[TowerTalk] FW: supporting a 40 meter rotatable dipole with trusses

Wed Aug 15 14:30:05 EDT 2018

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 at contesting.com] On Behalf Of terry
>> burge
>> Sent: Wednesday, August 15, 2018 1:06 AM
>> To: towertalk at contesting.com; Grant Saviers <grants2 at pacbell.net>; terry
>> burge <ki7m at 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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