To tune, or not to tune? Would you use an ATU?

Today I’ve been measuring the SWR of my 40m delta loop on different bands and frequencies using a borrowed MFJ-269 analyser.

40m
7.000 – 1.5:1
7.100 – 1.4:1
7.200 – 1.5:1

20m
14.000 – 1.6:1
14.100 – 1.7:1
14.200 – 1.8:1
14.300 – 1.9:1
14.350 – 1.9:1

15m
21.000 – 1.6:1
21.100 – 1.6:1
21.200 – 1.6:1
21.300 – 1.6:1
21.400 – 1.6:1
21.450 – 1.6:1

10m
28.000 – 1.5:1
28.500 – 1.3:1
29.000 – 1.5:1
29.500 – 1.9:1
29.700 – 2.1:1

My HF radio has a built in ATU and although it will quite happily operate over these entire bands without complaining about high SWR, the question is whether I should use the ATU or not.

Would you?

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6 Responses to To tune, or not to tune? Would you use an ATU?

  1. Rob M0VFC says:

    As long as the TX is happy (still giving 100w out), there’s no point using the external ATU (I’m assuming here that it’s physically close to the TX, and not remote at the feedpoint of the antenna).

    Whether you use the internal or external ATU, your feedline loss is going to be exactly the same, and the external ATU itself will introduce some loss.

    The only reason for using an external ATU is if the TX can’t match it satisfactorily (i.e. it’s backing off the power) or if you’ve got the ATU significantly closer to the antenna than the rig, therefore presenting the correct impedance on the bulk of the feeder, and therefore reducing the loss. (The loss of coax increases as the impedance moves further away from its characteristic impedance.)

    Have a read of http://www.arrl.org/files/file/Technology/tis/info/pdf/q1106037.pdf to find out more than you probably wanted to know!

    Cheers,
    Rob, M0VFC

    • Rob M0VFC says:

      Just realised I may have mis-read your question, when you may have meant “should I use the internal ATU or no ATU at all”, rather than my original reading of it as “should I use the internal ATU or external ATU”.

      I’d still go with the answer of “if you’re getting no fold-back without the ATU, there’s probably no point using it”. However, on rigs like the IC-7000, as soon as you get much above 1.5:1, it starts reducing its output power, so at 2:1 you’re probably getting 50w out instead of 100w. At this point, the extra loss of the ATU (0.5dB maybe?) is probably outweighed by the extra power you get by presenting 1:1 to the TX (3dB in this case), so I would do.

      Cheers,
      Rob, M0VFC

  2. Dave (G0DJA) says:

    Two things strike me…

    1. The VSWR looks suspiciously flat across a wide range of frequencies and
    2. I’ve used the MFJ-269 and wouldn’t trust it.

    1st Any load that appears to be at or near 50 ohms “flat” is a dummy load. Are you feeding this antenna with 50 ohm coax and, if so, how long is it and what type? Long, lossy, a 50 ohm coax run looks just like a dummy load to the MFJ-269.

    2nd The MFJ range is notorious for its build quality and I’ve had antennas that show as ‘good’ on them but as soon as you put more than a Watt or so up them, they don’t perform as the MFJ said that they would.

    So, my final points are these…

    The loop and feeder should only show resonance on certain frequencies. If you are very lucky, these might be on harmonically related bands, but I wouldn’t bet on it. So, should you use an Antenna System Tuning Unit at the radio end (it doesn’t matter if it’s inside the radio or not)? My opinion is yes, and no.

    Yes, they are OK to tune out slight miss matches such as you get, say, from the CW to the SSB end of a band when using a dipole but, no, they are not good at tuning out complex impedance miss-matches presented by many multi-band antennas, unless the antenna is specifically designed to show 50 ohms on various bands, like trapped dipoles or multi-band verticals with tuning for the various Amateur Radio bands.

    A loop at the end of a length of some sort of feeder should, unless you are very lucky, show some miss match on one or more bands. If it doesn’t, be suspicious of losses in the feeder.

    • g6nhu says:

      Hi Dave and Rob, thanks both for your comments.

      As it happens, the VSWR readings from the MFJ pretty much tie in with the readings from my old Daiwa meter which in turn match with those from a newer (and if I’m honest, lower quality) Watson meter. The antenna is a 40m loop cut and tuned to be a full wavelength on 40m and fed with a 4:1 current balun with RG-213 back to the shack. If you look at the blog post here (http://qso365.co.uk/2011/06/building-a-40m-7mhz-horizontal-delta-loop/) then you can see the full details of the aerial.

      Once I get outside the bands listed, the VSWR goes up dramatically. It really only does match on the bands which are harmonically related.

      Rob has touched on the point I was making – Am I likely to be hitting the level at which the wireless starts cutting back the power and therefore should I start using the internal ATU? I guess I need to spend a little more time testing at different spots in the bands with the key down and monitoring the power on an external meter.

  3. Mike says:

    Good morning and very good question and one I have often asked myself. I have an attic dipole (Alpha Delta DX-EE) and the internal tuner in my K3 comes in handy. As was already mentioned the feed line loss is not going to change. For me if the SWR is greater than 1.6.1 I use the tuner. The way I look at it…the tuner is there and to be used. It puts less stress on the radio if it see’s a better SWR.

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