The swr is higher than I had hoped for. I was hoping to use a duo-
band 15/40 meter wire dipole that could be fed directly with 50 ohm
coax connected to my Yaesu FT-450AT with internal tuner.
But the FT 450 AT internal tuner will only match up to a 3:1 swr, so
that won't be "do-able".
I think it *will* work (based on practical experience.. I've done this a
bunch of times).. you just need to fiddle with the length to get an
acceptable match in both bands. And, of course, you're not going to get
something which matches the bottom of one band and the top of the other.
I agree, it may very well work acceptably. I've been able to get a
similar 40-meter wire to match up OK on 15 meters, using the autotuner
built into my Kenwood TS-2000.
Another option would be to add some sort of matching element, either
at the antenna or at the rig, which would bring the feedpoint
impedances on both bands to within the ranges acceptable to the rig's
internal tuner.
If the antenna is close to 50 ohms resistive in the 40-meter band, and
is around 85.26 - J 186.3 in the 15-meter band, then you could
consider adding a series coil at the antenna feedpoint. If I'm doing
the numbers right, an inductance of 1.5 mH at the feedpoint would
cancel out the 186 ohm capacitive reactance almost exactly, leaving
you with an 85-ohm resistive impedance or an SWR of around 1.7:1, which
should be quite easy to drive.
This much inductance would change the feedpoint Z on 40 meters to
around 50 + J60, so the SWR would be quite a bit more than 1:1...
probably still within the range of your tuner, though.
Several ways to deal with this:
- Compromise, by using less series inductance... maybe 1 uH - this
wouldn't lower the SWR on 15 meters as much, but would raise the
SWR on 40 meters less. Calculate and fiddle a bit and you can
probably find an inductance which will result in similar (and very
comfortable) tuning SWRs on both bands.
- Instead of a plain L at the feedpoint, use a series LC - one which
is series-resonant at 40 meters (thus adding no reactance and not
changing the SWR at all) and has a net reactance of J186 at 15
meters.
All of these techniques will narrow the SWR bandwidth on 40 meters
somewhat, but with care you should end up with an easily-tunable
antenna.
--
Dave Platt AE6EO
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