How to use a 1:1, 9:1 UNUN and a tuner?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W4AFK, Jun 20, 2021.

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  1. W4AFK

    W4AFK Platinum Subscriber Platinum Subscriber QRZ Page

    What's the best way to minimize RF coming back into the room:

    Radio --> | window pass-through | Outside | 1:1 UNUN --> LDG RT-100 Tuner --> 9:1 UNUN --> Short HF antenna

    or

    Radio --> | window pass-through | Outside | LDG RT-100 Tuner --> 1:1 UNUN --> 9:1 UNUN --> Short HF antenna

    For low power ops from a condo (20-30W max)
     
  2. AA5MT

    AA5MT Ham Member QRZ Page

    First, get the rf out of the house, far out. Never run a random wire directly into the back of a tuner. It works, but rfi can be extreme. It starts radiating right there. When i ran into this, I put an extension cord on the rf to get it away. My rfi was horrible until I do this.

    My solution was to run a coax to the top of a tree. I then put ferrite beads right at the end of the coax. I then put a connector on. My plan was to use different lengths of wire for different bands. I put the antenna wire on a connector and screwed it into the coax. I walked off the length of random wire and estimated the length down the tree at the far end. It came up to around 250 feet. To my surprise, it resonated at 1840 KHz. I used it with a tuner on all bands, and never changed out the length. Sometimes you get lucky. So this is my version of a top fed inverted L. My favorite contact was with W1AW portable on 160 one night. I was very excited to work them, and received a 57 report from them. I was even more excited after the contact when i realized that I forgot to turn the amp on. I worked them with my drive power only, at 35 watts.

    Tom
     
  3. KG7WGX

    KG7WGX XML Subscriber QRZ Page

    Some recommend the CMC choke be reasonably close to the rig.

    Also, "short" HF antenna length must still not be a resonant length if you are using a 9:1 unun. There are tables, but 35 ft is one of the shortest lengths which works.
     
  4. W4AFK

    W4AFK Platinum Subscriber Platinum Subscriber QRZ Page

    Tom, I am limited to what I can use antenna wise in this condo. I'm on the 3rd floor and threw a ~35 feet insulated wire into a tree which is lower. I run a coax to outside the window via a window feedthrough, and then connect the coax to an outdoor LDG RT-100 tuner. The output of the tuner goes to a LDG 9:1 UNUN, then to the antenna. I found a LDG 1:1 UNUN (RF choke) in my junk-pile and wanted to add it to minimize RF. My question is to where it would be best? Before the tuner? or after the tuner and before the 9:1 UNUN?

    Per my experience, the 1:1 UNUN (choke) should be before the tuner as I want to minimize RF coming down the coax to the radio. Total coax length is around 20 feet. I don't really have any RF issues as I am running 5-20 W max.

    As for ops, I use my IC-7300 on FT8 at around 15W and a WSPR beacon at ~5W. It works fine. Mind you, I have a full 1kW+ remote system located in Florida (see my QRZ page). I am also preparing to setup a remote station around 30 miles from my condo. My home/condo use is to be able to have fun if the Internet is off-line or if I just want to enjoy the challenges of low power and less than optimized antennas :)
     
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    If you plan to put a coax Common-mode choke on the coax between the ham rig and the remote tuner, especially if that choke is closer to the tuner than to the rig, then there has to be a "ground plane" (counterpoise) connected to the chassis of the tuner, which is missing from your scenario.

    The antenna should be a Marconi, which is a 35ft wire driven against earth which would be close to a resonant antenna for 40/15m. As such, there is no need to have a transformer between the tuner and wire; let the tuner match impedance.

    If you choke the coax close to the tuner, you have a weird off-center-fed dipole, consisting of a few feet of coax shield (the CM path on the outside of the coax between the choke and the tuner) plus 35ft of wire. The tuner will likely tune it just fine, but that short piece of coax radiates as much as the rest of the wire, which is why you do not want it coming through the window inside the shack...

    IMO, dump the 1:9 (it just gets in the way and prevents the tuner from doing what it is designed to do). Put the CM choke at the window sill, and make the coax between the 1:1 choke and the tuner as long as possible, ideally 10s of feet. Remember that the control cable that powers the tuner has to be choked, too. Best would be to send power to the tuner up the coax using a bias-T.
     
  6. W4AFK

    W4AFK Platinum Subscriber Platinum Subscriber QRZ Page

    Thanks for the info.

    - I use the RF based LDG RT-100 tuner and power is through the coax via a Bias-T. I use clamp-on ferrite chokes on all DC and control lines.

    - The "antenna" is ~35 feet of wire that I threw into a tree near my 3rd floor Condo window. The tree is a bit lower so the wire is at around 45 degrees and pointing downward. Keeping power low to prevent RFI to neighbors and for near-field RF protection. The antenna needs to be VERY stealthy.

    - This IS a compromise setup but my choices are very limited. Luckily, I have my remote in Florida which I have lots of fun with. When I setup my SoCal remote it will give me even more options., I already have the radio, amp and ancillaries. The decision will be on the antennas, although I am edging towards a GAP Titan DX (like the vertical I use in Florida) plus a Horse Fence based inverted V for 75m.

    LDG 1:9 UNUN - You suggest that I dump it while others say that it's fine as it "supposedly" makes it easier for the tuner. I do have a pseudo-counterpoise, which is ~ 20 feet of wire wrapped around the metal window rail, this is guard rail like on a balcony, but it's offset around 2 feet from the window. The output of the RT-100 is a SO-239 so I used the LDG 9:1 UNUN to connect to the tuner and to get the wire connections. I have not had any issues with this. Mind you, I cannot vouch for efficiency apart from stating that my 5W WSPR signals are heard on 40m across the USA, Europe, Australia, South America and Antarctica, so it is radiating "something". With ~5W RF and the inefficiencies of my system on 40m, my ERP might be closer to ~1W LOL.

    LDG 1:1 UNUN/Choke - The coax run from the radio to the tuner is around 20 feet (no choice) and I do NOT have any serious RF issues. I noticed some on 30m and 20m at certain times, hence the idea to install the 1:1 before the tuner. I'll place it outside, just before it enters the room. I can add length to the coax but it just be coiled inside as the radio is not far from the window.

    ============================

     
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    What are your main bands?

    This suggests that it is better to put a 50:450 Ohm transformer between the remote tuner and the wire.

    In the model, Wire 1 is the 35ft sloped wire. Wire 2 is a place-holder for the feedpoint; the tuner followed by a transformer. Wire 3 is coax shield, the first 20ft of which is part of the radiating antenna between the tuner and the choke, while the part on the rig side of the choke is assumed not to radiate. This assumes a "really good CM coax choke", like a dozen turns of coax through the window of a FT240-31 core. Ferrite Clamp-ons do not provide enough choking; you would need twenty or more to get what one proper (coax wound on ferrite core) CM choke can do.
    upload_2021-6-20_15-47-59.png

    Here is what the tuner would have to contend with and without transformers.

    T is the transformer turns ratio variable. The transformer nominal primary impedance is 50 Ohms. S is the transformer secondary winding impedance in Ohms, which is a function of T. You can see what the transformer does to the R and jX seen by the tuner first with effectively no transformer (T=1, 50:50), then a 50:200 Ohm transformer (T=2), and finally with a 50:450 Ohm transformer (T=3):
    upload_2021-6-20_16-2-25.png

    The tuner would struggle with Z=1547-j1583 Ohms on 20m without either a 50:200 or a 50:450 transformer. The problem stems from the fact that 20ft of coax + 35ft of wire is approaching half-wave anti-resonance on 20m. Adding the transformer drops the R almost too low on 40m, but the tuner can deal with that better than the no-transformer case on 20m.
     
    Last edited: Jun 21, 2021

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