[TowerTalk] INSULATED MAST

[K7GCO@aol.com]

In a message dated 6/11/01 8:25:29 PM Pacific Daylight Time, k7awb@qwest.net 
writes:

<< 
 I recently bought an M2 eight element fm 2 meter beam  to install at the top
 of my mast on my tower.  Where do I purchase an insulated mast section for
 the 2 meter beam that I can connect to my 2 inch steel mast at the top of
 the tower?  I don't remember reading in the m2 literature  that I needed an
 insulated mast though.  But, I don't want to affect the pattern.
 Steve K7AWB
  >>
You didn't say if it was to be vertically polarized which I assume it is or 
you wouldn't be requesting the insulated mast.  Try the quad people for their 
fiberglass poles.

However there is another problem.  The coax has to be run in the same area as 
the insulated pole so there will indeed still be almost the same interference 
to the beams pattern if a metal mast is used connected to the boom center.  

There are two ways to solve this.  Support the 2M beam from the rear and run 
the coax to the back along the boom and run it down a supporting metal mast 
at least a 1/4 wave from the reflector.  The boom may have to be beefed up 
for an end support.  Or run a fiberglass support pole at 45 degrees from the 
mast.  The beam F/B will give a fair isolation from the mast in the rear.  
Add another band beam on the other side pointing the other way to balance it 
up.

At a Ham Convention in Sea Side, Or last week a vendor had 2 neat LP's for 
144 and 450 MHz only of about 6 elements each.  It was fed in the front.  He 
had a 19" (1/4 wave) piece of PVC pipe holding the coax out away horizontally 
to keep it from the vertical elements and then it dropped down and back to 
the mast.  He said "it maintained the 1:1 SWR doing this" which I didn't 
doubt.  I've seen this before, used it in a hurry 30 years ago with a single 
vertically polarized beam but haven't checked what it does in Eznec for sure. 
 (Since then I've always used 2 vertical beams on a horizontal boom since 
with both spaced away from the tower evenly and at a spacing that gives the 
cleanest pattern).  It does give a fairly good pattern but I'll run an actual 
pattern and also in Eznec on a 3 element quad and a yagi to see exactly what 
it looks like with the loop around coax.   

It's the only way one can run a single vertical yagi or quad center mounted 
on an insulated mast and get a decent pattern without major interference from 
the coax.  This coax support has to be fairly rigid to take wind and ice/wind 
loading. 

3 element vertically polarized yagis have been supported on the DE with a 
J-Pole support and feed.

The logs were mounted on a neat Y PVC pipe support on the boom and he was 
able to tip it either vertical or horizontal or 45 degrees.  k7gco

List Sponsor: Are you thinking about installing a tower this summer? Call us
for information on our fabulous Trylon Titan self-supporting towers - up to
96-feet for less than $2000! at 888-833-3104 <A HREF="http://www.ChampionRadio.com">
www.ChampionRadio.com</A>

-----
FAQ on WWW:               http://www.contesting.com/FAQ/towertalk
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com