Topband: Spitfire question

[Eduardo Araujo]

Many Thanks Tom for taking the time to respond with quite level and details.

I will definitely will give it another try from a different perspective.

I am just looking for a 3db gain, and if I fail, I will go for a second quarter wave tower and a 2 element array.

Regards... Eddie.... LU2DKT




On Monday, November 4, 2013 9:07 PM, Tom W8JI <w8ji at w8ji.com> wrote:
 
At first I was delighted because I got f/b at ground level of more than 20db 
across 25 Kc but surprise came when I
could not notice any improvement in the front gain switching on/off the 
elements to medium distances (Brazil) or long
distance like Europe where it was pointed to (and same line of Brazil).
I measured FS at 300 mts at ground level and I only detect 1db maximun 
change
Reading ON4UN book, It looks like because of its configuration, the main 
lobe is shifted upward a bit, so I could
think that because of that I measured very little at ground level but not 
noticing any detectable change at
Brazil (3000Km) nor Europe (>10000Km) it really makes me wonder if it is 
working at all.>>>>>

That antenna, with 25 radials on the center element, models to have about 3 
dB gain over a single element. That seems good to me for two sloped, close 
spaced, bent wire elements for director and reflector.

You can be sure it is sensitive to exactly how you construct it, and how 
good your ground below the antenna is. It does not mean we do not have earth 
loss just because we do not attach a wire to a radial system near earth, so 
it should be highly influenced by ground losses around each sloped element 
end.

In my model, I used 25 radials 100 feet long on the center element. It had 
very good F/B ratio, 23 ohms feed impedance, and 3 dB gain. I used normal 
ground and wire losses.

When I look at things, I like to look at something almost perfect and seem 
if it all seems logical or sensible. I would never expect much more than 3 
dB gain because of the wire folding, earth losses, and thin long wires for 
parasitic elements. After all, we only get 5 dB or so gain over a single 
element using a Yagi with full size thick elements when it is placed far 
above earth. A typical, real, four square is really about 4 dB over a single 
element. Certainly long thin wires near earth that are bent and sloped 
should not approach that amount, or the amount of a typical Yagi.

It would be pretty easy to go from 3 dB gain over a single vertical to 1 dB 
over a single vertical with any mistakes, and perhaps lose that much just 
through dimensional changes or earth problems. You are only 2 dB off, it 
usually does not take much to do that.

73 Tom 

_________________
Topband Reflector