Topband: ...feeding a tower...

[Donald Chester]

I have a somewhat different situation, but the results are interesting.  My 
tower is 127' of Rohn 25, series fed using a base insulator, with 120 
quarter wave radials buried about 2 inches below the ground surface.

I also use the tower to support the centre of a 130' dipole, attached at the 
119 foot level.  The dipole is fed with open wire line (#10 copperweld 
spaced 1 1/2 inches apart).  The open wire runs down through the inside of 
the tower, spaced at the centre of the triangle. Neither dipole nor feedline 
are connected directly to the tower at any point.  When using the tower as a 
vertical, the open wire line is left floating, using a DPST knife switch at 
the base of the tower to disconnect it from the ATU.

The base impedance varies from about 120 ohms at 1800 kHZ to somewhere about 
320 ohms at 2000 kHz.  There is considerable +j (I don't have my notes here 
in the house where the computer is located) throughout the band, lowest on 
the bottom end.

A quarter wave vertical should be about 36 ohms plus resistive losses, but 
the dipole is obviously adding substantial top loading, even though it is 
not directly connected to the tower.  Evidently, the feedline going up 
through the tower in close proximity provides the coupling, so my tower is 
really more like the old vertical "T" antenna than a true quarter wave 
vertical.  I have used it with great success for approximately 20 years.

As an added bonus, this arrangement is broadbanded enough that I simply set 
the L-network at the base of the tower to match 50-ohm line 1:1 at 1900 kHz, 
and it operates all the way from 1800 to 2000 with less than 3:1 swr.  I 
simply adjust transmitter loading into the 50-ohm line when I make a large 
change in frequency.

I'm not sure if the top 8' of tower is serving as useful radiator, but I 
attached the dipole at 119' because that is where the top set of guys is 
attached.  Each leg of the dipole terminates to a 25' wooden telephone pole, 
about 450' from the tower, using Phyllistran plastic guy line.  That makes 
the dipole more a true horizontal dipole than inverted vee (each end of the 
wire is 100' high).  I attached the dipole at the guy point to avoid damage 
to the tower in case one leg should fail during an ice storm.

The vertical is extremely poor for receiving, due to tremendous local noise 
pickup.

Don K4KYV

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