Topband: Antenna

[Herb Schoenbohm]

On 5/2/2012 9:21 PM k8fc.com wrote:
> Need some help.  Came home from Florida to find damage to most antenna systems here.  Have been using 50 foot high inverted L's on 160 and 80.  Want to see it shunt feeding tower will be better but no software to do that.  I have 52 foot of Rohn 25 with a TH6DXX and A3WS on top.  Already have 80 radials out under the L's.  The L's will go.  Can somebdoy plot this thing?  One thing I can not live with is a very narrow bandwith.  I run the station remotely via the internet in the winter.  Any help would be appreciated.
>
> 73's
>
> Marty, K8MV
> _______________________________________________
>
Marty,  You really don't need software for the configuration you 
described.   A cage feed of your grounded tower would give you the best 
bandwidth over a single shunt wire. Attach three four foot one inch 
pieces 120 degrees equally spaced around the top of the tower and drop 
three wires of #8 to the ground with three porcelain insulators at the 
bottom and three turnbuckles for good tension.  Run another wire to the 
top of each insulator and a short fly lead to the tuning box.  Your 
sweet point should be determined by experimentation but will probably be 
close to 38 to 42 feet ABG.  If you want to avoid trips up and down the 
tower you can use a omega match to electrically move the tap from the 
cage to the tower up and down.  You can also use a MFJ-259 to see what 
you are dealing with.  You probably will have about 500 ohms or more of 
inductive reactance that you will need to tune out with an equal amount 
of series capacitance.  A taped coil to ground will also give you a jog 
on the impedance for a perfect VSWR.  If you use and Omega capacitor it 
should go from the hot cage wire to ground.  If you don't have a MFJ-259 
you can find the best match with a VSWR bridge inserted in the coax feed 
near the tuning box.  A large plastic electrical box  would be fine for 
making the ATU which could be bolted to the base of the tower near the 
ground.  Hopefully you can reclaim some of your radials and put them 
down at the tower.  This is critical to have a good efficient 160 meter 
transmitting system.  With your set up you may be a bit short of 1/4 
wave electrical or resonant on 160.  However, resonance of the top 
loaded vertical is not as important as a good efficient feed arrangement 
to what you have.  It may take a bit of effort to get everything to work 
but when it does you should be very pleased with the cage fed towers 
performance *if* you have some radials down.

Good luck



Herb Schoenbohm, KV4FZ
St. Croix, US Virgin Islands