Milt, N5IA wrote:
"MUCH smaller bandwidth unless you want to do remote control of the shunt
capacitor. It is well worth the effort (coin and sweat) to make the
antenna a true Ground Plane."
==========
Not true if the shunt-fed tower is close to 1/4-wave tall. The 2:1 SWR
BW is just as wide as with a 1/4-wave series-fed vertical. At the other
extreme, very narrow BW is encountered if the vertical is 1/2-wave tall.
Although the tower need not be 1/4-wave resonant to shunt feed it, there
are several advantages if it is:
1) The shunt wire tap point is very close to the ground, so you do not
have to climb very high to attach it (about 15 to 20 feet).
2) The least amount of RF voltage is developed across the shunt ("gamma")
capacitor (typically about 700 volts with 1000 watts applied). The tuned
capacitor value will be about 600 to 650 pF.
3) The widest 2:1 SWR bandwidth (about 150 kHz) (without having to retune
the capacitor).
Other advantages for the shunt feed method:
1) No need to insulate the tower from ground.
2) Ground radial connection is easier because the radials are tied
directly to the base of the tower (with a series-fed tower, they must be
insulated from it but all tied together and to the coax shield).
3) Because the tower is grounded, a lightning strike is conducted
directly to ground, thus eliminating damage to anything (my tower in
Houston was hit many times with absolutely no ill effects. One of my
towers here in the desert was hit about 20 years ago with no damage
whatsoever).
4) A shunt-fed tower gets out as well as a series-fed tower.
73, de Earl, K6SE
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