We mounted a 30-meter full size dipole at 76' assembled from 204BA parts on
the side of a Rohn25 tower with coaxial balun.
It is spaced about a foot away from the nearest tower side rail, with a
matching hanger off the tower for a truss.
Has worked fine, been up 12 years. Fixed direction but would be fine on a
rotating tower.
SWR is less than 1.1 regardless of positions of the rotary beams above and
below it, no significant interactions.
Fotos showing details are on our QRZ page.
73, Don N7EF
On Sat, Mar 31, 2018 at 7:25 PM, Wes Stewart <wes_n7ws@triconet.org> wrote:
> It doesn't sound to me like the mounting plate would have a high enough
> shunt capacitance or loss resistance to bring about the effects you are
> seeing.
>
> Nevertheless, if you wanted to eliminate the possibility, you might
> consider better mounting insulators:
>
> http://www.stauffusa.com/pipe-clamps/
>
> or a non-metallic plate:
>
> http://www.eplastics.com/fiberglass-frp-channel
>
> Those guys seem to sell material from here: http://www.strongwell.com/prod
> ucts/structural-shapes-and-plate/
>
> I have a couple of 20', 2 inch square lengths of Strongwell tubing bought
> for an antenna project that never materialized that have been laying in the
> desert sun for ten years and show no sign of degradation.
>
> Wes N7WS
>
>
>
> On 3/31/2018 1:45 PM, Dave Sublette wrote:
>
>> I have a Rohn 55 tower that rotates. I recently built a rotary dipole for
>> 40 meters. It is full size. The center section tubing is 1.625 inches in
>> diameter. It is a split construction. I feed it with an RF choke and 50
>> ohm coax.
>>
>> Hoisting the dipole up to 100 feet or so and leaving it dangle, the
>> antenna
>> shows an almost perfect SWR curve. It has a 1.1:1 SWR at the bottom of the
>> band and rises gently to 1.5:1 at 7300. Since I operate mostly CW, I
>> didn't bother to shorten it.
>>
>> However, after I have bolted it to the mounting plate, the SWR goes bad,
>> raising to 5:1 at the band edges and being at 3:1 at about 7200 Khz. It
>> still works great and has a good pattern.(Nulls off the ends).
>>
>> My mount is made from two aluminum plates bolted to two pieces of Angle
>> steel, one on the top edge and one on the bottom edge. They run the
>> entire
>> width of the tower face. The edges of the angle are within an inch of the
>> dipole, so that makes it practically enclosed on three sides. I'm
>> guessing
>> the stray capacitance is killing me.
>>
>> I propose to redesign and get rid of the angle pieces, leaving a flat
>> plate. I presently insulate the dipole with a full length (tower face)
>> piece of schedule 40 PVC that is split and slides between the U-bolts and
>> the saddle clamps.
>>
>> The diameter of the tubing in the center is 1.625 inches. If I were to
>> replace the saddle clamps with insulated type, is there a product
>> available
>> that does
>> this, or should I just go ahead and make them? If so, What sort of
>> material? I am also thinking of ditching the metal plate and using Marine
>> plywood.
>>
>> The antenna uses dacron support ropes out about 25 feet from the center so
>> a lot of the stress is relieved by doing this.
>>
>> This thing works great. The change in SWR drives me nuts. So I'm going
>> to
>> try to fix it.
>>
>> Building stuff is not a problem for me, but I don't have a lot of
>> experience with tough insulating material. Any advice is welcome.
>>
>> Thanks and 73,
>>
>> Dave, K4TO
>> _______________________________________________
>>
>>
>>
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>>
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