T2FD Calculator

I love a good T2FD antenna and have built a good few. There’s a few variations but they all work well and are easy to construct. I’ve been asked a few time about the length and spacing so I made a small Excel sheet to work it all out and provide data for the 3 most popular variations.

Free free to download and use. The sheet is protected to prevent the formulas from getting messed up but if you would like to experiment with alternative formulas then please just contact me for the password. All I require is for my original credit to remain on any derivative works.

M0LMK T2FD Calculator

11 Replies to “T2FD Calculator”

  1. When I click on Frequency, I get a message,
    “Protected Cells cannot be modified”
    What do I do to make that work.
    I’m using OpenOffice.

  2. Hi Mike. I don’t know if it will work in Open Office. I use Excel. If you send me an email via my contact form then I’ll send you an unprotected sheet over to see if that works.

  3. I tried to ask from your comments page but somehow it didn’t send. Could you kindly send me a password for the file?

  4. The contact form should be fixed. Please try sending me a message again or email me direct matt@.co.uk and I’ll send a copy over to you.

  5. Hi Mike, thanks for your work, I’ve often wished for a T2FD calculator.
    Maybe I’m missing something but there’s nothing appearing (other than a row of Xs )under the ‘Feeding’ section; no info about the resistor or the impedance transformer.

  6. This is what it should say:

    Modern: Feed in the centre on the bottom with 50 Ohm coax to a 16:1 BalUn (you can also try a 16:1 UnUn with a 1:1 BalUn after if you can’t find or build a good 16: BalUn).Use a 820 Ohm non inductive resistor rated to at least 1/3 of your total power (I prefer to use a rating of at least 1/2 on the shorter 1/2 wave model) in the centre at the top.

    Traditional: Feed in the centre with 300 Ohm twin feed (any length). Terminate the twin feed in a 4:1 BalUn and run 50 Ohm coax to the radio. Use a 300 Ohm non inductive resistor rated to at least 1/3 of your total power (I prefer to use a rating of at least 1/2 on the shorter 1/2 wave model) in the centre at the top.

    I hope that helps.

    Matt
    M0LMK

  7. First of all, thank you for this, I’m starting a build on the 1/3 wave now. I have a pre-built 1/4 wave T2FD already up and I’m pleased with the performance, but you know, always reaching for more. My intention is strictly for only shortwave listening, using an SDR radio. I have a little challenge in that I have nothing to support the center, so I need what is airborne to be as light as possible. So, I am going with the traditional type with ladder line to a balun on a post where the ladder line would drop down. That said, I have some questions:

    1. 300 ohm ladder line seems to be only available as copper-clad rather than solid copper wire. I’m reading copper clad has more loss than solid copper wire. For a 20 foot drop, is this even an issue?

    2. On the balun, 4:1 voltage baluns are readily available, but 4:1 current baluns are hard to find. Do you have a preference for an SWL application?

    3. It seems to me like the antenna is presenting 300 ohms impedance, and a 4:1 balun converts it to 75 ohm. This makes me wonder if I would be better off using 75 ohm coax to the radio, but then again, the radio has a 50 ohm antenna port. The coax run will be about 25 feet. So, I am thinking about using 75 ohm coax instead, mainly due to cost, if the difference is negligible. Any thoughts on this?

    4. I can do this as a flat-top or a sloper, and not to get into too much detail, but as a flat-top I could have it at about 20′ height. As a sloper, I could make the higher end around 30′. All other things, time, effort material being equal, which would you go for sheer receiving performance?

    I appreciate your time, and I know these are a lot of questions. If you want to email me back, that would be fine too.

    Thank you so much!

    ~Phil KD0SCJ

  8. Hi Phil.

    I’m always happy to help where I can. I don’t consider myself a T2FD expert but I have probably built more than your average Ham and I have used them in commercial settings. Here are my thoughts for your application:

    1. I suggest that the difference in loss between solid and copper clad on HF with a 20′ run would be negligible. I would use whatever you can get easily.
    2. A current BalUn is the best option but you can work around it. They are pretty simple to build and I would look for the 4:1 Guanella style. You can use a 4:1 voltage BalUn and then add a 1:1 choke right next to it (Element wires -> 4:1 Voltage UnUn -> 1:1 Choke -> Coax to radio). It’s not ideal but it will do the job.
    3. For RX only, take 75 Ohm coax back to the radio and don’t worry about the small mismatch. You won’t notice any performance loss.
    4. I would always suggest going for the tilted, sloper configuration. It will offer better omnidirectional performance that a “flat top”.

    Come back and let us know how your experimenting goes!

    Matt
    M0LMK
    KI5GXV

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