Two more dumb antenna questions about a steel boat (tuner as dipole center connector?)

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KB0ATY, Mar 27, 2021.

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  1. KB0ATY

    KB0ATY XML Subscriber QRZ Page

    I don't know anything about radios so these questions may be dumb! I appreciate your patience.

    I am continuing my project to install a HF radio on my steel boat. I have a now bought a used Icom M802 and an AT-140 tuner. I have set it up at home to test it and it works!

    I know the two frequencies I am most interested in using: 7.268 and 14.300 MHz.

    The AT-140 antenna tuner will tune anything, and it is now connected to a random wire hanging from a tree to simulate my mast. But I was thinking...

    First question: If I'm operating at the same power, on the same frequency, with the antenna in the same position and configuration (hanging from a mast), a resonant antenna cut for the frequencies I want should perform better than a random length of wire tuned with the AT-140, other things being equal, right? (Is this true?)

    Some of the mariners on various forums think so and what they do is essentially use the AT-140 as the center connector for a dipole. They connect one wire of the dipole to the AT-140 "antenna" port and one wire of the dipole to the AT-140 "ground" port then turn on the AT-140 "THRU" function (deactivating the antenna tuner) when they are on the resonant frequency. They report success.

    So I cut two wires to what I thought was the right length and tried this, but I cannot figure out how to measure the SWR to trim the wires with the equipment that I have. I have an Ameritron AWM-35B SWR/Wattmeter (just bought it).

    When I key the mike to transmit at a low power level to measure the SWR with the AT-140 shut off ("THRU"), the SWR meter always starts to rise then it stops at about 1.2 and goes back down again. The Icom display then reads "SWR." The "REF IND" light comes on the SWR meter. I'm guessing the Icom is shutting off the transmission due to the SWR. My issue is that I don't know how to measure the SWR because I tried 8 frequencies spanning 2 MHz to 30 MHz and they all produce the same behavior and the same reading.

    When I have the AT-140 on, I don't read any SWR and the radio works.

    Am I misunderstanding how to measure SWR to trim the dipole or is there something wrong with my whole plan here that I am not seeing? Alternately, I was thinking I could take the AT-140 out of the center and use a more traditional dipole center connector, but OTOH it feels like I should test it as close to the actual boat configuration as I can.

    I have a patch cable from the Icom M802 to the SWR/Wattmeter, a 50' RG-8X running from the SWR/Wattmeter to a MFJ-915 line isolator (1:1 ferrite choke), then a patch cable to the AT-140 feed. The antenna wire is 14 AWG stranded copper.

    Boat layout in case relevant for antenna ideas:
    [​IMG]

    Another option would be I could raise two antennas and buy an antenna switch, having the AT-140 only on the non-resonant antenna. If that would be better for some reason let me know! I was trying to avoid two antennas.

    I've tried figuring this out on my own but I think I know too little about the area to make sense of the things I am reading/watching. The boating forums strike me as full of weird theories and conflicting information so I thought I would try the ham forums. I really appreciate any help!
     
  2. SM0AOM

    SM0AOM Ham Member QRZ Page

    From the dimensions of the wires and the boat, the first current resonance (1/2 wavelength) should be in the vicinity of 7 MHz.

    However, the resonance point is quite narrow, so you need to be within +/- 100 kHz or so to come within the 3:1 SWR circle.

    Further, the resonance frequency is affected by the proximity of the steel hull and deck in a somewhat hard to predict way.

    14 MHz would be in the vicinity of a voltage resonance which creates a large mis-match, which even may be difficult to match for the AT-140.

    My advice is to measure the SWR at frequencies spaced by 50 kHz, starting at say 6700 kHz up to 7400 kHz and find the lowest point. Then decrease the spacing to 10 kHz in order to find the minimum between two 50 kHz points.

    This is close to the lowest resonance frequency of the antenna,
    and you can then determine how much that must be trimmed off or added to reach actual resonance at 7268 kHz.

    The new length would also work at around 21 MHz, but would probably need the tuner.

    Main drawbacks of using the tuner is that it introduces some losses. For a medium-power tuner such as the AT-140, they can be estimated to be around 30% or - 2 dB on 14 MHz with the indicated dimensions.

    The losses need only to be considered in the transmit direction,
    as the noise level on a typical boat is far above the atmospheric environment.

    73/
    Karl-Arne
    SM0AOM
     
  3. PU2OZT

    PU2OZT Ham Member QRZ Page

    No «dumb» question here, questioning is a clever process.
    Never installed a dipole, so let others, and actually many, many others, resolving your station, making the Icom gem transceiver «happy» on the frequencies you elected, booming max power.
    Was a remote SWR-Power meter as the Ameritron an absolute need? only asking because you're then adding a non-marine element in your system.

    Oliver
     
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    Christian,

    In your drawing, is the 33' backstay and the 34' forestay effectively available as to be the antenna routing?

    Are the stays required to brace the mast mechanically?

    Are the stays free and clear with no other metal close by except the hull of the boat and the mast?

    Can you mount the remote antenna tuner at the top of the mast and run a coax feedline up the mast?

    Would it be easier to mount the remote tuner at the mast-top, the hull end of the forestay or the hull end of the backstay?
     
    Last edited: Mar 27, 2021
  5. KB0ATY

    KB0ATY XML Subscriber QRZ Page

    It could be removed certainly and I guess it adds loss? I put it there because the boat HF SSB installations I see on YouTube have an inline SWR/wattmeter -- probably for reassurance that everything is working OK and power is going out? I have also seen the explanation that saltwater corrosion and vibration constantly attacking everything makes it useful to have the inline SWR meter to help you notice abnormal reflected power -- an indication that something in the antenna system has come loose, gotten wet, etc.?

    Or did I misunderstand and you mean I should get a "marine" SWR meter?
     
  6. KB0ATY

    KB0ATY XML Subscriber QRZ Page

    Thanks -- Yes I should have said when I was thinking "dipole" my plan was to put the AT-140 at the top of the mast, that would be the center of the dipole and the forestay and backstay would be the wire elements.

    I was planning on trying just hanging the wire along the stays to start out and see how I do as I am not eager to replace the stays. I see on boat forums people can (reportedly) sometimes get excellent performance by just hanging wire along the stays. So I thought I'd just try hanging the wire first before replacing anything.
     
    PU2OZT likes this.
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    I added more questions to post #4, so respond to those.

    Are the existing stays made from a conductive material? If so, what?

    Are the existing stays electrically bonded (connected) to the hull and mast?

    If yes, are you prepared to electrically isolate then from the hull and mast by inserting some sort of "compression insulator" like these?

    https://www.3starinc.com/images/super/ROHN_502_800x600t.jpg
     
  8. KB0ATY

    KB0ATY XML Subscriber QRZ Page

    Thanks for your assistance, here are the answers:

    Yes I think so. The mast is very sturdy but on the other hand the stays are taut. They may be ready to brace the mast in difficult conditions. If I decide to try and replace the stays I will check with a rigger first (see below).

    Yes, stainless steel wire rope.

    There is no bonding strap or anything like that that I remember seeing but I guess you wouldn't need one? The mast is steel (unusual) and it penetrates the steel deck but stops short of the hull. The steel deck is attached to the steel hull. The whole rigging is stainless steel (lines, eye bolts, tensioners).

    There is a lightning protection system on the mast right now. I do not know what it is using for the ground terminal. I am away from the boat right now as it is still in storage for the winter.

    Sort of -- as I said in post #6 I could do that but I was thinking of trying hanging some wire first. I see a lot of advice about replacing the stays being mandatory to avoid the stays acting as parasitic elements. But there are many other reports that reply: there are so many parasitic elements in rigging and on boats that it is impossible to predict what will happen, just hoist a wire first and it will probably be fine. So I thought I would try that first.

    Also the mast is also a folding mast (to accommodate low bridges) so I am not sure how well an insulated backstay would work in this situation. But folding the mast down is very rare.
     
  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    From an antenna design and performance viewpoint, having both stays electrically connected to a metal hull at one end, and to a metal mast at the other end, is a disaster.... There are effectively two "loops", one involving the forestay, the other the backstay.

    My experience with loop antennas says that to recruit one of the existing "loops" as an antenna structure requires electrically "cutting a stay", inserting a compression insulator at that point to retain the ability of the stay being used as a "guy", and then feed RF current between the two cut ends of the opened loop.

    A preliminary simulation shows (putting a single compression insulator where the backstay connects to the mast top), and then feeding the resulting "loop" (backstay, aft hull/deck to mast, up the mast) between the electrically-isolated back stay and the mast top produces a reasonable radiation pattern on both 40m and 20m. The feed impedance (not resonant on either band) should be well within the matching range of the remote tuner. Feeding the aft loop also produces current in the foreloop, so both loops act as part of the "antenna".

    I am not optimistic that dangling wires parallel to (and close to) the exisiting grounded stays will result in anything worthy of being called an antenna...
     
  10. KB0ATY

    KB0ATY XML Subscriber QRZ Page

    Thanks for your reaction. Well I certainly don't know enough to argue as I know basically zero. I made this plan after noticing posts from people who do hang wires -- let's call them "the danglers" -- and they report that it works for them. I could not explain to you why it does, but I think these are honest reports. There are also marine antennas for sale that look like they are "dangler"-style. Here is an example: https://gamelectronicsinc.com/products/gam-mckim-split-lead-antenna/ (I mean that this looks like putting wires parallel to a stainless steel backstay cable.)

    There aren't that many steel hull boaters on the forums talking about HF but from what I can remember I think the general feeling I got from mentions of steel was not "oh no, there is so much metal around!" but rather it was more like "wow there is so much metal around you will get much better performance." I don't know enough to say why it would be one or the other. Perhaps it is because fiberglass boats reportedly often suffer from ineffective RF grounding?

    I think I am still going to try "joining the danglers" first as it is so much less complicated. If it performs poorly I will definitely reconsider.

    My questions still stand though -- trying to figure out how to cut a resonant dipole with the AT-140 as the center connection and/or if that whole idea is flawed!
     

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