Topband: Spitfire question

[Tom W8JI]

The 2 element hang from the top of the tower in opposite directions to an 
anchor point 40 mts from the base but suspended at that point at 3 mts from 
the ground and keep that hight returning to the tower as the final anchor 
point.>>>>

There are a number of compromises with an array like this. You probably want 
the current maximum someplace low, but not at or near ground level. I'm not 
sure what the typical optimum place is.

The parasitic elements are not what we might think. We can pretend like it 
is a half wave ground independent element, but it is really a sloped 
vertical with a single counterpoise near earth. They are really sloped 
verticals with a single, low, elevated radial.

1.) You want maximum parasitic element current as far away from the main 
element as possible. This dictates having maximum current at the base of the 
sloped wire, and as much ampere/feet in the sloped wire near the bottom as 
possible.

2.) The problem with the above in number one is the sloped element, which 
likes a low current maximum for best alignment and maximum effective element 
spacing, has a single low radial. The loss in this radial increases with 
lower position of sloped element current, and as the radial is closer to 
ground. This is contrary to the optimum of number 1.

The system should work better, or with more repeatability, if the 
counterpoise wires increase in number and area around the sloped parasitics. 
It would have less loss and more bandwidth. Then you could move current to 
the bottom more, and not have loss increase.

The trade off is unpredictability and loss of efficiency and 
performance-bandwidth for simplicity in design. There is no free lunch.

<<<I followed (I think) the tunning procedure suggested by the authors which 
consisted in keeping the driven element connected and using a vertical 
polarized signal from the back (700 mts in my case) tune the reflector to 
max attenuation, then the director for max attenuation and iterate both 
until maximum attenuation from the back is reached. That procedure is 
suppose to give a f/b centered in your chosen fcy (thats is what I got, more 
than 20db across 25Kc) and max gain >3db from 1800 to almost 1900. >>>>

There are several compromising adjustments in this antenna. There are:

1.) Effective spacing. Maximum effective spacing is with the maximum 
vertical current as far away as possible from other elements. This array is 
**always** spaced closer than distance out because the wire slopes. The 
slope angle is one compromise.

2.) Ground loss. This is a sloped vertical with single ground radial. This 
makes it ground dependent, and there is no imagined theory that can rid us 
of that. Radials have to radiate in  the near field around the radial, or 
they cannot be counterpoises. That means they always couple to earth, no 
matter what the design. What makes the vertical "better" makes the radial 
loss worse.

3.) You can get into poor tuning situations when you just optimize for back 
null. The reason for this is you can have poor current ratios in the 
parasitic elements. The ideal ratio would be 1:2:1 for gain and F/B. I've 
seen arrays be far from that and have good back null, and gain like two 
elements. I've always found it best to tune first for maximum forward gain 
back and forth several times, and then do a minimal change for peak back 
null (if that is the goal).

4.) All of this is worse because of the spacing and counterpoise compromise. 
The slope and single radial add more variables. So does the thin long wire, 
which can add element loss while decreasing bandwidth.

5.) Current positioning in the element will affect gain, because it affects 
losses in multiple ways. It makes the effective spacing closer or further, 
and it changes ground losses.

You could have any number of problems. I generally do not like unpredictable 
systems with many things that interact, but unless you want to add radials 
and make it less critical as a traditional sloped element array, you could 
try a different tuning method. I would tune for maximum forward gain first, 
going back and forth several times. Then, if F/B is important, I would tune 
a minimal amount from both elements for maximum back null.

Maximum F/B in a two step tuning method does not guarantee equal or 
near-equal parasitic currents, and the ideal 1:2:1 ratio. (Since the 
elements are sloped and have compromised grounds, it will not really be 
1:2:1 but it should be close.)

I also have doubt you can effectively tune the parasitic by resonance with 
the driven element or other elements functional. That makes no sense at all 
to me. Given the variability in effective spacing and driven element 
characteristics, I think every system will be different for resonance 
tuning. A real parasitic reflector, for example, has close to optimum gain 
and F/B when close to exact self-resonance. But that tuning requires no 
other elements be present. Mutual coupling changes resonance we see, so the 
other element characteristics change what we perceive as the element tuning 
through mutual coupling.

You have probably ruined the simplicity and beauty by actually measuring 
gain.   :-)

73 Tom