[Antennas] Model this idea

[Chris Boone]

No, that wont work....on FM/TV broadcast towers, nothing special is done
(not needed since the tower is already at ground potential). Only on AM
"hot" towers (series fed) do they need stubs to isolate the LMR antenna/coax
from the AM signal (this is done by isolating the feedline using ceramic
stand offs up to a spot on the tower that is 1/4wl of the AM signal
(provided the tower is taller than 1/4wl or 90degrees) and then bonding the
shield at that spot and above. This is a zero current point and thus NO
current flow should take place down the coax back to the LMR gear and
ground. This is not needed on folded monopole AM towers as the tower is at
ground potential (it has a multi-wire ring radiator built around the tower
like a cage and this part is the actual AM radiator; the tower is used as
support and because it is grounded, it can be used for commercial LMR and
other antenna support but MOST AM stations use a series fed, ground isolated
tower that is NOT DC or AC grounded; adding other antennas, etc to this is a
very complicated procedure; it can be done but its not cheap)

However, what I am trying to do is put 3 separate stacked vertical dipole
antenna array on one mast and tie them to one feedline...totally different
situation.
May have to go with a better designed/built triplexer design...most ham
versions are cheaply built using plain disk capacitors and small wire
inductors...
Not good when sitting on a "lightning rod"!    :(

Only tri-band antenna available is the Comet CX333 or Diamond equivalent. I
don't like the gain or construction of them to put them on a tall tower..did
it once on a 480ft tower back in the 90s..after 2 years, the antenna was
about ready to fall apart (especially the cheap radials)..and they are not
rated for high power use (because they use cheap disc caps inside on
matching the segments for proper phasing on the three bands)...this after I
had some beefing up to the antenna to better weatherproof it and it was side
mounted 60ft below the top..there making it less a target....luckily it
never took a lightning strike.

I swore NEVER again! Gotta be a better way using the hardened DB style
antennas.

Chris
WB5ITT
Vice Chairman and Frequency Coordinator, SBE chapter 134

-----Original Message-----
From: antennas-bounces at mailman.qth.net
[mailto:antennas-bounces at mailman.qth.net] On Behalf Of C.Whitaker
Sent: Thursday, October 11, 2012 6:50 AM
To: antennas at mailman.qth.net
Subject: Re: [Antennas] Model this idea

de WB2CPN
Chris,
Perhaps you should have a look at how commercial VHF and UHF stations
populate those broadcast towers.
Generally, you can put 1/4 wave and other size stubs all the way up and down
the tower.
Like everything else, there's a lot more to this.
73  Clete