There are detailed plans that turn a 2 ele XM240 or the earlier version
into a Moxon. I recall that it was pretty straight forward. You can
raise the antenna about 30ft or whatever is easiest in the air and check
it. The SWR curve will move when raised to full height.
W0MU
On 8/15/2018 11:51 AM, terry burge wrote:
Thank You Matt,
Your suggestion has sounded the most useful to me. I will say I do hate dealing
with Copperweld and have avoided it but we will see. I do have the stuff from
the 160 meter coil to turn a hytower into a inv-L which is copperweld. Talk
about HARD to cut! More likely I will try to use some kind of wire rope.
I still think the Dacron would work even if I need to go to larger stuff like I used
for guy wires on my crankup Rohn 45/25 tower in the past. Stretching could be an
issue there. More likely I would double up on the 3/16" Dacron.
I visualize something like 'H's for a linear loading arrangement but just how
much I can only guess. That in combination with a Capacitance Hat/cross piece
out beyond the linear loading I think.
Why am I not trying to do a Moxon? Because whatever I come up with it must be
pulled up above my existing Steppir 3 element, then tuned up by tilting
vertical to get to the adjustment slip joints. I can't see how that could be
done with anything with much of a boom (like a Moxan or 2 element
'shorty-forty). If I could figure that out I would try to put a Waller-Flag
above my Steppir. By the way, my steppir has about 30' of boom length and truss.
For the moment I am contemplating how to double up the thicknet of the 1.25" with adding
a 1.50" thick wall 0.120" slipped over it. Sounds like I've got some grinding of
rivets to do. Is making me wonder about just buying a Cushcraft 40 meter monobander or maybe
check out M2 if I think I might break down and spend some cash. JK while I love the look on
line at least I think would be too much money. Always looking for something on the used market
of course. But then there is the problem that I can't hardly get any large delivery truck into
my QTH.
Anyway, I will keep working on this and see what I can come up with. Don't know
is the rotatable dipole at about 124' will be an improvement over my 4-Square
ground mounted but want to see. Couple of mornings ago around 4:00AM I was
having a ball working JA's on 40 meters with great signals both ways. Got
reports like 59+20, etc. The 4-Square was really showing it's stuff that
morning. Had to check all for directions to make sure I wasn't going to cause
any grief to someone when operating.
Thanks to Matt and everyone for the suggestions. Please don't think I'm
ignoring or not reading them. Sometimes I just 'don't know what to say' to some
of the comments. I'm still learning what many of you have already forgot (hi
hi).
Terry
KI7M
On August 15, 2018 at 6:50 AM Matt
Hi Terry,
If you are going with light elements, as I believe I read in your posts,
then truss cables can be used to help stabilize the elements both in the
vertical and horizontal planes. There is a small diameter Phillystran
available that works very well for this duty. Truss cables are typically
supported from both ends of a horizontal bar mounted above and parallel to
the boom at each element. The opposing mechanical loads on the bar from
both sides of the element help cancel out the bending stresses on the bar.
Most designs I have seen use a bar length of 24" or more mounted about 6" to
12" above the boom. The cables are attached to the elements about half way
out from the boom to the element tips.
As I'm sure you are aware, popular shortening options include capacitance
hats, linear loading, and base coil loading - in typical order of decreasing
efficiency. I believe you will find that about 45' width is a practical
minimum for a shortened element on 40m. One interesting option is that
with linear loading, it's possible to support the elements with metallic
truss wires which do double duty for electrical loading without adding
weight to the ends of the elements, but be sure to use wire that won't yield
(stretch) over time such as Copperweld. This option will require some
short segments of non-metallic (typically fiberglass rod) component in the
element construction at the loading wire attachment points. The loading
wires will also need to be electrically isolated from the boom via the truss
support bar. Element tuning is accomplished by varying the position of a
shorting strap placed between the two support wires back on the boom side of
the arrangement.
The big tradeoff for shortened 40 is the decrease in performance
bandwidth... particularly F/B ratio, which can be rather dramatic if you
are trying to cover both the CW & phone segments with one antenna. IMHO,
the Moxon design provides a much better overall compromise in terms of
performance bandwidth for a shortened 40 antenna. There has been a lot of
discussion on this reflector about this subject in past years so you may
want to look in the reflector archives for ideas.
Hope this info is of help and good luck on your project.
73
Matt
KM5VI
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of terry
burge
Sent: Wednesday, August 15, 2018 1:06 AM
To: towertalk@contesting.com; Grant Saviers <grants2@pacbell.net>; terry
burge <ki7m@comcast.net>
Subject: Re: [TowerTalk] supporting a 40 meter rotatable dipole with trusses
Well, OK you got me redesigning my little project. Don't know yet how I will
change it or just start over again. I do have the 1.50" X 0.120 heavy wall
also 6' long. And I am considering how I could utilize some coils and/or
capacitance hat arrangements to shorten up the antenna. Closer to what I
suppose a shorty forty or something like that. One problem is I just haven't
been up close to a 40 meter beam to get a decent idea what I am really
dealing with. I know when I tried to raise this one above my head probably a
good 1/4 to 1/3 of the outer elements were still on the ground. That before
any kind of serious trussing. Might even go back to the design like in QST
antenna issue a couple years ago with weighted wires hanging off the ends.
But I would prefer something more stable than that.
Terry
KI7M
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