An inconclusive test.
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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. |
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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!