Posted by
VR2BrettGraham on
Oct 01, 2005; 1:07am
URL: http://elecraft.85.s1.nabble.com/Two-Band-Moxon-wire-beam-tp382311p382320.html
W1CG continued:
>To answer your question, I am using stranded, silver plated, teflon
>insulated #16 wire. It is plenty strong. I have a 3/16" Dacron line
>between the two poles to take the strain off the wires. This also
>decreases the vertical bend and completely eliminates the horizontal
>bend of the two poles.
Presumably you know the dielectric constant of that insulation,
verified that the wire you have is in the same neighborhood &
done all your modelling based on that figure?
If so, then you can see how ground dominates & if not, then
between both ground & insulation your elements are nowhere
near where you think they are.
>(To review, the antenna is a 40 meter Moxon wire beam with a 2
>element 20 wire meter beam inside. There are two supports, 22' long
>fiberglass poles at each end, spaced 45 '. There are two feed lines.)
>
>We put the antenna up today for our annual special event at for the
>Harvest Faire at the Norman Bird Sanctuary in Middletown, RI for this
>weekend. Listen for us on 20 and 40. Call is W1SYE, Newport County
>Radio Club. (See Sept QST, special events).
>
>The antenna is about 30' high on one end and about 25' on the other,
>from a portable mast to a tree. The Fo and Zo is not much different
>from what it was at 21'. Fo is about 14.00/7.00 MHZ . SWR is 1:1,
>50 ohms on both bands per MFJ 159B. 2:1 band width is (top
>frequency) 14.4 MHz and 7.25 MHz resp. I still want to move it up to
>14.2 and 7.25 MHz. It's a very nice beam; doesn't sag much and it
>is easy to erect. The weight is about 18# which includes everything
>supported.
Not many 2-element 40m beams that can be robust & only weigh
that much - not to mention less wingspan & far better F/R than a
yagi.
>The feed lines are two lengths of double shielded RG-58 (TRF-58). I
>am going to try simultaneous operation on 40 and 20 using 20 and 40
>meter bandpass filters by Array Solutions. The 40M filter has an
>insertion loss of 70 dB on 14 MHz and the 20M filter has an insertion
>loss of 35 dB on 7 MHz. I will use my K2/100 with a LDG Z-11 tuner
>and the club's Ten Tec Jupiter with its LDG tuner. I will try my
>other K2 with KAT2 in case the Jupiter overloads on receive. I don't
>know how this will work out. Calculations say it won't work at 100
>watts as there is not enough attenuation, but I will try it and
>report results. There must be some power level of the transmitters
>that will work. It would be handy for field day, but if it doesn't
>work we will just have to put the second transceiver on 80 meters
>using another antenna. Or else use separate receiving antennas. The
>K2 with the KAT2 can do this; I don't know about a K2/100 with a
>KAT100. Maybe someone can enlighten me.
With two 400 watt rigs into two bands of a tribander simultaneously,
those filters will work - though without stubs the 2nd harmonic of
the 40m station will render some number of dozens of kilocycles on
the higher band useless. Be sure to decouple the feeders at
the feedpoints, too.
Look at how each driver looks as you connect, disconnect or short
the far end of the feeder to the other driver. I would imagine that your
antenna probably has the two bands' drivers electrically closer &
therefore you will have far greater interaction between the two bands
than on my tribander. Hairpins seem to work pretty good on that,
but that means more trimming.
Anyway, I think the key point is the wire & like I said about my
bare wire Moxon, there's a bit of mucking about in practice to get
it to work. I would not make another Moxon unless I'm exactly
duplicating something else (right down to the wire) - otherwise
both elements need trimming & WA1X's approach (I think it was)
is probably the best way to go about it.
Enjoy!
73, VR2BrettGraham
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