[CRAPRS] Digi on a tower

[Jerry Pasker]

Ok, I need some help from the group.

I have a 150 ft tower.  I'm fairly confident that 40 watts on 144.390 
will not interfere with the fixed broadband gear on the tower 
(5.2Ghz, 5.7 Ghz, 2.4Ghz, and 900 Mhz... all running at 50mW).  But 
the only way to know for sure is to try it.  I'm not going to try it 
until this spring, but I would like to have the antenna squared away 
before I get to that point. The more time I have to shop, the better 
deal I can get.

Disclaimer:  While I've bolted up lots of things to towers, water 
tanks, grain legs, etc, I've never mounted VHF omni directional 
antennas on a tower before.

So I intend on mounting the APRS Digi antenna at the 125-135 ft point 
on the tower, due to lack of room at the top of the tower. Because of 
this I will need to use some type of standoff.

So I'm clear on the whole process with the exception of the antenna.

What type should I be looking for?

My thoughts are that a commercial VHF antenna would probably be the 
best, and one that was decommissioned from say, some farmer's grain 
leg on a now unused VHF radio system would be the most affordable.

Or should I be looking for one that's specifically designed for ham 
radio use?  There are stainless steel Jpoles available for less than 
$70, but they only have 3.7db of gain.

What antenna type is the best for mounting adjacent to a tower?

I've looked over data sheets such as this one -

http://www.antenasystems.com/pdfs/base_pdf/cat29-pg321-322.pdf

- that show radiation patterns on towers, but haven't been able to 
find good data sheets on J-poles, or fiberglass omnis when used in 
the same configuration (bolted to the SIDE of a big metal tower, as 
opposed to the TOP.

-Jerry