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An inconclusive test.

May 16th, 2009

Not quite what I was expecting or hoping for. I’d read a lot about how well a 44ft doublet antenna worked when fed with a balanced line. Unfortunately my little test this afternoon left me a bit disappointed because I certainly didn’t see the kind of performance I expected.

I’d taken the time to construct what I thought was a pretty well made antenna. I’d got the right kind of feed line and I’d found myself a good balanced tuner in the form of the SEM TranZmatch (pictured). I was hoping for great things. For one reason or another though the antenna was very difficult to tune on all but 17m.
SEM TranZmatch

SEM TranZmatch

I tried all the bands that the antenna is designed for with the exception of 40m. This includes all of 10m, 12m, 15m, 17m and 20m. I wasn’t able to try 40m due to time contraints and some pretty grotty weather (see yesterdays post).

Using a Yaesu FT-857, my transceiver of choice for portable work, I connected the radio to the TranZmatch with a separate SWR meter in line, connected the antenna to the TranZmatch with 20m of 300 ohm ladder line and set about 17m which wasn’t in great shape but did seem to have the usual early afternoon clutch of Asian stations (including VR2XMT, Charlie, coming in loud & clear as usual).

I tuned the TranZmatch for maximum noise on receive and then tweaked it with a bit of RF - quick & simple and a low SWR <1.2:1 across the band. I tried to reply to a few CQ calls but either wasn’t heard or just wasn’t getting out. I didn’t hang around trying to work stations though because this was primarily a test of the antenna/feedline/tuner combination so I moved on to see what 20m was like.

It was at this time that I got the first inkling that all wasn’t quite as I’d hoped. Getting a good match on 20m was really difficult and the lowest SWR possible was no better than 2:1. Now I know this isn’t terrible but I was hoping that the TranZmatch would do better. Maybe I was expecting too much because the same was then true on 15m, 12m and 10m. All of these bands had very sharp tuning and a minimum SWR no lower than 2:1.

I have a few questions in my own mind as to where the problem might lie. Is the feeder a ‘bad’ length - at twenty metres long it’s a full wave on 20m and might just present an extreme mismatch to the tuner. This doesn’t explain why other bands should be just as difficult to tune. The doublet wasn’t level - it was in an inverted-V configuration and the included angle was probably a bit small at around 100 degrees - 120 degrees is recommended. There was a lot of ’spare’ feedline just lying on the ground, loosely coiled - is it better to trim the feedline to prevent this?

For now the jury is out and I have more work to do before deciding one way or the other whether or not I’ll adopt the 44ft doublet as one of my regular portable antennas. And it’s just started raining again!

Dean Amateur Radio, Pictures , , , , , , , ,

  1. Andrew
    | #1
    Reply | Quote

    Just one brief observation whilst I have time. If you are using twin feeder it really needs to be in the clear and not lying on the ground or close to any other metallic objects.

    Also, which type of 300Ohm feeder are you using is it ladder or is it the stuff that they make the FM Tee antennas from, some of the non slotted twin feeders can be pretty lossy!! despite the amazing claims made about them. Ladder line is generally the better option.

    Regards

    Andrew - G7MNS

  2. admin
    | #2
    Reply | Quote

    Thanks for the comment Andrew.

    In answer to your question I am using the black ladder line with the alternating inch of gap/insulation between the conductors, not the cheaper FM aerial type.

    I have since posting my findings also read elsewhere that having the feeder lying coiled on the ground (even if only loosely coiled) is to be avoided. To get around this I have another version of the antenna with a shorter (9m) feed - this should allow me to hang the antenna at a height of 8m or so and then have the feedline come down to an operating position close to the portable mast in a pretty straight run. I hope to trial this at the weekend.

    Thanks again for the comments.
    Dean - G0RIF

  3. Andrew
    | #3
    Reply | Quote

    @admin
    Good Morning Dean,

    Fine on the Ladder line, I use 300 and 450Ohm ladder sometimes, it really is excellent stuff. The TranZmatch is also an excellent tuner for these types of antenna.

    I hope to be playing a bit of radio this weekend, am going to set up my Buddipole in the back garden and maybe try an inverted vee as well using my 5M Paint pole - V. Cheap portable mast, cost 14.95 from B&Q and with the adapter my Buddipole fits it too!!

    I hope your next test is more successful for you,

    Best wishes

    Andrew - G7MNS

  4. Dean
    | #4
    Reply | Quote

    Hello again Andrew,

    Ok on all the info and yes, got to agree about the TranZmatch being a great tuner. When I was first licensed back in 1991 I was loaned one of these from my local club (Telford & District ARS) and I had it hooked up to a 10 or 15 metre dipole (I forget exactly which) - it managed to tune that antenna to allow me to work the world on 10, 12, 15 and 20m (although there are very few log entries on 17m for that period).

    Good luck with the garden portable activities this weekend. As you may have read on the blog, I have the Buddistick and I love it to bits! I’ve previously tried to use the Par End Fedz as holiday antennas but they can be difficult to hang if supports aren’t conveniently close by. Being monoband antennas it’s also not always practical to take more than 1 with you. I got the Buddistick more than anything as a holiday antenna where it’s small size, decent performance and multiband capabilities tick all the right boxes. I’ve been pretty impressed with it so far.

    All the best,
    Dean - G0RIF

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