[Antennas] 1/4 wave vert,

Fri Aug 22 00:30:03 EDT 2008

At 08:28 PM 2008年08月21日, Joe WB9SBD wrote:
>Ok heres a question then, for the "Tower" section of this project,
>How about the style of a old farm windmill tower,, Like 10 foot 
>spacing on the legs at the base.
>I wonder how much this would shorten the antenna on 80,
>Plus how would feed such a beast? would feeding (connecting) to 
>just one leg work or it be like lopsided.
>>Now that really would make it ever weirder. How about the same on 
>10 meters? the crazy thing is a HUGE percentage of a wavelength wide!

I don't think it's too common these days, but I've seen both 
self-supporting and guyed AM broadcast towers that use "slant wire" 
shunt feed. It's really a variant of a gamma match. It consists of 
a wire tapped up the tower about 25-35% and brought down to the 
tuning unit on the ground about 10-15% of the tower height from the base.
The pattern has a little more signal in the direction of the feed 
wire, but it's usually less than a 1 dB F/B.
Eons ago, when I was in college, I worked as a transmitter operator 
at KOAC 550 (a great job for a starving engineering student). They 
have a two tower array with both elements fed with slant wires. The 
towers have wide bases and they were less than 1/4 wl tall (325 feet 
or 0.182 wl). In this case, I believe the towers were too short to 
be self-resonant.
The chief engineer told me that they sent a tower climber up with a 
copper cable and had him experimentally tap up on one leg of the 
tower while they measured the impedance with an RF bridge at the 
tuning box location. They made a permanent connection at the spot 
where the impedance was 50 ohms resistive plus inductive 
reactance. Then they tuned out the inductance with a series 
capacitor to leave 50 ohms. Of course, they had to provide switched 
matching to further compensate for the mutual impedances when the two 
towers were phased for day and night patterns.
Scott Fybush has some great photos of the KOAC towers and feed system:
http://www.fybush.com/sites/2007/site-070223.html
The width of a tower at the base has much less impact than the width 
at the top with regard to shortening the height for resonance and 
increasing the bandwidth. It's pretty easy to model this in EZNEC to 
get a good idea how this comes out. (If you have a particular tower 
in mind, I can model it for you.)
As for ten meters, you can still do this, but you'd expect some 
serious pattern distortion caused by the offset feed on a tower base 
that's >1/4 wl in width. Of course, if the tower is taller than 5/8 
wl on 10m, the elevation pattern will be full of high angle lobes, too.
73, Terry N6RY