like the balun you use and the characteristics of the balanced feed line you might find the 44' doublet works fine for you. But if the tuner struggles to find a good match on 40m then you might consider running longer doublet legs.

    The popularity of Cebik's 88' doublet and the 44' derivative are based on controlling pattern lobes on the higher frequency bands. That's a nice goal but doesn't matter much if you can't actually achieve a match to the antenna. Personally I'd try to use at least a 100' doublet to support 80m on the low end and 50' to support 40m on the low end.
     
  5. K8JD

    K8JD Ham Member QRZ Page

    I use a tuner made for balanced lines (MFJ974) and have a long run of 450 Ohm window cut twinlead and the tuner does the hard work, for instance on 60M I may have low Z maybe close to 50 Ohms at the feedpoint and on 30M it is high maybe up 2K or more Z. I use this 90 ft dipole on all bands from 5 to 18.1 MHz. with good results.
    I don't see using any tuner with unbalanced inputs and a simple "T" network for that ladder line fed antenna. Want to use A fixed ratio BalUn ? It just can't handle the very wide range of Z and X you will see at the end of the twinlead or ladder line >Going from band to band will give very different sets of numbers.
    . I have other dipoles up for 160 and 80M fed with coax lines. No tuner needed for them.
    I don't think your 44 ft dipole antenna will do very well below the 30M band.

    Good luck, it may work fine for you. Maybe build some more antennas and try them out too. Try some different designs and wee what works best for you.
     
    Last edited: May 21, 2018
  6. G5TM

    G5TM Premium Subscriber QRZ Page

    Great replies. I must admit I already use the 44 foot doublet with the ladderline going to a temporary shed/shack, into a good quality 4:1 Balun and the auto tuner. Can get a 1:1 match in 80-10 (I know 80 will be very poor radiator). Only 40 tunes to 1.5-2.0 SWR and I suspect the current 50 foot run of ladderline (almost 3/8 wavelength on 40) is providing a tough impedance for the tuner to work with). It works VERY well on 20 and 10 with 15 and 17 not being open for me to try at the moment. 40 is fine for around 1,000 miles and I'm ok with that as the doublet is in an inverted v shape with the apex at 30 feet. I'm able to work within the U.K on 80 and that band is simply a bonus. Thanks folks.
     
  7. W2WDX

    W2WDX Subscriber QRZ Page

    Understand that the impedance at the point where you connect the balanced line to the coax is going to have impedances all over the place. A balun is only going to be effective when the impedances are close to its design parameters. Most want to see 50 ohms on each side (for a 1:1 balun). If the impedance on the transmitter end of the balanced line is ranging from 20 to 1200 you will have very high losses on the coax at that length regardless of the balun transformation. And I am only talking about purely resistive impedance. Add to that the additional issues of reactive components. You may based on what you want to do is make the compromise of using a G5RV instead of thinking doublet. True multi-band doublets only work efficiently if the balanced line runs directly to the tuner (at the proper feedline length based on the doublet length) AND the tuner is an actual balanced tuner with no balun on its output. Otherwise, the better option is something like a G5RV.

    Maybe talk to the wife, beg, buy flowers and sweets ... something.
     
  8. NH7RO

    NH7RO Ham Member QRZ Page

    An all expenses paid trip to Mallorca is not out of the question here. Explain how her acceptance and understanding will make all the difference to you and you might succeed (emphasis on might).
     
    KC8VWM likes this.
  9. NH7RO

    NH7RO Ham Member QRZ Page

    Remember that it's OK to let the ends of a doublet hang down vertically---so maybe you could easily squeeze in that 55' length after all.
     
    KC8VWM and K7TRF like this.
  10. W1VT

    W1VT Ham Member QRZ Page

    A 48 ft doublet fed with 20 ft of 14AWG window line, like the G5RV, only smaller, makes a good compromise for 10, 20, and 40 meters. You can run a length of coax after the 1:1 balun that connects to the window line. Low SWR and good efficiency without a tuner is possible on 10 and 20 meters, while 40M will need a tuner.

    A 44 ft doublet is useful if you want to work the same path on 10, 20, and 40 meters, and can aim the antenna accurately in the desired direction. I don't think 10 meters is good enough to bother doing this.

    A longer doublet may have slightly less loss on 40M, while increasing the coax feedline loss on 20 meters.

    Zak W1VT
     
    Last edited: May 23, 2018

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