New aerial plans

I mentioned in the blog last year that I needed a new aerial and that I had plans for a Carolina Windom.  Although I’ve got the Windom I don’t think I’ll be putting it up.

I’ve recently become a member of the Hadley Wood CG and have had a lot of discussion with their very experienced members and it seems as though a good solution for my QTH here is a doublet.  That’s effectively a centre fed aerial with the two legs being equal length fed with 450 ohm ladderline feeder directly back to an ATU in the shack.  The aerial is balanced, the feeder is balanced and the ATU will have a balanced input.  Logic dictates that it should be somewhat better than the end fed long wire I’m using on HF at the moment and providing I can get the overall length to be around 100ft then it should tune on all the bands that I’m interested in.  I don’t have a suitable tuner yet but I think that something like an MFJ-974HB would do the job nicely.

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6 Responses to New aerial plans

  1. Nigel says:

    A doublet will work fine but an Auto ATU will not give acceptable performance on all bands. The best bet would be a good manual ATU. Vectronics are pretty good if you can get hold of one (bit like rocking horse poop though).

    If you make the overall length 102ft you would then have a G5RV in effect.

    Nigel, M0CVO

  2. g6nhu says:

    Hi Nigel,

    The ATU I’m looking at (the MFJ-974B/974HB) is a manual tuner and I agree to extent about the length being a G5RV. However a G5RV uses a drop of 300 ohm ribbon to match it and then changes to coax back to the shack, an unbalanced feed.

    I don’t want to use 300 ohm ribbon but just use 450 ohm ladderline from the centre point all the way back to the shack – a balanced aerial with balanced feeder into the input of a (manual) balanced line tuner.

    At least, that’s the current. plan, I am always open to ideas.

    73
    Keith.

  3. Nigel says:

    Of course, you could build your own tuner – two Jackson dual gang 365 + 365pF, a coil around a 35mm film canister with tapping points for the inductance and banana jacks for the antenna inputs and outputs. You will need a 9:1 current transformer (balun) in there too to reduce the impedance to 50Ohms.

  4. Scotty says:

    As discussed with Kieth – my aerial plans are rather similar – a doublet and tuner, and maybe a W3EDP. Nigel, where would I buy capacitors like that? I’m intersted in building a loop and the tuning circuit uses 2 similar to that – but no idea wher to buy them from!

    all the best

    Scotty M6OZI

  5. Rob M0VFC says:

    I’m slightly surprised by the “auto ATU won’t give acceptable performance on all bands…” comment above: in my experience, they’re actually very good indeed. (Sometimes a little more encouragement in the form of a little extra inductance is helpful to achieve a match on 160m, but that aside…)

    Moreover, they do have the advantage that you can then drop the feeder vertically downwards, put the tuner at that point, and run coax back to the shack, since the ATU needs no manual control.

    Balanced feeder is wonderful in free space, but proximity to other objects can decidedly un-balance it, at which point your nice low loss characteristics become more suspect. Coax of course is highly lossy when mismatched, but by putting the ATU outside, you then provide a known 50R impedance to the coax, which is now much less lossy, and much more tolerant of being run through walls / windows / etc. to the shack.

    The down-side of course, is that waterproof auto-ATUs are expensive, and non-waterproof ones need some kind of engineering (tupperware box?) to make them suitable for outdoor use. Not to mention, if you want to run >100W, you’re looking at silly money…

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