Brett,
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.
(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.
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.
Chas, W1CG
At 06:23 PM 9/29/2005, VR2BrettGraham wrote:
W1CG replied to K5KVH:
Thank you for the insight. I calculated the % the wire lengths were
off by % the frequency was off, and I also "moved" the model up in
frequency (by shortening wire lengths in the model) by the amount it
differed from the measurement. In other words, the actual was about
200 KHz below the model, so I moved the model up 200 KHz. The model
was at the same height as the measurements. Both techniques produced
results that were close. One thing I found was to not change the
wire spacing at the ends, but just to change the length of wires
between the main supports. I will wait until we hoist it to the
final height which I estimate to be near 30' and take some more
measurements then trim based on that. I think we can operate with
the existing lengths as the top band edge is within the 2:1 SWR
bandwidth at 21'; it may be a little off at 30' as the resonant
frequency goes down as the height is increased, but we don't have to
operate near the top end of the band. We have a LDG tuner, so it
should be able to tune ok.
I don't recall if you mentioned if you were using bare or insulated
wire.
Between the several percent that contributes & the coupling
between the elements, I think WA1X's (I believe it was) suggestion
on towertalk or somewhere a while back on how he trims
Moxons is the way to go.
With bare wire, I have done quite a bit of mucking around trying
to tune a 10m Moxon with effect of "ground" (reinforced concrete
roof top) a half wavelength up. A decent match with reasonable
pattern I believe is probably much easier to achieve by tuning
each element separately as WA1X described.
The coupling in a Moxon rectangle is pretty intense - it is a good
idea to maintain geometry after trimming the ends, though some
sort of directivity will remain if you don't (up to a point). I have
done all sorts of models of Moxons & all multi-band approaches
other than back-to-back look to be hard to achieve in practice
(nesting, mix with yagi elements, sleeves, etc). Nested Moxon
reflector for the higher band & sleeve around low band driver looks
awfully interesting, but would require a _lot_ of work to trim into
submission in the real world.
GL.
73, VR2BrettGraham
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