Topband: My Shunt Fed Tower

[Bob Garrett]

Greetings Listers,

 

I'm looking to this group for some input on my shunt fed tower loaded down
with antennas HI.

 

I have 85 feet of Rohn 25G with three sets of broken up guy wires that was
installed in 1999.  Probably 200 to 300 short radials that have been put in
since 1988 for the previous tower and added to over the years, all bonded to
the base of the tower.

 

On top, from bottom up I have - Optibeam 9-5 9 EL yagi, 5 EL M2 6 meter yagi
and at 100 feet, an Optibeam 30 - 40 rotary dipole.  All cables run inside
the tower and are grounded at the base.  I use an omega match made up of two
vacuum variables.

 

Now, this season, I wanted to have a few more options for contesting.  I
installed a 3 foot metal support from KF7P and put up a 80 meter inverted
VEE at 70 feet and a 60 meter inverted vee at 60 feet using the same
arrangement.  I did have to adjust the omega match after adding the two
additional antennas.

 

Regarding the shunt, I use a piece of RG8X with the attachment point at
about 48 feet and a spacing of 24 inches, a 1KPF variable in series and a
probably 400PF cap for the omega match.  No problem getting a perfect match.

 

Over the years, this arrangement prior to adding the two inverted vee
antennas this year performed very well.  Now I know about TB conditions so,
I am not making sweeping statements but, seems like I just don't get through
those pileups as quickly as before.  

 

My questions - Is there any fact to the statement that an 80 meter antenna
on a 160 meter shunt fed tower will degrade the performance?  Is it just one
happy amount of loading?  Anything else I should be considering?  The
problem is I live on a small lot and only have so many options to cover all
bands.  Any input appreciated.  73, Bob K3UL